GroupDIY

General Discussions => The Lab => Topic started by: clintrubber on May 17, 2008, 01:21:36 PM

Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 17, 2008, 01:21:36 PM
*** PLEASE HAVE A LOOK AT THE INFO IN BLUE BELOW ***


Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy
( http://www.groupdiy.com/index.php?topic=27172 (http://www.groupdiy.com/index.php?topic=27172) ) once the stuff has arrived.
Many months to go, but before we forget...

So let's try to combine the ACMP-preamp-related stuff here;
I guess we don't need another case of a thousand different threads about one and the same compreSSLor....  :wink:

In case a new 'wild' thread pops up we might remember this one and redirect the discussion to here.

Idea ?


And I propose likewise for the ACM-mics:

http://www.groupdiy.com/index.php?topic=27793.0 (http://www.groupdiy.com/index.php?topic=27793.0)


(http://img.photobucket.com/albums/v172/gyraf/797_Audio.jpg)






*** SUMMARY IN PROGRESS ***

This section in blue is meant to grow into a quick & dirty but convenient alternative to having to read
through all pages of this thread, nothing more.
While it'll never become as complete and make you as smart as when reading & digesting
the full thread, it'll have at least the function of getting your preamps in decent shape.
Enjoy !

Thanks go out to OKGB who collected the first batch of info.

Please all contribute/correct/comment ! Add your info at the end of this thread
& I'll try to keep the info in this first message up to date

(so check the last page of this thread as well to see new additions).

Please add your contributions in a ready-to-paste format. Thanks !


Note that not all preamp-types suffer from these and some to varying degrees,
some fixes are relative. Preamps are designated by 73/81/84, indicated where relevant.
Again, for more info you WILL have to dig through the thread,
NOT add to it with questions that have been answered already IN the thread.


Let's have a disclaimer:
All mods at your own risk, please work safely & take care when testing & modding your
preamp while live power needs to be present, etc etc...


Here we go:

Most obvious & serious problems of the chineve preamp/eqs concern the EQ-disturbances &
the loud pop on the gain-switch.

Some of these may be accumulative for some people with no one fix taking care
of everything, although the EQ transistor & shielding go a long way, maybe enough.

Do also realize that when in doubt/curious...., you can have a look at the original N-schematics.
Note that the 81 happens to be a 'lucky Frankenstein' in that it uses the class-A stage of the 73/84,
so the original N-schematics don't completely describe the 81-clone.  




<< EQ-NOISE >>

#001
On the 81 units incorrect transistor pairs are on each EQ pcb,
marked as Q4  & Q5 (the output transistors of each of the four EQ-amps).  
This will solve the most obvious noise problems
(possibly revealing the other problems after that)
Suitable replacements are  BC xxx  
I used  2N2907 & 2N2222 respectively,  others transistors are possible.  
Be sure to check pinout is correct for transistors you are using  (some types need legs to be bent).
In case of metal-can types take care you don't make shorts with other metal stuff nearby.

#002
Lossen and twist the torriod power transformer (while listening to the audio out of unit),
until you find the point of least radiation from xfmr onto EQ pcbs. Some have flipped the xfmr
upside down , once this is as good as possible move onto next fixes ,
(may or may not help, varying reports). Again, work safely, don't get zapped.

#003
Shielding of power transformer with some kind of Mu metal, as well as shielding of inductors
on EQ boards, no common method up to you really.

#004
Replacing inductors, see Alex's posts & investigation  [ insert link ] suitable replacements from
Cinemag & Carnhill. All other fixes should be performed first to possibly save you money.

#005
81: Hiss on EQ: replace BC557 transistors X [insert designation] on all four EQ-amp pcbs with lower noise type
(BC2*** or BC560C)


<< PREAMP-CLICKS >>

#101
Popping on gain control between line & mic settings, a couple fixes but quickest & easiest
is to lose one position by lifting resister X [insert designation], see full thread for other methods.



<< SAFETY >>

#201
Check the fuse-rating and adjust when needed (according to your local mains-voltage).



<< SIGNAL-TRANSFORMERS >>

#901
As reported (thanks Roddy), replacing the signal transformers by more expensive types won't improve as much as one
would hope/expect. Unless the stock ones in your unit happen to be faulty/ringing, they're decent.





      [ first bunch of info from Greg, edited & added stuff, last update: 2010-06-04 ]




For now, let's compile questions & things we're wondering:


What is it:

* sound ?

* can we have a look at the three final schematics upon which these units were actually built ?
(especially for the hybrid-81: does it indeed have the cl-A output ?)


Mod-candidates ?

* quality of iron ?

* quality of caps ?

* quality of switches ?



etc...



Cheers,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 19, 2008, 09:24:29 AM
With the payment-deadline done I guess Chance & Randy deserve a well deserved rest for the upcoming months....  :thumb:

With those payment-arrangements now out of the way & before everybody goes back to doing his/her own things, it now may a good moment to try to sort out the remaining open questions as much as possible.

I'd appreciate it if for instance the tech-behind the scenes would chime in with more info about what's actually under the hood of the three preamps. Could the used schematics perhaps be shared ? Or could some questions be passed on to him ? etc...

Thanks,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 19, 2008, 10:20:57 AM
Are there schematics  ? or is this a " refined " design offered by the
manufacturer . Does any other company put out the same three preamps ?
 I would think at least better photo could be offered .

 Likely any of the compenents could be replaced wit higher quality parts ,
but which would make the most improvment ?
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 19, 2008, 10:44:11 AM
Quote from: "okgb"
Are there schematics  ? or is this a " refined " design offered by the
manufacturer . Does any other company put out the same three preamps ?
 would think at least better photo could be offered .

The 'picture' is not fully clear to me (in both ways  :wink: ); as we understood they're based on what an US-tech near Chance prescribed, but from the info from Jakob one would conclude they're a Chinese initiative.
If it's the former one would hope/think schematics are available and could be shared (since the stuff it's all based upon is public, and the mods/approach won't be rocket-science).  

Quote
Likely any of the compenents could be replaced wit higher quality parts ,
but which would make the most improvment ?

Apart from the obvious replacement of the lockwashers (http://www.groupdiy.com/index.php?topic=27806) I guess the TXs... but that's an too-easy guess... Those boxes first need to be heard as is of course, and at least for me the aim is not that they should sound like 'them'; I just want a nice fat sounding preamp.

At the moment I'm actually the most interested in what's exactly in there (especially since I went for the '81, as probably displayed above: most liklely this one will deviate the most from...).

Regards,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 19, 2008, 10:50:52 AM
lockwashers , polarity & power up , on or off ?

Yeah a schematic would go a long way to getting prepared ,
of course then one would listen with their eyes , but i think many
people here are experienced enough to know
, not to mention the several chips that seem to be in there .

And with the amount of money spent , you'd think chance could get the
 schematic or better pictures
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on May 19, 2008, 01:36:41 PM
Like I said in the blk mkt "The xfrmrs will not be China made". I have a HUNCH that they're US made. I am submitting the order tomorrow (Tuesday) I'll get/post the schems as soon as I can.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 19, 2008, 02:35:06 PM
Quote from: "CHANCE"
Like I said in the blk mkt "The xfrmrs will not be China made". I have a HUNCH that they're US made. I am submitting the order tomorrow (Tuesday) I'll get/post the schems as soon as I can.

Thanks Chance for chiming in, much appreciated  :thumb:
Cool, being able to have a look at the schematics & discuss them would make the picture complete.

Regards,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 19, 2008, 02:59:52 PM
Wonder if they'll be the Altran ones talked about here...

Chance, any 'chance' you could get your tech to comment on the gain stages in the various pres?

Thanks
-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on May 19, 2008, 03:56:05 PM
Me and someone else have left voicemails, and it's been 2 weeks and he hasn't responded. He did however pay for his pre amps
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 19, 2008, 07:54:16 PM
seems to be progress , thanks
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 22, 2008, 12:54:19 PM
Just an FYI.

This is the chosen colour I believe...
(http://www.spottedmuse.com/pres/orange.jpg)
-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: hejsan on May 22, 2008, 01:29:47 PM
I wish it was red with wite lettering like the pico 500..
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 22, 2008, 04:06:14 PM
the Orange , may say 70's but it wouldn't be in my first 10 choices
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 22, 2008, 07:11:07 PM
Wouldn't be my first one either, but as long as it hasn't become that Heino-style white-with-red-legending I'm fine  :cool:

Tom, where did you got the 'chose-one' info from ?

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Jim50hertz on May 23, 2008, 03:48:09 AM
Quote from: "okgb"
the Orange , may say 70's but it wouldn't be in my first 10 choices


Yes, like brown cordruroy curtains and flock wallpaper but hold on, is that not rust? or terracota? or perhaps burnt earth? :wink:
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 23, 2008, 05:28:55 AM
Agreed, orange not my fav.

I believe that is a photoshopped example, and we all know anodising is a very unpredicatable process.. so it may look different.

Red, White or Teal would have been my preferred choices.

Peter, I read the news over at PSW.
-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on May 23, 2008, 08:24:21 AM
That color shown is not the color. The color sample I sent to them was very dark bronze.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 23, 2008, 08:26:19 AM
Any word on schematics or pix ?
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on May 23, 2008, 11:27:59 AM
I have a long list of potential mods and fixes.  And the last one, on the bottom of the list, is sell it!  We'll see how far down the list we will have to go.

:grin:
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 23, 2008, 12:10:09 PM
Quote from: "TomWaterman"
Peter, I read the news over at PSW.
-T

Thanks Tom, I should take a look over there from time to time.


Quote from: "tommypiper"
I have a long list of potential mods and fixes.

Cool  :thumb:

As it happens I heard of a nice place (http://www.groupdiy.com/index.php?topic=27791) to post that list  :wink:

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 23, 2008, 01:11:30 PM
Quote from: "CHANCE"
That color shown is not the color. The color sample I sent to them was very dark bronze.


OK cool. I'm sure I read it was going to be the burnt orange, but as the man himself has said - dark bronze it is.

Should look nice. Thanks Chance.

-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on May 23, 2008, 02:58:22 PM
Actually I still haven't recieved an "OK" from them on the color, but don't worry it won't be pink.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: hejsan on May 24, 2008, 09:30:36 AM
Red is the warmest color, leading to the warmest perceived sound you know.. :green:

Check this out:
http://www.groupdiy.com/index.php?topic=27790

That's one of the prettiest frontplates I've seen round here..
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 24, 2008, 03:56:15 PM
Quote from: "hejsan"
Red is the warmest color, leading to the warmest perceived sound you know.. :green:

Nice try  :wink:  :thumb:
But I do like the colour combination of the Brent A.-neighbour.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 24, 2008, 06:11:31 PM
yeah i don't think it will change now , i didn't
vote for leaving to fate either
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 26, 2008, 08:36:56 AM
Hey Chance , any update ?
Title: [ACMP investiupgradifications] All things PREAMP
Post by: gyraf on May 27, 2008, 03:41:06 AM
Quote from: "CHANCE"
Like I said in the blk mkt "The xfrmrs will not be China made". I have a HUNCH that they're US made. I am submitting the order tomorrow (Tuesday) I'll get/post the schems as soon as I can.


Then this is probably not the unit I saw (and took photo above) at the Frankfurt Music Show. Those units were explicitly claimed to be all-chinese parts by the vendor, 797Audio.

Jakob E.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 27, 2008, 05:41:36 AM
W.r.t. "The xfrmrs will not be China made", dunno, isn't it about the used materials and designs...? As long as they're well-made by highly motivated and skilled workers I'm fine with any place of manufacturing :wink:

I understand 'Made with pride in Bobbejaanland/Legoland/the US or wherever' sounds better than 'with a few Chinese junk-TXs thrown in', but OK, let's not let that be the single criterion.


And thanks Jakob for that pic you made available, it has given lots of info and wil have helped people with deciding on what to order
  :thumb:

... or it will have given us 'false' info and we all end up with boxes with uA741-gainstages  :twisted:  :cool:  :grin:  :wink:

Looking forward to using these & I expect we'll have some nice fun ahead when testing & modding & tweaking.

Best regards,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 27, 2008, 08:23:49 AM
honestly i don't expect xfmrs would get shipped in from anywhere ,
 that would up the cost and hurt the profit but after a few generations
you hope they have gotten better in whatever design they are using ,
refining everytime another order is made .
who knows maybe in the future , the Chinese vintage will be the thing
 in mid-FI sound , getting back to the 2000's !
 Chance , do you have any solid info to say that the transformers have
some american connection ?
 Again at the least i would think this may be a " stock " design , hopefully
at current " state of the Art  " for china .

I agree gyraf's pic was encouraging , hopefully at least that quality if not
better , still no pic or schematic from Chance ......................
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on May 29, 2008, 05:01:37 AM
These pics surfaced on GS. Pictures taken by Alan Hyatt.
Unfortunately the resolution on the schematic is real low. But I guess it's better than nothing.

(http://img.photobucket.com/albums/v646/red_3000/how_to/ppa_797EQ2028229.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00969.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00970.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00971.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00972.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00973.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00974.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00975.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00976.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00977.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00978.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00979.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00980.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00981.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00982.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00983.jpg)
(http://img.photobucket.com/albums/v646/red_3000/how_to/DSC00984.jpg)
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 29, 2008, 06:26:33 AM
Quote from: "tarnationsauce"

(http://img.photobucket.com/albums/v646/red_3000/how_to/ppa_797EQ2028229.jpg)


Good find. Now, let's see if we can get a better copy of the schematic and check for any issues.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: gareth.h.rees on May 29, 2008, 07:34:21 AM
Its worth having a look at everying going on over at gearslutz and homerecording - Its not confirmed in any way that these are the same units.

..Not that I'd be disappointed if they are!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 29, 2008, 08:34:54 AM
yeah someone is trying to stir things up ,
thanks for the schematic , the effort is appreciated
even if it may not be the same , interesting none the less
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on May 29, 2008, 08:40:25 AM
Quote from: "rodabod"
Good find. Now, let's see if we can get a better copy of the schematic and check for any issues.

It is possible those schematics are of a different revision and/or different alltogether as Gareth said.
Though Alan PM'd me on GS and said he has all the schematics but the file sizes are too big to upload to the forum.
I asked him if he could email them to me so I can host them so others can see.

I really hope Alan pulls through, he's been catching a lot of flack (see GS / HomeRecording for all the drama).
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 29, 2008, 12:19:50 PM
I'll be surprised if these are any different from the 797 design. I did wonder why Chance never managed to provide any documentation regarding the schematics.

However, this does not really bother me at all. 797 Audio appear to be one of the better Chinese manufacturers.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on May 30, 2008, 02:29:50 PM
PM´d you

Matti
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 30, 2008, 03:01:58 PM
Courtesy of Alan Hyatt and Matti, here is the '73 schematic from 797 which we believe is the only one we have available.

I'd just like to say, can we please leave the arguing over at Gearslutz and Homerecording? We don't need that here.

JPEG:  http://img407.imageshack.us/img407/517/scem73gd2.jpg

PNG:   http://img294.imageshack.us/img294/1774/scem73wv9.png

Cheers,

Roddy
Title: [ACMP investiupgradifications] All things PREAMP
Post by: sintech on May 30, 2008, 05:12:42 PM
That's a pretty encouraging looking schematic :thumb: thanks guys
Title: [ACMP investiupgradifications] All things PREAMP
Post by: playboss on May 30, 2008, 05:44:48 PM
so what about this chamlab iron? I think I put the mic input one into the 33609 redo as interstage,lol. The switch assemblies look like the standard cheap stuff you can pick up at every diysite for ladder attenuators.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 30, 2008, 07:56:39 PM
Not much time to really look at the schemos, but first thing I noticed:

Whats up with the excessive damping capacitance on the mic trafo secondary. Standard 10468 requires 180pF not 1000pF.

Wonder if the chinese trafos have substantial ringing and overshoot?

Line input and line output values look right or ballpark.

As said previously the switches look like Chiefdom, which are used by Joe Malone... maybe not as good as an Elma or Grayhill but likely better than a Lorlin.

-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on May 30, 2008, 09:34:13 PM
There was a picture of Chameleon Labs trafos here with the same
1000pF on the input one

Matti
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 30, 2008, 09:47:59 PM
Yeah I have that pic too. It's obviouly not a schematic error... so I'd guess these trafos needed controlling more so than the originals.

One place to investigate when they arrive...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Svart on May 31, 2008, 12:47:02 AM
another interesting thing is that the pic of the *possible* preamp that Gyraf found shows IC opamps while the schematic on this thread shows discretes..

I wonder which one of these designs these preamps will be.

Kinda makes me wish I had ordered some..
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on May 31, 2008, 04:01:40 AM
This schematic doesn´t show any auxiliary circuits like
power supply or metering & leds ( for wich the opamps are used here ),
so I´d say these are the same 797 preamps, 81 in Gyraf´s pic.

Matti
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 31, 2008, 09:42:55 AM
Quote from: "TomWaterman"

Whats up with the excessive damping capacitance on the mic trafo secondary. Standard 10468 requires 180pF not 1000pF.

Wonder if the chinese trafos have substantial ringing and overshoot?


Probably. When we get them, I can test them against the Carnhill equivalents and also see if they've got the best values chosen there. Also, I wasn't sure if the original transformers would have a 12K strapped across the secondary too.

I'm not going to get in the mode where I decide everything is likely to be rubbish and decide what's going to be changed already. I'll test out what's there and see if anything is really holding it back. Just wait until people start discussing coupling caps...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Baltimore on May 31, 2008, 09:51:54 AM
well, if these are similar to the chameleon labs stuff, and it looks like they are, they should sound pretty f'in good right out of the box.  i've used the CL stuff in a few different rooms and can't complain...and i thought that stuff sounded excellent for the price...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Svart on May 31, 2008, 10:50:30 AM
Quote
This schematic doesn´t show any auxiliary circuits like
power supply or metering & leds ( for wich the opamps are used here ),
so I´d say these are the same 797 preamps, 81 in Gyraf´s pic.


Look at the cards, there are opamps on each board:

(http://img.photobucket.com/albums/v172/gyraf/797_Audio.jpg)
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on May 31, 2008, 10:53:18 AM
Yes, there are but not in the AUDIO circuit

Cheers

Matti
Title: [ACMP investiupgradifications] All things PREAMP
Post by: ioaudio on May 31, 2008, 10:54:23 AM
overload/clip led for each stage. transistors in the back.

funny way of marking the caps in the schematics.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 31, 2008, 12:34:06 PM
From looking at the schematic, I thought they might have been a bit more subtle with their Neve-inspired preamp... Still, I'd rather have that than anything else I suppose.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on May 31, 2008, 12:41:25 PM
As said there are enough transistors in the photo to form the discrete amp stages and there are so many clip leds on this device. God knows why but that is what the IC's are.

I agree with Roddy, there's not much point deciding to rip them apart before they get here. Chameleons do sound really good, so I don't think these will sound bad at all.

Roddy mentioned coupling caps... I don't see many tantalums but must listen first before judging.

I'll try and organise a shoot out with some real Neve on a variety of sources when they get here. Maybe piano, drums, ac guitar and vocal.

-T
Title: [ACMP investiupgradifications] All things PREAMP
Post by: gareth.h.rees on May 31, 2008, 01:41:19 PM
Quote from: "TomWaterman"

I'll try and organise a shoot out with some real Neve on a variety of sources when they get here. Maybe piano, drums, ac guitar and vocal.


That would be amazing! Except it may make me wish I ordered more!

Cheers
Gareth
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Jim50hertz on June 01, 2008, 06:55:39 AM
Quote from: "TomWaterman"
I'll try and organise a shoot out with some real Neve on a variety of sources when they get here. Maybe piano, drums, ac guitar and vocal.

-T


+1 on that.  Tom, that's a fine offer and I think would be very useful to the folks who've bought these.  Many thanks :thumb:

Cheers

Jim
Title: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 01, 2008, 04:39:53 PM
Looks like 3Q3 got left hanging. Doesn't inspire a lot of confidence in the schematic.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: EEMO1 on June 02, 2008, 03:54:05 AM
Quote from: "rodabod"
I'd just like to say, can we please leave the arguing over at Gearslutz and Homerecording? We don't need that here.



 2nd that.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: playboss on June 02, 2008, 10:48:53 AM
2nd what?
anyway, tought I post this
http://tinyurl.com/46dfn7

 :green:
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 03, 2008, 07:12:52 AM
2nd , not turning this into a *****fest ............which i 3rd
how may i help
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on June 06, 2008, 09:41:09 AM
Been away for a week, amazed a bit by all the action here (and indeed lame to see all those other discussions 'elsewhere').

Cool the schematics popped up, thanks  :thumb:
I guess it'd be safe to say now that the output-stage of the ACMP-81 is identical to the -73 & -84 (so class-A, '3055, gapped TX bla bla bla) or what do you guys think ?

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on June 06, 2008, 09:55:26 AM
Quote from: "clintrubber"

I guess it'd be safe to say now that the output-stage of the ACMP-81 is identical to the -73 & -84 (so class-A, '3055, gapped TX bla bla bla) or what do you guys think ?


I reckon this is probably likely.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: W DeMarco on June 06, 2008, 12:19:45 PM
edit/delete

sorry read the original thread Im up to speed now
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on June 16, 2008, 03:51:01 PM
This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though. In fact, it's awful. I think it's fairly obvious what's going on though (despite the number of people who seem to think otherwise).
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on June 16, 2008, 04:36:39 PM
Quote from: "rodabod"
This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though. In fact, it's awful. I think it's fairly obvious what's going on though (despite the number of people who seem to think otherwise).

Thanks for monitoring.
Looks like a few people over there decided to fill the time until the gear arrives with filling page after page over there with endless non-conversations...  :cry:

I wouldn't mind if AH posted all he knew - let's be informed.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Jim50hertz on June 16, 2008, 07:16:08 PM
Quote from: "rodabod"
This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though.


Roddy, Aye.

Quote from: "clintrubber"
[I wouldn't mind if AH posted all he knew - let's be informed.


Clintrubber, I'm with you on that, let's have it...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on June 16, 2008, 11:40:16 PM
Well, it seems AH feels he designed the whole thing.  And he's not saying it directly, but seems he's 1) slighty pissed off that 797 is selling it now and, 2) peeved that Chance was either misled by 797 or purposely misled buyers into believing it was his (Chance's tech's) design, or both.  

We've seen the schem.  If we get the same thing that's in the schem, then it's mostly going to be a matter of component quality, not design quality.  AH has made noises in this direction several times by mentioning inductor resonance problems, that the iron is Chinese-made, and other general comments.

I expected this.  When I saw the open hood photo, I could see it was more or less a N**ve, and probably closer rather than farther from the real thing.  And I posted that several times in the Black Market thread.  And I predicted there would be other questions like layout, details, components, etc could potentially be trouble.  We'll see.  If it's a good foundation with not too much to replace or change I'll be more than pleased.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on June 17, 2008, 04:08:48 AM
Quote from: "clintrubber"
I wouldn't mind if AH posted all he knew - let's be informed.

BTW, let me add to that that with this I don't mean to join either 'side'- if there's a matter of choosing sides anyway, which I don't think so or at least don't want to get into, just appreciating the efforts of Chance et al. - whatever confusion has popped up or was made to exist.

Quote from: "tommypiper"
Well, it seems AH feels he designed the whole thing.
The obvious 'how about RN then?' could be added here, but I could imagine he (AH) has at least spent considerable effort on preparing it for the present way of implementing etc.


Anyone perhaps spotted schematics yet of the two other types ? ('*1, '*4)


Cheers everyone,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 17, 2008, 07:43:50 AM
i'm surprized that no one has mentioned the obvious , and not to flame
 this cause bottom line no one expects that this is not a chinese preamp .
But if there was a prototype who wouldn't have taken pics? other than
what looks like a b&w pic of a brochere ?
 Not sure if AH justs wants someone to say he is right or why he persists
or why the two haven't talked . neither seem 100% open but sticking to
the story , other than technical i won't post more of this cause that would
be helping someone else

 If he cared , he could cough up the schematics , Al that would be nice
thank you .
He does offer one possibility that the inductors may need to be changed ,
hope they used a common value
Title: [ACMP investiupgradifications] All things PREAMP
Post by: W DeMarco on June 17, 2008, 09:32:18 AM
Wow, Thats some thread of BS.  Glad that, for some reason, group DIY is so calm!  Im so excited to get my 81 that I can hardly contain myself, let not to mention my ribbon.  And unless Rupert designed it I really don't care who did or didnt.  Good work on the GB Chance.  I hope all the bullsh*t floatin around over at that other forum doesnt affect you permanently.  

Onto the TRANX.  Anyone found some good iron to swap yet?
Title: [ACMP investiupgradifications] All things PREAMP
Post by: reg presley on June 17, 2008, 10:02:29 AM
Quote from: "rodabod"
This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though. In fact, it's awful. I think it's fairly obvious what's going on though (despite the number of people who seem to think otherwise).


!!!!!ouch!!!!! makes for a painful read.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on June 17, 2008, 11:35:24 AM
Yes, really unpleasant over there.  I'm not following the politics or care much who is right or wrong.  Just waiting to work with the box when it arrives.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on June 24, 2008, 09:37:12 AM
I was just checking out "Tamura" xfrmrs ( http://www.eifl.co.jp/index/export/tamurat/tamura.htm ) that Scott Dorsey has used in the C-3000 mic. While I was there I was also checking out their 6072 tubes. 5000 for one! I hope that is yen. When I was there in the 70's it was 360 Y = $1.00. Anyone know todays echg rate?
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on June 24, 2008, 09:49:35 AM
Chance, the class-a output will require a "gapped" output transformer. Not sure if any of those Tamuras are.

Either way, there's nothing to say that the stock transformers won't be any good.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on August 24, 2008, 01:59:03 PM
for all interested heres the owners manual. We left in the part from China for laughs (Close your eyes etc)

http://www.compasspointstudios.com/TNC%20ACMP%20Manual.pdf
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:00:43 PM
Let's proceed here in this thread:

Quote from: "aortizjr"
Here are some ACMP-81 gut shots:

http://homerecording.com/bbs/showpost.php?p=3023296&postcount=5808


Note that below I've copied a few tech-posts from the Black Market to keep the tech-discussion complete (since the inner-'81-pics above).

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:02:26 PM
Quote from: "ChuckD"
Maybe some others can put there 2 cents in after me on these photos. Especially those ,like me, would built the 1081 circuit already.

I can say that yes this is a 1081! At least a cheaper version of a very very simular circuit. Here are the main differences just after glancing at the photos

1. using discrete ... well newer 5532 type opamps in the EQ section
2. input and output transfomers are China made.
3. different inductors like the transformers above.
4. big change to the output as it looks like the Class A output of the 1073. Much like Vintech does in there clone.

If you changed "all" of the above to actual 1081 specs especially the output to a  class A/B  B340 opamp. Then you would have something really close I think!  <= my opinion...  

The input transformers would have to be relocated but everything else could essentially "drop in"

Damn I should have gotten one. Oh well I have some nice mics coming instead to take apart.


-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:02:54 PM
Quote from: "wmtunate"
Quote from: "ChuckD"

1. using discrete ... well newer 5532 type opamps in the EQ section


Based on the schematic that was posted here a few months back of the 73 (I think it was), the ICs are only used to drive the LEDs, and aren't part of the audio path.  That's probably the case in the 81 as well.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:03:22 PM
Quote from: "wmtunate"
Quote from: "ChuckD"

1. using discrete ... well newer 5532 type opamps in the EQ section


Based on the schematic that was posted here a few months back of the 73 (I think it was), the ICs are only used to drive the LEDs, and aren't part of the audio path.  That's probably the case in the 81 as well.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:03:41 PM
Quote from: "ChuckD"
I see now the opamps of the EQ section are fixed on the board like the 1073 opamp stages are done. Then no unless you can bypass these and replace with the correct  B338's for each then it will never be quite the same.


-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:04:03 PM
Quote from: "clintrubber"
Still a lot of discrete transistors on those pics though, at least 4 on each 'coil-PCB'... don't have the info here, how many of these should a real '81 have ?

But nice I'd say, a sweaty class-A gapped-TX outputstage for the China'81, we like that, right ?  :thumb:
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:04:24 PM
Quote from: "clintrubber"
Quote from: "aortizjr"
Here are some ACMP-81 gut shots:

http://homerecording.com/bbs/showpost.php?p=3023296&postcount=5808


Thanks for spotting that they're there  :thumb:
And if you're reading this, thanks drBill !  :thumb:  :thumb:



But now this: see the four letter word there on the left  :oops:     ROHS.....
 
http://img.photobucket.com/albums/v248/drBills/Group%20Buy%20Gear/148-4829_IMG.jpg

Do we need to take that into account for eventual mods ?
Simply ignore ?

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 17, 2008, 09:04:59 PM
Quote from: "ChuckD"
The 1081 uses a simple RLC "tank" circuit scheme for it's EQ. Switching between the caps and the resitors of the tank. The B338 at the top of the vertical strip making the tank give you the boost for that selected filter.

A good comparison to start would be simply to count the caps for each tank. The opamps are  different then the B338. How that effects tone well ... opamps are strange that way.

-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 18, 2008, 03:59:54 PM
Hi Peter,

I'm looking into this just now, and trying to do some investigative work with what information we have so far. I will report back if I can figure anything out from the pictures, although my Neve knowledge is limited!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: plumsolly on October 18, 2008, 04:34:31 PM
Quote from: "reg presley"
Quote from: "rodabod"
This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though. In fact, it's awful. I think it's fairly obvious what's going on though (despite the number of people who seem to think otherwise).


!!!!!ouch!!!!! makes for a painful read.



http://www.theonion.com/content/news/local_idiot_to_post_comment_on

-Ben
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 18, 2008, 05:57:51 PM
Quote from: "plumsolly"

http://www.theonion.com/content/news/local_idiot_to_post_comment_on


I love the Onion. So true.


Well, here are my (not particularly valuable) thoughts so far.

Having compared the Chinese '73 clone schematic from Alan Hyatt, it appears to be a direct copy from what I can see from a few minutes of comparing.

The '81 appears to have adopted the class-A output section from the BA283. I'm fine with that. Good.

Now to the rest of the circuit, it appears that they have not used the same component numbering for the '81 as they have with the '73 schematic. So thia does not help us here. I'm not sure about this suggestion of the ICs seen on the board being used as replacements for the BA338 and other amps. You can see that the transistors are laid out the same on each separate EQ board. In total, I count the correct number of transistors for each opamp in the original circuit.

What I'd like to know if anyone else knows off the top of their head is, what is the purpose of the two BA306 opamps in the original schematic (http://audio.kubarth.com/ten81/1081bd2.jpg)?

There is a twelve transistor arrangment on the low-pass/high pass board in the Chinese '81 unit, and I'm wondering if these could possibly account for two BA306 circuits.

Right, that's enough mad speculation for the moment!

Roddy
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 20, 2008, 06:52:00 AM
Quote from: "rodabod"
What I'd like to know if anyone else knows off the top of their head is, what is the purpose of the two BA306 opamps in the original schematic (http://audio.kubarth.com/ten81/1081bd2.jpg)?

There is a twelve transistor arrangment on the low-pass/high pass board in the Chinese '81 unit, and I'm wondering if these could possibly account for two BA306 circuits.

Hi,
Little bit of RE for now, I checked the BA306-schematic (it's for instance in the 1081 manual here (http://www.ams-neve.com/Sites/8/Files/Documents/Outboard/1081_User_Manual_Issue3.pdf)) and twelve BJTs is exactly right for the hi/lo-pass-section  :thumb: :

each BA306 is a dual unity-gain buffer: 2 BJTs for a dual-output current source and 2*2 BJTs forming the actual buffer-stage (Sziklai-topol.)
So four buffers in total.

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 22, 2008, 06:59:53 PM
Not quite enough detail, but here is the '81 schematic courtesy of Chance, and enlarged, flipped and sharpened to get a little more detail:

http://www.sendspace.com/file/1iiv6n

I'm going to compare in a minute. Looks promising.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: ChuckD on October 22, 2008, 07:46:23 PM
Nope still can't make out any values in this one either.

-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 22, 2008, 09:39:42 PM
Guys, there seems to be a noise issue with the '81 preamps:

http://recforums.prosoundweb.com/index.php/m/380587/13216/#msg_380586

Can anyone offer any help? Perhaps Chance could ship to a buyer who has some background with 1081 channel strips. I'd be happy to look over one myself, but there are many people here who have built and serviced N*eve gear who would be more efficient.

Roddy
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 23, 2008, 04:45:50 AM
Quote from: "rodabod"
http://recforums.prosoundweb.com/index.php/m/380587/13216/#msg_380586

Hmm, curious to this but the link won't give anything else than a blank page... problem at my side ?
Either way, thanks for spotting Roddy.

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on October 23, 2008, 05:38:36 AM
The link is working again

Matti
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 23, 2008, 06:59:10 AM
Quote from: "MATTI"
The link is working again

Matti

Thanks, works here as well now.

Let's discern between 'noise' (hiss) & 'hum' (AC-induced-nasties), as I understood now it's the latter.

Cheers all,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on October 23, 2008, 01:49:41 PM
it looks like there is room to [ drill a hole ] move the xfmr an inch or so,
although may make no difference , anyone try turning the xfmr ?
 Or as mentioned [ and never a bad idea ] shielding the inductors ?

 we all know what they cost and where they came from , at worst
could convert to external xfmr / supplies
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on October 23, 2008, 06:56:30 PM
I contacted China and they can't duplicate the buzz we are getting (that figures) They sent me a very detailed schem in a "RAR" file that has many PDF's in it.  I'd be glad to share it with anyone.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: ChuckD on October 23, 2008, 07:03:52 PM
Chance ,

Chuck here, we met on the weekend when I picked up some mics. Please send me the schematics for the 1081 to cdickinson66 (at) hotmail (dot) com
I built a stereo 1081 as I mentioned before as well as a stereo 1073 preamp. Maybe I could have a look at the design in detail.

-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on October 23, 2008, 08:08:29 PM
Quote from: "ChuckD"
Chance ,

Chuck here, we met on the weekend when I picked up some mics. Please send me the schematics for the 1081 to cdickinson66 (at) hotmail (dot) com
I built a stereo 1081 as I mentioned before as well as a stereo 1073 preamp. Maybe I could have a look at the design in detail.

-Chuck


Thanks Chuck. I'll send it now


I'm going to the wearhouse tomorrow
If anyone is POSITIVELY coming over on Sunday for your gear PM me by noon Friday and I'll bring your gear back with me.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 23, 2008, 08:35:53 PM
Here are the schematics which I have converted to JPEG:

"Amp" (http://img206.imageshack.us/img206/1286/811hr8.jpg)

"Filter" (http://img206.imageshack.us/img206/6905/812iv0.jpg)

"Out Power" (http://img206.imageshack.us/img206/6717/813bx2.jpg)

"Upper" (http://img513.imageshack.us/img513/8759/814ll0.jpg)

"Bass" (http://img259.imageshack.us/img259/5106/815aj7.jpg)

"Low" (http://img259.imageshack.us/img259/1953/816ez5.jpg)

"Treble" (http://img259.imageshack.us/img259/5557/817xe7.jpg)
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on October 23, 2008, 08:39:31 PM
Roddy, thanks.  But I can barely read the values at that scan size, at least on my screen.  I've asked Chuck for a copy of the PDFs.  In meantime, any chance you can post them with higher line screen?

cheers.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on October 23, 2008, 08:45:30 PM
Ah. I've got a large screen so overlooked that. I could double the resolution if that helps.

Roddy
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Svart on October 23, 2008, 09:02:30 PM
hum can be caused by a number of reasons, mainly poor decoupling and/or poor bulk capacitance.  It can also be caused by poor grounding or large currents flowing over ground.

If anyone wants to donate a preamp to me, I'll figure out what's wrong..

 :green:
Title: [ACMP investiupgradifications] All things PREAMP
Post by: ChuckD on October 23, 2008, 09:59:17 PM
Looking at the schematics now. Not much time spent on them yet.

But I can say ... The EQ section is a close 1081 EQ copy.


They have copied the B338 correctly it seems however they are using BC557 and BC547 for the constant current emitter follower output stage. Also they are using 1N4148 diodes instead of the correct ones BAX13 and the AA144

I don't like all that LED junk in there! 50% of the components are to support the different LEDs. You don't need that crap in there. Plus it is tapping off the output signal in each stage I think noise can get injected there easy.

Has anyone gotten the center frequency of the hum? 120 or 60 maybe?


More to come later

-Chuck
Title: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on October 23, 2008, 10:04:09 PM
http://www.sendspace.com/file/osjykd
81
hope this helps

Matti

P.S. didn´t notice these were already in use ;-)
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 24, 2008, 06:06:36 AM
Thanks for those added schematics  :thumb:  Good to see it's close to the original AND this way of drawing is actually more convenient than the original documentation at the same time.

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 24, 2008, 06:15:31 AM
Quote from: "ChuckD"
They have copied the B338 correctly it seems however they are using BC557 and BC547 for the constant current emitter follower output stage. Also they are using 1N4148 diodes instead of the correct ones BAX13 and the AA144

For the current sources I guess it'd be hard to discern between 'original' and 'generic NPN & PNP' here, but it's an easy mod if one wants to 'correct'.

I haven't looked more detailed, but that AA144 should be in reverse during normal operation so can't imagine it'll affect 'sound'.

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 24, 2008, 07:41:50 AM
FWIW: ... I saw the gain-stage for the treble section does have BAX13 diodes though, but likely these will be forgotten diode-properties when converting the original schematics to the ones they were actually going to make...

... and there's of course no guarantee that the schematics & semiconductor-types in the actual units will actually exactly match, but that's true for more brands...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 24, 2008, 08:13:54 AM
What's the deal with the dual +24V supply section ? Obviously spreading the load, but the schematic doesn't specify which supply goes where. No problem though.

W.r.t. how alike it is to the original (as can be with the different output-stage), obviously this one doesn't use tant-caps but 'normal electrolytics'.
If it really matters I dunno, but it might be another mod-candidate.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on October 24, 2008, 08:46:12 AM
I'm getting the detailed schems for thr 73 and 84 too if anyone wants them.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 24, 2008, 08:48:52 AM
Quote from: "CHANCE"
I'm getting the detailed schems for thr 73 and 84 too if anyone wants them.

Sure, that'd be nice  :thumb:

And without wanting to overask, but while at it, if they have the various condenser-mic-schematics available then that wouldn't hurt either  :wink:  

Thanks
Title: [ACMP investiupgradifications] All things PREAMP
Post by: mfdu on October 25, 2008, 05:43:22 PM
so theres been a noise issue with the '81.  any problems reported on the '73's?

chris.mfdu
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 25, 2008, 06:21:36 PM
Quote from: "mfdu"
so theres been a noise issue with the '81.

No!

Let's call it a hum issue.

Or is that just me ?    

noise = hiss,
hum = mains-nasties in the audio

nitpicking, sure  :wink: but it keeps the conversation clear.

Bye,
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on October 25, 2008, 09:00:11 PM
A friend of mine who worked with Paul at Manley Labs said that they once had a simular problem and it was paint insulating the cover to the chassis. These ACMP's have a heavy coat of black wrinkle paint on the cover. I doubt that this is the problem, but you never know. I'll check later.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Svart on October 25, 2008, 09:28:49 PM
Don't be so sure.  I just figured out a huge hum/hiss issue and it turned out to be poor grounding in a computer due to powercoating and lack of good contact between sections..
Title: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on October 25, 2008, 11:39:02 PM
even more strange:

i just figured out some hiss coming from my canon printer! though i use professional balanced cables there's lot of hiss when the cables are somewhere near the printer.

best,
flox
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on October 26, 2008, 08:07:44 AM
Most printers are just a plastic shell.. Theres no shielding in them. I ended up moving mine away from the studio and just print to a network printer.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on October 29, 2008, 10:46:48 AM
With regards to the buzz problem we are having, this is from another board:


"Ok everybody, I'l speak...

First of all the modules sound fantastic. Granted 99.999% of the time I am mixing, so I haven't checked out the preamp portion, but the line amps / EQs are great.

It's interesting that all of the modules use the single ended class A output stage based on the BA283 card ( same as what's inside the N£V£ 1064, 1066, 1073 1272, etc) even the "81".
In the N£V£ 1081 the output is a class A/B push pull design.

The "81" uses a A/B amplifier in the EQ buffer amp cards, so it has a bit more of the 1081 sound.

In all cases the units produce a nice amount of 2nd and 3rd harmonic and the top end sounds sweet.

I ran some minimally miced drums through these and A/B'ed an identical setting on a popular plugin N£V£ emulation.
The sound was quite similar, but the long term impression was that when listening through the TNC..
the room that the drummer was in seemed like it was 20˚ hotter and the drummer sounded like he was smiling.... I know.. Rolling Eyes Ok?

I HIGHLY recommend that you run some pretty hot signals through these when you first get them and exercise every switch and pot. I had one or two that didn't pass signal until it got LOUD. This is a symptom of new switches with anticorrosive coatings on the contacts which are intended to self clean through use, not a problem.

Now to more serious matters. I have the same buzzing in my 81s that a few of you have reported. I disassembled the 81 and ran some tests.

Here are my findings so far:

It does not appear to be caused by power transformer induced EMI.

I ran the unit off an external supply and had the same problem.

The "73" and "84" have their inductors equally close to the power supply as the "81" and have none of these problems.

Re-routing the the audio cables (which go to the output pot and phase switch) away from the PSU made the noise floor higher.

Moving my hand in the proximity of the EQ cards made the noise floor higher.

There is a transistor on each of the EQ amp cards that is running very hot (Q4).

I suspect that there may be ultrasonic oscillations in the EQ amp cards. Every symptom I found and every other reported symptom lead me to this hypothesis.

If this is indeed the case, the solution will be much simpler than if it was power supply related. It will require a few very very small capacitors be tack soldered across a few components. I'll run more tests when I am able. Unfortunately, I don't have a working Oscilloscope here and am in the middle of producing one record and mixing two... plus having major dental work performed this morning.. ugh."
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on October 30, 2008, 09:44:33 PM
Quote from: "CHANCE"
With regards to the buzz problem we are having, this is from another board:

Quote from: "ZMIX"

"Ok everybody, I'l speak...

First of all the modules sound fantastic. Granted 99.999% of the time I am mixing, so I haven't checked out the preamp portion, but the line amps / EQs are great.

It's interesting that all of the modules use the single ended class A output stage based on the BA283 card ( same as what's inside the N£V£ 1064, 1066, 1073 1272, etc) even the "81".
In the N£V£ 1081 the output is a class A/B push pull design.

The "81" uses a A/B amplifier in the EQ buffer amp cards, so it has a bit more of the 1081 sound.

In all cases the units produce a nice amount of 2nd and 3rd harmonic and the top end sounds sweet.

I ran some minimally miced drums through these and A/B'ed an identical setting on a popular plugin N£V£ emulation.
The sound was quite similar, but the long term impression was that when listening through the TNC..
the room that the drummer was in seemed like it was 20˚ hotter and the drummer sounded like he was smiling.... I know.. Rolling Eyes Ok?

I HIGHLY recommend that you run some pretty hot signals through these when you first get them and exercise every switch and pot. I had one or two that didn't pass signal until it got LOUD. This is a symptom of new switches with anticorrosive coatings on the contacts which are intended to self clean through use, not a problem.

Now to more serious matters. I have the same buzzing in my 81s that a few of you have reported. I disassembled the 81 and ran some tests.

Here are my findings so far:

It does not appear to be caused by power transformer induced EMI.

I ran the unit off an external supply and had the same problem.

The "73" and "84" have their inductors equally close to the power supply as the "81" and have none of these problems.

Re-routing the the audio cables (which go to the output pot and phase switch) away from the PSU made the noise floor higher.

Moving my hand in the proximity of the EQ cards made the noise floor higher.

There is a transistor on each of the EQ amp cards that is running very hot (Q4).

I suspect that there may be ultrasonic oscillations in the EQ amp cards. Every symptom I found and every other reported symptom lead me to this hypothesis.

If this is indeed the case, the solution will be much simpler than if it was power supply related. It will require a few very very small capacitors be tack soldered across a few components. I'll run more tests when I am able. Unfortunately, I don't have a working Oscilloscope here and am in the middle of producing one record and mixing two... plus having major dental work performed this morning.. ugh."



That was my post over at the Pro Sound Web...  I've done some more investigating.

The major difference between the 1081 and the TNC81 is that the EQ line amps in the 1081 use a complementary BC461 / BC441 pair and the TNC use a complementary BC547 / BC557 pair .
  The BC441 and BC 461 are rated at 2A Ic and the BC547  / BC 557 are rated at 200ma Ic and they run quite hot.

I've got some BC441 and BC 461s on the way and I will know exactly what's what when I swap them in to the TNC81.


-CZ
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 31, 2008, 08:50:22 AM
Quote from: "zmix"
The BC441 and BC 461 are rated at 2A Ic and the BC547  / BC 557 are rated at 200ma Ic and they run quite hot.


Silly us that we didn't spot that when the schematics popped up... nice you spotted that !  :thumb:
Those BC441 and BC 461 in the Neve-DOA's even use heatsinking for them, won't be without reason... and then we/they expect that a simple plastic TO-92 would do as well...  :?

Should check the pics, but what's your idea so far, is there room on the PCB to do an elegant replacement ? (including at least some small clip-on heatsinks for those TO-39's)  

Regards,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on October 31, 2008, 09:43:49 AM
Here is the board layout for the amp:

http://recforums.prosoundweb.com/index.php/fa/10261/5734

(http://recforums.prosoundweb.com/index.php/fa/10261/5734/)

And the schematic for the amp:

http://recforums.prosoundweb.com/index.php/fa/10262/5734

(http://recforums.prosoundweb.com/index.php/fa/10262/5734/)



Why aren't the images appearing in the post??
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 31, 2008, 09:54:20 AM
Thanks. That should easily fit. If I'm not mistaken the pinouts are identical as well (EBC).


Apart from that & this hot-hot-issue & the hum, it'd be interesting to keep one preamp with only the necessary mods and compare it with one that tries to come as close as possible to a real one (without swapping the likely dominating audio-TXs): replacing the ceramic caps with polystyrene, using tants etc...
Title: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on October 31, 2008, 10:33:33 AM
Quote from: "zmix"

Why aren't the images appearing in the post??


The URL has to end in ".jpg" or another image format for the IMG tag to recognize it.

You can add .jpg to the end of those urls, like so:

(http://recforums.prosoundweb.com/index.php/fa/10261.jpg)

(http://recforums.prosoundweb.com/index.php/fa/10262.jpg)
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on October 31, 2008, 11:31:35 AM
Quote from: "wmtunate"
Quote from: "zmix"

Why aren't the images appearing in the post??


The URL has to end in ".jpg" or another image format for the IMG tag to recognize it.

You can add .jpg to the end of those urls, like so:

(http://recforums.prosoundweb.com/index.php/fa/10261.jpg)

(http://recforums.prosoundweb.com/index.php/fa/10262.jpg)


Not seeing pictures..!?!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on October 31, 2008, 12:05:46 PM
I don't know.  I see them fine.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on October 31, 2008, 12:15:58 PM
I was able to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.


(http://img366.imageshack.us/img366/3349/picture45xl2.jpg)


(http://img147.imageshack.us/img147/2564/picture48cl0.jpg)


[EDIT] Changed schematic per Zmix's request.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on October 31, 2008, 12:21:57 PM
Quote from: "tarnationsauce"
I was abl;e to see them by adding .jpg and using the address bar...


Maybe that's why I can see them in the forum...  I pasted the URL (with the .jpg addendum) first before I created the post with the IMG tags.  The photos were probably already loaded in my cache.

Regardless, there they are.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on October 31, 2008, 02:23:02 PM
Thanks!!  Could you please post this schematic instead? It has the same part designations as the PC board photo:

http://recforums.prosoundweb.com/index.php/fa/10264/5734/


Quote from: "tarnationsauce"
I was abl;e to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on November 01, 2008, 01:14:19 AM
Quote from: "zmix"
Thanks!!  Could you please post this schematic instead? It has the same part designations as the PC board photo:

http://recforums.prosoundweb.com/index.php/fa/10264/5734/


Quote from: "tarnationsauce"
I was abl;e to see them by adding .jpg and using the address bar, but it didn't work in the forum. So I put them on imageshack for ya.

Done!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 01, 2008, 08:56:28 AM
(http://img366.imageshack.us/img366/3349/picture45xl2.jpg)


(http://img147.imageshack.us/img147/2564/picture48cl0.jpg)

Fantastic!!

Now that we can all see the schematic and the layout... any thoughts?
 
I removed the boards from the chassis yesterday and it was RF city.. I have never heard more simultaneous radio stations...  it's also important to note that even when it's in the chassis, simply touching the top of Q4 qnd Q5 will allow you to hear local radio stations.  
I've got some BC441 - 6 and BC461 - 6s and heatsinks on their way to me now.  I hope that this will clear up the RF induced buzz.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 03, 2008, 06:33:50 PM
I subbed in some output transistors with the proper ratings and voila..! NO RF...  Therefor no HUM .!.!.!

http://recforums.prosoundweb.com/index.php/fa/10273/5734/
Title: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on November 03, 2008, 07:03:34 PM
Quote from: "zmix"
I subbed in some output transistors with the proper ratings and voila..! NO RF...  Therefor no HUM .!.!.!


Super-duper. That's great.

Shame for the many people who will kind of be a bit donald-ducked, but it's better than having a flawed PCB design.

Roddy
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on November 03, 2008, 09:59:13 PM
Well,, Chuck is our hero. Here is what he found


"
The TnC 81 hum problem was exactly as I had theorized:

The amplifier circuits in the EQ sections were oscillating, leading to RF induced hum and overheating.

Chance sent me some NTE substitutions for the for the parts I requested, I swapped them out and everything is running absolutely perfectly.

I do NOT recommend that these parts (NTE-186 and NTE-187) be used under any circumstance except with an expert repair person installing them because the pinouts do NOT match the actual part and it requires some pretty dicey crossing of the leads to fit them into the board... not a good bet for reliability.


To Sum up:

NOT powersupply related
NOT grounding related
NOT layout related
NOT induced EMI related
NOT wiring related

Also,  NOT manufacturer error... the part in the schematic is the part used, and it is simply the wrong part for the circuit. the fix will be to order the correct parts for each of the 4 EQ boards ( Q4 must be a BC461 - 6  and Q5 must be BC441 - 6).  The input board and HP / LP boards are fine as they are..



Let me know if I can be of any further assistance in sorting this out."
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on November 04, 2008, 07:37:31 AM
A few other TO-39 candidates came to mind as well, which will broaden the options and ease of finding: I expect that the BC161 & BC141 (or even BC160 & BC140) would be well suited as well. So:


Suggested replacements (to be confirmed by actually soldering in & testing):

Q4/PNP  BC461-6  -->  BC161-16
Q5/NPN  BC441-6  -->  BC141-16


Ratings, current-gain, min-fT are identical, as are the pinouts.

H*ck, I guess that for even more ease of finding you could even slam in the 'usual' BD13x-16 types, most likely the same dies, but different packaging.

BTW, various people use these BC-devices also in their 2520's (although I don't recall the current-gain range for now)

And last but not least, thanks guys for fixing this problem  :thumb:

Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on November 04, 2008, 07:58:25 AM
Quote from: "maxwall"
Will these original Q (BC)461/441 parts need to be selected for gain, matching , or performance or just plug in and solder up and its all done ?

Note that a range of current-gain is already specified: '-6'.
More matching wouldn't hurt but I've never toyed around with these actual circuits. I do think though that it wouldn't really be required - escpecially for this circuit since it's not the 'usual' push-pull but (simply said) Q5 is a current source for Q4.  

Quote
Will the BC 441/461 need to be heatsinked like original BA type amp boards ?

We'd like to run then at the same voltages & current, same TO-39 package (identical thermal resistance), so the same heatsinking-need I'd say. But I have no idea how big the need for heatsinking was in the originals - maybe just there because it was a more crowded DOA. The TNC-version seems to have ample room for it though, and there are some nice types out there so it'd look cool as well  :wink:

Quote
Alternatives/ Subs below ?

BC441 = 2N3053 or 2N5320
BC461 = 2N4037 or 2N5322

FWIW, I just posted some other candidates one post up.


Bye,

  Peter
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 04, 2008, 08:30:42 AM
(http://recforums.prosoundweb.com/index.php/fa/10273/0/5734/)

Chance sent me some NTE substitutions.

I do not recommend that these parts (NTE-186 and NTE-187) be used under any circumstance except with an expert repair person installing them because the pinouts do not match the actual part and it requires some pretty dicey crossing of the leads to fit them into the board... not a good bet for reliability.


http://recforums.prosoundweb.com/index.php/fa/10273/0/5734/
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 04, 2008, 08:32:20 AM
I wanted to post an update about the weird negative cycle distortion I was experiencing with one of my 81s when the EQ was in circuit.


 Here is a screen shot, and this was only present when the input signal was above -36dB:


http://recforums.prosoundweb.com/index.php/fa/10246/0/

After I cured the oscillation issue, I was dismayed to hear this same distortion in that 81 when the EQ was engaged.

 I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

Lucky for me these two unrelated problems were in the same unit...!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 04, 2008, 10:46:29 AM
A list of test voltages and test points should be established so each person can check their own units for any defective transistors or other components in other sections of the circuit besides the known ones from the Factory, just in case.
Title: TnC 81 Alternate Hum Cure....
Post by: zmix on November 05, 2008, 12:28:48 PM
Please Note:

If your 81 has a HUM rather than a BUZZ... it may be a simple fix...

[/b]
The difference in hum is very important. If the hum has a raspy, chainsaw like quality, then this may indicate an oscillation in the circuit. In addition, if the signal sounds distorted when the EQ is engaged, then the unit will require a different approach than the one I will outline here.

If the hum is a mellow, sine wave + overtones type of hum, then try this solution.

You will need a #1 and a #2 phillips screwdriver and an 8mm nutdriver, and a pair of rubber gloves.


Step 1:

Unplug the power cable.

Step 2

Remove the lid of the preamp using the #1 Phillips screwdriver..
There are two screws on each side, two in the back and two screws on the top.

Step 3

The power supply transformer is located in the rear right corner of the preamp as you face it from the front.  It is fastened to the bottom of the chassis by a large phillips head screw and an 8mm nut.  Put the nutdriver on that nut and the #2 phillips screwdriver on the screw on the bottom of the chassis. Gently loosen the screw, do not remove it, just loosen it by a quarter turn or so. so that the power transformer can be moved without too much force.

Step 4

prepare the unit as follows:

Input Gain: Minimum
EQ: IN
LF: 330hz, gain fully up (clockwise)
LMF: 220hz HiQ IN Gain fully up
HMF: 1.5khz Hi Q IN Gain fully up.

Output fader fully up.

Connect the output of the preamp to a suitable monitoring device (console, etc).

Power up the module and connect it to the monitoring device.

Step 5

Turn up the gain until you can hear the HUM plainly.

Put on your rubber gloves.

Grip the power transformer and begin to rotate it. You will hear the timbre of the HUM change as you rotate the transformer.

Rotate the transformer to the point where the HUM is at it's minimum level.
Put the cover on temporarily and check the hum level.  The cover will influence the magnetic field of the transformer, so this may take a few trials.

Step 6

Tighten the center bolt and replace the top cover.


Done!!
Title: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on November 05, 2008, 03:03:13 PM
I just tried Mouser and Digikey and neither have the -6's
Title: [ACMP investiupgradifications] All things PREAMP
Post by: alanhyatt on November 05, 2008, 05:33:44 PM
You can also use Mu Metal to surround the transformers to prevent hum. You can find Mu Metal pretty easily and it is affordable. Just mold by hand or cut pieces and hot glue...the place it over the transformers and glue it to the PC board but that will not correct the wrong components used.

Rotating the transformer is something each and everyone of you should do no matter if you have noise or not, as it can make a difference in the noise.

Alan Hyatt
PMI Audio Group
Joemeek - Toft Audio Designs - Trident Audio Developments - Valley People - Studio Projects - CLM Dynamics
1845 W. 169th Street
Gardena, CA 90247
toll free: 877-563-6335 fax: 310-323-9051
url: www.pmiaudio.com
email: [email protected]
Title: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 06, 2008, 03:08:55 PM
I think that I have discovered the LAST of the TnC 81 issues..


Here goes:

 The Line input of the TnC 81 is in antiphase ( ø )  with the input signal...  the mic input is not.

I tested all of the other modules I have... the 84 and the 73 check out fine. Both their mic and line inputs  maintain polarity.

The rear connector PC board is traced and labled correctly so the inversion must be happening at the preamp board.. which is odd because that mic pre / line amp board is the same part as the 84 and 73 use.


The solution I used was to de-pin red and white wires from the wire connector that plugs into "BCZ2" on the input / output jack PCB and swap them.
Title: [ACMP investiupgradifications] All things PREAMP
Post by: Knarleybass on November 08, 2008, 02:30:25 PM
So, I have a few  NTE-186 and NTE-187's in my drawer, but I can't figure out what substitutes what? what is Q4 supposed to be and what is Q6 supposed to be?

 I can't find any BC461 - 6 and BC441 - 6's either so I figured these NTE"s would would according to Chance's post.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 13, 2008, 09:27:19 AM
would would they? ;)

bump for new forum software....
Title: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 13, 2008, 09:21:04 PM
Just received my ACMP81's

New problem discovered

replaced 4Q4, 4Q5 on the Upper EQ Board which fixed buzzing oscillation issue on this board.
Payed careful attention to pin orientation between the BC547,557 and 441,461 when soldering back
into the PCB. After power up , I noticed a buzz oscillation coming from Lower EQ board. It also has a
output complimentary NPN/PNP pair so I replaced them ( 5Q4,5Q5 ) like the upper eq pcb board but this did not
cure its oscillation problem. So as it stands there is a possible oscillation problem on the lower eq pcb board
that is evident when engaging the rotary switch from 220 -1200 hz followed by its associated 10K
sweep fader. when the rotary switch is dis-engaged to the off position the oscillation is gone - eq amp out of path.

Looks like I'll have to replace the comp transistor pairs for the following boards below, which I have not
done yet. It may be possible that the oscillation is being passed by another eq amp , not sure.

3Q4,3Q5
6Q4,6Q5

Any ideas on this being another transistor issue on the 220 -1200 hz lower eq board ?

I have some test equipment handy but don't know where to start dropping the scope probe ?
maybe I'll start checking transistor emitters for broken waveforms.

The noise I'm getting is not a ground hum or 60 cycle low pitch hum , its sounds very close to the oscillation
that I heard on the upper eq board before I fixed it but a different pitch.

By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on November 14, 2008, 04:05:59 AM
By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.

I don't have them handy here to compare them to TO-39, but would using BD13x-16 (TO-126; say BD139-16 & BD140-16) help here maybe ? I expect these will be electrically close - most likely even the very same die, just different package.

These might need drilling the holes a bit because of their thicker leads, but I expect they can be mounted closer to the PCB-surface.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 14, 2008, 09:47:42 AM
This is certainly not a 'new problem".... ::)   RTFF..!   :)

ALL of the EQ amp boards have the wrong transistors and ALL of them will oscillate.

This is exactly what I stated in my initial assessment. I specified that ALL of the output transistors on the 4 EQ boards need replace(http://)ment.

Oscillation contaminates the power supply and will therefor appear in all connected circuits.

Once you have replaced all of the Q4 and Q5 on the 4 EQ filter boards this oscillation will go away.

PS: Look at the parts I put in mine (photo below) certainly no better than using the correct BC461 BC441 parts, yet no oscillation problems!

Just received my ACMP81's

New problem discovered

replaced 4Q4, 4Q5 on the Upper EQ Board which fixed buzzing oscillation issue on this board.
Payed careful attention to pin orientation between the BC547,557 and 441,461 when soldering back
into the PCB. After power up , I noticed a buzz oscillation coming from Lower EQ board. It also has a
output complimentary NPN/PNP pair so I replaced them ( 5Q4,5Q5 ) like the upper eq pcb board but this did not
cure its oscillation problem. So as it stands there is a possible oscillation problem on the lower eq pcb board
that is evident when engaging the rotary switch from 220 -1200 hz followed by its associated 10K
sweep fader. when the rotary switch is dis-engaged to the off position the oscillation is gone - eq amp out of path.

Looks like I'll have to replace the comp transistor pairs for the following boards below, which I have not
done yet. It may be possible that the oscillation is being passed by another eq amp , not sure.

3Q4,3Q5
6Q4,6Q5

Any ideas on this being another transistor issue on the 220 -1200 hz lower eq board ?

I have some test equipment handy but don't know where to start dropping the scope probe ?
maybe I'll start checking transistor emitters for broken waveforms.

The noise I'm getting is not a ground hum or 60 cycle low pitch hum , its sounds very close to the oscillation
that I heard on the upper eq board before I fixed it but a different pitch.

By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 14, 2008, 11:05:18 AM
Quote
I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

RTFF ? - yeah a lot to F*%^*$ reading already.

Zmix ,
I read your assessment above looks like you had problems in a different area Q2.  Chance mentioned the 4 EQ board amps needing possible replacement , maybe he cc'd this info from you, dont know. The HP/LP Board obviously does not contain any comp 'Q' pairs so an isolated issue with Q2. What method was used to check distortion on Q2 and how ? can you explain ? some other members might learn something here.

Why when I switch the 200-1200HZ rotary to 'off ' position on the lower eq board amp the oscillation disappears ? When I switch to 'off' does'nt this isolate the problem to the audio path of this eq module or does the oscillation travel thru the power supply or ground path from a different eq board and find its way thru to the lower eq board when switched to 'on' position ?

I'm still going to to replace the other eq's complimentary 'Q's pairs regardless. I report back my results.

Also love the modular design and layout , very easy to remove boards and work on.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 14, 2008, 02:45:21 PM
Just replaced all output transistor comp pairs with BC461,441's on all 4 EQ boards
and the oscillation buzz is still there. A little on the upper mid board EQ and really pronounced on the lower mid EQ board. WTH !!! This ain't right

The buzz is gone when the rotary switch is in the off position. All the other EQ's are fine except the Upper mid EQ. It stil has a little buzz in it when the rotary switch is engaged.

I'm going to get to the bottom of this , for sure !!!

I'm going try swapping in another board from another 81's and see what happens.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 14, 2008, 02:52:13 PM
Chance was quoting my posts on the Pro Sound Web....
Quote
I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

RTFF ? - yeah a lot to F*%^*$ reading already.

Zmix ,
I read your assessment above looks like you had problems in a different area Q2.  Chance mentioned the 4 EQ board amps needing possible replacement , maybe he cc'd this info from you, dont know. The HP/LP Board obviously does not contain any comp 'Q' pairs so an isolated issue with Q2. What method was used to check distortion on Q2 and how ? can you explain ? some other members might learn something here.

Why when I switch the 200-1200HZ rotary to 'off ' position on the lower eq board amp the oscillation disappears ? When I switch to 'off' does'nt this isolate the problem to the audio path of this eq module or does the oscillation travel thru the power supply or ground path from a different eq board and find its way thru to the lower eq board when switched to 'on' position ?

I'm still going to to replace the other eq's complimentary 'Q's pairs regardless. I report back my results.

Also love the modular design and layout , very easy to remove boards and work on.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 14, 2008, 04:14:21 PM
Problem solved !!!

swapped in another eq board to rule out other component failures, ie inductor , cap  etc.
the problem stays with the box . The eq board I pulled out from another 81 , had the same buzzing
noise problem when engaged in the test platform . I even replaced the transistors again like before BC441, 461 and still the buzzing noise remains.

Before Testing the inductor , I removed the toroid power transfomer and moved it slightly away from the eq boards and this instantly romoved the buzzing hum problem from the mid upper and lower eq sections when engaged. (Consequently, these are the only two boards which contain the only two inductors) This is somewhat strange because I'm used to hearing a low 60 cycles hum
but this usually points to a ground loop noise, not a transformer EMI noise like what is evident here.

I found that if I rotated the transformer left or right in its mounting location the noise did not change significantly enough to call it a fix. so again I found that the transformer needs to be relocated pretty far away from the eq boards to get the signal quiet like it should be.

It appears the transformer will have to be moved outboard of the box to get a good clean noise free signal. It may also be that these transformers put off a field that is not found in higher cost toroids, not sure. I don't have any Mu Metal to cover the donut shaped transformer to see if shielding will help. maybe the inductor on the eq boards needs sheilding instead. I read thru the Prosound web forum and this problem has already been encountered, this is what led me to at least trying it. and it worked !!!

Looks like the best solution is to mount the transformer and maybe the power supply in a outboard box.
Its a hassle but I don't think there is any other choice for getting a clean signal out of this ACMP 81.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on November 14, 2008, 04:53:13 PM
Nice work ,
 generally speaking torriods pass allot more hi freq sh*t than normal pwr xfmrs
curious though only the 81's suffer from this ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 14, 2008, 05:06:33 PM
okgb,

81's , definitely.

It seems likely the others would get effected as well, but I have not heard any reports yet.

I might also add that this preamp sounds damn good for the price , really damn good.

That is when the power transformer gets relocated and the transistors get switched out for the BC441, 461's.  it will make for a really decent recording tool.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 14, 2008, 07:43:29 PM


Can you provide measurements? What's the noise floor with the EQ in and EQ out?

okgb,

81's , definitely.

It seems likely the others would get effected as well, but I have not heard any reports yet.

I might also add that this preamp sounds damn good for the price , really damn good.

That is when the power transformer gets relocated and the transistors get switched out for the BC441, 461's.  it will make for a really nice recording tool.


Thanks
Chance
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on November 15, 2008, 07:02:38 AM
Look at the parts I put in mine

Thanks for the various info.

Just wondering, how is the PCB-quality ? You've done several mods'n'swaps now,
have the circuit-traces been a bit sturdy so far ?

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 15, 2008, 08:57:09 AM
Look at the parts I put in mine

Thanks for the various info.

Just wondering, how is the PCB-quality ? You've done several mods'n'swaps now,
have the circuit-traces been a bit sturdy so far ?

Bye,

  Peter

Peter,
 I had an opportunity to do some more extensive repairs to one of mine after I had accidently swapped two side by side identical wire connectors on the preamp board. This neat trick resulted in 2 popped electrolytic caps and 2 BC184 transistors.  I would rank the quality of the PC boards as being as good as any modern epoxy / fiberglass boards, certainly better than the boards in any of my 1960s and 1970s gear.  As you can see in the photo, I needed to fit those large leads from the NTE parts into the 0.1 holes in the board. I was concerned about lifting the donuts around the holes, but I used my normal care when desoldering and no traces were harmed during any of the repairs. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 15, 2008, 12:01:16 PM
Zmix,

I think its safe to say that when the EQ is engaged the noise floor does increase. But to what level ? not sure ,  technically I think a audio analyzer would work nicely for this but I often use my ears because in the end the final test is listening to the results. If everything is quiet and the headroom is
big and harmonics smooth, its good for me.

The niceness in this amp is that most all the caps are Rubycon 105's throughout and transistors are Fairchild , very delighted to see this level of quality. the film caps in the passsive sections seem to be nice too.

I'm going to make some stainless steel covers out of tubing for the inductors and power transformer to see if I can't eliminate the buzzing from the power supply. I would really like to keep the power supply
self contained within the chassis if possible.


Here is a snapshot of the inductors in the Eq section , BC transistors upgraded already.
I noticed they don't get hot to the touch, rather on the warm side. The heat tends to increase on the metal case when the EQ is at different settings. i don't think they will require heatsinks so far.

Out of curiosity, I'm tempted to wind my own Neve inductors using the pultec toroid cores and sub them in for these just to compare....keeping the Neve inductance specs the same of course. Might be interesting.

(http://img.photobucket.com/albums/v611/maxwall/ACMP81_1.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on November 15, 2008, 01:59:04 PM
Um, I'm having problems with this new forum software.  I don't know where to post these problem comments.  Anyone?

Maxwall, et al, thanks for posting, guys.  I am only surprised that it's taken this long for someone to discover problems coming from the toroids.  When I first saw the photos I suspected they would be causing noise.  I was planning to shield them, or remove them, and possibly shield the inductors, it was the first item on my list of things that would probly need fixing.  Unfortunately, I still don't have a shipping invoice so I can't comment on actual experience and join your fun.  Keep the updates coming, it's very helpful!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on November 16, 2008, 10:06:04 PM
Um, I'm having problems with this new forum software.  I don't know where to post these problem comments.  Anyone?

Maxwall, et al, thanks for posting, guys.  I am only surprised that it's taken this long for someone to discover problems coming from the toroids.  When I first saw the photos I suspected they would be causing noise.  I was planning to shield them, or remove them, and possibly shield the inductors, it was the first item on my list of things that would probly need fixing.  Unfortunately, I still don't have a shipping invoice so I can't comment on actual experience and join your fun.  Keep the updates coming, it's very helpful!

Well, as I've posted and / or been quoted before the TnC 73 and 84 use the same inductors  and PSU  toroids without issue.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 17, 2008, 11:38:06 AM
Zmix

The Tnc 73,84 inductor boards are they the same distance and orientation away from the transformer as the 81 ?

Can some one snap a photo so we can see the board layout in a 73,84 ?


Also for those who intend to replace the BC557, 547 transistors with BC 461,441 , there are substitutes
which can be used like the 2N5320 or 21, 2N5322 or 23, but these are a little pricey.

If you want a cheap substitute use TIP31B or C(NPN) and TIP32B or C(PNP) both rated at B=80V C=100V, 3A. These parts will work and are overrated in their specs to carry out their jobs as subs. The BC441,461's are 75V, 2A rated.

IMPORTANT : the Legs on the TIP31B or C, and 32B or C are not pin compatible with the BC 557,547's. The legs will need to be shaped/bent to match the Collector , Base, Emitter pin arrangement exactly. The TIP transistor legs will need to be thinned to fit in the pcb pads holes as well. View available data sheets on web to familiarize yourself with both the original and replacement transistors regarding Emitter , Base and Collector pin differences before attempting installation. Also the gain on these transistors are rated lower than the 2N and BC parts

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 22, 2008, 11:15:48 AM
Has anybody confirmed that the power toroid transformer in this ACMP 81 does not have a electrostatic shield in it.
I don't believe it does, and this may be part of the problem with China transformers.


Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Its just the damn toroid power transformer ruining the party. Its too close. I sheilded the inductors and it still picks up noise from the toroid power transformer, regardless.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 23, 2008, 03:39:38 PM
Anyone have any suggestions regarding the "killer pop" when turning up the gain? It seems that the pop exists on at least two of the models. Someone on gearslutz said they have it on their 81s, and it's on both of my 84s.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on November 23, 2008, 11:05:14 PM
Ask and ye shall receive:  Inside the ACMP-73
(http://www.organissimo.org/pub/acmp-73.jpg)

I have not had a problem with hum nor oscillation of the EQ.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on November 25, 2008, 01:36:39 AM
Rocket Attack,
Your absolutely 100% correct with the Brutal pop in the 81 gain switch, mine even cuts out for a few seconds then comes back very slowly. Sounds like the preamp goes dead for a second or two. Is this a non-shorting switch ? I'll have to check it to see wether it is or not. I'm hoping not. It should be a shorting type. Yes, ?


b3groover,
The effected area in the BUZZZZ zone is highlighted in yellow below. On a 73 you wont get it because the eq boards are far enough away from the transformer flux field to avoid the EMI.  There only needs to be about  4-5 inches of empty space between the transformer and boards then the buzz is almost non-existent. On the 81 , the same area highlighted below in yellow is occupied by the low mid and low eq boards and no extra space exists to be free from the transformer flux field. The 73 and 81 share the exact same case, but the 81 has more boards.  The 81 owners are stuck with the problem ,  its a simple fix but a pain in the arse. Mount the transformer and power supply outside in its own box. It will probably make it quieter anyhow, as long as the lines are kept fairly short ( 3-4) feet and generous guaged wires are used utilizing safety ground by default.

(http://img.photobucket.com/albums/v611/maxwall/ACMP73.jpg)

I just picked up some Cannon multipin connectors to start the process on my 81's for remote placement of the power supply. The power connector on the back of the 81 can be removed and a plate installed in its place for use with the Cannon multipin power connector. Yes , its a pain is the arse...... The only thing I have'nt tried is a Mu-metal barrier
between the transformer and afflicted boards. MU-Metal might be another easier alternative.

Why is it when I lift the PS grounds I always get a AM radio station in a foreign language, why can't it be Michael Savage from the Savage Nation. Sheeple
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 25, 2008, 08:46:59 AM
Rocket Attack,
Your absolutely 100% correct with the Brutal pop in the 81 gain switch, mine even cuts out for a few seconds then comes back very slowly. Sounds like the preamp goes dead for a second or two. Is this a non-shorting switch ? I'll have to check it to see wether it is or not. I'm hoping not. It should be a shorting type. Yes, ?

I haven't noticed a cut, but that isn't to say it doesn't cut out.

Has anyone noticed it on the 73? Or is it only on the 81 and 84?


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 25, 2008, 10:41:10 AM
Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on November 25, 2008, 10:42:16 AM
The gain switch does not cut out or pop on the 73.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Crash on November 25, 2008, 11:11:55 AM
Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on November 25, 2008, 12:32:57 PM
I hemmed and hawed about which preamp to buy; looks like I made the right choice between the pops of the 84 and the hum of the 81.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 25, 2008, 12:34:17 PM
Somone else had posted about this besides me as well, so it is more than an isolated incident.

It does seem to affect the 81s and 84s. If it is the preamp section, then I'm surprised it doesn't affect the 73 as well, but B3groover didn't notice it on his units. It happens with or without the EQ section engaged, and is noticeably worse if you turn up the gain on the last HF filter on the 84. That causes the meter to bounce. Without the HF turned up, the meter doesn't bounce, but the clipping light comes on.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on November 25, 2008, 01:22:22 PM
I'm surprised, too... but no pops, no hum with the 73.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on November 25, 2008, 02:15:21 PM
Has anyone investigated the switching to see if the amp is temporarily going to open-loop gain at this point? That might cause the symptoms that you're speaking of I guess.

I'm going to take a look at the schematics again tonight to see if I can gather anything - I assume the mic amp stage is that same as that of the '73?

Roddy


Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on November 25, 2008, 03:14:26 PM
Chuck (zmix) figured that the pop position is exactly where on the original Neve the "off" position is
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 25, 2008, 03:25:33 PM
I'm going to take a look at the schematics again tonight to see if I can gather anything - I assume the mic amp stage is that same as that of the '73?

I thought this was a true statement before, but maybe it's not since no one with a 73 has reported the problem yet.
 ???
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on November 26, 2008, 02:10:14 AM
Oh.. the 73 has the pop problem in the gain switch. It really doesnt bother me much as I generally set the master out all the way down when Im messing with the gain stage so I didnt notice it till I started hearing about the problem and checked to see if mine had this problem. They did.
I havent looked deeper into this because I didnt really feel the need to.
I dont think the gain switch is of the shorting variety due to some of the noise I hear when clicking through it though as I said, I havent checked it out.
I didnt hear any humming noise in the 73 but I havent done any real critical listening yet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on November 26, 2008, 02:42:55 AM
Here are some noise floor tests of the 73, 84, and 81. I also did the converters alone as well as a SCA N72 as a control.

Keep in mind a SCA N72 has superior power supply and shielding. It utilizes a fully regulated PWM power supply and full Mu metal shielding.

Note: This is a non-scientific test, just a reality check. This was done without top covers on the ACM preamps! Also the scaling is slightly different from graph to graph.

Green █= EQ Out
Pink █ = EQ In

(http://img.photobucket.com/albums/v646/red_3000/how_to/73noisefloor.gif)

(http://img.photobucket.com/albums/v646/red_3000/how_to/84noisefloor.gif)

(http://img.photobucket.com/albums/v646/red_3000/how_to/81noisefloor.gif)

______________________________________________________________________
(http://img.photobucket.com/albums/v646/red_3000/how_to/scan72noisefloor.gif)

(http://img.photobucket.com/albums/v646/red_3000/how_to/converternoisefloor.gif)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on November 26, 2008, 09:30:28 AM
Oh.. the 73 has the pop problem in the gain switch. It really doesnt bother me much as I generally set the master out all the way down when Im messing with the gain stage so I didnt notice it till I started hearing about the problem and checked to see if mine had this problem. They did.
I havent looked deeper into this because I didnt really feel the need to.
I dont think the gain switch is of the shorting variety due to some of the noise I hear when clicking through it though as I said, I havent checked it out.
I didnt hear any humming noise in the 73 but I havent done any real critical listening yet.

This post made me curious, so I went back and checked mine again.  On one unit, there is a slight noise when switching gain, but nothing more than my Quad Eights, which have a stepped attenuator on them.

However, on that particular unit, stepping the gain back down from the highest point to the next position below did cause a slightly louder noise, almost a tone.  But only when coming down and only in one spot.

On my other 73 (yes, I was able to fix it), there is the slight noise as in the other one, but that's it.  However, that one does a weird thing when you first turn it on that the other doesn't:  The meter pegs itself all the way to the right and then slowly goes back to zero.  While it's doing this, there's no noise or anything coming out of the output.  Weird.  The other 73 I have doesn't do this.

Quality control! :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on November 26, 2008, 09:49:15 AM
B3Groover, were you doing those tests with mic input? My 73's make a loud "pop" oscillation like others describe. But onlu with mic input.
I have noticed that when the 73 is first turned on the meters move all the way to the right and slowly fall. I think that's just a coupling cap charging. Maybe this was the case for you?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on November 26, 2008, 09:51:05 AM
Both of mine did the VU meter peg when first started up, but even then it passes audio normally. The pop on the gain switch was mainly in 1 spot, Just past 9 o'clock, there is a loud pop going in both directions.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on November 26, 2008, 10:08:52 AM
I went and did a bit of listening. Heres the setup.
ACMP - Line in,mixer - Headphone out on mixer.
Have preamp all the way down,
Swith on the EQ High band with the boost all the way up on that channel, output all the way up, I can hear what sounds like 60 cycle hum. But only on the first 3 eq settings. I might try playing with the transformer or shielding it up to see if that will solve my problem.
 Might also try the transistor replace if that will do it too.

Edit---
Anyone have a source on the  BC441, 461's
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 27, 2008, 06:28:08 PM
Both of mine did the VU meter peg when first started up, but even then it passes audio normally. The pop on the gain switch was mainly in 1 spot, Just past 9 o'clock, there is a loud pop going in both directions.



fwiw, neither of my 84s had the VU meter issue.

Did Chance ever post what the issues were with the pre's that were held up due to QA issues?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rocketattack on November 29, 2008, 02:29:09 PM
Crash posted this over at the home recording forum. I didn't see it on either of the ACMP threads (this one or the group buy one) here.


Quote

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

Yes this is exactly the spot where the additional gain stage gets switched in.
7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

While that explains how it behaves similarly to the pre's it's modeled after, such a loud pop seems to be a
design flaw here.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Brad McGowan on December 02, 2008, 12:49:12 AM
I'm noticing the hum/noise problem in my ACMP-84 when the EQ is engaged and the midband is set to either 360Hz or 700Hz.  The hum is there regardless of the amount of boost or cut and goes away if I switch to a higher frequency or switch the midband to off.  I am experiencing this on both 84's that I have.

Has anyone else noticed this as well? 

I'm guessing the toroidal inductor is the culprit?  Any thoughts?

thanks,
Brad
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Crash on December 02, 2008, 03:48:15 PM
Crash posted this over at the home recording forum. I didn't see it on either of the ACMP threads (this one or the group buy one) here.

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

Quote

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

Yes this is exactly the spot where the additional gain stage gets switched in.
7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

While that explains how it behaves similarly to the pre's it's modeled after, such a loud pop seems to be a
design flaw here.

Just so the right person is given credit for this, I quoted Zmix, that is his quote. I have no business making any sort of statements like that, I more of an end user of these things. ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 03, 2008, 02:38:02 AM
Brad,

Loosen the single retaining screw holding your power supply toroid and move it around  a inch or two towards the back of the chassis while you have the mid band engaged with the hum present and report back your results here.

you won't be able to move the inductor since its soldered to the board.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on December 03, 2008, 11:44:37 AM
Are these the correct parts to fix the 60 cycle hum ?

http://export.farnell.com/magnatec/bc461/transistor-pnp-to-39/dp/1208598
http://export.farnell.com/magnatec/bc441/transistor-npn-to-39/dp/1208596
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Brad McGowan on December 04, 2008, 03:53:17 PM
Brad,

Loosen the single retaining screw holding your power supply toroid and move it around  a inch or two towards the back of the chassis while you have the mid band engaged with the hum present and report back your results here.

you won't be able to move the inductor since its soldered to the board.

There's a user on Gearslutz that has done just this.  It's definitely an inductor shielding / proximity issue:

http://www.gearslutz.com/board/low-end-theory/187097-acmp-73-neve-73-clone-45.html#post3706676

Brad
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on December 04, 2008, 04:13:41 PM
well, I would change the transistors before any other conclutions. -Still waiting my units

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Brad McGowan on December 05, 2008, 04:23:04 PM
I was under the impression that the transistors were only in error on the 81's. 

Brad
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 05, 2008, 05:09:15 PM
It won't hurt but
 i think maxwell had the experience of changing the transistors and still having the problem
until moving the power transformer [ which won't hurt either  ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 06, 2008, 12:35:52 PM
Thats correct okgb, I'm confident the transistor change may have fixed another issue far more subtle than the EMI/RFI interference I still have with my 81's.

Brad, If you conclude its the inductors without moving PSU transformer then your just speculating.

Pot core inductors don't need shielding, even Cinemag pot core inductors for a pultec are not sold with shielding unless specified. It just matters how close you put them to EMI/RFI devices like a power transformer. Like anything.

You need to get your hands dirty to understand the problem.

Besides part of the shielding is seperately tapped inside the winding , and that would require more than just throwing a mu metal jacket over them. I tried the so called shielding trick by using a vacuum tube shield with  nickel coating and no change at all. I even grounded the shield to chassis. still no change. The location of the PSU transformer is the culprit.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on December 06, 2008, 01:52:27 PM
I wasn´t following glose enough, it seems. It´s kind of good you sort this out before I
get mine...

Thanks

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on December 07, 2008, 05:32:01 AM
since i received my acpm81 (thanks again Franck).i did a quick test.
here my report
First i ve changed the 8 filters boards transistors with 2n5321 and 2n5323 as i suggested to maxwall for a close matched replacement for the bc 441 bc461.
it works great.
troubles i ve had
first a dead line mic switch. replaced with a toggle one.
btw thoses china "alps copy" switches are real poor quality. if a led fails and you ll have to open the switch  good luck to re mount it and keep it working.
i doubt they will do long use.
So it will be a good idea if "chance "can stock an amount of thoses switches cos illuminated switches like thoses could be hard to find.
btw there s no traces for the led on the line mic switch .it could have been implanted with some mods.
I have the pop (well it s not a pop but an uggly shortcut) between 6 and 7 position.
as stated before it s the 73/84 mic off position which have been ommited. bad design.
no way to sort out easily without mod the board and loose one position..
switch to line before change 6 to 7 position is the simplest thing to do.
the 81 preamp is not a 81 preamp but a 73.(see ba284 sections)..
the 39 ohm resistor (line switch) was dead too. quite tricky to find as it was the lead which was broken at the resistor body junction.... replaced it and everything was ok..
regarding the HUM thing
well i didn t notice it on the oscilloscope
1) i m a lucky guy
2) i run the unit within 220V 50hz (all the hum troubles reports seems to come from people with 115v 60hz)
definitively the psu traffo seems a bit short to feed the unit..
somes mods to improve the psu board can be done (adding pot to fine tune the 24v and 48v adding some diodes for the 24v regulators etc...) but it work as is....
3) i did not do the "good" eq combinations setting to have the hum appears on the oscillo...
please elaborate
i did nt test the 81 in real so i can t comment about the audio
when my 1084 (with carnhill tx and inductors) will be finished (almost done) i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 07, 2008, 09:51:48 AM
Thanks for the update , good to see you got your's
i think clint would be curious what you paid in duties & shipping .

More & more this would seem to be the chinese
selling us a design [ rejected ] that they already had
but i still think it is easier to mod than build one from scratch
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on December 07, 2008, 08:19:52 PM
For some strange reason I stopped getting new post notification E-mails. I received a strange box from China with 10 ext PSU's and 210 pots. I got no E-mail from them but am waiting for a reply to the E-mail I just sent them
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 07, 2008, 11:44:23 PM
That's funny , i was thinking serious power supplies
likely not expect any help from them [ they got the money ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on December 08, 2008, 09:23:15 AM
I just received a reply. "It was a mistake, please return"
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on December 10, 2008, 11:20:48 AM
i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge

This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on December 10, 2008, 11:30:26 AM
This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

That's interesting to hear, cool that people have access to the 'other clones' as well for comparison.

Quote
Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.

That's FAQ#7  ;) , but here goes: they're not in the signalpath but used for the 'signaling-LEDs'. No need to upgrade these chips (some people might be inclined to).

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: CHANCE on December 10, 2008, 11:32:16 AM
i will compare them with an sca1272 as reference...( maybe a bit too much gearslutz? no??)
audioforge

This is what I have been comparing them with as well. While I think the SCA n72 has a bit more depth, the 73 does hold its own. Its bit hissier than that n72 but thats not too hard to control. I like the way the EQ sounds, If only I could get the buzz out of it. I tried making a shield around the mains transformer but that didnt make but a really small difference.

Anyone know what the TL072s are for? They're not listed on the only schematic I have for this.

Are you referring to the 73, 81, or 84? I have detailed schems for all three. I thought I already posted the 73, maybe not here.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on December 11, 2008, 04:04:05 AM
The 73, The only one I have is a 1 page PNG file but it only has the signal path. There no power section or anything outside of the signal path.

http://goose-tracks.com/thegeekgoose/images/scem73wv9fi2.png
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 11, 2008, 08:40:10 AM
Could repost all three [ whoever has them  ]
some on the other forums are reporting increased
noise when engaging the eq [ on the 84 i believe ]
you get what you pay for  [ not a bad deal in this case ,
well haven't got it yet ] but was expecting to upgrade components
and still be ahead of the game .
the guys building real 73's & 81's were too serious for my brain

on another note , perhaps some lpf'iltering on the torriods may help
[ easier if you are moving these to an external box ] as the torriods
pass allot more Hi freq garbage than the traditional power transformers
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on December 18, 2008, 09:54:56 AM
Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Hi maxwall,

I've got a handful of each of the non-suffix-6-BC4x1's.

If you are using the non suffix 6 BC parts what type of hfe measurement would be ideal?

The BC4x1-6's are rated for hfe of 115-250
The BC4x1's are rated for hfe of 40-250
The supplied BC557 hfe of 125-900 and BC547B hfe of 200 -450

Thanks!
-jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 20, 2008, 01:02:04 PM
Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Hi maxwall,

I've got a handful of each of the non-suffix-6-BC4x1's.

If you are using the non suffix 6 BC parts what type of hfe measurement would be ideal?

The BC4x1-6's are rated for hfe of 115-250
The BC4x1's are rated for hfe of 40-250
The supplied BC557 hfe of 125-900 and BC547B hfe of 200 -450

Thanks!
-jonathan

Jonathan

mine were hfe 90 , and they sound fine, no issues.



Chance

Whats the gameplan for fixing the gainswitch problem. I have read thru various forums and all I see is
comments but no solutions. Are the 81 owners going to have to live with this problem with no solutions ?
Tried emailing you directly thru the musicainsworkshop email address but 'zero' response. a simple reponse is appreciated either way.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on December 20, 2008, 03:24:17 PM
Hi maxwall,

is the switch the only remaining problem for you? Did you fix your EMI/RFI issue? How? I hope you didn't have to move the power trafo outside the box??

As far as the switch goes, it's strange that it would only cause problems in one spot. I mean if it were non-shorting it would pop all over the place right? Is it possible the switch could be wrong in another way? Comparing Chance's 1073 schematic to the original 1073 schematic the switch part seems pretty close to the original.. At least the 6 and 7 positions look the same. I hope the schemo is correct, that is, I hope that it reflects what is, in fact, in the box, and there aren't any surprises by our Chinese friends.
One thing I did notice comparing it to the 1073 schemo on Dan A's site is that the 2x 10K resistors (R31 and R32) are not in the switch section. Could this make a difference?

If there is something mechanically wrong with the switch, hopefully Chance can convince the manufacturer to send out a batch of new ones, although we probably have very little leverage at this point.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 20, 2008, 03:43:14 PM
Mitsos,

I'm interested in your assessment of the gain switch schematic comparison. It would be helpful for you
to post pics here of the missing '73 10K resistors in comparison to the China 81 layout , and highlight
the schematic locations so everyone here can see , learn , compare and debate for all of us to get to the bottom of this lousy gainswitch problem.

I do agree with you that a non shorting switch would cause popping in most if not all gain selections. But my preampliterally saturates, or blacks out then comes back slowly. It sounds terrible. It actally sounds like I'm killing it everytimeI use the gainswitch. I'm just waiting for a component failure to happen next.

the two issues in my '81 are power transformer noise and gain switch. And yes, moving the PSU transformer
seems to be the likely fix, this is very unfortunate. Obviously the ones who sold us these '81 never really
evaluated them before sending them out. How nice of them. but its entirely possible that there was a design error in the manufacturing process that has not been identified yet. It may certainly take a skilled individual to find it.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on December 20, 2008, 06:54:06 PM
Hey Max,

I'm not the right person to be assessing anything. I know near nothing about Neve stuff so I'm just throwing something out there that I noticed was different on the two schematics, but it could be that it's not needed or that it's in a different place. Anyway here goes..

Here is the TnC 1073 schem:
http://goose-tracks.com/thegeekgoose/images/scem73wv9fi2.png

and attached is the one from Dan Alexander's site, with the two 10K resistors marked (If I don't attach this right please see the link below and look for R29 and R30).
Original 1073 schematic:  http://www.danalexanderaudio.com/neveinfo/Neve1073schem.jpg

Also, I noticed that R26 is 220 Ohms and in the Neve original is 120 Ohms, and there are a couple of resistors out of order: 3K9 and 12K (positions 12 and 13), as well as an additional 12K resistor at position 15. I don't know anything about any of this, just that the two schems are different, and I do wish someone with more experience with Neve circuits would take a look.

Thanks so far to all of you for checking into this. I've had many tests of patience recently and this is proving to be one for all of us. But with everyone working together on this, we'll get it sorted out sooner or later. (cheesy or what?)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on December 21, 2008, 09:09:06 AM
I don't know how accurate that ACMP73 schematic is. It was given to out while the preamp was still in pre production. Plus you can see there are some parts not even connected to anything.
Here are some schematics of the ACMP-81 that are newer and more complete. As far as I know these could be the revision of the schematic used in the production preamps.

(http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_amp.gif)

(the rest of the ACMP-81 schematics listed bleow)
http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_amp.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_amp.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_filter.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_filter.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_treble.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_treble.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_upper.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_upper.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_low.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_low.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_bass.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_bass.gif)

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_lout-power.gif (http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_lout-power.gif)

[edit] Almost forgot the Output / Power schematic.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on December 21, 2008, 09:14:59 AM
Are the rest of the schematics available also? Maybe posted in another forum? Anyone seen them? How's Chance been these days? He's been pretty quiet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 21, 2008, 09:33:07 AM
with all the problems it's hard to know if it was a previously rejected design ,
I'm suuuure they'll correct it for whoever buys the next versions of it
Still what schematic did they use for design ? seems to be a little
cut & paste going on , you could imagine how they'd just slap a ba 283 board on
copying the pcb layout and change up the different versions .

Still for the price it will nice to get it , it'll allow me to have something at home
while my better pres are rented out .

thanks for the schematics
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on December 21, 2008, 08:17:42 PM
Are the rest of the schematics available also? Maybe posted in another forum? Anyone seen them? How's Chance been these days? He's been pretty quiet.
Chance put those ACMP81 schematics on a forum (or maybe they were emailed) a while ago in .pdf format. I converted them to .gif for easier display on forums.
Those schematics were requested by Chance from China when zmix was troubleshooting the 81's hum with the EQ engaged.
BUT I do think I heard Chance has the other schematics also. It's be nice if he can make them available to us. But I hate bothering Chance because he's so busy.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 21, 2008, 09:39:15 PM
Yes, I think Chance has taken a beating. I just read he's on 1400 shipments right now. So I have to calm down and be patient for this problem to play out and get resolved. Chance will probably need some R&R after its all over . I'm sure then with all the units out, the focus on solving problems will most likey intensify.

Like I said, I'm no designer so. Without inside info like Chance or Zmix . My ability to find the problem on tha gain switch is limited. All I can do is trace out the schematics and compare with the actual unit to see if there are missing links or trace errors. not super hard to do just time consuming.  If there is a design problem with the preamp board, then this is not going to be a quick fix. There are brighter EE people here who might have answers.

yes, I agree there are traces to mic/line switch ( led specific) that are not even there. I pulled my preamp board and have seen it first hand. jumpered dc power wires can be used in place, but does the
power supply meet the current requirements for it, especially if multiple leds are on at the same time  ?

I also noticed some small leds near the eq sections that do not appear to do anything. There not the leds in the pushbutton switch. There located on the lower right side of the sweep pot in each eq section.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on December 22, 2008, 12:44:38 AM
Yes, I think Chance has taken a beating. I just read he's on 1400 shipments right now. So I have to calm down and be patient for this problem to play out and get resolved. Chance will probably need some R&R after its all over . I'm sure then with all the units out, the focus on solving problems will most likey intensify.

Like I said, I'm no designer so. Without inside info like Chance or Zmix . My ability to find the problem on tha gain switch is limited. All I can do is trace out the schematics and compare with the actual unit to see if there are missing links or trace errors. not super hard to do just time consuming.  If there is a design problem with the preamp board, then this is not going to be a quick fix. There are brighter EE people here who might have answers.

yes, I agree there are traces to mic/line switch ( led specific) that are not even there. I pulled my preamp board and have seen it first hand. jumpered dc power wires can be used in place, but does the
power supply meet the current requirements for it, especially if multiple leds are on at the same time  ?

I also noticed some small leds near the eq sections that do not appear to do anything. There not the leds in the pushbutton switch. There located on the lower right side of the sweep pot in each eq section.
The LED's by the sweep pots are clip OverLoad LED's. They will light if that part of the EQ circuit clips.

It has been suggested that the gain switch "pop" position on the original design is where the extre off positions are. For example: http://benversluis.com/wp-content/uploads/2008/06/1073.jpg (http://benversluis.com/wp-content/uploads/2008/06/1073.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on December 22, 2008, 09:48:29 AM
This whole thing makes those posts by Alan Hyatt on the homerecording.org forum a year or so ago a little more prophetic, eh?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 22, 2008, 11:16:02 AM
Yeah , i never had a problem with that being the case
as all the info didn't seem so forth coming right from the start
and expected to mod it anyway , I'd just like to get mine

the post about a delay from some of them not passing QC was curious ,
someones excuse for a delay ? and they just shipped them anyway ?

I don't think these were already build but seeing how the Chinese operate
it's also curious that no one else has bought the design yet ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on December 22, 2008, 02:42:47 PM
I've been seeing this as China's way of "beta testing."  I've always found interesting that many people on this forum seem paranoid about someone stealing a design developed here and reaping a profit.  But you can steal a desing from anywhere.  I would think the most expensive part of product development is testing. And China is getting it for free. In fact, we are paying (discounted prices) to be beta testers. As to why no one's bought the design yet, I'm pretty sure most other, interested "manufacturers" are aware of Chance's group buy and are watching this unfold. I would be if I owned a company considering offering this product.  Wait for this trial phase to be done and I think then you'll see people like PMI (maybe not, but who knows who else) offer these exact pres under their own brand.

Anyway, I can't wait to start working on these. I've enjoyed learning from everyone on this forum, and I'm taking this as a learning experience as well. Cheaper than community college, and much more satisfying... just less girls in the classroom...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on December 22, 2008, 03:17:51 PM
Franck
i ve received your PM
what sort of troubles you have with the gain sxwitch.??
is it only in line mode? or mic? or both?
please describe
this gain switch is an adaptation of the original.
11 position against 22.so the mic/line switch "1k2".
original is 5db step both line mic as far as i remember.
tnc is11 pos. for line against 7 positions in original so a 3db step.against 5db.
mic :11 pos. against 14 positions on the original and a 6 db step.against 5db.
this explain the resistors differences.
as i didn t do the math i cant confirm the tnc choosen resistors values are the correct ones....
as i said in my oscillo test i ve had trouble with a brocken resistor lead in the line switch.
my advice would be follow the signal with the oscillo in line mode then in mic mode (eq out of course)and see what happen.
if you have trouble for both mic and line ,whatever positions ;check the amp card transistors first.
but i remember zmix have to change a dead resistor too so check for a broken lead or cold solder....

btw there no way to avoid the short between 6/7 position in mic mode( when the b284 ampcard add the extra gain stage) so flip to line before switching or build a new 3 deck 22 pos.attenuator.....
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on December 22, 2008, 05:06:33 PM
btw there no way to avoid the short between 6/7 position in mic mode( when the b284 ampcard add the extra gain stage) so flip to line before switching or build a new 3 deck 22 pos.attenuator.....
audioforge
Hi Audioforge, I'm curious,( I know next to nothing about neve circuits) how did the originals prevent the pop when the extra gain stage got switched in?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on December 23, 2008, 01:32:17 AM
Hi Audioforge, I'm curious,( I know next to nothing about neve circuits) how did the originals prevent the pop when the extra gain stage got switched in?
[/quote]


As far as I understand it .. they didnt. There was a off position on the gain switch at that same position where the additional gain stage kicks in. I dont have an original 73 but If I had to guess.. The unit powering on and off with the line to the mixer unmuted would probably result in a pop.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 23, 2008, 12:52:45 PM
Audioforge

Thanks,

Which resistor failed on your amp board - give location (example r29) ?

I will put my oscilloscope to work and trace signal flow with 1khz sine wave.
should all gain selections be checked or only the ones that give trouble ?


Is the shorting betweeen pos. 6-7 posing a risk of component failure on the amp board
wether it be a transistor , cap, resistor, IC or diode ?


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on December 23, 2008, 08:16:31 PM
Is the shorting betweeen pos. 6-7 posing a risk of component failure on the amp board
wether it be a transistor , cap, resistor, IC or diode ?

I just tried it and after ~10 seconds 7R8 (47ohm) got hot and began to smoke/discolor. If I would have held it longer the resistor probably would have failed.

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_lout-power.gif
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 24, 2008, 09:15:25 AM
 better that the phase reverse is on the output ,
wonder if those 1000pf to ground are Needed ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on December 24, 2008, 12:00:59 PM
Tarnsauce,

Good note.

I've built Neve BA's before from etched boards and parts and never seen that resistor run as hot as you describe.

What happens to the temp of that resisitor when you change the mic/line push botton switch to either position
( in/out ) ?

Next thing would be to get a temp probe on that resistor to see just how hot it gets
and compare with my unit. I have a temp probe here so I will get that info and post it.
and if the sucker burns then maybe the 5K adjuster pot (7W1) near the 2N3055 might need to be trimmed so that section won't run so hot. Not sure how they have these calibrated for crossover distortion with (7w1) set. But, its worth tracing the gain path to that area to see what is happening.


Audioforge,

did resistor 1R35 ( 39 ohm ) on the line gain switch fail in your gain switch resistor matrix ? you mentioned this
in a earlier post but not sure which location on amp board your referring to.


OKGB,

1k pico caps on the output XLR are certainly unconventional along with the phase switch , I not sure about those either. They seem unneccessary , I have not seen this technique used in Neve classic designs. Its usually on or before the input transformer. Those 1K pico caps are also on the mic and line input xlr board as well.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 24, 2008, 12:41:26 PM
After any of the other fixes are done  [ whenever i hopefully eventually get mine ]

I'll try pulling off those caps , agreed you would think there no need for them ,
unless it was a cheap fix / precaution for  oscillations
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on December 24, 2008, 02:27:19 PM
franck
if you switch to line before gopng to the bad "mic "shortcut position you disconect the mic matrix. so no pop and no shortcut when switching back to mic.
doing that will prevent damage on r7 as described by tarnationsauce.
r39 was dead on the unit you sent me.
 "will put my oscilloscope to work and trace signal flow with 1khz sine wave.
should all gain selections be checked or only the ones that give trouble ?"
if only one position give you trouble check the resistors and solder joint relatative to this position on the matrix.

myrlithra
there s no big pop on an an original 1073 when you "engage " the B284 extragain stage after the off position.
audioforge
happy xmass guys
.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 28, 2008, 09:07:03 AM
Here's a kick in the crotch

http://www.groupdiy.com/index.php?topic=31105.0

apparently these got guys got it together  and  / or
benefited from the chance grp beta buy

I Don't imagine we'll get anything from the chinese now
in terms of replacement parts [ well maybe on the next
order but we'll always be an order behind on getting the right things ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on December 28, 2008, 10:02:07 AM
Bo from Golden Age already had his prototypes when I ordered my gb 81´s, wanted the eq...
He offerd me one of those to buy back then but as I just had ordered...

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on December 28, 2008, 11:12:16 AM
As audioforge points out , it appears to be the same design
so why they'd give us ones without the off postion ???
going backwards , unless these were sitting around ,

not surprizing for the chinese to copy things and resell
them [ sometimes making improvments along the way
or as someone else buys it ]

curious question would be who started this design ,
Bo's seems to take into account some of the problems
of the acm ones
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 08, 2009, 01:45:02 PM
btw there no way to avoid the short between 6/7 position in mic mode( when the b284 ampcard add the extra gain stage) so flip to line before switching or build a new 3 deck 22 pos.attenuator.....
audioforge
I just saw that part of your post.. oops. that's why I asked my stupid question after.
Is there a source for that switch? I bought some similar ones on ebay for my PM660 but never saw a 3 deck one. Ideally China would send those for FREE, but I won't hold my breath... so if anyone knows where to buy this (for cheap) I'm looking for two.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 08, 2009, 09:07:16 PM
Hey Maxwall, maybe a new problem/solution for '81 owners?  Check out dgatwood's posts on page 284 and 285. He seems to think he doesn't have the transistor problem but that the noise it is mostly induced. And then some board connector problems too, it seems. Hopefully he'll find some new, helpful, info. Good luck all.

http://homerecording.com/bbs/showthread.php?t=180964&page=285
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 09, 2009, 03:43:10 AM
Added observation as copied from Black Market:

If anyone has received preamps here, or has read about the hum problems, if you have time, then could you contribute any thoughts you have on fixes?

I just put my unit through its paces last night, and the um is bad.  I'm sure others have seen posts elsewhere about this, but the main issues are that with the EQ engaged there is a nasty ground loop buzz when the notch filter selection is set to 360 Hz.  It seems to be across each frequency selection for that switch.  I also noticed that even the hi pass filter has hum problems as well.  As you continue to move the switch towards 300, the hum gets progressively louder.  The worst problem appears to be a nasty pop that occurs when increasing the gain on the gain switch.  Somewhere around 8 o clock on the dial it pops hard.  It's so loud that it hurt my ears with headphones on.  If it had gone to my monitors, there's a good chance it could have damaged the speakers.  It's that bad.  So keep that in mind when you get your units.

Does anyone have a schematic for this?  When I got the unit I pulled the cover and noticed that there is no star grounding scheme for all the card connections internally.  I'm thinking of redoing the internal grounding, but I'd love to see a schematic so I can do it properly.  I'm sure I can suss it out just by tracing wires and such, but a schematic would make it much easier and would save some time.

Chance, is there any way you can make schematics available for these units?  I hear the ACMP-81 and ACMP-84 units have the same hum problem.

BTW, question for schematic already answered.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 09, 2009, 05:04:21 AM
I'd hesitate to use the term "ground-loop buzz" until it's confirmed that the hum is caused by a ground-loop. That said, it's certainly potentially a problem.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 09, 2009, 08:48:30 PM
Two notes from my side.

1.  Hum/Buzz

After replacing the transistors in one of my 81's with the BC's I still had hum/buzz... Rotating the transformer changed the quality of the buzz but did not eliminate the fault.

I made up a 10 conductor test extension cable to remote the power xformer and that has solved all of the buzz issues. Nice and quiet with EQ engaged.

I tested the remote xformer with an 81 that has the original trannies and still have some oscillation.

My conclusion is that for the best and most reliable results the power transformer needs to be outboard and the transistors need to be swapped.

I'm going to look into putting together a small project box, some 10-conductor-16awg cable, and dedicated 10-pin connectors to connect up the power transformer for each of my units... As well as replacing all of the eq board driver transistors with the recommended BC's..



2. The OFF-POSITION POP / Second stage shorting issue...

Just wondering if anyone thinks that removing the highest gain setting from the switch/unit, inserting an OFF position at the point where the second stage kicks in, and respectively moving up the resistor network one position would eliminate the "pop"... I personally don't mind losing the highest gain setting to accomplish this. That would be some fairly easy res-older and jumper work if it would resolve/remove the pop sound.

I'm also wondering whether the switch is actually a 12 position switch with a mechanical stop at the 11th position and it might be possible to rebuild it with an off position and all of the original sensitivity positions. The sensitivity switch does look the same as four others which are six position 3 deck switches.... I'm guessing that those might have a stop at the 6th position. Any thoughts about that?

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 10, 2009, 12:53:00 AM
good news... not an ideal solution for '81 owners but.... it's a step in the right direction. Thanks for doing the tests. Let us know if this solves the second units noise problem.

sorry, no help on the switch thing... I was planning on getting a whole new switch like the Golden Age pre... is there a source for that that anyone knows? Or anything similar (cheap chinese switch, not Elma, etc)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 10, 2009, 07:44:49 AM
The Rotary switches on the 81's (prolly the 73 and 84 too) are modular multi-deck 12 position switches (14 pads for each deck)..

All of the rotary selectors on the 81's use the same switch with stops and the number of decks configured appropriately...

On the preamp board it looks like 13 of the 14 pads are used The 12th position/14th pad is unused.

On another note, these switches may be relatively inexpensive for use in other projects.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on January 10, 2009, 09:44:06 AM
The Rotary switches on the 81's (prolly the 73 and 84 too) are modular multi-deck 12 position switches (14 pads for each deck)..

All of the rotary selectors on the 81's use the same switch with stops and the number of decks configured appropriately...

On the preamp board it looks like 13 of the 14 pads are used The 12th position/14th pad is unused.

On another note, these switches may be relatively inexpensive for use in other projects.

Cheers,
jonathan
I just took the gain switch from my 81 apart. The stops are not adjustable. It'll require a Dremmel and you'll need to make sure you don't remove too much.

I was hoping to gain the extra position, then just move over the 2nd gain stage resistors by one position and point to point wire the highest gain resistor.
I think the "off" position will need a resistor to B-, or ground. I'm not sure how the original 73 was wired.

Probably just easiest to sacrifice either the highest 1st gain stage position, or the lowest 2nd gain position for the off position.

(http://img.photobucket.com/albums/v646/red_3000/how_to/S1052286.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 12, 2009, 07:57:13 AM
From Homerecording forums:

Quote from: DGatwood
Further analysis on the hum:

I'm definitely seeing some oscillation in the preamp circuit. Replacing the transistors probably will fix the problem, but I'm not entirely convinced that this amplifier circuit oscillation is inherent with the parts chosen. I'm seeing too many obvious problems with the board layout to jump to that conclusion—places where they could easily have dropped in an additional star grounding via a case screw but didn't bother to do so, for example.

I'm also seeing ground traces that are no bigger than the V+ and signal traces. They really should be. On a board like this, I would have expected every board to have a ground plane. That means a double-sided board in which one entire side is a giant ground trace, with every screw passing through a plated through hole connected to that plane. Sure, that ground plane might have the occasional V+ trace running up through it, with gaps around components where the leads go through—maybe the occasional extra trace where jumper might be used with a one-sided board—but otherwise, pretty much a solid copper ground plane on the entire face of the board.

When I looked at this board and didn't see any traces wider than about an eighth of an inch, I quickly became concerned. On further analysis, the entire 24V ground bus for the circuit is only grounded on one end, and that's the end closest to the humming toroidal monstrosity. Having more than a foot of ground between one end of the circuit with a trace width of 1/8" means you're probably between 0.05 and .1 ohms of resistance, depending on how thick the traces are. (No, I'm not going to measure them.) When you're dealing with circuits in the presence of that much EM, even that tiny little bit of resistance counts. A lot.

Needless to say, this is a big reason that the circuits are oscillating. When circuits like this oscillate, what's happening is that some of the output (which contains 60 Hz hum) is bleeding into the voltage lines for whatever reason. The result is that the signal below some frequency is getting boosted massively---equivalent to the maximum gain for the transistor (as opposed to a controlled feedback loop with a resistor in it). Now all the boards have capacitors on the voltage lines to cut down on this, but they need an excellent ground for this to work.

So as a ten second test to see if the grounds suck, I tried hooking up a .47 uF capacitor temporarily across the +24 and 24V ground connections on the right end of the rightmost jumper board. It didn't do anything. Then I tried the same thing, only using a wire wrapped around a case screw for the minus side and the hum dropped in half. So basically whoever did the board layout on this thing screwed up. There simply should not be that big a difference in ground potential between two electrically adjacent boards.

Grounding the foot-long ground bus between the upper and treble boards made the biggest difference. By itself, this cut the hum fairly dramatically. Soldering a capacitor at the right end of the right jumper board between the +24V and the 24V ground and a grounding wire between the 24V ground and a case screw knocked it down further. At this point, with proper cable placement, I'm at probably about a tenth the hum. It's still humming, but the level is close to that of the 73s. Getting closer.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 12, 2009, 08:06:53 AM
Thanks for posting.

In all respect, in general & imho DGatwood is overemphasising the importance of BIG groundtraces.

As long as it's not in common then a bit of additional resistance wouldn't hurt.

In general and if possible, w.r.t. crosstalk etc it's better & more effective to approach grounding by wisdom (separate branches & star-star-star, might be relatively small) than by brute force (big wires, ground planes).

When you can't separate, yes, then go for The Big Wire.

Regards,

  Peter 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 12, 2009, 08:10:24 AM
Peter, I've replied over there. Basically suggesting lifting the board grounds and replacing with a star ground.

Also, pointed out that oscillation often manifests itself as hum in the audible range (when it's not motorboating or whooshing). Sounds like the transistors should still be swapped too though.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on January 12, 2009, 08:13:04 AM
Nice work everyone.

So far I'm thinking that the transistor swap, sorting the grounding and moving the PSU trafo out of the box is the best plan for silence and not too intensive as a mod.

The gain switch is a bit of a ***** though. Maybe that board can be redone with a new switch.

-T
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 12, 2009, 08:58:39 AM
Tom, there were some tests made to see how much the PSU was actually responsible, and it's not really clear yet from what I can tell. It's certainly hard to say when the units are oscillating since it leads to so much hum itself (which can easily be triggered or retarded by moving cables and the transformer itself).

I really want to get hold of one of these so I can put some time into figuring out what's what. For instance, if someone could swap the transistors and run off a bench PSU, then we'd have a better clue as to what's going on.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on January 12, 2009, 09:11:36 AM
Absolutely mate, just going on 0dbfs post above, sounds like moving the toroid solved some issues. I've not had time to really follow the threads so am not 100% on the real faults etc.

The minute I get one I'll try it on a bench PSU.

-T
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 12, 2009, 09:30:37 AM
Hey Roddy, thanks for consistently reporting on this. I too am looking forward to you getting your hands on one, as well as the rest of the hard core guys here, since I trust you guys will get to the bottom of this.

I am wondering though, maybe these toroids are bad? I remember reading in the EZ1290 thread about 3nity fixing Bluzzi's pres. They had hum which was caused by a bad toroid. Maybe these cheap China toroids are emitting more crap than a decent toroid would/should and maybe swapping it with another would help (instead of taking the PSU out of the box)?  In final costs it would maybe work out about the same, if you add up the separate enclosure, multi-pin connectors, cable, etc... and would be more elegant since it would be in the same box. Wishful thinking?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 12, 2009, 09:54:02 AM
I agree, it might be worth trying a different transformer. We'll just need to see what comes from testing these boxes when we gte them.

A lot of the broadcast audio kit at work uses toroidal transformers in mumetal cans. It might be an option to use a smaller model in a can in this case.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 12, 2009, 10:29:23 AM
1.
I've got 4 81's and have swapped the transistors in all four of them (ie; removed original TO-92's and replaced with recommended TO-30's).

2.
I tested the location of the toroid using three tests:
A-> Inside the box.
B-> Hanging out the back of the box.
C-> About 2 ft from the box with a 10 conductor jumper.

As expected, A was the worst, B was in the middle somewhere and C is the best.


Quote from: DGatwood
Further analysis on the hum:
Grounding the foot-long ground bus between the upper and treble boards made the biggest difference. By itself, this cut the hum fairly dramatically. Soldering a capacitor at the right end of the right jumper board between the +24V and the 24V ground and a grounding wire between the 24V ground and a case screw knocked it down further. At this point, with proper cable placement, I'm at probably about a tenth the hum. It's still humming, but the level is close to that of the 73s. Getting closer.

3.
Just now I jumpered the 24Vdi connection on the rightmost jumper-board to chassis/mains gnd and bypassed +24V and 24Vdi (also at the rightmost jumper-board) with a .47u cap while the toroid was still in the box (original position toroid with upgraded transistors):
There is a very slight change in the hum for the better but not much. :(
I'm interested in keeping an eye on and testing other upgrades to the grounding to find the best combination of all.

4.
The writing on the toroid appears to label it as a:
PRI:
0-115
0-115

SEC:
16-0-16
0-26
0-26
0-45

That's what it measures unloaded as well (at least on the 115 mains setting).

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 12, 2009, 10:47:11 AM
Thanks for doing all these tests as well as for the toroid info..
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on January 12, 2009, 12:06:51 PM
A post I made on another board:

I had been messing with only 1 of my 73s up to this point. I read in another post where a guy was wiggling connectors and got some of the noise to go away so I thought Id look into it.

I took the cover off the one I had been messing with and starting poking around with it on and humming. When I touched the 1000uf 25v Cap in the back of the mid EQ board, I hear a pop. I took the board out of the chassis and found the soldering job on this cap was horrible. I just had to touch my iron to it and the whole solder joint got sucked up on my iron. I cleaned and re-soldered the part. Beware, there may be some poor soldering jobs on some of these.

I put the cover back on and this time decided to try both at the same time. The one I had not been messing did have a very minor hum (almost un-noticable) but was not humming like the other one. I suspect this will be mostly fixed by Brad's shielding. (which Im still in for BTW) Something else I noticed. I had both units, one on top of the other on my lap. The one that didnt hum all of a sudden started humming when I turned on the power of the 2nd unit. This to me sounded like something 2 fold going on here. The inductors are picking up EMI, but it also sounds like power transformers may not be consistant from unit to unit. My next test when I get some time will be to take the EQ board out of the one that doesnt hum and place it in the one that does to see if it inherits the hum or not and vise versa.

/end old post

I still havent gotten around to that test yet but I willng to bet the EQ board from the one that doesnt hum,will start once its in the other case. I think the power transformer in this case is most likely causing my hum.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 15, 2009, 07:13:37 PM
More notes on my 81's:

1.
Line input sounds heavily filtered (no low's, exaggerated hi's) and attenuated. Suspect bad line-in transformer.

2.
This is a comparison of a 400hz square wave from line in connection to the secondary of the xformer (maybe you can imagine what that sounds like?):
(http://diy.dbfsforge.com/TNC81/81-line-xformer.jpg)



3.
The same test using the mic in xformer both looks and sounds much more as expected. Like this:
(http://diy.dbfsforge.com/TNC81/81-mic-xformer.jpg)



4.
This is the same for all four 81's that I have.

Thoughts?

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: playboss on January 15, 2009, 08:06:48 PM
arent those xformers supposed to be the same as chameleon labs?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 15, 2009, 08:50:29 PM
Does the line-in sound very, very tinny? You can't see any sign of the fundamental there, let alone any much other low frequency content. You could try taking the trafo out of circuit by lifting the secondary.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 16, 2009, 12:03:22 AM
Not sure if they are the same as CL's or not. There are no markings on the IP trafo's.

The symptoms with the line in turned out to be wiring related.

Unfortunately the analog outs on the motu 24io and 2408 aren't really balanced + and -.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 16, 2009, 01:29:03 PM
I added an OFF switch-position to the gain knob in the #7 position where the second gain stage first kicks in. Now switching to the next position (#8) does not cause a pop.

I chose to test it out by replacing a single 680R resistor (R43) with a 1.5k, disconnecting one end of that resistor and jumpering it to the position between R42 and R43.

This pretty much replaces the #7 position sensitivity setting with OFF and keeps the original remaining 4 higher gain settings.

You could do this with a different value (replace R43 closer to 620 ohms) and replace R42, 41, and 40 with different values to make the gain steps more uniform up to the original full gain if you want.

Here are some snaps. Sorry for the bad cell phone pics (wife is out of town with the camera).

(http://diy.dbfsforge.com/TNC81/81-OFF-ORIGINAL.jpg)

(http://diy.dbfsforge.com/TNC81/81-OFF-MOD.jpg)

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 16, 2009, 01:36:34 PM
Nice... Thanks once again.. cheaper than a new switch that's for sure!

When your better half gets back home, I'd love to see some better pics.. If you can do macro that would be best.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 16, 2009, 02:15:41 PM
Hey Mitsos,

I'll post some better pics after this weekend.

I'm going to go ahead and finish my remaining units and try to even out the gain steps so that what was position 7-11 is now 7-OFF -and- 8-11 with smoother gain steps leading up to full-open-gain.

Just another couple notes:

1.
These units have spacers/washers in the boards between the chassis screws that appear to orient the 48V and polarity switches into proper alignment for the front panel holes. In other words, after dis-assembling and re-assembling them several times the push-button switches will become "stuck" if you don't get the alignment correct on re-assemble... Making the holes slightly larger might be a solution if you have them apart and don't want to worry too much about the washers.

2.
The issue I had before with the i/p trafo was not due to the motu. I was using an unbalanced TS cable to send line level to the 81..  :-[

3.
It appears that there are currently a handful of mods to get these units in better condition. Best to try and address them in one go if possible. Here's my running list:



Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: druedger on January 16, 2009, 03:15:53 PM
Added observation as copied from Black Market:

If anyone has received preamps here, or has read about the hum problems, if you have time, then could you contribute any thoughts you have on fixes?

I just put my unit through its paces last night, and the um is bad.  I'm sure others have seen posts elsewhere about this, but the main issues are that with the EQ engaged there is a nasty ground loop buzz when the notch filter selection is set to 360 Hz.  It seems to be across each frequency selection for that switch.  I also noticed that even the hi pass filter has hum problems as well.  As you continue to move the switch towards 300, the hum gets progressively louder.  The worst problem appears to be a nasty pop that occurs when increasing the gain on the gain switch.  Somewhere around 8 o clock on the dial it pops hard.  It's so loud that it hurt my ears with headphones on.  If it had gone to my monitors, there's a good chance it could have damaged the speakers.  It's that bad.  So keep that in mind when you get your units.

Does anyone have a schematic for this?  When I got the unit I pulled the cover and noticed that there is no star grounding scheme for all the card connections internally.  I'm thinking of redoing the internal grounding, but I'd love to see a schematic so I can do it properly.  I'm sure I can suss it out just by tracing wires and such, but a schematic would make it much easier and would save some time.

Chance, is there any way you can make schematics available for these units?  I hear the ACMP-81 and ACMP-84 units have the same hum problem.

BTW, question for schematic already answered.

Perhaps I should have elaborated here.  I need the FULL schematic, not just the signal path of the 73.  The 81 power schematic is significantly different than the 73 board, and in particular I need to know what the 9CZ7 connector is providing.  I think it might be +48 phantom, and I'll be scoping this today to try to suss it out.  Basically I am going to completely replace the power supply section and custom build my own.  I think I can do it with some PS parts I have lying around that will allow me to completely redo the supply and still keep the IEC plug and mains transformer in the box (I prefer this for convenience).  I have a switching power supply that provides + and - 12v, so I just need to add the +24 and +48 outputs, which I can custom design a PCB for pretty easily.  Having the power board schematic with all the connector marking for the 73 would save me some diagnostics and guessing, but for the most part I know what I need to do.

I'll definitely keep everyone posted how the hum is affected after I integrate my new supply.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on January 21, 2009, 11:55:16 AM
great to see folks are experiencing the same issues I have in my ACMP81's

now how to figure out the corrective actions ? I have had zero time to work on mine as I have been too busy with my regular line of work, but I'm following this closely.  GOOD WORK odbfs, thanks for your efforts !!!

I really would like to add a Elma 4130 gain switch and wire it to the board to maintain all
the proper gain selections ( resistors on switch, bypass resistors on the board) . Toss the cheap gain switch. This option comes second to the noise in this box. currently I have a 100% failure rate on all my boxes. The noise issue is the primary problem. Good to see that moving the transformer confirmed my findings in most of the noise.

Sure wish we were forewarned about these problems before the purchase. The boxes are unworthy for studio recording. Disclosure disclosure disclosure = integrity, right mates.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 21, 2009, 12:35:25 PM
I have tried a couple temporary shielding options in lieu of an external toroid.

I don't have any mu-metal to experiment with but have found that some perforated aluminum works better than steel or solid aluminum as well as nulling the toroid and mounting it at a slight angle so that it's not parallel to the chassis... Still, it's audibly better with the trafo OTB.

If a shielded toroid were available (avel lindberg?) with similar voltage and current ratings then that might well be a best "ITB" solution. Ie; remote box's and connectors start to add up depending on how nice you want it to look (for remote of the toroid) and a shielded trafo might cost similar at the end of the day. The included toroid does not indicate current ratings (at least in english! :) anyone know a method to compute current ratings on a given trafo?

The inductors are not shielded either. I've got an altec-4722-trafo can which may be mu-metal and would fit nicely around the inductors. Have not yet been able to empty the can to test that yet and i'm not sure whether I want to sacrifice 8x 4722's for that if it does help (in my case).. I was planning on using those for a couple four channel one-bottles possibly re-engineered for 12ax7's :)

I briefly googled the elma 4130 and it looks to be 103mm deep at first glance... There is about 55 mm behind the current/included switch before the mic and line input trafo's would get in the way. Of course anything is possible.  ;)

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 21, 2009, 12:43:59 PM
I've got the schematic saved here, and it gives the following ratings for each secondary of the transformer:

2 * 15V (CT) / .1A
26V / .2A
26V / .2A
45V / 20mA
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on January 22, 2009, 06:34:48 AM
no need to try and sheild the inductors. Pot core inductors by design are supposed to inhibit noise fields. Cinemag sells their Pultec inductors without shielding and they too are pot core designs. you can also compare the factory Neve inductors which are not shielded either.

On the other hand, a custom wound shielded power torroid might work well. But not one made in China.

Ever polish a turd ? well thats what this China preamp is , a turd. try to polish it and its still a turd.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 22, 2009, 08:46:24 AM
It's not uncommon for preamps to have seperate power supplies ,
often just to make it easier to sell into other countries , but as well
to  eliminate the noise caused [ radiated ] especialy in cramped
designs .

Although there was disscussion about it being better to feed D.C. from the extrenal supply because of the same a.c. radiation ,
IT is easier and in this case for a retro fit , to
Simply extend the secondaries [ anyone know how many  ? maybe not ] move the power xfmr and make an extension cable

So A.C. or D.C. ?

And on the torriods some kind of low pass filter won't hurt eirther
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 22, 2009, 11:39:25 AM
So A.C. or D.C. ?
I'd say that AC would be better, meaning the reservoir caps inside the preamp-box. But it ain't no
real big-big poweramp, so...

If there's significant regulation and/or additional cap-stuff inside then it may not matter and
then passing DC from external supply to the preamp-box would even avoid
bringing in big 50 (60) Hz signals. But as I think to have understood it's the TX-strays,
not the AC-sec-wires, that cause the nasties, right ?
Then the AC should be doable.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on January 22, 2009, 01:32:45 PM
I think we may be on to something, but it's hard to say without doing an actual test. Does anyone have a Gauss meter? Is this something that could be checked with one? It'd be nice to test this toroid and a known good one for EMF. Maybe check inside a few pieces of equipment. I don't know if there are normal levels, but if I had to guess, I'd say there are limits to the amount of EM garbage a toroid can put out in, say, a medical device used around people with pacemakers..

anyway an easy test could be if someone had a working JLM 5 rail PSU and could bypass the whole PSU+toroid in the box to see if there is a change. I don't think this has been done yet? I hope my 84s come to me perfect(wishful thinking) but I will try this if the PSU gives me problems. If there were a noticeable improvement, I don't think it would be too off the wall to ask Chance to arrange for new toroids.  Not to be a dick, but at the quantities we're looking at, they'd be cheap. Wishful thinking with China I guess, but Chance did offer a warranty and to make things right in case of problems. Has anyone had any contact with him at all lately?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on January 22, 2009, 01:45:04 PM
...I don't think it would be too off the wall to ask Chance to arrange for new toroids.  Not to be a dick, but at the quantities we're looking at, they'd be cheap. Wishful thinking with China I guess, but Chance did offer a warranty and to make things right in case of problems.

Yeah right.  Good luck with that.  The warranty Chance is offering is "send it back to China".  He's not standing behind these at all.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 22, 2009, 01:47:00 PM
Chance did mention on the PSW thread that he would honor and deal with warranty issues after shipping is complete. I haven't heard anything else yet.

I don't think a toroid with these secondaries is a stock item but maybe China will honor a replacement.

I'm waiting to see how that plays out before spending the $$ on making remote boxes.

I think China should pony up a replacement (shielded or whatever)..

No gauss meter here but maybe a coil connected up to an amp-meter would provide a reference point against some other typical situations?

FWIW, I even threw in the kitchen sink with no difference  :P:

(http://diy.dbfsforge.com/TNC81/acmp81_kitchen_sink.jpg)

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 22, 2009, 02:42:21 PM
Did you remember to earth the kitchen sink?

Have you swapped out the transistors already?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 22, 2009, 03:15:19 PM
I did earth the kitchen sink... :)
Same results.. :(

This is with the new transistors...

Move the toroid out of the box 6 inches or so and the source of the field is far enough away to not be picked up by the inductors.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: playboss on January 22, 2009, 03:17:03 PM
what about an 1U case under the 2x 1U eq , holding both supplies? Someone was about to offer expansion cables no?
That 1U case might house some ledmeters ( I sell  ;D )
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 22, 2009, 03:26:12 PM
I wonder if one external PSU (or just a torroid for that matter) is enough to power two 81s?  If I can get away with powering both of mine (whenever they arrive  ::)) from one external supply/transformer, that would be ideal.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 22, 2009, 04:05:30 PM
Someone gave some current figures and it [ the transformer looked low ]
and knowing the chinese figure on underpowered as a cost saver
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 22, 2009, 04:28:45 PM
I wonder if one external PSU (or just a torroid for that matter) is enough to power two 81s?  If I can get away with powering both of mine (whenever they arrive  ::)) from one external supply/transformer, that would be ideal.

Might be doable, very doable. But why not solve the issue completely while at it ?
You might otherwise end up with potential ground-loops, at least additional thinking about it required.
And you won't be able to use those two units far away from each other. You might save a few tens of your fav. currency but introduce things you need to be aware of at the same time (but if you'll always use these in pairs then it'll be less of a potential hassle)

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 22, 2009, 05:07:53 PM
I think he was asking  [ or please clarify ]
if one of the stock transformer had enough current
to power two units
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 22, 2009, 05:25:00 PM
Either way, if wmtunate uses the original Chinese TXs or a golden one with endless power and signed by Mr. Rupert himself, he'll introduce the issues of one supply box that's common to both boxes. That one-box supply is obviously exactly what he asks, but perhaps the consequences I mentioned weren't immediately clear ?

Related: if the orig Chinese TX 'just' disturbs but is of adequate capacity then you could of course put both in one box, to supply two preamps. You could even connect them in || if you want (I'd keep them isolated, both electrically as physically but that's just my preference).

But hey, no boxes here yet, not even humming ones, so what am I talking about ?!  8)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 22, 2009, 05:43:53 PM
I may try a different trafo with a shielding can. I'd need suitable voltages though...

I was thinking a JLM 48V PSU might be an option too.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 22, 2009, 05:45:55 PM
You haven't had too much of that strong Euro coffee today have you ?

Yes the grounding is the big issue , and there may be times when
you want to use the units separately  , otherwise one A.C. feeding
multiple bridges is not out of the question .

I got two greens yesterday , so hope your next
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 22, 2009, 07:34:19 PM
Now that I look again at the torroid's current specs, it does seem pretty low.  I was just curious if anybody had approached the external PSU from that standpoint yet.  I know the Chandler power supply will run two different pieces of gear at the same time.  It was just a thought that occurred to me.  Saves a little bit of space behind my rack.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on January 22, 2009, 10:10:38 PM
Chance did mention on the PSW thread that he would honor and deal with warranty issues after shipping is complete. I haven't heard anything else yet.

Yes, he did.  But when one of my ACMP-73 arrived non-functional (ie, not passing audio) and I emailed to ask about a replacement, he replied that I should send my unit back to China.  That's not standing behind the product, imo.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 22, 2009, 10:26:14 PM
Clint has been asking how far we are in the process , but
it's not all shipped yet
My prediction is that the chinese will offer chance a
 new improved model which will be a new sale
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 23, 2009, 05:02:06 AM
You haven't had too much of that strong Euro coffee today have you ?

Tokkin' to-mie ?  ;) If it was about my previous entry sounding a bit harsh perhaps then ignore;
I never post sarcastic stuff.

Quote
Yes the grounding is the big issue , and there may be times when
you want to use the units separately  , otherwise one A.C. feeding
multiple bridges is not out of the question .

Might very well work indeed, but under weird circumstances some weird consequences are not to be excluded
(the units are DC-linked). Again, while at it I myself wouldn't cut the corner of saving one secondary winding,
but it's not up to me how everybody should make his/her boxes, I'm just suggesting.


Quote
I got two greens yesterday , so hope your next
Hey, congratulations, I'm happy for you (not in jest) ! Enjoy !

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 23, 2009, 07:49:07 AM
IF the chineve xfmr COULD supply enough current for two pres ,
that may be slightly convenient but having the second psu xfmr
as a backup would be wise , otherwise one psu going down
could put two units out of commision , so make two anyway .

Hey Clint next are you going to tell you don't smoke those
Stinky Euro cigs either   ;)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 23, 2009, 08:39:10 AM
If it was about my previous entry sounding a bit harsh perhaps then ignore;
I never post sarcastic stuff.

(http://img.photobucket.com/albums/v358/AlexClarke/Starfighter/sarcasm_detector.jpg)

"It's off the charts!"
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 23, 2009, 08:58:09 AM
 
:D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on January 26, 2009, 02:26:55 PM
I chose to test it out by replacing a single 680R resistor (R43) with a 1.5k, disconnecting one end of that resistor and jumpering it to the position between R42 and R43.

This pretty much replaces the #7 position sensitivity setting with OFF and keeps the original remaining 4 higher gain settings.

You could do this with a different value (replace R43 closer to 620 ohms) and replace R42, 41, and 40 with different values to make the gain steps more uniform up to the original full gain if you want.
Thanks!!! and that goes out to 0dbfs and everyone who's been posting here.  I've been following this thread with interest and trying not to post, due to my chronic ignorance condition, but after enduring that pop as many times as I now have, I'm compelled to ask:

1) by "jumpering it to the position between R42 and R43" do you mean taking the lead of the resistor that's farther away from the switch (I'll call it the "far lead") and soldering it to the same place as the far lead of R42, so that R43 runs diagonally away from the switch, with the near lead soldered to the original pad, and the far lead soldered to the same pad as the far lead of R42 (or to the far lead of R42 itself, assuming there's not room on/in the pad for both leads)?

2) Since different values for R43 will serve in predictably different ways, if I'm not worried about elegance or uniformity, would it work to merely re-wire the existing resistor at R43 into the new configuration, instead of replacing it?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 27, 2009, 02:12:36 AM
Received the first ACMP81 here is Oz. Another one to come and a ACM1200 mic still.

The shipping, which seemed to be Fedex, is obviously split up into 3 boxes.
Which is a good thing, because the value for customs was around USD 400 for the one pre. (much inflated)
Shipping ended up being around 70% of purchase price USD to Oz.

Anyway - the first ACMP81 out of the box - like others, I get :

- rack on  outside looks quite solid. I had to JAM it into my rack though - MUCH wider than normal.

- preamp section is very quiet. I mean *really* quiet at <-90dBu. This is with input and output connected to my Motu
  2408MkII balanced in/out, input gain at unity, output gain at unity and signal at -4.5dBFS on Cubase level meters.
  A 'normal' working level for me.

- EQ is a real stinker  >:( 
  Adds between 30dBu of 50Hz harmonics when all bands flat and eq engaged
  Lo/Hi pass - no effect on noise
  Lo and Lo-Mid add about additional 10dBu of noise  each
  Hi-Mid and Hi add about 15 dBu each.

- Mic switch engaged st the '-80' marking creates a Full Scale blast of noise - ear/speaker popper for sure!

Fix Mode ON
- rework the input selector
- change the eq transistors
- play with power grounding for kicks, then replace with external PS : toroid, JLM PSU and dc connect to preamp
FIX Mode OFF

Should be OK then.

Good thing we live to DIY around here.
Wouldn't want to have bought 8 of these and not be ready to do significant work!   :o

PS - makes me feel better about the stuff I throw together!  Quieter than this, for sure  :D
       I'll eventually be able to compare with the 2 81's I'm making from Tommy's boards with Carnhills




 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 27, 2009, 08:57:04 AM
That's great Alex. Thanks for posting.

How did you go about converting to use the JLM PSU - did you use the 48V model?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 27, 2009, 09:07:28 AM
Hey Alex are you 50 or 60 hz line fequency in Oz ?
curious if this is general line contamination and
perhaps grounding issues
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 27, 2009, 09:56:42 AM
I'm guessing 50Hz as they use PAL video.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 28, 2009, 05:09:00 AM
Hi Guys

I'm on 50Hz here in Oz. The most noise is at 50Hz and 100Hz decreasing at 150Hz and 200Hz.
Just starting up the PSU replacement.

Using a single (non-China!) potted toroid 50VA 24-0-24VAC  feeding (in parallel)  2x JLM Powerstations
each giving DC rails of +48V, +24V, +/- 12V and one negative rail unused.

A 5 core cable (rated at 2A each core) on 5-pin XLR socket out of an external rack from each
of the Powerstations to each of the ACMPs. I have to check out the grounding and figure out
the best way to do it.

Just received the parts today, including BC441s and BC461s (non "-6" variants, I'm afraid!)

Should have something tested this weekend, I hope. I'll post my findings as soon as I have something good happening.

Actually not so much of a hardship, as I already am building a 2RU 'DI' box to feed these ACMPs.
Uses JLM FET DIs (and also has a couple of Edcor based reamps and  Haufe based DOA style additional DIs for fun).
So, I just upped the VA of the toroid and add a couple of Powerstations and connectors.

With a bit of luck, these puppies will sing yet!

As a preamp, they sound pretty good. My normal reference mix (Stone Roses Second Coming)
sounded pretty full - no obvious frequency response problems. Knobs all worked fine.

Also, received the ACM1200 and other pre. The mic looks reasonably good - some
denting on the top of grill as others have noted and the pad/lo-cut switches pretty nasty too.
But the rest looks good. Will be burning it in and testing this weekend.

I tell you - I'm just happy to finally start moving to a more familiar DIY phase of this project.

Thanks to Chance - I believe he really worked hard to get this GB done.
I wish him luck in future GBs and thank him for his efforts.


See you


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 28, 2009, 08:28:50 AM
thanks for the nice detail, i haven't seen the schematic lately
but you figure a 5 conductor cable will do all the dc rails ?

D.I. ahead of the preamp input xfmr , nice option to have
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on January 28, 2009, 09:25:53 AM
I'm compelled to ask: ...
My compulsion got the better of me and I went ahead and rewired the one lead of the existing resistor at R43 to the leg of R44, and it seems to work for me -- thanks again!!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 28, 2009, 10:22:45 AM
I'm compelled to ask: ...
My compulsion got the better of me and I went ahead and rewired the one lead of the existing resistor at R43 to the leg of R44, and it seems to work for me -- thanks again!!
Awesome!
I put a little heatshrink around the end of mine to prevent intermittent connections with the previous solder pad.

Cheers,
Jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 28, 2009, 10:31:09 AM
Hey Antichef [ what'd food ever do to you ? ]

it's be good if you referenced r42 thing as it comes out of context
are you talking about the gain switch ?

tia just trying to get ready for mine when they come
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on January 28, 2009, 12:22:29 PM
Hey Antichef [ what'd food ever do to you ? ]

it's be good if you referenced r42 thing as it comes out of context
are you talking about the gain switch ?

tia just trying to get ready for mine when they come
sorry - that was indeed a few steps removed from 0dbfs's original post:

http://www.groupdiy.com/index.php?topic=27791.msg381731#msg381731

yes - the gain switch.  I did it to both of my 81s and both of my 73s, and I'm thinking the numbering may have been different on the 73s, but the layout was the same. -- anyway, here's my picture, fwiw:

(http://www.recursor.net/popfix.jpg)

(I like food...)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on January 28, 2009, 01:16:09 PM
Because the value for customs was around USD 400 for the one pre. (much inflated)

Really 400USD for one preamp? Thats more than excessive, good work on the other fixes here though guys.

-Tom
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on January 28, 2009, 01:23:52 PM
Using a single (non-China!) potted toroid 50VA 24-0-24VAC  feeding (in parallel)  2x JLM Powerstations
each giving DC rails of +48V, +24V, +/- 12V and one negative rail unused.

A 5 core cable (rated at 2A each core) on 5-pin XLR socket out of an external rack from each
of the Powerstations to each of the ACMPs. I have to check out the grounding and figure out
the best way to do it.

Hey AlexC,

If you get a chance I would like to know whether you still experience hum with your potted 50VA 24-0-24 toroid inside the chassis. Not sure if that's a test you can do easily while you're at it or not.

If it turns out that a diff toroid in the case with a dedicated 5-rail-JLM-Power-Station works well, I might go that route instead of remoting the original toroid..

Thanks in advance!

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 28, 2009, 03:05:28 PM
Because the value for customs was around USD 400 for the one pre. (much inflated)

Really 400USD for one preamp? Thats more than excessive, good work on the other fixes here though guys.

It might be nice if Chance could sends ours as "going to be repaired" or similar in the tax declaration seeing as they don't bloody work! Or maybe there still is another way... We could always pay a US middleman to forward them!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: playboss on January 28, 2009, 03:35:12 PM
as pennyless i am i already talked to him about this and he said to remind him once the green route is prepared  :D. I used to buy <50 usd rack gear from usa ebay and it comes thru without hassle, but the seller has to indicate it . So I guess its ok to tell him only once if yo have the greenie.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 03:10:26 AM
OK - so I have 1st mod results for the ACMP-81

- changed Q4,Q5 on 4 EQ bands to bc-441 and bc461 (non "-6" variant)
- replaced toroid with my usual China 30VA 24-0-24VAC unshielded
- replaced PSU with JLM Powerstation (+48V, 24V, +/-12V, Gnd)

My results are :

- basic noise floor of my converters < 90dBu 50Hz harmonics. Bal In/Out to AD/DA.

- with EQ OUT, noise floor slightly higher than before but 50Hz harmonics all < -90dBu  :o
   very little (almost none) added noise at all gain settings

- with EQ IN, all bands 0 gain, OFF frequency  : very little change to 50Hz harmonics    :D

- with EQ IN, Hi and Lo filters ON, all frequencies: very little change to 50Hz harmonics    :D
                  Hi       band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, <-90dBu mid hash    :D
                  Mid-Hi band ON  Max + gain, all frequencies   : very little change to 50Hz harmonics, <-90dBu mid hash    :D
                  Mid-Lo band ON  Max + gain, all frequencies   : 20dBu of 50Hz and mid-freq hash added   >:(
                  Lo       band ON  Max + gain, all frequencies   : very little change to 50Hz harmonics, <-90dBu mid hash   :D

- with EQ In, all good filters and EQ bands in and max + gain, all frequencues : < -85dBu 50Hz harmonics, < -90dBu mid hash  ;D

So - nearly there. Seems internally mounted toroid + new PSU + replace transistors is *almost* great.

Now to work on the remaining offending Mid-Lo EQ and then try out externally mounted toroid + PSU.   :-\

See you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 03:18:22 AM
Should have added :

- haven't checked 2N3055 bias
- was done at unity gain for a 'normal' level of about -4dBFS on Cubase meters
- didn't change Grounding scheme : case grounded at XLRs and JLM PSU 100R+47uF between chassis and signal star
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 29, 2009, 03:39:28 AM
Thanks Alex, that's really helpful.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 04:00:37 AM
Glad to help (after taking away so, so much from this forum    ;D)

Further futzing about on the offending Mid-Lo band shows that
the amp section is not noisy - definitely the filter part.

And it doesn't look like 'regrounding' the section does anything,
so I think it is induced noise.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 05:10:16 AM
Another part of the puzzle

- subbed out the offending lo-mid band board with one from my second acmp-81
  No change to the noise

- swapped position of mid-hi and mid-lo boards and the noise moved with the board
  There was still in the mid-lo position a mid-hash at -85dBu but all the 50Hz gone
  The Lo-Hi position now showed the 50Hz noise and higher mid-hash

So it looks like :

- the Mid-Lo board 50Hz harmonic noise is not position dependent but inherent to that board design
- the Mid-Lo position has some inherent mid-hash noise induced

Also, putting a 12ax7 tube shield on the inductors reduces mid-hash  by 3dBu (mid-hi) and 6dBu (mid-lo)

OK. Now what? Try to debug the Mid-lo board ? Check layout for bad or misplaced parts?
Look into replacing the inductor?

Now to externalise the toroid and PSU or 'learn to live with some noise in the mid-lo band'

That's it for tonight!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 29, 2009, 05:45:52 AM
Mid-Lo band ON  Max + gain, all frequencies   : 20dBu of 50Hz and mid-freq hash added   >:(

By the way Alex, did you actually mean +20dBu? Ie, your converters were either close to or clipping? Or did you actually mean that the noise floor rose by a factor of 20dB?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 05:53:06 AM
I mean an additional 20dBu to take the noise from around -90dBu to say -70dBu.
Not close to clipping.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 29, 2009, 06:10:31 AM
I mean an additional 20dBu to take the noise from around -90dBu to say -70dBu.
Not close to clipping.

So, an additional 20dB with respect to the noise floor; I think +20dBu confused me as dBu is a unit of voltage level as opposed to a ratio. That's maybe not awful, especially since we might not take that boost frequency to maximum often. I do like things to be working as best as possible though!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 06:18:11 AM
One more bit

- subbed in a mid-hi board from second ACMP81 into the mid-lo position.
  So I now have 2 mid-hi boards installed in the mid-hi and mid-lo positions

  No 50Hz harmonic noise at all. And the overall mid-hash dropped a couple of dBu too.
  That rules out basic position dependent induced noise.
 
  It's either the layout differences between mid-lo and mid-hi that is the problem
  (they are about 90% the same excepting the filter section differences)
  or it is the inductor.

Tomorrow I'm going to :

- test the second ACMP without the toroid and PSU changes but with the transitor mod
- swap a mid-hi inductor into the mid-lo board to see if it is the inductor
  or the board layout of the mid-lo that is the problem
- do the external PSU and toroid

Now I'm going to bed.
I have to deal with real world instrumentation system QC issues tomorrow
(the ones that you get paid for!)




Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 29, 2009, 09:32:25 AM
hey alex, over at the homerecording thread, dgatwood said that the electrolytic caps on the V+ rail local to each filter board were different values in the 81 than they were in the 73.  I guess the 73 has them somewhere around 1000uF at each filter board, but the 81s are in the neighborhood of 100uF.

Also, he mentioned that the ground traces on the 81's filter boards were much smaller than those of the 73.  Where the 73 has a huge ground plane over the entire board, the 81 has tiny traces.

here's some of the relevent posts:
http://homerecording.com/bbs/showpost.php?p=3088534&postcount=7317 (http://homerecording.com/bbs/showpost.php?p=3088534&postcount=7317)
http://homerecording.com/bbs/showpost.php?p=3090116&postcount=7376 (http://homerecording.com/bbs/showpost.php?p=3090116&postcount=7376)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on January 29, 2009, 10:53:03 AM
Today I was going to try your test of moving the Mid-hi board from the 73 I have that buzzes to the 73 I have that doesnt. For me, they both buzz with the EQ engaged and the Mid hi set to .36 but one does it far, far worse than the other. Instead of moving the Board, on a hunch, I moved the Transformer from one to the other. Orieted them the same as they were in the previous chassis and the buzz ended up moving with the power Transformer. On the one that use to buzz badly, (wearing a leather glove) I rated the Transformer around till it was almost all gone. On the one that didnt previously buzz but now does, I did the same thing, I could hear the buzz changing as I moved the transformer but It never really subsided, just changed harmonics. I do have some sheilding Im getting for the inductors and I anticipate that will mostly take care of the remaining buzz in the one that doesnt buzz, but I suspect Ill have to change the Transformer out on the one that does buzz now to get it to stop.

Something else I noticed. I had one on top of the other on my lap while testing these. I did notice the one that didnt buzz, started buzzing badly when I turned the other one on.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 29, 2009, 06:37:34 PM
Yeah grounding noise and induced noise is a real Alice in Wonderland thing.

Move this and check that. Notice 'this' is good and 'that' isn't.
Fix 'that' Now 'that is good' and 'this isn't'
Repeat 1000 times until you become psychopathic.
Then 'learn to live with it'
Give up music recording forever cause you can't stand the hypertension.

Like the clap, best thing is not to have it in the first place.




Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on January 29, 2009, 11:48:13 PM
Dont forget to wave the Voodoo Chicken Stick
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 02:59:00 AM
>Dont forget to wave the Voodoo Chicken Stick

That's a given!

OK : another bit

- swapped the inductor from the good mid-hi board into the noisy mid-lo board and put it into the mid-lo position
  EQ IN, mid-lo all frequencies, max + gain : 50Hz harmonics -90dBu and some mid-hash  -80dBu
  tube shield on inductor reduces mid-noise hash by 3 to 5dBu

  The noise level is much lower now than with the mid-lo inductor

So, it appears :

  The mid-lo inductor itself is contributing to 50Hz harmonic noise quite a lot
  The mid-lo inductor itself is contributing a little to the mid-noise hash
  The mid-lo board     itself is contributing a little to the  mid-hash noise

I would guess that a 'better' mid-lo inductor would mostly reduce 50Hz harmonic noise
                              a 'better' mid-lo board would mostly reduce the mid-hash noise


Now to the other ACMP81 which is stock PSU and toroid but with mod transistors and reset wiring away from toroid

Line IP, set to unity gain IN and OUT (about -4dBFS on Cubase meters), bal I/O to Motu with basic noise floor of <-90dBu

- with EQ OUT, noise floor very low with  50Hz harmonics all well < -90dBu, no mid hash noise     :D

- with EQ IN, all bands 0 gain, OFF frequency  : very little change to 50Hz harmonics, no mid hash noise :D
- with EQ IN, Hi and Lo filters ON, all frequencies: very little change to 50Hz harmonics, no mid hash noise   :D
                     Hi         band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, no mid hash noise   :D
                     Mid-Hi  band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, mid hash noise <-90dBu   :D
                     Mid-Lo  band ON, Max + gain, all frequencies   : added 20dBu 50Hz harmonics, added 20dBu mid hash noise <-90dBu   >:(
                     Lo         band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, mid hash noise <-90dBu   :D

- now sub in the inductor-replaced mid-lo board
  The 50Hz harmonics have decreased about 8dBu and the mid-hash has decreased about 5dBu.
  Has not decreased as much as before
 
So I conclude that :

- the onboard replacement toroid and PSU have contributed very little improvement except perhaps some on the mid-lo board maybe 5dBu or so.
- the wiring reset has also done very little
- the transistor mod fixes everything except the mid-lo band which stubbornly remains NFG
- the mid-band inductor is itself the main culprit on 50Hz harmonic noise and some mid-hash noise
- the mid-lo board itself is main the culprit on mid-hash noise and some 50Hz harmonic noise

Leastways, that's how *I* sees it.

Next - externalification of the toroid and PSU. This will probably take a few hours.

See you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 03:56:49 AM
And again ...

Externalification of toroid and PSU done. Here we go again. ACMP81 with transistor mods.
No change to ground scheme

Line IP, set to unity gain IN and OUT (about -4dBFS on Cubase meters), bal I/O to Motu with basic noise floor of <-90dBu

- with EQ OUT, noise floor very low with  50Hz harmonics all well < -90dBu, no mid hash noise     :D

- with EQ IN, all bands 0 gain, OFF frequency  : very little change to 50Hz harmonics, no mid hash noise   :D
- with EQ IN, Hi and Lo filters ON, all frequencies: very little change to 50Hz harmonics, no mid hash noise    :D
                     Hi         band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, no mid hash noise     :D
                     Mid-Hi  band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, mid hash noise <-90dBu   :D 
                     Mid-Lo  band ON, Max + gain, all frequencies   : added 25dBu 50Hz harmonics, mid hash noise <-90dBu    ???
                     Lo         band ON, Max + gain, all frequencies   : very little change to 50Hz harmonics, mid hash noise <-90dBu    :D

FLYING F****G SON OF A B****

Subbed in the mid-lo board with the swapped inductor and the 50Hz harmonic noise decreases to about half.

Well at least it shows that the PSU and toroid aren't the problem regarding mid-lo 50Hz harmonics.
Externalising definately got rid of the mid-hash noise. I only have the 50Hz harmonics now.
Probably half due to the inductor, half due to mid-lo board.

Well - that's all folks.  - Your mileage will probably vary!

I'm now looking at mid-lo board layout tweeks.



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on January 30, 2009, 04:16:35 AM
I only have the 50Hz harmonics now.
Probably half due to the inductor, half due to mid-lo board.
Just an idea: as a temporary/troubleshooting utility perhaps an emulated coil (gyrator & caps) could be used ?

I didn't do the maths here, but this should be well feasible. It won't be the most time-efficient perhaps,
but could sure help for a structured approach to the troubleshooting (it'd skip questions like whether the coils are shielded enough or not)

Regards,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 04:53:55 AM
Could be helpful - been a while since I did a gyrator.

I actually have a couple of Carnhill inductors - I may try one of these.

But I'm thinking that the issue is a layout  things now.
It doesn't seem like an induced noise issue as much as a good ol' on-board layout thing.

I'll probably start removing components in the hope of narrowing down the issue.

When I unplug the Gain pot, no noise. Unplugging it simply removes the whole filter
network from the feed-forward and feed-back path of the whole board, unless I read the circuit wrong.
This leaves the amp in circuit. So the amp seems OK. I'll do some signal tracing to confirm that.

It may wind up being a case of foregoing the use of mid-lo freqs 220Hz - 390Hz at anything more
than 1/3rd + gain. That reduces the effect to a sort of passable level.

On the upside, most of it works with the suggested transistor mod and I don't think external
power supply is really worth doing. I could live with the around -80dBu mid-hash this removes.

Please feel free to make any observations on my postings!

See you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 30, 2009, 08:09:15 AM
How about publicly thanking you for all the work & effort

Thank you

Bummer as i often like to cut a narrow notch around 200 ish ,
but [ not totally serious ] perhaps a passive mains notch ,
could we get it steep enough , Ha !
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 08:35:26 AM
No worries Greg - I just wish I could find a solution, tho'. 

So far the 'Zmix' solution alone has done all the good stuff.

I've just been following in the foosteps of dgatwood and others on HomeRecording regarding grounding problems,
as suggested earlier in this thread.

I can certainly confirm his observations about high-resistance ground wiring across the unit. I too see 6mV DC
and up to 80ohms to signal star along the ground wiring.

So now I'm doing as he suggests and upping the filter cap on each board as well as adding a heavier guage
ground wire from the ground of each board to star ground.

I've reduced the dc voltage and the resistance but not yet checked with analyser to see if it has helped any.

I'll have some more results tomorrow. Day 2 of work *finished*

Bye for now
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 06:17:31 PM
Some more...

Upped the +24V filter cap from 100uF 35V to 1000uF 35V as well as added heavy guage grounding
wire from the -ve cap leg (ground) to star signal ground on each of the EQ boards.

No change to noise on mid-lo board.   :(

Noticed something odd.

The noise level at output of unit (contributed almost exclusively by the mid-lo board) reduces
as you increase the input gain selector to max.   :o

At max input gain, the mid-lo noise is almost gone.

Am I missing something here? Why should the noise level decrease as you turn up the system volume?


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 30, 2009, 08:08:52 PM
It's bad gain staging that's just attenuating the full signal ?
maybe there's a path that loops to ground or has more resistance at high att and raises [ or lowers ] as the gain switch is turned up

Talking out of my hat cause i don't have the schematic [ where i can find it ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on January 30, 2009, 08:15:55 PM
I think 81 schems were here some months ago, or if not I can send them even if I can´t host them

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 08:28:44 PM
The preamp Line Out noise (in this test almost all lo-mid noise contribution) reduced when the IP Gain is increased.
Also when the signal input is increased at the generator.

And another ...

I removed the inductor from the mid-lo board and jumpered the pcb inductor holes at pin 1 and 5.
This has the effect of allowing me to keep the filter in circuit but remove the the inductor.

Obviously it is not  filtering the same anymore, but it does keep the gain  pot, resistors and caps of the filter in circuit
(for Frequency positions 2,3,4 200Hz, 270Hz and 330Hz - the worst ones)

No noise. I checked that the gain knob still boosts as before (but not in the way of an RLC filter , just RC)

So that indicates amp is OK, the filter bank is basically OK and not introducing any inherent ground or induced noise.

To me, it's the inductor. It just likes to buZz or HuuMM or whatever, at least in the presence of this board layout,  in this unit.

I'm now subbing in one of my Carnhill VTB9047 inductors. It's not shielded or very special, but it is the one used in other home brew 1081s.

Later..
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on January 30, 2009, 08:41:29 PM
You are getting there, it seems  ;)

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 09:30:13 PM
Finally ....

Carnhill inductor doesn't buzz (excessively).

That's right - mid-lo inductor board with Carnhill VTB9047

220Hz, full + gain, 50Hz harmonics <-90dBu and mid-hash <-90dBu.

Checked carefully lest mine eyes deceive me.   ;D

This is in stock ACMP81 with the EQ boards Q4, Q5 transistor mods only.
Same toroid, same PSU, onboard.

To be sure, it still adds some noise, overall about the same now as the mid-hi. Perhaps 10dBu above converters.
But definately not the same madness as before.

I'm now going to clean things up and put in permanently and do some more testing,
but I think it's in the ball park now, where things like external toroids and sheilding
and ground improvements will have some reasonable impact.

Disclaimer - these things have a way of changing inexplicably, so I'll monitor for a while and report back.

See you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on January 30, 2009, 09:35:08 PM
So when you upgrade these with Carnhill transformers you should think of the inductors also

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 30, 2009, 09:41:01 PM
Here is pics

(http://www.users.on.net/~alexc/res/acmp1.jpg)
(http://www.users.on.net/~alexc/res/acmp2.jpg)

More testing now.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 30, 2009, 10:39:02 PM
I guess Colin is going to have a run on those !
nice work
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 01:50:45 AM
Thanks Maxwall

I'm glad to add to the efforts of everyone on working out how to polish these puppies.

--------------------------------------------

My opinion on mods to the ACMP81 is :

1. replace the Q4 and Q5 transistors on all 4 EQ boards to BC441 and BC461
2. replace the Lo-Mid EQ board inductor with a Carnhill VTB9047 or equivalent

Based on my observations, this should  take the unit to a minimum working level and I expect <-80dBu total
noise with all bands engaged and max boost and individual spectral components <-85dBu.

--------------------------------------------

External mounting of toroid and PSU I expect would give another 5-10 dBu improvement.
I can't comment on whether the BC441-6 and BC461-6 variants will improve because they are not available anywhere I can find.

Further investigations I'm doing are :

1. check mic input
2. check 2N3055 bias
3. check for distortion
4. frequency sweeps at the EQ bands and gains
5. effect of using the correct values of mid-lo band inductor resistors  R18-21 (3K0,3K3,3K3,3K6) instead of ACMP 3K3 for all


Finally, I will "taste of sweetness sense of music" and  "let each note be injected into my soul"
     
So far it feels like this unit has injected something, but it ain't sweetness. And not into my soul!

I'll post my results as I get more.

Suffice to say, I feel a whole lot better.   But I'm still cautious.




Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on January 31, 2009, 03:08:27 AM
Boy, I need to check with Cinemag. They can do pot core inductors. They have the ability to encase them in Mu Metal cans just like the Pultec Cm150's they offer. The metal can is a option that adds a additonal $20-30 to the end price each. But may help in this case.

Looks like these inductors may have been wound poorly, very poorly.

Unfortunately my units are shelved for now until I can find time to get to them, but I want to fix the annoying gain poping and overloading first , just can't stand this issue will all my units.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 03:29:47 AM
Just been testing the modified ACMP81 in my main rack.

Placing it in my rack next to other units has added about 5-10dBu of noise, so my figures have become :

Line Mode, Unity Gain, Bal In/Out
IP Gain at Position 7, Output Gain 11 o'clock

EQ In, Filters Out, Mid-Lo EQ Band  On, Max + Gain -> Overall Noise Floor -85dBu, individual components < -90dBu 

EQ In, Filters Out, All EQ Bands On, Max + Gain -     > Overall Noise Floor  -72dBu, individual components < -76dBu 

That's a bash test, pretty unlikely and so, my basic worst case test.

So, here's where the shielding and externalising would  improve things, I think.



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 03:52:30 AM
Now for the fun stuff ..

Playing my Tele Thinline into a clean, hi quality DI and into the ACMP81 Mic In with 48V on.

Sounds really good. No noise that I can hear. The EQs are pretty powerful. Its sounds great on guitar  ;D ;D
As far as guitar sounds go, I'm pretty fussy. I'd have no worries using this to record clean guitars.

Bass - fabulous. Hofner heaven  and JBass paradise   8) 8)

Acoustic Guitar jim dandy! The eq really works well on this. Crisp highs all right.

I'm going to have a go with the ACM1200 now. Start burning it up.

At last, some good soul-injecting going on
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 31, 2009, 07:27:29 AM
For as heavy as everyone says they are , it sounds like a steel case
so putting one on top of another makes them more noisy in a
grounding way ? or ?

does one rack space of blank make a difference or enough ?

Of course now Alex , as if you haven't worked hard enough
we want to hear your gtr playing  , lol !
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on January 31, 2009, 12:13:00 PM
AlexC

As I am gathering
Seems the only way to fix the problems is re-design the boards, which is'nt an likely endevour for a lot of owners here. It also seems that if the boards can be re-worked the noise floor will not be eliminated like we hope for, only reduced. But this does not guarantee we're left with a good recording tool.

Am I exaggerating or is this on target with the price tag. Because the amount of effort going into making these useful equates to just making one from scratch using tried and true design techniques which would result in a better sounding preamp.

Looks to me , we've been had.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on January 31, 2009, 01:20:24 PM
AlexC

As I am gathering
Seems the only way to fix the problems is re-design the boards, which is'nt an likely endevour for a lot of owners here. It also seems that if the boards can be re-worked the noise floor will not be eliminated like we hope for, only reduced. But this does not guarantee we're left with a good recording tool.

Am I exaggerating or is this on target with the price tag. Because the amount of effort going into making these useful equates to just making one from scratch using tried and true design techniques which would result in a better sounding preamp.

Looks to me , we've been had.

So far, the fixes to get me 80% of the way to a no-issues unit are easy and cheap - rotate the toroid, replace the output transistors on boards 4 and 5 (I used 2N2222's and 2N2907's), and shield the toroid and the inductors. That's an hour or two's work (mostly for cutting and forming the shields), and $20 of hardware.  My only remaining issues are residual hum (though greatly diminished) from board 5, and general circuit noise at max gain from the preamp board, and to a lesser extent from the eq boards in general.

I still have more fixes to try: replace small signal transistors with lower noise units, possible grounding issues, and of course, the input gain "pop" (though there is a workaround for this.)
So the story isn't finished yet. And though some of us are spending some considerable time and effort on this, I think in the end the sum total both in time and $ of the fixes for individual units will be minimal.

So will we be left with a "good recording tool"? That will depend on individual needs.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on January 31, 2009, 04:06:28 PM
Some seem to have units that are totally fine , [ or are they ? ]
if that's the case then it's fixable

maybe a combination of things
bad inudctor with board layout

but it you have to replace the whole thing , hey
maybe we can get the next version on warrenty
[ not bloodly likely , how about a deal & less fixes , ha ! ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 05:52:19 PM
I think that the performance is varying for different people. Some people's work well, some have grounding issues
and some have other issues.

Shielding of various kinds will certainly make a difference. As crazydoc said, it is relatively easy to do.

It's not uncommon for additional noise to be added when the unit is in a rack. I see it all the time
in my units. It can add several dBu easy.

My experiences with the unit show me that there is definately an issue with the inductor.
It resonates beautifully at frequencies at which it operates ie. round the area of 50Hz harmonics.
The problem followed the inductor in all my tests. Nothing else made a significant difference.
Shielding never showed the potential of fixing the inductor noise.

I am preparing a set of analyser screen shots showing the performance I'm getting.

I do think the unit is worth persevering with. When it's not buzzing like a bastard, its sounds great.
For me the cost so far is around 20aud for transistors and 30aud for an inductort to get it where it is now - at the minimum level of good performance.

As to being had - I'm OK with it. But then I luv DIY and I didn't buy lots of units.

If I wasn't an EE type and expected a working unit out of the box, I'd be mightily bummed.

If it had of arrived performing like it is now, I'd have had no problem at all with it.

That said, there is a reason why a real one costs 6Kaud and this one cost me 350aud - not much more than an
effects pedal.

Overall - stick with it. It's a really good sounding unit.

Yes, I think I will post some guitar noodlings thru the unit. I was really impressed with the sound.
Played thru yesterday for hours, which is a great sign that it is doing some unexpectedly nice.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 31, 2009, 06:26:58 PM
I think I owe you a few pints Alex. Thanks for continuing with this.

I'm interested in how using the Carnhill inductor helped; I just don't exactly see why. As far as I understand, these inductors are simply a coil wrapped around a core - why would the Chinese model perform worse with respect to EMI (I'm assuming the problem is inducted EMI)?

Cheers,

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 07:21:59 PM
No worries Roddy - I like to be part of a community and DIY is my favourite place.

I'd be lost without it.

I started in EE when I was a kid to learn how to build  a guitar amp cause I couldn't afford one on my McDonalds wage.
Then I built those dodgy kit mixer kits and stuff only to find they were pretty awful, so I gave up diy audio.
Focussed on my day job to afford commercial gear and left DIY behind.

Then a bunch of years ago, a friend told me about this place. Now this is *real* audio diy.
I rediscovered the love of EE - back to my roots, you might say.
7 units later and 15 more in the works!

But I'm still mystified by the black arts of grounding, noise and impedance matching!
And  also neat wiring!

----------------------

As far as why the inductor may be dodgy? I don't know. The other one is very similar in construction and
its board is very similar. Yet it doesn't buzz. Mainly because it operates at frequencies not near 50Hz harmonics.
On the dodgy one, when you operate it at the higher frequencies, the noise is not there.
It's that 10% care that makes 70% difference.

The Carnhill reduces it a lot. More than 20dBu on the 50Hz harmonics.
If I was inclined to go all the way, I'd get a shielded inductor from Sowter at 100aud.
But overall, I'm happy with the Carnhill for the cost.

This unit is like Eliza Doolittle. You can clean it up and it will be pretty and passably mannered..
Buut it will never be an aristocrat.

Which is fair enough for the cost.

Would I recommend it to a non-DIY friend. Nope. Glad I didn't. I'd be up for reworking theirs too!

On the upside -  As a guitar/bass/acoustic box, you cant beat it (with mods) at the price.


-------------------

What mystifies me is why I can't see any improvements when improving grounding, as others have reported.
It's clearly a problem, yet when I regrounded and upped the filter caps - nothing. I watched on the analyser as I did it.

In fact on that unit, 3 of the EQ boards mysteriously died within 5 minutes of each other, after testing for 1hr. I still haven't identified the
failure. Nothing obvious. Changed Q1, Q2 and Q3 with no fix. I don't know if it's related, but only that unit has had any failure.
And  2days of constant messing around with no issues before I regrounded them.

---------------------------

Bottom line is that with the mods, it's basically  OK * for me*.
Your mileage may vary.
Others are reporting good results with shielding and regrounding alone.

Me - I will try keep the Carnhill, maybe add some shielding.

Then I'll focus my energy on my 1081 builds.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on January 31, 2009, 07:26:08 PM
Check this out:

http://homerecording.com/bbs/showpost.php?p=3092577&postcount=13 (http://homerecording.com/bbs/showpost.php?p=3092577&postcount=13)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: jdbakker on January 31, 2009, 07:40:25 PM
Check this out:

http://homerecording.com/bbs/showpost.php?p=3092577&postcount=13 (http://homerecording.com/bbs/showpost.php?p=3092577&postcount=13)

Running extra ground wires to "nearby ground screws" as well as the PSU ground screw sounds like a recipe for lots and lots of ground loops.

(Hm, maybe this should go to the HR thread)

JDB.
[no ACMPre here yet, so no way to do anything other than being an armchair EE]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on January 31, 2009, 09:14:52 PM
jdbakker ,

I agree , ground loops galore.

These units are going to look more like a EE project than anything which is'nt all bad for the learning experience ( in favor of that) but when talented minds are sratching there heads over this , its like AlexC said not for the weak handed.

Rodabod,
Your inductor question raises some valid questions
for example, like I learned with Pultec cores, the chinese pot cores may be a cheaper recipe
than what is used with Carnhill pot core recipes. But how do you check for inductive EMI
compared to Non-inductive EMI ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on January 31, 2009, 09:45:20 PM
Could it be that the Chinese inductors have too much resistance in the winding? Ie. currents induced forming greater voltage than with a low resistance coil?

Alex, could you compare with the Carnhill possibly (unless I am talking nonsense!)?

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 11:24:03 PM
Yes I can compare. When I fix the bad EQ boards, I'll capture and add to my big file of analyser screenshots.

There was absolutely no doubt that in my case, the inductor was  major noiser compared with the Carnhill.
It was clear as my shouts of joy when I replaced it.

I tell you, I spend half my life looking at buzzes, ground loops and so on.
In my home studio, all I did for the first 6months was measure it and check for good combinations.
It was a standing joke with my professional studio owner friend!

I guess it is pot core material, winding guage, purity of the copper (or other!) wire.

The Carnhill taps DC resistances are 1036, 840, 611, 500 ohms
The 'soul-injecter buzz-o-matic' inductor shows  455, 227, 176, 82 ohms

That's a fair difference. I reckon if you set the other one up to operate boost in the 50Hz harmonic region,
it too would buzz.

Also the pot core seemed pretty conductive and no pot core clip ground as there is on the Carnhill.
Maybe significant or not. It certainly buzzed more when you touch it.

The Carnhill is just thrown in there with 5 wires hanging around. No special construction, it's quite
small and ordinary. Yet it instantly and repeatably removed 2/3rds of the noise.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on January 31, 2009, 11:43:17 PM

Still doing more. It shows the unit + transistor mods + Carnhill.
Rest is stock.



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 01, 2009, 01:48:06 AM
AlexC,

I'm trying to get a macro view of the problem before I narrow my sights on something.

Has anyone tried removing all the different board sections out of the power ground signal buss. Then starting with the preamp board ,listen, and add the eq/filter boards in one at a time until the origin of the noise gets isolated. I not sure if this preamp has a modular capability or all boards must be in place to pass signals. I hav'nt tried this yet, just brainstorming.

If its even correct to call it a buss its that backplane that runs along the backside of all the boards.
seems to carry power supply voltages and grounds.

Is it possible that we have a floating ground bleeding over into a power ground or vice versa. Its then feeds the unit a ground loop that gets filtered thru the inductor and comes out sounding high pitched only on certain eq switch settings. Then disappears as the eq is switched to other settings. becasue we know that certain buzz hum noises are not found in a majority of eq settings. just particular ones.

zzzzz time for sleep.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 01, 2009, 03:43:57 AM
It is definitely a bus. Parallel lines all the way across the unit.

It's a little difficult because the power is distributed in one direction - right to left while the signal goes in the other direction.
You could do it by jumpering the power but that's 5 rails to jumper across lots of connectors.

I'm not sure that would clarify things.

--------------------------------

What I have seen is very little contribution to noise from anywhere except the 2 mid bands. And the mid-hi doesn't contribute to
worst noise, that is 50Hz harmonics, because it is not boosting in that part of the frequency spectrum.  ie. 700Hz and below.

As far as different frequencies giving different noise - yes that is correct for the mid-lo band. The closer you select to 200-700Hz
and boost, the more pronounced the noise.

The lo band, of course does boost that region, but it has no inductor to pick up the noise.

Certainly switching frequencies causes all manner of pops, farts and squeals. No surprises there.

And I also have seen some 'twitching' in the noise floor - the spontaneous 'clicks' that have been reported.
I think that is from the power supply - didn't see it in the other unit with replaced psu but I haven't focused on it yet.

Until that mid-lo mains noise is tamed, nothing else matters.

In my rack, the unit is 3 rack spaces from  an amp below. Turning on the amp alone adds 5dBu of additional mains noise.
Rack unit 3 above, a little less. Without the case lid, you get about 2.5dB worse.

This inductor is an exceptional antenna. The Carnhill significantly less so. I expect a shielded Sowter less again.
If you then remove the toroid externally, you'll get a couple more dB. Rotating the toroid is good for 2dB.

To summarise, in order of significance the road map as I see it.


Out of Box ----> Embarrassing Crap ----> Change Transistors ---> Mostly Acceptable  sans mid-lo --->

                 ----->  Change Mid-Lo Inductor  ---> Minimum Good Performer   ----> Shield mid-lo Inductor  ---->
               
                -----> Getting Semi-Pro   ------> Change and/or shield mid-hi Inductor  --->   Almost Studio Ready    ---->

                -----> externalise toroid and/or psu   ---> Sounding Good!  --->  rework grounding

Achieve full potential to become a full-fledged project-studio box with a big sound

Oh yeah - and do the input gain switch mod so as not to punch a hole in your ears or speakers. Repeatedly.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 01, 2009, 08:18:45 AM
I'd think some of the random noise to be additive ?
but that star grounding the individual boards
[  cutting their ground bus traces ] shouldn't be risky
6 or 7 lines back to the input or psu ground point
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on February 01, 2009, 09:32:22 AM
Alex this is superb work.

I have found some of the Chameleons pick up buzz on the MF inductor only when it is set to resonate at freqs approaching mains A.C.

Remote PSU and a new inductor sounds like a good plan - however this is really turning into a lot of work if having to do multiple units.

To be fair here, Chance claimed all pres were heard by a tech and they were good to go before leaving china, and a batch of 1081s had been pulled as they didn't pass QC specs.

I'm starting to wonder if I made an all mighty mistake getting involved here with a large order for friends... based on some internet statements which appear to be BS.

-T
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 01, 2009, 10:20:16 AM
I'm starting to wonder if I made an all mighty mistake getting involved here with a large order for friends... based on some internet statements which appear to be BS.

-T
Well, you know the rule: No good deed shall go unpunished.  :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TomWaterman on February 01, 2009, 10:58:07 AM
Tell me about it!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 01, 2009, 11:11:18 AM
I still think it was a stock design that they already had [ possibly rejected by someone else ]
Hey we want a english design preamp with inductor eqs ,
O.k. we make that for you
Can you change that ?
Yes we will change that
Can you fix that ?
Yes it will be o.k. thanks for payment we send them now thank you for business
we can make more again if you pay again

One possible advantage here is that modding things one at a time , will still leave you with a working
unit and be cheaper than building from scratch [ maybe ], depend who you are and what skill you have .

but there always was a vagueness and lack of concrete info , oh maybe this is the one Calmillion labs was working on , and what will the chinese do about warrenty & QC now ? fa i think
offer to sell a improved  , fixed version is my guess . I never expected it to be as good as a neve but did think they'd work
just not to a 2k perf level

but thanks again to the good people doing good work here
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 01, 2009, 01:21:03 PM
TomWaterman,


Quote
To be fair here, Chance claimed all pres were heard by a tech and they were good to go before leaving china, and a batch of 1081s had been pulled as they didn't pass QC specs.

You took the words right out of my mouth.

I to am stuck in a similiar situation based on Chances statements and units on hand for friends.

Complete BS.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on February 01, 2009, 01:41:38 PM
 "Every fatterer lives at the expense of the one who listens to him" .
the crow and the fox.
Jean de la fontaine 1668.....

nothing new under the sun...
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 01, 2009, 06:57:50 PM
I think that a 'repair' kit is required, consisting of :

REPAIR KIT
=========

- 4x BC441 transistors
- 4x BC461 transistors
- 2x shielded inductors

would just about get this to a reasonable state with about 3hrs work, consisting of :

i)  open up the unit (top, back, side screws and remove front panel selector knobs/nuts)
ii)  for each of 4 EQ boards
    iii) remove boards (screws, unplug connectors, unplug bus)
    iv) de-solder Q4/Q5 and resolder correct parts
    v)  prepare shielded inductors and replace stock ones
    vi) replace board (screws, connectors,replug bus)
iii) modify resistor(s) in the gain selector switch to remove pop
iv) close up unit (ensure no push-switch binding)
v)  test


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: jdbakker on February 01, 2009, 07:06:14 PM
[...] star grounding the individual boards
[  cutting their ground bus traces ] shouldn't be risky
6 or 7 lines back to the input or psu ground point

The individual boards have unbalanced ground-referenced signal I/O. If you star ground each board, the current consumption fluctuations of each board multiplied by the inductance of your star grounding get added to the signal. Best case this injects both PSU noise and signal-related components (possibly overtones); worst case it turns the system into one big oscillator.

You need either ground-follows-signal (current situation) here, or a whole bunch of interstage transformers.

JDB.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 01, 2009, 08:25:03 PM
I got rid of almost all the hum from that low freq inductor! Read all about it: http://homerecording.com/bbs/showthread.php?t=275506 posts 22 - 25

 ;D ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 02, 2009, 06:26:32 AM
Another couple of analyser shots.

http://www.users.on.net/~alexc/res/acmp81_mid-lo_inductor_test.pdf (http://www.users.on.net/~alexc/res/acmp81_mid-lo_inductor_test.pdf)   (208KB)


ACMP81 with Q4/Q5 mod, mid-lo inductor Stock and Carnhill
Note 50Hz harmonic at 200Hz down 15dBu in the Carnhill.

Based on other's comments, shielding Carnhill inductor should reduce maybe another 8dBu

I'm hoping to try a shielded approach soon.

PS - my dead EQ boards all had the Q4 BC461 pnp failure. Real mysterious. No shorting of power
or anything like that. Literally I was looking at the scope then dead. One after the other clocked off.
Oh well - back in business now.

PPS - I can confirm that the 'clicking' also occurs in my toroid+PSU replaced on-board unit.
so it's not that.

So I think that there is definitely a forward path here. High quality MuMetal seems to be easy to get in
a number of formats in the US. Maybe a group buy of some mu metal  ;) or damn the torpedoes and go Sowter/Cinemag

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: zmix on February 02, 2009, 12:31:55 PM
Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.

So sorry for the late reply, I've been off this forum for the past few months!!!


This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. This is where the additional preamp stage gets switched in..

7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

The additional stage gives another 20dB of gain to the circuit, allowing the -80 position. It's a standard N£V£ circuit trick.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 02, 2009, 01:39:37 PM
Good & bad report

Recieved two of my three parcels today ,
[ 2 73's , 2 amc - 100's , 1- amc 900
and 1 amc 200 [ subed for the 300 i ordered ]


One of the 73's seems good out of the box , the other has a slight broadband buzz
almost like flourescent lighting [ will do the xister mod ]

It's a little goofy the way they jammed the hi pass filter close to the
LM gain control so you think they're related at first because they're closer together than
the gain control is to the lm freq control .
on the good one there may be some  slight buzz & hash but it's under the system hiss
level , so seems like nothing good gain staging wouldn't get rid of .

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 02, 2009, 02:52:13 PM
crazydoc,

the photo's posted over at your links only work for registered users on that forum.

Could you post links here that don't require a password and username just to see them ?

thanks

______________________________________________________________________

Also talked with Cinemag today. I will be shipping the 81 inductors down to them for analysis and recommendations on having new ones made up that should reject noise a lot better than these
China examples. If were successful in getting the right solution , then I will be offering a group buy  on quantity order of high quality inductors to solve this problem. I will have mu metal can sheilding option added as well to the design. First , some prototypes will be tested before production of the replacement inductors.

any comments or suggestions will be gladly taken from members at this time.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 02, 2009, 03:41:17 PM
Hey Maxwall - that's a great initiative. I'm certain lots of 81 owners could
benefit from a good quality shielded unit with the correct taps.

I think the frequency points on these is a little all over the place.
Am still confirming certainly at least some don't seem right.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 02, 2009, 03:51:37 PM
crazydoc,

the photo's posted over at your links only work for registered users on that forum.

Could you post links here that don't require a password and username just to see them ?

thanks
Sorry - the pics were uploaded to the hr.com site

I'll try to re-create the postings here.

________________________________________________________________


I've been able to do some work on the 81 this weekend.

I grounded each board a la dgatwood, including board 1. I also added a 22uF filter cap to board 2 (which was what I had on hand.) Though all their ground traces are now within 0.1 mV of chassis potential, I hear no difference in the hash noise from the preamp board and eq boards, or hum from that damned lo freq inductor on board 5.

Here's a pic after grounding, with Mumetal shields in place on inductors and PS transformer.

(http://home.att.net/~crazydoc/ACMP/ACMP81-_020109_2.jpg)

So I removed the inductor from board 5 to make it remote, to see if that diminished the hum. There was so much noise from the 3 feet of unshielded wiring I couldn't tell.

(http://home.att.net/~crazydoc/ACMP/ACMP81-_020109_ind-long-cab.jpg)

I then thought to try to reorient the inductor with respect to the PSU transformer. This helped a little, but what made a major difference was being able to extend the shielding on the lower part of the inductor, so it is now enclosed in a long cylinder. (I'd tried capping the tops of all the shields last week - it made no audible difference.)

(http://home.att.net/~crazydoc/ACMP/ACMP81-_020109_ind-reorient.jpg)

So now I've got the major noise under control - Zmix's suggestions for reorienting the power transformer and swapping the transistors on the eq boards (I only did boards 4 and 5, and used different transistors), shielding the inductors, and now being able to extend the shield on that pesky low frequency one.

I've socketed the 1Q1 and 1Q2 transistors in anticipation of receiving a bunch of low noise transistors this week, to see if replacing these, or some on the the eq boards, will help resolve the remaining hash noise at high gain.

But I think what I have now is very usable.

(http://home.att.net/~crazydoc/ACMP/ACMP81-_020109_sockets.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 02, 2009, 04:05:27 PM
 Err , maybe you don't want to send Cinemag the chinese ones
[ or at least not to copy ] but send the specs of the sowter and
Carnhill

Yeah sorry of course i didn't think you were going to copy it ,
suppose it does make a good example of how not to make it ,
my point was , incorperate the good things of the carnhill & sowter

and yes , good energy Maxwell , we all shall surely benifit
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 02, 2009, 04:31:06 PM
okgb

I'd be sending these to Cinemag to get the footprint copied and correct inductance points for the taps. And We might get to find out why these China inductors are nothing but crap compared to a quality one.

I was advised that if the China inductors are made using certain common pot core material then they will be noisy by design, and no way to clean them up unless replaced with something better.

Crazydoc
thanks for reposting the photo's .
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 03, 2009, 07:16:04 PM
Talked with Cinemag again today......

Seems the Chinese recipe for good transformers is part of everyone's bad inductor experience so far. And the use of the Carnhill inductor already supports that evidence.

However , Is there any evidence to suggest that we are getting a oscillation problem when the inductor hits its saturation level at a particular frequency ?

Also, Is there any evidence to suggest that the power transformer is not being run without sufficient reserve current or power rating to prevent it from inducing more noise than a transformer being run with a conservative power reserve rating. I saying that if the transformer is run to its near limits or ratings it will be a noise generator as compared to one for example that is being run at half its power and current rating. hence giving lower noise or EMI to the mid upper and lower eq section of theis preamp.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 03, 2009, 09:20:47 PM
My first impression on opening the lid , was that the power transformer looked a little puny , so that & chinese cheapness
i wouldn't bet on much current reserve / headroom
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 04, 2009, 12:18:57 AM
Makes more sense now does'nt it., Greg

AlexC , whats your thoughts on this ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 04, 2009, 02:59:33 AM
On the question of the toroid running close to it's VA limit and so adding noise, I do not believe this is the case.
It is puny and possibly underpowered but I don't think it is at it's limits (in the Q4/Q5 modded case)

One of my units has a 30VA toroid and JLM PSU and it exhibits all the same noise. It is definitely no where near it's limits.

On the question of whether the the inductor is saturating and causing oscillation, I do not believe this is the case,
at least on noise floor tests, where there is no applied signal.

Running the unit at no signal , full Input Gain, all EQ bands on, max boost and full Output gain does not seem to change
the spectra of the noise, just adds more, as would be expected. It doesn't seem like the noise is 'unstable' or
jumping around, as I would expect if it were oscillating.

There is some oscillation but I don't think it is due to saturation.

Changing the frequency of operation away from the 50Hz harmonic region gives a steady decrease in the
noise - no sudden reductions which I would expect if it was oscillating at some frequencies and not others.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: hejsan on February 04, 2009, 07:48:44 AM
I think that a 'repair' kit is required, consisting of :

REPAIR KIT
=========

- 4x BC441 transistors
- 4x BC461 transistors
- 2x inductor shields  (or replacement mid-lo & mid-hi inductors)

would just about get this to a reasonable state with about 3hrs work, consisting of :

i)  open up the unit (top, back, side screws and remove front panel selector knobs/nuts)
ii)  for each of 4 EQ boards
    iii) remove boards (screws, unplug connectors, unplug bus)
    iv) de-solder Q4/Q5 and resolder correct parts
    v)  install and afix shield OR desolder and replace inductor
    vi) replace board (screws, connectors,replug bus)
iii) modify resistor(s) in the gain selector switch to remove pop
iv) close up unit (ensure no push-switch binding)
v)  test



Don't forget that the kit should also have a switch assembly that works. I don't know if it can be retrofitted, or if the whole preamp pcb would have to be replaced.. maybe a switch with resistors soldered on it's tags could have leads running to the pcb.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 05, 2009, 02:38:15 AM
Sorry for the potentially obtuse question... I've been following this thread closely with the understanding that most of the mods discussed here address the ACMP-81: the bad transistors (zmix's fix), the poor grounding, the poor shielding.

The pop in the 7th position of the gain switch is common to all 3 preamps, as is the need to rotate the power supply to minimize the hum.

Do the other ACMP-81 fixes also address the ACMP-84?

Maxwall, is it known whether the Cinemag pieces you're pursuing for the -81 will also fit the -84?

Are there other shortcomings of the ACMP-84 that have not been investigated?

--
matt.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 05, 2009, 04:31:19 AM
The preamp pop fix is supposedly the same for all units though the actual part label may be different from model to model.
The rotate transformer fix is for all 3

I dont believe either the 73 or the 84 have the transistor problem. (the 73 doesnt even have the transistors inside that were specified. I only have the 73 so dont take this as gospel.)

The 50/60 cycle hum in all units looks mainly to be caused by a combination of the crappy field the power transformer is putting out and the crappy inductors on the EQ board.
For me this was verified when I moved my transfomer from the one that buzzed to the one that didnt and the buzz went the transformer.
Countless others here and at another board I frequent have verifed the inductor problems. For me, it was when I had one unit on top of the other on my lap (cases all buttoned up), had them both hooked up and was flopping back and forth between their mute button on the console. One had a slight buzz to it till I turned the other on. The other buzzed regardless of which unit was on. Yes the buzz was a little more intensive on the one that did it when I powered on the other, but the change was insignificant. I then slid the units around on one another and the loudest buzz I ended up getting was when the transformer from one unit was right over the mid high inductor on the other. I did have the transformer over the mid low inductor but suprisingly , it did make it buzz but no where near what the mid-high inductor was doing.

For me,  the fixes will end up being.
1. Replace power trafo (or Power supply) on the unit that buzzes. ( probably end up doing both)
2. Replace inductors with properly shielded inductors.

I still havent decided which inductor to get yet. Check this out:
http://www.sowter.co.uk/eqinductors.php
Would the following entry be a part compatable replacement for the mid high inductor?
9810 Neve T1295 10/7/3/1.3 H 1000/800/490/310 ohms

If so, I might just order one and see what the differences are. Once Cinemag has a replacement, I might do that. Ive been watching the progress on that here and if a group order would be placed, Id be in for it.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 05, 2009, 05:41:48 AM
The DIY 1081  inductors :

Mid-Hi : Carnhill Part No.  vtb9048 with inductance taps at 205mH, 296mH, 504mH, 767mH
Mid-Lo : Carnhill Part No. vtb9047 with inductance taps at 1.43H, 2.06H, 3.56H, 5.11H

Looking at the Sowter parts, there doesn't seem to be an off the shelf part to suit.

They have a custom service and would wind them specially, I guess.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 05, 2009, 08:13:10 AM
Rats... I dont have the equipment needed to measure inductors either and unfortunately, the values arent listed in the schematic. Any ideas on what the values are to get someone to make them. Chance, if you're still checking in here, any way of getting the manufacturer to give us this information?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 05, 2009, 10:21:55 AM
Maybe some proper Mumetal shields (http://uk.farnell.com/oep-oxford-electrical-products/a262can/screening-can-transformer/dp/1172345?crosssellid=1172345&crosssell=true) might work with the original inductors.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on February 05, 2009, 10:28:12 AM
Further investigations I'm doing are :
...
2. check 2N3055 bias
...
swap out the 2n3055 and the mic input and i bet the things sing.
I'm at the point where I need to stop messing with my units ("if it ain't broke, don't fix it" -- decidedly not the mantra on this board, I know :D), but since there was some early attention to the 2N3055 in the 797 photo way back when, I've been wondering about it -- any opinions on what's there in what we got?

(and btw - I've got the 81s and the 73s, but not the 84s - I can confirm that the "transistor swap" with BC441[-6] and BC461[-6] doesn't apply to the 73s because transistors you want to replace aren't there to begin with -- all I've done to my 73s is shield/rotate the toroid and fix the gain switch pop)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on February 05, 2009, 11:18:04 AM
Rats... I dont have the equipment needed to measure inductors either and unfortunately, the values arent listed in the schematic. Any ideas on what the values are to get someone to make them. Chance, if you're still checking in here, any way of getting the manufacturer to give us this information?

If it's not known (perhaps some original N***-documentation ?) it could be calculated from the capacitance & intended frequency
(  freq = 1/(6.28*sqrt(L*C))  ), but the circuit-topology might be a clouding factor here.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 05, 2009, 12:03:55 PM
Values are listed in the '81 schematic. Otherwise, take them from the originals.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 05, 2009, 12:10:30 PM
Cinemag quote $25 per 500 piece order.

Mu cans available. Numbers must hit target and order must be prepaid but are refundable
if target is not met.

Cinemag feels the China inductors are inferior with certainty.

81 and 84 inductors will be different.

Modeling the replacement inductors after the China ones may not be the correct path.
I have a good LCR analyzer so I can measure the 81's
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on February 05, 2009, 12:16:03 PM
I'd love to have those inferior crappy inductors here, Here, HERE !  :P

But no green yet...


Sorry.....  ;)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 05, 2009, 12:33:51 PM
81 and 84 inductors will be different.

Is there any benefit to setting up a single group buy for both models? (or possible 3 models, if the ACMP-73 uses a third variety?)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 05, 2009, 12:51:35 PM
If the Mid Hi board in the 84 has the following frequencies:

.36
.7
1.6
3.2
4.8
7.2

Then the inductor for the 84 and the 73 will be the same. I believe it is. Is there someone here who has both a 73 and an 84 who can verify? If not, Ill take a hi res photo of my Mid Hi board for someone to compare with an 84.

Of the 2 '73s I have, one buzzes much worse than the other and I had been focusing on the worse of the 2. Today, I looked at the one that doesnt buzz so badly. I ended up making a small shield out of some cardboard and several layers of copper tape for the mid hi inductor. That got rid of almost all but the faintest of buzzes and that was with the output gain maxxed. With the gain at normal levels, I couldnt hear a buzz anymore.

On the one that buzzes badly, the shield made no difference. Im pretty certain that the cause of the buzz on this one is the power transformer but I wont be 100% sure till I replace it.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 05, 2009, 01:39:12 PM
Just for reference ,
 there are references to Alan Hyatt saying they had inductor resonance
problems and that they may have to be changed
this at least as far back as june last year .

Anyone have a link handy to those i was just looking but i can see my own
post saying , i hope it's a coomon inductor to change


got my 81's today and definitely an unusable amount of  Hum  / buzz
just by switching in the eq in without any gain

thanks for the legwork Maxwell , obviously if there were any common
inductor values among the chineve eqs it would be easier to hit 500
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 05, 2009, 04:10:44 PM

Thanks Maxwall for the Cinemag input.

> Cinemag feels the China inductors are inferior with certainty.   :P

500 target per piece sounds pretty unachievable?

I guess that leaves talking to Sowter or using Roddy's excellent suggestion of
OEP mu-metal shielding cans (didn't think of that!) or other mumetal solution.

Or awaiting some action from the GB organisers (Step 1 - wait. Step 2 - don't hold breath)

For us cheaper seats, sounds, like the Carnhill + OEP can.




Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 05, 2009, 05:20:34 PM
I could handle the grp buy on the 81 inductors and leave the 73'a and 84's to other members to setup, that is if the inductors have different henry values. Physically I think they are the same footprint and lousy core material.

I don't have any of the 73'and 84's just he 81's. Glad I did'nt get any more crap units from the same source.

The 73's and 84's should be easy to get as they are already available. Not sure on the 81's

I can 't see why we all could not wind our own inductors on high quality pot cores. it cant be any harder than winding a Pultec Toroid
inductor. But the carnhills and others are inexpensive enough just to buy outright.

Well 500 would be the cheapest price, but as the target number goes down the price goes up. So were probably looking at
$60 each at least, which is more than Carnhill. Cinemag just will not negotiate with me, I don't have the political pull and contact friendship like others may. And it only takes Cinemag a few minutes to wind one on a computerized winding machine. How easy is that.

Cinemag should be a little more price friendly, I agree , but I'm not a big player so the small guys take the hit. And they know it.

If I was Fender , Neve , Harman etc , or some other big company then I'd be getting samples thrown at me for free.

Either way this mess gets fixed, its gonna cost extra $100 or more to do something about it. Which is'nt welcoming news. But if its done right its not a show stopper either.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 05, 2009, 05:47:41 PM
OK - I thought the 500 was the target to wind any at all. That's better.

At higher prices, it still the best option.

And no doubt a fully sealed mumetal unit will shield better than a DIY solution.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 05, 2009, 05:56:22 PM
Maxwell did you explain the story to them ? they have been good to us in the past ,
for grp buys on the api outputs
Either David or Tom   , usually David
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 05, 2009, 06:09:53 PM
Sowter might be worth speaking too. They'd be guaranteed to be very high quality at least.

It would be nice if we could find a mumetal can source which is close to the industor size.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 05, 2009, 06:12:33 PM
Right Chae here at prodigy distributes Sowter correct ?
they may accomedate a grp buy if numbers were good
it would be responsable shopping to get three quotes  but

I just emailed David at Cinemag with a little more info
and he'll work up some numbers for us
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on February 05, 2009, 09:12:58 PM
Another company worth looking into is Tab-Funkenwerk. http://www.tab-funkenwerk.com/id2.html
I think Terry and Chance have a foot in the door with them since they are making custom transformers for the ACM mics.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 05, 2009, 10:26:16 PM
I'm not sure if what they are doing is making xfmrs for the pcbs that
they are making , that can be used with many mics including the TNC
Is it an exclusive ?

doesn't hurt to see what they say , so whoever has the best shot at getting a realistic price but they may be too busy with their own products [ the microphone stuff doesn't seem to be happening too quickly ]
why don't you propose the idea to chance
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 06, 2009, 12:36:56 PM
Sorry, another rookie question -- what's the best way to test for preamp noise? Is it valid to run the gain up to max with nothing plugged into the input? I've seen other posts that mentioned shorting two of the pins together; is this necessary/recommended? Can I just jam a paperclip into the female end of an XLR cable, or do I need a special shorting plug? Or should I have a quiet mic plugged in (in a quiet room)?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 06, 2009, 01:12:20 PM
200 ohm resistor between pins 2 and 3 of the mic input then you can mess with the gain to see where noise is.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 06, 2009, 05:16:40 PM
Okgb,

I spoke with David at Cinemag. Nice guy I've done business before , but maybe I'm not the right person to
do the negotiating on these inductors. No clout/pull with Cinemag yet.

Let me know how you do with it, and/or PM me otherwise.

Roddy,

Sowter would be ok if I was dealing in pounds ,but the exchage rate is never the selling point in USD.
Can's would be easy maybe cheaper, but your still left with a chinese made inductor so not sure about that.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 06, 2009, 05:57:33 PM
Hey Maxwell , email me
but i believe we'll be o.k.
[ find a way  ]

Where is anybody getting the transisters ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 06, 2009, 06:44:52 PM
Where is anybody getting the transisters ?
If you mean the Q4's and Q5's for the eq boards, I used 2n2222 and 2n2907's instead of the recommended ones - they work just fine, and are only about $0.50 apiece in TO-18 cans at Mouser.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on February 07, 2009, 02:15:21 AM
I got the BC441 and BC461 (but without the -6 extension) at newark.com -- more than what crazydoc paid.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 07, 2009, 03:36:44 PM
Hey guys  , what do you make of this
while investigating my ACMP 81 pre [ the eq section ] noises
i stumbled on a gain setting where all the noise
 disappears , has this been reported before ?

it's on the gain control of the board 5 with the  220 -1,200 freqs
at max gain the noise changes and gets a little more nasty
but right before that point there is a little area where it gets quiet
but still passes signal , you can use this frequency band but stuck with that
[ or use the off position ] gain setting all other bands become o.k.
max gain cut produces a simular effect
but the noise doesn't completely go away . regardless of the freq setting or off pos .
Also the board 4 becomes a little less noisy  [ still unusable ] when i touch
the 3 conductor cable that goes to it's gain pot .

ideas?


Curiously enough [ and i believe coincidental ] some of the inductor boards
on the 81's have had loose screws , hmmm
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 07, 2009, 04:46:26 PM
Hey guys,

been travelling for a bit without much internet and not keeping up with this thread.. Glad (sort of) to see it's still going, and just wanted to mention that another source for inductors would be Altran. They are quite a bit more economical than Cinemag on trafos so I would guess inductors would be the same. Plus they are really good guys and try to help out. I've been meaning to call them lately so I could ask about this too, but someone would have to tell me what exactly to ask for cuz I'm quite clueless on inductors.

Thanks to everyone who's been helping out here.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 07, 2009, 05:34:17 PM
The 1081 inductors should be :

8 pin package (5 connected), core material not specified
Shielded can

Mid-Hi :  inductance taps at   767mH,  504mH,   296mH, 205mH
            resistance at taps      105R,       57R,       46R,   24R

Mid-Lo : inductance taps at 5.11H,   3.56H, 2.06H, 1.43H
            resistance at taps  1036R,    804R,  611R,   500R

Source : Carnhill part # VTB9048, VTB9047 measured resistance
             DIY 1081 inductance specification
           
Physical package is :
circular outside diameter 28mm
circular pins arranged on diameter 26.2mm

Would need to send a sample to show physical arrangement.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 07, 2009, 05:39:33 PM
Interesting observation re the gain setting.

This is with the transistor mod?

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 07, 2009, 06:07:44 PM
No  , totally as i recieved them
it's almost like you find a null point , but it
must have something to do with the resistence
value at that point , if you still have a stock one try it ,
Imean it's be a drag to lose a band but it does allow you to use
the eq

fwiw , the Carnhill input transformers are bigger than the chinese ones
and don't look like they fit on the PCB , but the 73 has enough space
 on the back panel , if that isn't too close to the output xfmr
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on February 08, 2009, 04:38:03 AM
Yes - I remember before the transistor swap, I could set the gain on the two inductor boards at a point just above all the way down, and the hum (apparently that part of it from the board I was operating) would greatly diminish -- but then if I turned it all the way down the hum  came back some.

I made an mp3 of it - not that you'd want to listen to it - in the first post of this thread:

http://homerecording.com/bbs/showthread.php?t=275506

After the swap, of course, it became a moot point for me.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 08, 2009, 08:08:13 AM
the transisitors totally cleaned up yours?

the hum / buzz totally goes away but for me it's at the just before MAX gain
[ or right as roddabod points out maybe only the symptom

just got them this week so , of course i will do the xister mods
just gotta get some
thanks Zmix

as i say i wouldn't trust them for a paying gig , but good enough for home
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 08, 2009, 09:29:58 AM
Hum/buzz disappearing at certain points doesn't necessarily mean that he oscillation has stopped, despite the hum/buzz not being noticeable.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: antichef on February 08, 2009, 07:49:24 PM
the transisitors totally cleaned up yours?
I have two 81s and two 73s --

On one of the 81s, replacing the transistors in the two inductor boards *and* shielding/rotating the toroid completely got rid of the hum -  the other had some board related hum even after the treatment of the inductor boards, so I did the transistor swap on the two other boards with a similar config, and then along with rotation/shielding of the toroid, the hum was completely gone (I went ahead and treated the other two boards on the first 81 for good measure).

On both 73s, a mere shielding/rotation of the torioid got rid of all the hum (which is good, because the transistors are different, and you can't do the same swap).
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 08, 2009, 09:34:32 PM
The 1081 inductors should be :
................
Would need to send a sample to show physical arrangement.
Or maybe post a couple small pics??
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 09, 2009, 02:37:44 AM
Here are a couple of pics

- mechanical sketch
http://www.users.on.net/~alexc/res/inductor.jpg (http://www.users.on.net/~alexc/res/inductor.jpg)

- snaps
http://www.users.on.net/~alexc/res/inductor1.jpg (http://www.users.on.net/~alexc/res/inductor1.jpg)
http://www.users.on.net/~alexc/res/inductor2.jpg (http://www.users.on.net/~alexc/res/inductor2.jpg)
http://www.users.on.net/~alexc/res/inductor3.jpg (http://www.users.on.net/~alexc/res/inductor3.jpg)

That would help to get started. I can send one of each of the mid-lo and mid-hi inductor.
They are completely useless to me   :D
(not quite true - you could make a very useful 'gauss meter' out of these to test
for radiated mains noise. They are exquisitely tuned for that)

(failing getting replacement made, I will use Carnhill + OEP cans (which seem to fit OK))

Mid-Lo and Mid-Hi have same pin-out (looking at bottom of inductor)

Starting at Pin 1 and tapping to Pin 5 (max Inductance, Max Resistance)
Pins 6,7,8 are not connected.

Inductance and Resistance restated :

Mid-Hi :  inductance taps at   767mH,  504mH,   296mH, 205mH
            resistance at taps        105R,       57R,       46R,   24R

Pin 1   - Start of tapped inductor
      2   - 205mH,  24ohms
      3   - 296mH,  46ohms
      4   - 504mH,  57ohms
      5   - 767mH,  105ohms and end of tapped inductor


Mid-Lo : inductance taps at 5.11H,   3.56H, 2.06H, 1.43H
            resistance at taps  1036R,    804R,  611R,   500R

Pin 1   - Start of tapped inductor
      2   - 1.43H,   500ohms
      3   - 2.06H,   611ohms
      4   - 3.56H,   804ohms
      5   - 5.11H,  1036ohms and end of tapped inductor

And a pin to the shield would be nice - to connect to ground.

I'm pretty sure that's correct, but always nice for a second set of eyes!

Hope this helps....

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 09, 2009, 08:48:42 AM
On my lo freq inductor, pins 7,8,and 1 were no connection, IIRC.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 09, 2009, 11:37:15 AM
Dave from Altran sent me their normal shielded inductor package. Trying to attach it here.  It has up to 8 taps for different inductances,  but it is mechanically different, so it won't fit perfectly. I will email for pricing and to see if there are any other packages available, even unshielded, that could maybe fit better. 

Am I to understand that the Carnhill fits the PCB without modification or did you wire it in?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 09, 2009, 04:09:00 PM
On the pinout - depends on where you decide 'pin 1' will be.

I labelled 'Pin 1' to be the first connected pin and counted from there.

The Carnhill part does not drop-in replace the stock inductor, physically.
It is electrically compatible. Indeed, I think it will be closer to spec'd frequencies.

I have it wired with wires. See pic earlier in thread.
The OEP can will fit the board but it is strictly a DIY affair. Hack the can
lid, put leads to the inductor, insulate inductor and mount in can  etc

Messy, but around 50aud in cost (ie. relatively cheap - an aud is not worth s**t these days)
Cheaper than buying mu-metal sheet for me.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 09, 2009, 05:12:59 PM
Can someone recommend a part compatable replacement for the Mid Hi inductor for the 73? I dont have the equipment to measure the ones I have to find a replacement.
Thanks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 09, 2009, 05:41:19 PM
On the pinout - depends on where you decide 'pin 1' will be.

I labelled 'Pin 1' to be the first connected pin and counted from there.

The ceramic form has the pins labeled 1 to 8 - that's what I was going by. And it is important if you're going to get drop-in replacements, as the pin spacings are different. You'll notice they are in two groups of 4 with equal spacings, but wider spacing between the end pins of the two groups.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 09, 2009, 05:54:43 PM
BTW, here's my hack for this.

(http://home.att.net/~crazydoc/ACMP/inductor.jpg)


(http://home.att.net/~crazydoc/ACMP/inductor_clamp.jpg)


(http://home.att.net/~crazydoc/ACMP/ACMP81_done.jpg)

Not elegant, but it works.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 09, 2009, 06:27:34 PM
> The ceramic form has the pins labeled 1 to 8 - that's what I was going by.
> And it is important if you're going to get drop-in replacements, as the pin spacings are different.
> You'll notice they are in two groups of 4 with equal spacings, but wider spacing between the end pins of the two groups.

True enough.

When I measured it, I found the resistance (and assume inductance) increases as I have shown.
I wired it that way and it works as expected.

However I could be wrong - I'll check again and post back.

Thanks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 09, 2009, 07:40:20 PM
I'm not saying you're wrong - you're right that the pins can be numbered starting anywhere. I just wanted to be sure you realized there is a much wider spacing (in your numbering scheme) between pins 1 and 2, than between 2-3, 3-4, and 4-5, which is not clear from your drawing. This would be important if manufacturing or buying a replacement to drop into the board.

Thanks Alex for all your work on the 81 - it's been very helpful. (You wouldn't know of any schematics for the 73, would you?)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 09, 2009, 10:13:36 PM
Thanks Crazydoc - I too have followed your work with great interest.
Your input has been very much appreciated.

I like your shielding solutions, particularly on the toroid. The double layer
with cardboard spacer is really good! And the fact it has shown results
makes me hopeful.

Truly, I haven't done much - just discovering the same as you and others!

On the pinout, I just want to make sure I have it as I think.

On the schematic, the inductor entry point is marked as '1' and progresses to '5' with
inductance increasing as you go. So I think I wanted to be consistent with the schematic

- but this does not match the part pin numbering and also crosses the 'pin grouping divide'
  as you noted.

Sorry - I don't have the schematics for the ACMP73 or the ACMP84.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 09, 2009, 10:24:33 PM
it's worth noting those curious tabs around the pwr xfmr ,

doc did you first try an L shape around the xfmr then the full circle

And Roddabod can confirm i first mentioned twisting the xfmr long ago ,
if only that were enough , he makes a good point that even if you can't hear
the oscilations from the Xisters can still be there
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 10, 2009, 06:32:32 AM
First off:
http://goose-tracks.com/thegeekgoose/images/scem73wv9fi2.png

Its in Chineese and doesnt have the metering or power sections on it.  Ive asked for the 2nd page which should have that on it but Ive never receieved a response at any rate, its the only one I know of. (I posted it quite a few pages back BTW)

Now a couple of more pics:
(http://goose-tracks.com/thegeekgoose/images/midhieqacmp73.jpg)

Note the way the inductors are laid out. As if they could be 2 physical parts. We know thats not the case in these units cause theres only 1 inductor on this board. This confused me a little why I was having a hard time finding a part compatable replacement.  I went back to the source and found this:

(http://goose-tracks.com/thegeekgoose/images/1073midhieq.jpg)
In the original, they were 2 parts. Sowter has them. I might try replacing the 1 inductor with the 2.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 10, 2009, 08:27:15 AM
I can email the 73  [ chinese as recieved somewhere in this thread


(http://i83.photobucket.com/albums/j319/okgb/scem73gd2.jpg)

thanks for the good energy so far everyone


edit by clintrubber on Gregs request

thanks it was almost freezing my computer as an attachment , i can email it
to anyone who can't find it  , gboboski at shaw.ca
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 12, 2009, 10:04:30 AM
Finally got my stuff. Had arrived months ago at some family who just came to visit! Got 2 mics and 2 84s.

I tested one 84 to see what was going on with it, it had hum and buzz at first, so I played with the toroid a bit (ended up flipping and rotating), now the hum is gone, and the buzz is only there when I engage the mid band (I thought it was audible when engaging the low band as well at first, so this is an improvement I guess). Anyway, it is highest at .36, a bit less at .7, just audible at 1.6K and I think it's there at 3.2K but not sure... maybe I'm hearing things. It is more audible with the high-Q button depressed in all cases, and disappears when this frequency selector switch is set to OFF.

The low band is not inductor based, so maybe it was hum I was hearing in that band. Anyway, curiously enough, the low and high pass filters are inductor based, but they do not cause any buzz.

I wonder if a new inductor would help in mid-band case. Will try to find some copper foil and try shielding this one first.

Also want to add that the gain switch pop is still there when the pre is set to line-in, just much less of a pop.

I'll try to get to testing the second unit later.. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 12, 2009, 10:45:36 AM
The impedance selector , maybe only because of the lower gain  / ratio
makes a difference too the pushed in setting will give less Hum
i don't think it is humbucking or anything like that .
twisting the pxr xfmr made no difference on mine but maybe i'll try
flipping it , did you do that with the wire attached ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 12, 2009, 11:44:04 AM
Didn't try the impedance switch... oops. didn't even think of it cuz it's on the back.  Anyway, I did all this with the thing turned on, listening to the hum/buzz through headphones and trying to find the sweet spot. Here is a crappy pic I took with my laptops built-in camera, since I haven't seen any 84 pics yet.  I wonder why the other inductors don't cause problems.. Just the distance? 

Let me know if flipping helps.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 12, 2009, 09:05:33 PM
I opened up my only ACMP73 today to check it out. Here's the first thing I saw:

(http://home.att.net/~crazydoc/ACMP/ACMP73_buss-connector.jpg)


When my shipment arrived in December, I had an 81 that wouldn't pass signal, and this was what I found:

(http://home.att.net/~crazydoc/ACMP/ACMP81before.jpg)

That's 2 of the 3 I've opened so far (one 81 to go.) Mighty impressive assembly quality and QC on these.



The 73 at least doesn't have the eq oscillation, as the circuits are different. There is some 120Hz hum with eq switch engaged, which is improved, but not eliminated, by rotating the toroid. Both inductors have hum when engaged, especially the mid-low on 0.36 with boost. This is considerably improved with inductor shielding, so I think that shielding the toroid and the inductors is going to make this serviceable.

Noise/hash is equivalent to the 81 at high mic gain, and of course the ever-present gain switch pop is there in full force.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on February 13, 2009, 06:15:19 AM
Mighty impressive assembly quality and QC on these.

This is Mr. Lee speaking, the factory replesentative of the Chinese crone-fab.

Prease accept our sincele aporogies, but it ain't neccesaliry a QC ol assembly issue.

How about just poor cheap components & lude postmen thlowing alound in wild fashion with these boxes ?

Fol that leason we don't accept any lesponsibirity, have a good weekend, thank you  ;)

  Mr. Lee
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 13, 2009, 06:50:05 AM
Dont want any lude postmen around MY house!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on February 13, 2009, 07:44:02 AM
i ve build a two channel 1084 with carnhill inductors. and i ve got inductor oscillations (50hz) on the ba182c board (the hight pass filter). i ve tried shielding the power traffo the inductor , add L alu piece of sheet to separate the psu from the boards etc...with no real good succes.
only remove the psu cure the trouble.(btw the other ba182c inductor don t oscillate and is 20cm away from the psu traffo.(even the ba194 with a t1280 just above the 182board don t oscillate))
so now i have to do an external psu box.....
this to say that even a carnhill inductor can t solve all oscillations troubles induced by a power psu traffo.(i ve tried several one thoroid or not..)
i now know that there a good reason ams neve and others don t put their psu within the same rack as the EQ...
ps i ve tried my 1081 in the studio (only the transistors mod and mic "pop" resistor change (thanks 0dbfs)and a 620 ohm output resistor termination on a switch.
As found on the oscillo ,i don t have buzz on the eq. 'at least at usuable level for mixdown) 50hz here.
i hope i can soon compare the diy 84 and the tnc81 eqs ....
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on February 13, 2009, 11:20:13 AM
I wonder what kind of effect moving the inductors as far to the front-left by pigtailing the inductors.
And drill a new hole to move the mains transformer to the far right-back.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 13, 2009, 12:47:34 PM
I wonder what kind of effect moving the inductors as far to the front-left by pigtailing the inductors.
And drill a new hole to move the mains transformer to the far right-back.
I don't think moving the toroid a few inches will make any difference - the first thing I did was I move it over the edge (as far as the wires would allow) and it made little difference.

I extended the low-mid inductor with about 3 feet of wire well outside the box, (see post# 354 this thread) and the wire picked up so much EMI it was even worse. Maybe if the wires were shielded... (which is what I did for the 3" or so I used to move the inductor the way I did, though for that short distance it didn't make any differece if the shield were grounded or not.)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: esiu on February 13, 2009, 04:18:55 PM
Folks: I'm thinking of 'consolidating' some of the info in regards to fix/mods...etc. on TnC stuff (on a web page or something).  I want to gauge if there's interest in this effort (use the info, or help compile/validate info).
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 13, 2009, 05:39:37 PM
I'm sure many of us would be interested in helping with this. There's a guy at Gearslutz (maybe it's you?) that's interested in doing this too - you should get together to avoid duplication of efforts.

http://www.gearslutz.com/board/3911056-post1785.html

It would be nice if TNC were to take an interest in this too.  ::)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: esiu on February 13, 2009, 05:53:04 PM
I'm sure many of us would be interested in helping with this. There's a guy at Gearslutz (maybe it's you?) that's interested in doing this too - you should get together to avoid duplication of efforts.

http://www.gearslutz.com/board/3911056-post1785.html

It would be nice if TNC were to take an interest in this too.  ::)

Yeah that's me - I just cross-posted on GS/HR/PSW/PP today ^_^

slaveern...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 13, 2009, 08:18:11 PM
I'm sure tnc wouldn't mind if this just takes care of itself
so other than your time what could be bad about consoldating the info ,
 for anyone who hasn't followed the tech thread,
and good  the different grps info gets together

sure , thanks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 14, 2009, 11:16:23 AM
Thanks to Chance, here are the ACMP-84 schematics (archived together with the ACMP-81; I'll add the 73 when I find them):

http://recordinghacks.com/tnc-audio-preamps/

UPDATE: I plugged in an SM57 and recorded some clips of room sound. Two of my four ACMP-84s are pretty quiet. One has buzz. The other was DOA (hoping it's a loose cable inside). I put a "quiet" clip and a "buzz" clip at the URL above. The quiet units are quiet even with the EQ engaged; the humming unit hums with or without EQ. And yet even the quiet units are a lot noisier than my DAW's pre's, so I'm interested in cap/inductor/transformer/shielding/grounding upgrades. :)

--
matt.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 15, 2009, 12:15:45 AM
just to say I got my other 84 opened up today, and it was the same as the other one. Hum/buzz, then flipped the toroid and it got way quieter. Usable, yet I will try to find a 100% solution.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 15, 2009, 11:46:49 PM
I was just made aware by a friend of the Group Buy Clone Preamps and the enormous amount of frustration the purchasers are having with hum and buzz in their units.

When I was VP Engineering at Jensen Transformers I designed all the JT-series audio transformers and some EQ inductors as well. I make my living today modifying and upgrading pro audio gear -- mostly vintage, but I also design inductors and power supply transformers, both toroids and conventionally laminated for a California transformer company that speciallizes in small to medium run custom transformers.  We have designed and manufacture many power transformers for audio manufacturers.  I will disclaim right now that I am not up to speed with everything that has been tried to fix the hum in these preamps, but I want to add some technical expertise to the analysis of the problem.

The Chinese toroid is likely generating significant magnetic field, which is inducing hum into the rest of the circuitry.  We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together.  The dreadful QC of some Chinese mfgs is the reason we still make everything here in the US, including winding our own cores.  The clone preamp toroids appear to vary widely in quality, but they all appear to induce hum due to excessive stray magnetic field.

It is a myth that a toroidal transformer automatically generates less stray magnetic field than a conventionally laminated transformer.  The amount of stray field is much more related to the way the transformer is designed.  For Audio use, one wants the power transformer to have minimal stray field, not only to prevent induced hum in the equipment it powers, but in order to prevent the power transformer from infecting neighboring devices with induced hum.  It should be a "friendly neighbor" for the units racked above and below it.   It appears that these Group Buy preamps suffer from both afflictions.

The place I would start in fixing these units would be to reduce the stray field inside the box by retrofitting a new power transformer specifically designed to have low leakage flux. An "audio grade" low stray field design requires more turns and/or a larger core than a general purpose (cheaply made) power transformer.  One manufacturer of lunch box type amplifiers commented to me that the transformer we made for his rack was quieter than his laboratory bench supply! A good design, properly made can have VERY low stray field.

Regarding the inductors:  A properly designed and executed pot core inductor is self-shielding and can be very consistent in inductance, resistance and suseptability to pickup of hum. I don't know what is going on with the Chinese inductors in these units, but if the DCR is low compared to the old Neve units, then they are using fewer turns of fat wire (cheap) on a who-knows-what core.  The inductance can be controlled quite well by purchasing high quality precision-gapped cores.  They should be able to be made 5% tolerance without too much difficulty.  The DCR built into the coil will effect its Q.  Low DCR makes for less damping, so ringing and resonance problems might occur.

Regarding EQ hum pickup: I have not personally seen the PCB layout of these preamps, but I would bet (if I were a betting man) that there are significant loop areas in the layout of the PCB. An example might be a ground trace that goes around the circumference of the pcb. Such a loop acts like a transformer secondary in the presence of stray field from the power transformer, and generates hum like crazy.
Sometimes the problem can be solved by a strategic trace cut or two, but other times, the PCB must be laid out by someone who understands these things.

Regarding pops from switches:
Has anyone checked out the switches to see if they are make-before-break types, especially in the frequency selection area?  If they are the wrong type for the circuit, that could be a show-stopper due to needing to replace the switch.

I am exploring the possibility of making available for sale a properly-designed replacement power transformer with low stray field.  Perhaps I might be able to offer inductors, as well, if there looks like there's some interest.  I was originally thinking of offering to mod these units for a flat fee, but realizing the various and severe QC problems that the buyers have encountered, it seems that just replacing the power transformer would be just the beginning to really make these things professional.

Please let me know if anyone out there would be interested in a new, better power transformer for their unit. It seems that if these preamps could be improved to have professional hum levels, that one would still be ahead in value for dollar spent.  A new transformer may make an unuseable device a working tool in one's bag of tricks.  Maybe a "group buy" for a new power transformer is the way to make these boxes work correctly.

Steve Hogan
The Sound Steward







Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Bluzzi on February 15, 2009, 11:54:33 PM
Yes I think there would be enormous interest in these. Anyone that has built a NEVE clone will agree that they are more sensitive to stray magnetic field pickup. I'm sure if the price is right many of us would use them for all their DIY.

Jim
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on February 16, 2009, 03:38:26 AM
FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15   
0--26         0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 16, 2009, 04:03:20 AM
We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together. 

Eek!

Quote
The DCR built into the coil will effect its Q.  Low DCR makes for less damping, so ringing and resonance problems might occur.

I didn't consider this point before, yes.

Thanks for your input, Steve. That was helpful. I'm sure there would be many cutomers interested in what you might be able to offer.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: gyraf on February 16, 2009, 04:29:51 AM
We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together. 

Eek!


Don't eek that - think of it as an air-gapped transformer. Less sensitive to mains DC offsets also..

 ;)

Jakob E.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 16, 2009, 04:50:31 AM
Regarding the low DCR and lower damping - that's a good point.

But when I look at the frequency sweep in the mid bands with the inductor I see very broad curves.
5KHz or so of peaking bandwidth - way broader than I expected.

And not that much boost - around 8dB only at max (specs say 18dB)
HiQ doesn't do much that I can see.

The freq switches are more the 'pop before squeal' variety.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 16, 2009, 09:57:07 AM
FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15   
0--26         0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.

Thanks for the heads up.  I could design 2 different transformers or maybe just have one and wire the 2nd 0--26 Volt secondary in parallel with the first when used in the 73/84 clones.

For me to design a replacement transformer, it would be helpful for someone to make some accurate measurements of the existing transformers for me as follows:
1. The overall height.  Since this is a 1 RU box, I also need to know the maximum height I can make the replacement transformer.
2. The overall diameter.  The pics show quite a bit of space around the transformer, so I presume a larger OD would be fine.
3. The DCR of each winding, including separate measurements of each primary. These resistances must be measured with the transformer disconnected from the circuitry.  (easy if the trafo is connectorized).
4. The open circuit (disconnected, no-load) voltage of each winding when the primary voltage is held at 115 Volts with a Variac or whatever.
5. Mounting details -- does it have an epoxy center or does it just use rubber washers and a round disk to hold it in place?

I would make the replacement transformers 50/60 Hz. (Low flux density at 50 Hz requires 20% more turns or core than 60 Hz, but the resulting 60 Hz operation has even lower stray field)
I would think that a 120V/240V nominal primary would make more sense nowadays since utilities are trying to hold voltages closer to 120 than 115 to lower losses.  With more reserve, the transformer will have better regulation than the original(s).  I would design it for +/- 10% (108/216 to 132/264 Volt) operation.  If we really want to get carried away, a faraday (copper foil) shield between primary and secondaries will prevent capacitive coupling of AC line noise to the circuitry and the chassis.

In the published schematics did I see capacitors intentionally wired from the IEC input connector line and neutral to chassis?  EEK!  What a way to make more Pin 1 problems with excessive AC chassis currents. The faraday shield is a much better approach.






Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 16, 2009, 10:27:50 AM
Can anyone see why they bothered with the two 26V secondaries? They both seem to be providing +24V after regulation.

I was also wondering if it might be possible to use an off-the-shelf 120+120:15+15+15+15 for those who are not able to buy from the USA. Off the top of my head, that would give 15+15 for the bipolar 12V rails, 30V for the 24V reg. (heatsink the reg.), and 45V for the phantom power.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 16, 2009, 11:22:12 AM
Can anyone see why they bothered with the two 26V secondaries? They both seem to be providing +24V after regulation.

I was also wondering if it might be possible to use an off-the-shelf 120+120:15+15+15+15 for those who are not able to buy from the USA. Off the top of my head, that would give 15+15 for the bipolar 12V rails, 30V for the 24V reg. (heatsink the reg.), and 45V for the phantom power.

"Off-the-shelf" power transformers are never low-flux density, because they must compete price-wise with all the other off-the-shelf power transformers.  They are all, therefore, made with the smallest cores and the least amount of wire that will result in published specifications. Some manufacturers are better than others.  Some of the off-the shelf toroids in plastic PC mount cases have appalling, totally unuseable stray field. Since we already know that the EQ inductors and maybe the PCB layout is very sensitive to stray field, I doubt if an off-the-shelf replacement transformer will help much.

That's why a custom-designed low stray-field design is necessary to really solve the problems in this preamp correctly.
The only other good alternative is to remote the transformer and power supply regulator boards from the box and pass the regulated DC to the preamp via a cable.  (Just remoting the power transformer means having long cables with high-current rectifier pulses outside the box to infect other wiring).
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 16, 2009, 11:42:13 AM
 Would be interested in seeing some
" how much for how many " numbers

Cinemag is looking into the inductors ,
but curious to see what you can come up with

nice to " see " you again steve
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 16, 2009, 01:01:28 PM
FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15   
0--26         0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.

Thanks for the heads up.  I could design 2 different transformers or maybe just have one and wire the 2nd 0--26 Volt secondary in parallel with the first when used in the 73/84 clones.

For me to design a replacement transformer, it would be helpful for someone to make some accurate measurements of the existing transformers for me as follows:
1. The overall height.  Since this is a 1 RU box, I also need to know the maximum height I can make the replacement transformer.
2. The overall diameter.  The pics show quite a bit of space around the transformer, so I presume a larger OD would be fine.
3. The DCR of each winding, including separate measurements of each primary. These resistances must be measured with the transformer disconnected from the circuitry.  (easy if the trafo is connectorized).
4. The open circuit (disconnected, no-load) voltage of each winding when the primary voltage is held at 115 Volts with a Variac or whatever.
5. Mounting details -- does it have an epoxy center or does it just use rubber washers and a round disk to hold it in place?

I would make the replacement transformers 50/60 Hz. (Low flux density at 50 Hz requires 20% more turns or core than 60 Hz, but the resulting 60 Hz operation has even lower stray field)
I would think that a 120V/240V nominal primary would make more sense nowadays since utilities are trying to hold voltages closer to 120 than 115 to lower losses.  With more reserve, the transformer will have better regulation than the original(s).  I would design it for +/- 10% (108/216 to 132/264 Volt) operation.  If we really want to get carried away, a faraday (copper foil) shield between primary and secondaries will prevent capacitive coupling of AC line noise to the circuitry and the chassis.

In the published schematics did I see capacitors intentionally wired from the IEC input connector line and neutral to chassis?  EEK!  What a way to make more Pin 1 problems with excessive AC chassis currents. The faraday shield is a much better approach.







1) The current ratings are on the schematics (? accuracy)

2) Transformer dimensions (to 1/16 inch): diameter 2 1/2 inches, height 1 inch

3) The box's inside depth is a tad over 1 5/8 inches

4) The transformer is a doughnut, mounted with rubber washers above and below, and a steel disc above.

5) There is a dimple into the bottom of the box for the mounting hole, so this might make remounting the transformer in a different location slightly problematic. There is 1/4 inch of space between the transformer and the wires coming off the mains connector (if you bend them a little,) so the transformer could be 3 inches diameter in its current location.

I'll post some pics in a bit. Hope this helps.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 16, 2009, 01:18:03 PM
Here's the pics:

(http://home.att.net/~crazydoc/ACMP/toroid1.jpg)

(http://home.att.net/~crazydoc/ACMP/toroid2.jpg)

(http://home.att.net/~crazydoc/ACMP/toroid3.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 16, 2009, 01:52:57 PM
On a different note, I've been trying to deal with the gain switch noise between positions 6 and 7. Its not just a pop - it's a low frequency oscillation if you hold the switch between those positions (not for too long though - you'll fry the 3055's emitter resistor.) It looks like this on the scope (at the junction of 1R3, 1R14, and 1C1 - that is, the preamp input) - about 0.5v PP and 2 Hz. It is present in the mic or line input position - you just don't hear it (much) when switching through it in the line mode. Of course it is present on the output of the stage too, just hugely amplified.

(http://home.att.net/~crazydoc/ACMP/switch-pop-scope.jpg)

The switch is make-before-break, so in essence, the preamp's input and output are connected together through R40, 41, 42 and 43 at this point, apparently turning the preamp first stage into an oscillator.

(http://home.att.net/~crazydoc/ACMP/ACMP81_amp-switch-pop.jpg)

I tried substituting a 470uF cap for 1C1 in an attempt to lengthen the time constant for the oscillator, and allowing the switch to pass through that point before the first wave could be produced, but this didn't work - it only changed the relative duration of the negative and positive excursions of the wave, without changing the frequency.

So I've currently cut the trace between the two poles of the switch, making this an off position. I'd like to be able to have this an active position, if anybody has any ideas how to prevent the oscillation. I'm afraid my knowledge of electronics theory (Ohm's Law) just isn't enough to get me through this.  :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 16, 2009, 05:50:35 PM
Steve, thanks for joining the fray! I'd be interested in a new toroid. Worst case scenario, it makes no difference.. but I can still use it in another project.

As far as making one toroid or two (for the extra winding in the 81) I would vote to make only one (the 81) if we'd benefit from economies of scale.  Or maybe make a single higher current 26V secondary that could be run in parallel to both inputs on the PSU for the 81??? Any inherent problems with that?

Inductors.. would you be able to match the footprint? 

Thanks again for offering to help! 


and Crazydoc: Thanks for the find.. Hopefully one of the Neve gurus around here can shed some light.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 16, 2009, 06:38:38 PM
Another inductor hum hack I've done - raise the inductor up on stilts so it is centered in the shielding cylinder. (This is on the 73.) This is moderately helpful in further decreasing the level - the shield can only extend to the top of the 1U box, of course. Mounting it horizontally as I did in the ACMP81 is probably marginally more effective.

(http://home.att.net/~crazydoc/ACMP/inductor-stilts.jpg)

In any case, the hum on either unit is now only heard in the line-in position, with the output gain of the pre maxed and the input gain of the listening/recording device maxed. It is inaudible at any microphone setting, as the baseline noise of the preamp's gain stage is far greater than any inductor induced noise. I wish there were a way to decrease this: I've substituted several types of low noise transistors without any notable improvement.

To summarize, I've gotten my units functional by (in this order):

1) Swapping the Q4 and Q5 transistors to stop the oscillation (ACMP81)

2) Rotating the toroid power transformer to the position of least hum in the mid-eq inductors

3) Shielding the toroid and the inductors

4) Cutting the trace between the two poles of the 6th position of the input gain switch (not an optimal solution - loses a gain setting)

This has cost me about $30 per unit for 4 units ($12 for transistors and $100 for mumetal, though there is enough shielding left over to do a few more.) My only remaining issue is just the baseline preamp hash/noise at high mic gain, which shouldn't be a problem unless I'm recording a very soft voice/guitar with a ribbon mic. Thanks for everybody's help, energy and good will in working on this, with more to come as necessary.

Hopefully, Steve Hogan's entry into this will give some better options for those who want to pay a little more (and wait a little longer.  :))
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 16, 2009, 09:08:51 PM
swapped transisters in one of my 81's , warning
if you get the info off the internet double check the ebc pinout
an DON'T heat em up too much , they do lift easy
[ i pushed against one of mine to break the solder connection ]
my local active store had some 2n2222 & 2n2907's that were in
metal cans marked audio / VHF Amp  don't imagine they sound any better but it cleared up most of the problem
thanks Z

also noticed that the pcbs are dated 2007 and although most of the
soldering had a slightly dull look , the inductors pins were shiny & fresh
[ it would seem  ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 17, 2009, 06:52:29 PM
New photos of the ACMP-84 insides:
http://recordinghacks.com/tnc-audio-preamps/

Here's the overview; 5 closeups and hi-rez versions can be found above.
(http://recordinghacks.com/images/mic_extras/tnc/acmp-84.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 17, 2009, 07:14:46 PM
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 17, 2009, 07:44:57 PM
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?

I know you've asked this before and no one's responded. I can't think of a good reason either, but: why would they design a PS with extra parts/expense if there were no reason. Maybe with the 81, there were even more oscillation/hum/noise issues with the output amp being on the same supply as the eq boards (or at least they thought so.)

Anyway, a good question in light of possibly designing another transformer for the power supplies - simpler and cheaper to have only one 26V secondary. If I get the time (and the nerve), maybe I'll short the two supplies together and see what happens.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: peteb on February 18, 2009, 03:51:30 AM
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                     BLACK         YELLOW+GREEN
                     RED             PURPLE + RED

Cheers,
peteb
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 18, 2009, 04:13:49 AM
One of my 81 units has all +24V supplied from a single regulator  on series connected secondary windings via the JLM Powerstation PSU board.
30VA transformer with 24VAC-0-24VAC secondary to +48V, +24V, +15V, -15V  all located on-board.

I haven't noted  any problems at all so far (small T220 heatsinks on each of the regulators), after several weeks of usage.

I didn't notice any noise performance benefit either. (No additional shielding on the power transformer. Used a 'reasonable' quality chinese model)

Mounting externally reduced the noise from the inductor boards in the 1KHz-6KHz region by about 6dBu (from -89dBu to -95dBu components)
and made no impact on the mains supply harmonic noise resulting from the mid-lo inductor,
which now seemed to be picking up noise from surrounding rack units (as shown by switching surrounding units on/off and measuring)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 18, 2009, 07:48:03 AM
Hey alex did you say removing the torriod still helps a little
with the carnhill inductors ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 18, 2009, 09:16:33 AM
Alex.. that kinda sucks.. I hoped a toroid swap would significantly reduce the noise... Have you used the trafo you are using now without problems?   Maybe someone has a really nice quality trafo they could test?  I know Altran makes high quality toroids, but I don't know if they would have a 24-0-24 stock (so as not to pay setup fees)... I was going to try that myself, but hearing your results, not sure it would help much...

Maybe there are other factors as Steve Hogan mentioned..
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 18, 2009, 09:48:49 AM
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?

I know you've asked this before and no one's responded. I can't think of a good reason either, but: why would they design a PS with extra parts/expense if there were no reason. Maybe with the 81, there were even more oscillation/hum/noise issues with the output amp being on the same supply as the eq boards (or at least they thought so.)

Anyway, a good question in light of possibly designing another transformer for the power supplies - simpler and cheaper to have only one 26V secondary. If I get the time (and the nerve), maybe I'll short the two supplies together and see what happens.

Don't short the outputs of the 2 supplies together.  You won't be happy with the resulting damage.  See my notes for
modifications below.

Having now spent several hours with the help of Mylithra, Matt at recordinghacks.com, and others here,  I now have more info and already have a preliminary replacement power transformer design.

I'm not sure why the manufacturer of the 81 clone used two separate supplies, but it seems clear that the one supply has enough current to drive the mic/line amplifiers, and the second regulated supply powers the EQ section.  The separate regulated supplies will offer some isolation.  If I make a transformer with two 25 volt secondaries, then they can be wired separately for the 81 and in parallel for the other units.  Or the winding can just be left unused.  Anyone have both an 81 and a 73/84 to see if there is much of a size difference between the two transformers?

The size of my preliminary design is 3.625 to 3.8" OD x 1.4 to 1.5" high. This large size allows EXTREMELY low stray field, even lower than my usual low stray-field designs.  I want the transformer to fit in the original space.  I know it will fit in the 84 chassis, so I would appreciate it if those of you with 73 and 81's can measure to see if that would fit in place of the old one.

Here are some potential modifications that I recommend from quickly studying the schematics of the 81 clone.
(sorry, I don't have an answer to the gain switch problem yet)

1. 81 Schematic error:  Filter caps C2 and C6 are upside down. The + terminal must go to ground on the negative supply.

2. C19 (100uF/63V) main filter cap in 48 V supply should be 100 Volts. At high line the Voltage here can exceed 63 Volts, and an exploding cap inside your preamp is not a pretty sight.

3. The LM317 regulators do not have the required protection diodes. Add 2 each 1N400x rectifiers, one in parallel with R2 and another in parallel with R4. Cathode (banded end) connected to the output and the anode side connected to the LM317 adjustment terminal.   Add a third 1N400x protection diode Cathode (banded) end to Input of LM317 regulator and Anode end to output of regulator.  Add a fourth 1N400x protection diode to the other regulator.

4. Changing C11 and C16 (10uF/35V) to 100uF/35V will reduce the HF noise of the LM 317.

5. Phantom power circuitry:
     A. 1R58 and 1R59 (6.8K) must be matched within 0.4% to meet DIN specs for Phantom.  Absolute value is not critical, check the resistors with an Ohmmeter and if they aren't matched, replace them with a matched set.  Mismatched resistors will add Low frequency THD to Mic Preamp input transformers and microphone output transformers.
     B.  1R60 (1K) is too big for use with 48Volts.  High current demand, modern mics will be starved.  Replace with 100 Ohms for stock supply, or if you get my new transformer, you will be able to get 52 Volts from the 48 V supply and you can use 200 Ohms here.
     C.  1C23 (100uF/63V) would benefit by being increased to 220uF/63 V, especially since 1R60 must be lowered in value.
     D.  Switch contact 4 of K2 "48V" switch should be connected to the same ground point as the bottom of 1C23.
and not left floating.  This discharges 1C23.  Without grounding the contact, the phantom stays on the XLR until the cap is discharged by a plugged-in microphone.

6. RF bypass caps C15, C16, C17, C18 (1000 pF) represent significant capacitive loads on any microphone or line level device.  At 33pF per foot of good mic cable, this represents the capacitance of an additional 30 ft of mic cable.  I have not checked to see if the original Neve circuits had these caps, but if they did they should be of good quality and well matched.  Otherwise I would get rid of them and let the input tranformers deal with the RF.

7. Warning!!  Half-wave rectifiers are used in peak lights and metering.  The metering and peak lights do not employ full-wave rectification of the waveform. When  monitoring highly assymetrical waveforms like male speech,for example, the meters will respond quite differently depending on the absolute polarity of the signal. In one polarity the meters/clip lights may light up like crazy, and in the other polarity they may hardly register.  Be forwarned that the clip lights and meters may be lying to you in regard to levels, unless you end up modifying the precision rectifier circuitry to full-wave.

That's all for a quick glimpse.

Hope it helps.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 18, 2009, 10:07:33 AM
By the way, has anyone put a scope (AC coupling, line triggered) on the regulated DC output voltages to see if the power supply regulators are putting out low-ripple power?  A bad PCB layout on the power supply can easily increase the ripple 20 dB or more over a proper one by allowing the sense terminals of the regulators to be attached to a ground trace carrying the high-current rectifier pulses from the filter caps.  Since the Neve circuitry is single-ended 24 Volt, a low-ripple supply here is a must.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: ioaudio on February 18, 2009, 10:37:34 AM
also noticed that the pcbs are dated 2007

i thought it was costum build in 2008?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 18, 2009, 10:39:16 AM
also noticed that the pcbs are dated 2007

i thought it was costum build in 2008?

Hah! Okgb, maybe you should add that to your post over at R/E/P.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Jim50hertz on February 18, 2009, 10:41:55 AM
also noticed that the pcbs are dated 2007

i thought it was costum build in 2008?

Yes, curious
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 18, 2009, 10:55:28 AM
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                     BLACK         YELLOW+GREEN
                     RED             PURPLE + RED

Cheers,
peteb
My working unit is the same, with two unconnected lugs to the left of the black and red wire connections. Switch set to 115.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on February 18, 2009, 11:12:57 AM
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.

Is 100mA large enough?  There's probably more than 100mA of inrush current on these things.  Maybe try a slo-blo fuse?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 18, 2009, 11:28:22 AM
2007.12.20  to be more exact  ,
 but we'll leave that to the other forums
and just worry about making it right here
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 18, 2009, 11:48:49 AM
Don't short the outputs of the 2 supplies together.  You won't be happy with the resulting damage.  See my notes for
modifications below.
Why not? I'm maybe too dense to find the explanation below?  ;D

Quote
The size of my preliminary design is 3.625 to 3.8" OD x 1.4 to 1.5" high. This large size allows EXTREMELY low stray field, even lower than my usual low stray-field designs.  I want the transformer to fit in the original space.  I know it will fit in the 84 chassis, so I would appreciate it if those of you with 73 and 81's can measure to see if that would fit in place of the old one.

As I posted about a page ago:

2) Transformer dimensions (to 1/16 inch): diameter 2 1/2 inches, height 1 inch

3) The box's inside depth is a tad over 1 5/8 inches

4) The transformer is a doughnut, mounted with rubber washers above and below, and a steel disc above.

5) There is a dimple into the bottom of the box for the mounting hole, so this might make remounting the transformer in a different location slightly problematic. There is 1/4 inch of space between the transformer and the wires coming off the mains connector (if you bend them a little,) so the transformer could be 3 inches diameter in its current location.
I'll post some pics in a bit. Hope this helps.

There's not enough room in the 81 for that size, without remounting it using a different hole location - then you have to deal with the raised dimple from the old location.

Thanks for your time and effort in this - it's a great help.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: peteb on February 18, 2009, 05:19:27 PM
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                     BLACK         YELLOW+GREEN
                     RED             PURPLE + RED

Cheers,
peteb
My working unit is the same, with two unconnected lugs to the left of the black and red wire connections. Switch set to 115.

Thanks Crazydoc. Are the two unconnected lugs bridged?
Cheers,
peteb

wmtunate, I'll try a couple of 125mA fuses. The operational manual that came with the units says 100mA but a pdf I've come across on the net says 125mA.
Cheers,
peteb
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 18, 2009, 05:32:12 PM

Thanks Crazydoc. Are the two unconnected lugs bridged?
Cheers,
peteb

Cheers,
peteb
Yes, they are. I hadn't noticed that. But I see it now on the schematic also.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 18, 2009, 09:22:29 PM
>Hey alex did you say removing the torriod still helps a little with the carnhill inductors ?

Haven't explicitly tested that. I think it would make a small difference in the 1KHz-6KHz
region perhaps 5dB or so. But I haven't tested that combination.
My tests were with ACMP81 Q4/Q5 mods and (3 feet away, DC connection) externally mounted Altronics toroid and JLM PSU.

In my case, I saw the overall noise benefit of replacement PSU and toroid onboard or offboard
as not being significant unless the major noiser inductors was elseways tamed. This test was done in the rack with
other units on 3RU above and 3RU below.

It was unequivocal for me - no mains noise benefit at all. Some mid-hash noise benefit across all the filtering bands.

However - your milage may vary.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 19, 2009, 02:12:50 AM
My latest rev of the new transformer design looks like 1.4" max height and about 3.4" to 3.5' max diameter. Making a smaller OD is certainly possible, but not with the same good regulation and extremely low flux density even at 50 Hz.  It appears to be able to fit nicely into a 73 and the 84 chassis, but the 81 box appears to have the most real estate covered with circuit boards and the least amount of available extra space for the larger tranformer. I need to get an 81 in house to work on  I will comment on the regulator connection tomorrow, when I'm not so tired.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: peteb on February 19, 2009, 02:28:08 AM
Both my preamps (73s) are instantly blowing fuses.
Haven't had them working at all yet.
Before I plugged both of them in I set em to 230v (For Australia) , tried 100mA then 125mA.
Any advice appreciated.
What's the max fuse I could try?
Cheers,
peteb
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 19, 2009, 02:42:26 AM
On the '73, I measured the current draw (with an RMS meter so I dont know the peak value) off the primary tap to be only .064A. The original fuse is a 250ma fuse so I think you should start there.
At this point, If I were you, Id take my meter to the power transformer and start looking for shorts.

Winding DCR
15v 14.2R , 7.5R to ct   
26v 6R   
48v 52.1R   
115v [email protected]   [email protected]

here are the DC resistences on the windings from the Transformer (taken from the one that had the least noise with an RMS meter). Id make sure you're at least in the ball park. (or cricket park as the case may be  ;D)

Hope it helps.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 19, 2009, 07:47:30 AM
Start by measuring the DCR between the L and N blades on the EIC301 Mains Inlet with the transformer switched to 230.  It should read about 188 Ohms.  If so, the transformer primaries are series connected as they should be.  Intitial inrush current drawn by a toroidal transformer can be very high, depending on where the flux in the core was when it turned off and where the AC waveform is when it turns on.  Worst case surge is limited only by the DCR of the primary winding due to the complete saturation of the toroidal core during intial turn on.  In this case: 240V x 1.414 = 340 Peak Volts/188 Ohms = 1.8 Amps for first several cycles, which are 10 msec each instead of 8.3 msec per cycle since you are 50 Hz instead of 60 Hz.  If the small fuses you are using are fast-acting types, they will blow for sure. Slow blow is required Littlefuse 313 series or equivalent in whatever size fuse is being used in the AC inlet.  If you have access to a Variac you could try ramping up slowly to make sure that there are no transformer shorts or mis-wires, but it seems unlikely that 2 units would have the same problem. One other possibility is that the primaries may be wired in reverse polarity.  That would put a 188 Ohm short across the 230VAC line at all times due to the reversed polarity cancelling out the primary inductance of the transformer leaving only the 188 Ohm resistance at all times, not just at initial turn on.  If the line voltage selector switch were correctly wired, then the 115 volt selection would also blow fuses if the primaries were out of polarity with each other.  The DCR measurement won't catch that miswire.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: gyraf on February 19, 2009, 09:04:53 AM
Remember to use a "T" type (slow blow) fuse. A "F" will blow instantly.

Jakob E.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 19, 2009, 10:14:55 AM
Hey Jakob ,  that first picture that appeared of the 81 ,
It was actually taken by you at a trade show ?
Do you remember when ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on February 19, 2009, 10:18:02 AM
There was a spare original fuse in the fuse holder.. Did you go through that one as well?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 19, 2009, 10:28:08 AM
2) Transformer dimensions (to 1/16 inch): diameter 2 1/2 inches, height 1 inch

3) The box's inside depth is a tad over 1 5/8 inches

4) The transformer is a doughnut, mounted with rubber washers above and below, and a steel disc above.

5) There is a dimple into the bottom of the box for the mounting hole, so this might make remounting the transformer in a different location slightly problematic. There is 1/4 inch of space between the transformer and the wires coming off the mains connector (if you bend them a little,) so the transformer could be 3 inches diameter in its current location.

There's not enough room in the 81 for that size, without remounting it using a different hole location - then you have to deal with the raised dimple from the old location.


Presuming that the 81,73 and 84 chassis are all identical, I have just made up a 3.5" OD "paper doll" to scale of my new transformer with max OD of 3.5".  It appears to me that it will fit over the original dimple just fine.  The new transformer's center hole is large enough to move it forward in the box to clear the line voltage switch and the AC inlet faston connectors.  It also clears the 3 unused circuit board standoffs in that area -- 1 at about 5 o'clock, 1 at 8 o'clock and 1 at about 10 o'clock.  Perhaps in your effort to eliminate the hum induced by the toroidal power transformer you loosened the securing bolt and moved it as far back in the chassis (away from affected EQ boards) as possible.  There is no question that one will have to carefully dress the new soldered connections to the Voltage selector switch.  The Old switch is wired with wires and heat shrink sticking straight out from the switch.  The switch terminals themselves, however, as far as I can tell from photos, are short enough to clear a 3.5" OD transformer.  One needs to wire the switch from the top and bottom instead of straight out towards the transformer.

I have now read an additional 10 pages of posts in this thread and I am up to speed on the Q4, Q5 transistor problem, the fact that the gain switch omitted the "OFF" position that Neve implemented to prevent positive feedback in the first amp and the discussions about remoting the toroidal power tranformer to get it away from the sensitive circuitry.  I also read about the power tranformer passing high frequency garbage from the line, so I will comment on that.

Because of the way a toroidal transformer is wound with layers of primary wire wrapped around the core and then the secondary wires wrapped around the core on top of the primary, there is excellent magnetic coupling between primary and secondary due to the windings being long and flat. This gives toroidal transformers low leakage inductance compared to conventional EI core transformers.  This construction also results in high capacitive coupling between primary and secondary. Thus high frequency noise on the power line (from switching power supplies, computers, etc.) can more easily get into the secondary.  This capacitive coupling can be minimized by careful arrangement of the primaries making the "groundy" neutral end of the primary winding end up on top, closest to the secondaries.  For best eliminination of this problem, however, a grounded copper foil shield can be installed between the primary and secondary. This essentially eliminates the capacitive coupling of high frequency garbage from the line into the secondaries. This type of shielded power transformer is used on a much bigger scale for "balanced power" installations.
The original Chinese transformer does not have a faraday shield, but I have designed one into my proposed replacement.  The shield is a separate issue from the low-flux density, no-hum magnetic design, but together they make a killer solution for these boxes IMHO.

Someone asked if anybody has a gaussmeter in one of the posts.  I do.  I have a 73 unit on the way to me.  I have a tranformer on the way to me.  I guess I really need to get someone who wants the new transformer in his/her 81 to send me that box so I can have it as a test bed and can measure everything in place for fit and electronic function.

A big part of the problem with just remoting the tranformer is that it radiates enough magnetic field to induce hum into other gear.  IMHO a much better solution is to attack the problem at its source by replacing the transformer inside the box with one that doesn't cause problems. That eliminates the offending radiator of stray magnetic field from the picture entirely, and it can't infect its own circuitry or the circuitry of the other gear racked on top and bottom of the preamp.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 19, 2009, 10:35:04 AM
There was a spare original fuse in the fuse holder.. Did you go through that one as well?
And they are marked "T 250ma"
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 19, 2009, 10:58:33 AM
Presuming that the 81,73 and 84 chassis are all identical, I have just made up a 3.5" OD "paper doll" to scale of my new transformer with max OD of 3.5".  It appears to me that it will fit over the original dimple just fine.  The new transformer's center hole is large enough to move it forward in the box to clear the line voltage switch and the AC inlet faston connectors.  It also clears the 3 unused circuit board standoffs in that area -- 1 at about 5 o'clock, 1 at 8 o'clock and 1 at about 10 o'clock.  Perhaps in your effort to eliminate the hum induced by the toroidal power transformer you loosened the securing bolt and moved it as far back in the chassis (away from affected EQ boards) as possible.  There is no question that one will have to carefully dress the new soldered connections to the Voltage selector switch.  The Old switch is wired with wires and heat shrink sticking straight out from the switch.  The switch terminals themselves, however, as far as I can tell from photos, are short enough to clear a 3.5" OD transformer.  One needs to wire the switch from the top and bottom instead of straight out towards the transformer.
Thanks Steve - you are right. I just measured the hole location in the 73 and the 81 - identical, so I presume the 84 uses the same chassis also. And I hadn't taken in to account re-connecteing the voltage selector switch wires with a different orientation - it will certainly fit with millimeters to spare, especially if the new transformer can be moved forward somewhat.

Were you ever able to get ahold of Chance to get some units to work on? I'd be glad to send you one of my 81's (from Northern CA.)

Here's a pic set at a radius of 1 3/4".

(http://home.att.net/~crazydoc/ACMP/ACMP73-trafo-radius.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 19, 2009, 04:04:22 PM
Thanks Steve - you are right. I just measured the hole location in the 73 and the 81 - identical, so I presume the 84 uses the same chassis also. And I hadn't taken in to account re-connecting the voltage selector switch wires with a different orientation - it will certainly fit with millimeters to spare, especially if the new transformer can be moved forward somewhat.

Were you ever able to get ahold of Chance to get some units to work on? I'd be glad to send you one of my 81's (from Northern CA.)


I did reach Chance by phone.  He said that he was concentrating all his efforts on getting the last units shipped out, and that he would then be able to focus more attention on fixing the various issues as soon as that was done.

At this point in time, however, I believe it is best to just get examples of the three different units from owners who want the transformer upgrade and use those units to prove out the design, etc. Therefore, I would be delighted if you can send me one of your 81 units since I already have a 73 and an 84 in transit to me. It makes the most sense to send me a stock unit, without the shielding that you have added to your experimental unit.  That way I can correctly judge the impact made by simply changing the power transformer.

With the clearance issues resolved, the final specs for the replacement transformer(s) will be:
ID = 1.25" approx.
OD = 3.5" Max typically 3.4"
Height = 1.5" Max typically 1.4"
Faraday shield with separate lead
Ultra-low flux density design
Nominal Primary Voltage = 120/240 Vac
Min. Voltage to maintain regulation = 105/210 Vac
Max Voltage = 132/264 Vac

  I have designed the new transformer for 11 kGauss at 50 Hz and 9.2 kGauss at 60 Hz.  My regular low-noise audio power tranformers are typically 12 kGauss at 50 Hz, so these are exceptionally low stray field.  The winding resistances of my transformer design are roughly half of the original, so the regulation will be twice as good.  I am trying to set the secondary output voltage so that all the supplies will stay in regulation until 105VAC line and will not overvoltage the filter caps until 132 VAC line.  I also want to put enough volts on the phantom winding to be able to raise the regulator to 52 volts instead of 48 which will better power modern mics.

Because the transformer has less voltage droop than the old one, it will be necessary to up the voltage of the main filter cap for the Phantom supply to 100 V instead of 63.  The pre-regulator Filters in the 24V supplies should be 50 Volts instead of 35V to prevent over-voltaging the filter caps at high line voltages.  With the actual units in house, I can make adjustments to the secondaries to get it right.


 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on February 19, 2009, 05:51:13 PM
So you seem to be pretty far along with this. Could you give an estimation of what we can expect in terms of cost for this mofo?

Thanks for all the work. Pretty excitin'.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: peteb on February 19, 2009, 11:00:10 PM
There was a spare original fuse in the fuse holder.. Did you go through that one as well?
And they are marked "T 250ma"

Thanks,
Crazydoc, Mylithera, Gyraf, Steve Hogan and anyone else I missed,
It was as you guys said. I put a couple of 100mA Slow Blow fuses in and they're running well.
Hopefully I'm a better engineer that a tech.
Well as you might have guessed I'm not a tech at all.
I didn't use the original 250mA ones because I was on 240v.
Was following instructions to the letter. Except for the Slow Blow bit that is.
Interestingly one 73 has the loud input position noise and the other is very much quieter through the same position.
They both have the eq buzz but it's not in an area that I use very much so I can live with that.
I like the sound of these pres.
Thanks again for a great response to my query.
Cheers,
peteb
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: gyraf on February 20, 2009, 12:02:15 AM
Hey Jakob ,  that first picture that appeared of the 81 ,
It was actually taken by you at a trade show ?
Do you remember when ?

It was taken at the "797 Audio" booth at Frankfurt Music Show - 13. March 2008 at 13:51 CET

Jakob E.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: pohaku on February 20, 2009, 01:08:06 AM
Steve,

I have a stock unmodded 81 I can send you.  Where should it go?

Great work BTW.

Thanks!

Henri
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 20, 2009, 12:28:36 PM
Sounds like the power xfmr will be a good general use
stocking kind of item .
As you've done a few of these you must have an  approx idea on cost ?
what would it be for this unit ?

And now that i try to use mine , the transister fixed the overall
eq hum / buzz so now you can tell the hum that is contributed
from those bands [ unusable ]

thanks & regards Greg
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 20, 2009, 06:17:16 PM
Sounds like the power xfmr will be a good general use
stocking kind of item .
As you've done a few of these you must have an  approx idea on cost ?
what would it be for this unit ?

thanks & regards Greg

At the present time I think that the transformer cost (as a separate part) will be in the $75 ballpark.

Once the preamps arrive, and I get a chance to get my scope probes on them, and get a chance to look at the PCB and cable layouts, I will have a lot better sense of both the "must fixes" and the optional improvements that can be made to really polish these preamps.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 21, 2009, 02:33:16 PM
I personally appreciate the time your giving to this preamp fiasco. I hoping for the best.
Although something tells me this is not a typical DIY project. Its more of a engineers
puzzle than a simple circuit fix. But educating, nonetheless.
Thanks for the kind words. Since I upgrade and modify pro audio gear for a living, and end up fixing many problems that were originally designed into the gear, my approach is hopefully very scientific.  The transistor swap problem, for example is one that I want to see for myself.  It is true that substituting different transistors may have stopped the apparent oscillation, but I want to see the PCB layout, and figure out what is REALLY going on.  The power dissipation in these Q4, Q5 transistors isn't very high (unless it is oscillating), so a 2 Amp rated transistor isn't really necessary here. Q5 is just a current sink load for Q4 and it is set to 8.5 mA.  Q4 should only dissipate about 125 mW when it's working right.  All that to say that the large, metal can BC441 and BC461 are perhaps overkill to resolve this oscillation problem.  I have spent considerable time researching beefy transistors that have similar capacitance, gain-bandwith product and Hfe as the BC441/BC461, but will fit easily into the existing PCB.  I am not inclined to go for the TO-126 package as a solution (mechanical nightmare) when there are many appropriate TO-92 and TO-92L (tall package for audio driver) transistors that may work perfectly, and be a very close mimic to the Neve BC441/BC461 parts, which are hard to get and unnecessarily expensive.  I am seeking the very best, if you will, engineered solution.   I will let you know what I figure out as soon as I get my hands on the preamps that are in route now.

I received my first power transformer from a 73 yesterday and today I put a 79 turn test winding on it which enabled me to calculate the exact number of turns on the transformer.  Based on the number of primary turns, (1975T at 115V), and estimating the core to be 1" ID, 2" OD and 0.5" High, the 50 Hz flux density can be as high as 20 kGauss at high line, and about 19 at 115V. This is right at the limit of the core material, which is what you do when you are trying to squeeze maximum output from a tiny transformer.  Fine for an appliance, but for audio?  Compare that to 11 kGauss at 50 Hz on my new transformer.

The maximum field on the transformer is directly below and above it and it measures about 100 milliGauss at the bottom of the transformer (opposite the lead break out).  This is pretty much in line with an off the shelf design, which is very cost sensitive due to competition.

Since the highest concentration of stray flux is on the top and bottom of this transformer, I was thinking that the bottom of the steel chassis will conduct magnetic lines of force.  I am wondering if perhaps part of the problem of the inductors and/or potential loop areas in the PCB picking up hum may be due to the steel box acting as a giant pole piece to conduct the stray flux from the transformer to other areas where it causes grief.

Reducing the overall stray flux from the power transformer is a great place to start, but there may be some conduction that we may want to address as well.

More Later as the project progresses.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 21, 2009, 03:28:26 PM
I swapped Q4 , Q5 for 2N2222A & 2N2907A
which were in metal cases and indeed the hum / buzz
noise went away now revealing the hum as the freq bands
on the inductor boards get close to mains freq , there also
sounds like harmonics in there as well .

It may be worth noting that there seems to be varying
degrees of these same problems with some people
I've got a 73 that mostly seems fine and another
that has a slight broadband buzz to it
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on February 21, 2009, 05:15:17 PM
The maximum field on the transformer is directly below and above it and it measures about 100 milliGauss at the bottom of the transformer (opposite the lead break out).  This is pretty much in line with an off the shelf design, which is very cost sensitive due to competition.

Since the highest concentration of stray flux is on the top and bottom of this transformer, I was thinking that the bottom of the steel chassis will conduct magnetic lines of force.  I am wondering if perhaps part of the problem of the inductors and/or potential loop areas in the PCB picking up hum may be due to the steel box acting as a giant pole piece to conduct the stray flux from the transformer to other areas where it causes grief.


FWIW, when I was testing proximity of the toroid on my 81's I found that if I tilted the very front of the trafo up 1/4"-to-1/2" that the inductors picked up much less of the field. It was still pretty bad though.

It was as if the lobe of the field was somehow more focused on the LM inductor when the trafo was parallel to the chassis and focused elsewhere when slightly angled up to the front..

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on February 21, 2009, 06:09:19 PM
Hey Steve. Are these fixes you're sussing out going to be available for the DIY crowd. These things are mother heavy and sending 8 of 'em to you from NY could be frickin' expensive. In another post, you'd mentioned doing the installs yourself and I was wondering if you were thinking that was the only way.

Thanks again for all your work.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 21, 2009, 07:14:43 PM
Hey Steve. Are these fixes you're sussing out going to be available for the DIY crowd. These things are mother heavy and sending 8 of 'em to you from NY could be frickin' expensive. In another post, you'd mentioned doing the installs yourself and I was wondering if you were thinking that was the only way.

I believe I will be able to make the tranformers available as a separate item along with a handful of MUST install power supply parts. At present the procedure that I'm considering includes the following items:

1. Remove old transformer and voltage selector from box.
2. Remove heat shrink and unsolder old transformer from selector switch.  Install Heatshrink over primary leads, solder primary wires to switch entering from top and bottom and/or the side to provide clearance for larger transformer.  Remove Molex shells from old transformer and transplant them to the new transformer leads with molex pins already attached.  Attach green faraday shield lead of new transformer to a circuit ground point yet to be determined.
3. Remove PS PCB.  Replace 48V supply Main filter cap with 100V cap (63V is too low).
4. Probably replace 24 Volt main filter caps with 40 or 50 Volt units instead of 35V (too low again for High line conditions)

Optional, but highly recommended additional power supply improvements include:

5. Add 2 protection diodes to each LM317 24V regulator
6. Change 10uF ref cap to 100uF (fixes rising HF noise in 317 regulators)
7. Upsize the output caps in the 24 Volt supply.
8. change 48V regulator to output 52V (not sure best way to do that yet).
9. Replace 1N400x diodes with Diotec-USA SRP-206 Fast, soft-recovery rectifiers. This eliminates tiny bursts of RF that occur when the slower,  standard recovery rectifiers turn off.
10. Add RC Network across at least 1 transformer secondary to damp the power tranformer when the rectifiers shut off.  This further reduces any spurious emissions due to tranformer ringing.

Other mods include:

1.Replacing the problem transistors (or other fix yet to be determined to fix oscillations in the EQ stages)

2. Modify the gain switch to eliminate the positive feedback pop. I like moving the resistor instead of cutting the trace so far.

3  Replace the 1K series resistor in the phantom circuit with 200 Ohm (1k will starve high-current mics, especially with only 48 Volts from the supply. The 100uF/63 cap should be upsized to 220uF/63V and the phantom switch should be grounded when not in use to discharge the phantom feed cap thru the 200 Ohm resistor when phantom is turned off.

4. Check and/or correct polarity from XLR to XLR.  Some units have line inputs wired backwards.

5. Likely that 470uF instead of 100uF decoupling caps on the EQ stages may improve LF stability.

6. Other mods yet to be determined once I get these units in house.

All these things can be done by an experienced DIY, but for those that aren't handy, I was planning to do all that stuff for a fee.  Most likely I will offer a complete kit of parts with all the right caps and parts and the transformer in a bundle.  I haven't yet seen a box in person, and we are just now making the cores to make the new transformer prototypes.  Watch this space as I figure it out.






Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 24, 2009, 01:03:15 AM
Chance sent me the ACMP-73 schematics.

Top of page: http://recordinghacks.com/tnc-audio-preamps/
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 24, 2009, 06:55:38 PM
Looks like Tom at Cinemag will have a good price for us
on the inductors [ 20 ish or less ]  stay tuned [ in  ]

regards Greg
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 24, 2009, 10:41:54 PM
...we are just now making the cores to make the new transformer prototypes.  Watch this space as I figure it out.

The cores for the new transformers were wound this morning, are being annealed tonight and will be coated tomorrow.
I expect to wind the first articles tomorrow.

Today I opened up Matt McGlynn's '84 unit.  I am amazed at the general quality of the parts used in this preamp.
One would be hard pressed to make the chassis alone with the machined front panel for the money that these cost.  I see decent PC boards, a lot of decent parts including Wima and Roederstein caps. Most electrolytic caps are Rubycon 105C types.  My initial impression is that there is an awful lot of stuff in this box for the money.   All this is before I powered it up. -- Lighted pushbuttons -- impressive. there is no way you could even buy the parts in this box for what they cost.

OK, so my initial impression is that this box is well worth putting some additional money into it to really upgrade it into a very decent mic preamp.

Here's what I found so far in the power supply.  First of all, the Power supply PCB layout is OK on the 48V and 24 Volt outputs.  The Bipolar 12 Volts had some ripple which doesn't immediately make sense, since the PCB layout appears to be OK.  There is one connection that doesn't seem right to me.  The 24V and 48V supplies are grounded to the chassis via the screw nearest the power transformer. The Bipolar 12Volt ground, however, is attached to a different chassis standoff.  This may cause a problematic loop, because the Bipolar 12 Volt supply does connect to the 24 Volt circuitry and the grounds should be grounded in the same place.  It looks like the ground connectin is made with a zero Ohm resistor (jumper) so it looks like it may be easy to get the bipolar supply on the same ground as the rest of the preamp.  More experiments needed.

At 120Volts input, the Phantom supply filter cap has about 61 Volts on it.  At 10% high line it measures 69Volts and the cap is 63 volts.  I will be replacing that 100uF/63V input filter cap with a 220uF/100V Nichicon HE long life, low impedance 105C cap.  I will be using some Nichicon HE series caps and some Panasonic caps in this project because the Panasonic FM caps I usually use are not available in all the values/voltages/case sizes that I need to properly fit in the power supply PCB.  The Nichicons are excellent and inexpensive.  The main filter caps in any power supply take the most abuse due to the high-current pulses from the rectifiers and the heating caused by the ripple current, so these premium (but still very reasonably priced) caps are the right choice in this location.  The Factory Rubycons are 105C but are not the long-life types.  The remaining factory Rubycons don't get stressed nearly as much as the main input filter caps, so they will work fine when they are just acting as filtering on the regulated supply.

The Filter caps on the 24 Volt supply run at 34 Volts at 120 Vac line and reach 39 Volts with 132 Vac high line.  I will be replacing these 35 Volt caps with the long-life 105C 50 Volt parts, which is the next voltage up from 35.  There will be less total capacitance on the input filter caps, which is actually a good thing for reasons I will explain below.  On the 84 (and probably the '73) there are two 2200uF/35 Volt filter caps (4400uF total) which will be replaced with a maximum of three 1200uF/50V (3600uF total).  This is actually way more than is necessary due to the fact that these caps are pre-regulator.  Having really high capacitance in this position makes the current pulses from the rectifier extraordinarily high, which may be another factor in the hum being picked up by the circuitry.  Smaller filter caps have more pre-regulator ripple, but the current pulses are longer and lower in amplitude.  It is a common mistake for those who are trying to upgrade their gear that they increase these main filter caps to many times the original values.  This is really hard on the transformer and rectifiers because they must deliver the current in very, very short, very, very high current bursts.  Usually it's a lot better to use medium capacitance here to have reasonable  pre-regulator ripple, but much lower amplitude current pulses. 
In audio circuits, IMHO, the best place to have large capacitors is post-regulator.  The '84 supply has huge (4400uF) capacitance pre-regulator, but only 220uF/35V after the regulator, where huge would be good.  I will be upping that capacitor value to at least 470uF-- maybe even 1000uF if I can make it fit nicely.

Bottom line is that I am going to recommend changing out about half the caps on the Power supply board to run it more conservatively and to protect it from accidents.  My philosophy is that quality sound starts with a solid power supply, and this one can be made quite solid by simply changing out a few caps and adding a few extra parts.

I have a new question for 81 owners:
   On the 81 power supply,  the photos show a 4700uF/35V electrolytic on each of the two 24V power supplies.  these caps are lying down on the board.  Can someone please look to see if there are places for 3 total filter caps under the cap that is lying down, similar to the '84 and '73 power supply PC boards.  This cap must be replaced as well, because 35 Volts is not safe as an input filter cap with as much as 39 Volts on it.  I want to place an order for replacement caps tomorrow, so a prompt answer by someone on this forum will be greatly appreciated.

Stay tuned . . .






Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 25, 2009, 08:10:21 AM
Hey Steve
if you can post pix of the progress that would be great ,
facinating to see what really makes things work
[ i;e good design  ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on February 25, 2009, 09:22:11 AM
Quote
I have a new question for 81 owners:
   On the 81 power supply,  the photos show a 4700uF/35V electrolytic on each of the two 24V power supplies.  these caps are lying down on the board.  Can someone please look to see if there are places for 3 total filter caps under the cap that is lying down, similar to the '84 and '73 power supply PC boards.  This cap must be replaced as well, because 35 Volts is not safe as an input filter cap with as much as 39 Volts on it.  I want to place an order for replacement caps tomorrow, so a prompt answer by someone on this forum will be greatly appreciated.
There are no other mounting holes other than the two for the large caps, C9 and C14. Board is blank underneath them.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 25, 2009, 09:47:06 AM
I will try to shoot some pics, as requested, but right now it would consist of my hand moving my meter test leads and my oscilloscope probes around.

Testing the '84 unweighted noise level as delivered:  Tektronix AA5001 THD analyser, Level, RMS detector.
Preamp settings: (These settings were what gave me the highest hum level.  Other settings were better.)
Gain = max (-80)
Line/Mic = Line (no input termination -- just nothing plugged into either mic or line input).
EQ in , HI Q in
All pots to max, both EQ and Output Gain
All EQ's OFF except low shelf 220 and mid peak 0.36kHz
Scope trigger = AC line.

Clearly the hum is being induced by the power transformer. High current magnetic pulses are clearly visible in the output waveform.
As delivered:  Output level = -45.1 dBu   35 milliGauss magnetic field at mid inductor.
Power transformer turned upside down --  leads up instead of down = -51.5 dBu = 25 milliGauss
Power tranformer hung outside box as far as leads allow: -57.4 dBu = 7 milliGauss
Whether or not the EQ circuit is especially sensitive to stray magnetic flux, it is clear that lowering the stray flux will definitely lower the hum.  There may be other steps that can be taken to lower the sensitivity to stray field, and that will also lower the background hum, but lowering the field will directly reduce the hum.  By the way the field strength meter drops to zero when I turn off the unit so there is no other significant stray field on the bench.

I believe that the hum induced from the peak currents being drawn from the power transformer will be reduced when I reduce the value of the 24 Volt supply input filter caps.  I know that it seems counter-intuitive -- smaller cap = less hum, but because the very short, high amplitude magnetic pulses are inducing the hum, making it more "buzzy", the smaller filter caps will help lower that peak amplitude.

To verify my theory, I removed one of the 2200 uF filter caps from the 24 Volt Supply, thus reducing the total capacitance from 4400uF to 2200uF.  The scope showed a significant reduction of the highest 120 Hz rep rate peak in the hum waveform.  And the RMS hum levels went down approximately 1.5 dB at all three transformer locations.  The field strengths were the same, but that is an averaging meter.  This means that losing capacitance when I change the filters from 35 Volts to 50 Volts will actually benefit the situation.  As a matter of fact, I may further reduce the size of those caps to an optimum value.

As first delivered, the ripple voltage on the phantom supply (100uF) = 300 mV PP
The ripple on the 4400uF 24 Volt supply was 110 mV (which is unnecessarily low) 500 mV to 1 Volt would be fine.  On the other hand, the PP ripple on the (too small) 100uF Bipolar 12 Volt input filter caps is 3.6 Volts.  There is significant current drawn by the 12 Volt supply -- all the LEDs etc.  I will be changing the input filter caps on the Bipolar 12 Volt supply to 470uF/35 Volts.  That will reduce the ripple to about 750 millivolts, which is about right.  Hopefully, I will end up with a more balanced set of cap values for the entire power supply, correctly sizing them to balance ripple and excessive transformer/rectifier current pulses. and increasing the size of the post-regulator caps, which will produce sonic benefits.  By the way, I can fit a 1000uF/35V output coupling cap to the 24V output with no problem, so that cap will get a 4.5 x increase in size over stock.

More later ---





Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 25, 2009, 09:56:07 AM
There are no other mounting holes other than the two for the large caps, C9 and C14. Board is blank underneath them.


Thanks, Crazydoc, for the remote look-see!

Based on my latest experiments detailed in the above post, a single 50 Volt cap with reduced capacitance from 4700uF should actually produce less hum.  I will spec out a replacement that is the same case size, and whatever capacitance you get in a 50 Volt part.  Maybe even less, but I'm not there yet.

Thanks for the prompt response -- your 81 has not yet arrived.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 25, 2009, 11:45:05 AM
Looks like we'll be able to reuse the little inductor cup
[ as long as the replacement is small enough , or it just
sits over the side ]
(http://i83.photobucket.com/albums/j319/okgb/Picture092.jpg)
(http://i83.photobucket.com/albums/j319/okgb/Picture095.jpg)

Anyone have the real Neve 1073 , 1081 schematic ?

anyone have a bunk acmp 73 to volenteer the inductors ?
the chinese schematic i have doesn't seem to state inductor values
[ only " L" labels ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on February 25, 2009, 02:55:59 PM
Looks like we'll be able to reuse the little inductor cup
[ as long as the replacement is small enough , or it just
sits over the side ]

Anyone have the real Neve 1073 , 1081 schematic ?

anyone have a bunk acmp 73 to volenteer the inductors ?
the chinese schematic i have doesn't seem to state inductor values
[ only " L" labels ]

Re-using the cup seems like a good idea. See the Kubarth website for 1081 details. The Chinese '81 schematic notes inductance values, and Alex C measured these I believe. The '84 and '73 use different inductors obviously.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 25, 2009, 04:06:34 PM
Looks like we'll be able to reuse the little inductor cup
[ as long as the replacement is small enough , or it just
sits over the side ]

Anyone have the real Neve 1073 , 1081 schematic ?

anyone have a bunk acmp 73 to volunteer the inductors ?
the chinese schematic i have doesn't seem to state inductor values
[ only " L" labels ]

Download the Neve schematics at the AMS/Neve website.  The manual for the 1073, 1084 and 1081 have complete schematics in the back.

Why, exactly are we re-doing the inductors?  Do we know that they are the wrong values/resistances, etc. compared to the original Neve EQs?  These inductors are pot cores, and they are inherently self-shielding (up to a point). If additional shielding is required once the power tranformer stray flux is reduced, I know Jensen sells empty annealed 80% nickel transformer cans that could possibly be slipped over the inductors right down to the circuit board, and held in place with a dollop of Dow 748 electronic grade RTV.  The 80% nickel material won't work in very strong magnetic fields because it will saturate, and once that happens, it no longer acts like a shield.

If accurate measurements need to be made of either original Neve or Chinese inductors, I have a GenRad 1659 RLC digibridge that can do a decent job.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on February 25, 2009, 04:46:43 PM
Hi Steve,

>Why, exactly are we re-doing the inductors?

If you wish, you can have a look at the description I posted earlier in this
thread relating to basic noise identification tasks that I performed showing the noise
contributor being the mid-lo inductor.

Although it is hardly an exact analysis, I found it useful in focussing my efforts for
mitigating the performance issues I experienced with the units in hand.

I look forward to your noise measurements and observations on the 81 when you have
had an opportunity to examine one.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on February 25, 2009, 06:21:17 PM
Alan Hyatt had brought up claims of this being a prior design
which he had something to do with and rejected ,
forwarning of inductor problems they had which seems to have
more credibility now , you'd have to dig through the threads
but if the chinese skimp on xfmrs what confidence does one have
in their inductors ?

And i don't know about anyone else but when i tried mine through
the hum , the actual Q button " felt " funny , maybe unrelated
and/or my imagination .

And thanks for the redesign work , who would think to reduce the capacitence !?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on February 25, 2009, 10:10:30 PM
Here is Alan Hyatt's comment about inductors:
Quote
When I read another post where it was said this project was a year long project, again I came in to make a correction.
Why, because I have seen these units, and have been involved in fixing resonance peaks on the inductors when we were going
to do it under the SP ENVY IN June of 2007, but I eventually passed on them.

In context: http://homerecording.com/bbs/showpost.php?p=2943222&postcount=4910

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: jwk1 on February 26, 2009, 11:26:47 AM
I wonder if anyone could help me with a question about the values of the resistors across the secondaries of input transformers on the acmp73, when upgrading the transformers to carnhills?

The acmp73 has 2k2 (R2) on the line input secondary and there is 12k (R1) on the mic input.  From looking at the original schematic at

http://www.danalexanderaudio.com/neveinfo/Neve1073schem.jpg

the 2k2 on the line input seems to correspond to R1 on the original neve schemo.  However I can't find a resistor on the original schematic which corresponds to the 12k (R2) on the acmp73 schematic.

Should I conclude from this that the 2k2 on the line input should be kept, but that the 12k on the mic input should be dispensed with, when I swap transformers over?

Thank you for any help, as I am not too knowledgeable in these matters...

Josh

EDIT: I think that I just found the answer to this question here...
http://www.auroraaudio.net/dcforum/DCForumID1/131.html
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on February 26, 2009, 05:46:29 PM
I agree 100% with the following



    Steve Hogan
1. The power supply must be made solid, this is the foundation of any recording tool.
    thanks for the re-assurance, testing  and improvements. I believe this
    issue contains about 80 percent of the noise issues and other design problems.


    Greg Boboski
2. The inductors must be investigated tested and fixed as needed.
    I suspect that if the inductors have the same level of quality and reasearch as do the
    toroid power transformers then this leaves me with absolutley no confidence in trusting they
    will perform as needed. However, this should be done after the power supply has been
    properly fixed.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 28, 2009, 10:13:00 AM
Results of supplied ACMP Preamp Chinese Power transformer tear-down:

Actual core size is 1.1" ID x 2" OD x 0.6" High.  At 115/230 Volts (50 Hz) the flux density is 16.34 kGauss. At 120/240 Volts, which is much more likely in the USA today due to utilities maintaining higher voltages to reduce transmission line power loss, the 50 Hz flux density is 17 kGauss. As I predicted, this level is right at the maximum operating level of the core material.

The quality of the winding, taping, leading, was OK.  Taking off layer after layer of secondary windings was tedious and uneventful.

The real interesting part was the primary and the core.  It is clear to me, based on the construction technique used for the core, that the transformer manufacturer makes one standard 115/230 Volt primary in a certain VA rating (I suspect this is their 10VA rated primary).  I am very sure that they make these universal primary windings by the 10's of thousands all the same.  They then make semi-custom transformers by adding various secondary windings to the stock primary+cores to make whatever model transformer is required.  The primaries are wound for maximum utility of the core, minimum number of turns, minimum core size to accomplish the VA rating required when operating the core at maximum operating flux density.  Reasonable commercial practice, but not suitable for use around audio circuitry if you want low hum levels.

Here is the really interesting part.  Remember that in one of my earlier posts, I said that we had torn down some poorly-performing Chinese toroidal power transformers and found that they had made the cores out of multiple scraps of strip steel instead of one continuous strip of grain oriented silicon steel?  Well, because the primary of my sample ACMP transformer was varnished to its core, I could not unwind it, but I had to just cut and peel it away.  It was a waste of time to remove it all, so I removed a portion on each side of the core 180 degrees apart so I could get calipers on the core to measure its size.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/ChineseACMPTransformerCoreSmall.jpg)

 In the one small portion of the core where I stripped it down to the steel, I found the end of one strip. about 0.125" in from the OD.  This core in this particular transformer was therefore made from at least two pieces of steel. If I tore the rest of the primary off, maybe I would have found other end gaps that would indicate more than two strips. What's the chance of me finding that out from uncovering 1/2 inch of one core from one transformer, if it were not common practice to make the cores from whatever scraps of steel they could find?  See the attached photos of the uncovered core and a close-up showing where the gap is in the core from where an inner strip of steel ends.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/ChineseACMPTransformerCoreGap.jpg)

If the core quality in these Chinese transformers is a total crap shoot (pun intended), that may explain why some folks are having terrible hum from their preamps and others not so bad.  Even if you luck out and get a core made from a single strip of steel, the flux density is very high for including in a 1 RU audio preamp with unshielded inductors.

The cores in the transformer I will be making available is made from one continuous strip of grain-oriented silicon steel.  We wind, anneal, and coat the cores in house, so we maintain absolute control over the material used and the processing.  The photo below shows the Chinese core, the (green) core used to make the replacement transformer and the first prototype transformer that I finished yesterday, but have not yet tested in a preamp. The extra Green/Yellow wire connects to the copper foil faraday shield inside the replacement tranformer that keeps AC line noise on the primary from capacitively coupling into the secondary.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/TSSACMPTransformervsChinesesmall.jpg)

I also obtained, with the kind assistance of Dave Hill at Jensen Transformers, a collection of various sized hydrogen annealed, 80% nickel cans and covers, with which to experimentally shield the inductors in order to quash any remaining hum and buzz.

To be continued:

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 28, 2009, 05:56:32 PM
Looking forward to seeing the results of this in a preamp. Waiting and hoping your toroid will fix most issues...

OT about the faraday shield.. just wondering about the process. Do you wind the primary, then wrap the shield, then wind the secondary on top?


Thanks again for doing all this!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on February 28, 2009, 07:26:14 PM
OT about the faraday shield.. just wondering about the process. Do you wind the primary, then wrap the shield, then wind the secondary on top?

That is exactly how we do it, although one could put the secondaries on first and then the shield and then the primaries. If the transformer were a step-up instead of step-down, I might put the primary on the outside.  In the case of toroids, the coupling between windings is really good whether the winding is on the top or the bottom.

Steve
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on February 28, 2009, 09:18:46 PM
Thanks for the clarification... Would you say that is generally a good thing to do if one is winding toroids? Or is it generally preferred, not necessarily needed?

From time to time I have to have trafos wound and I wonder if I should have the shop add a shield. Guess it can't hurt?  Any spec for material? I need to give these guys pretty specific instructions...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 01, 2009, 04:54:30 AM
Thanks for the clarification... Would you say that is generally a good thing to do if one is winding toroids? Or is it generally preferred, not necessarily needed?

From time to time I have to have trafos wound and I wonder if I should have the shop add a shield. Guess it can't hurt?  Any spec for material? I need to give these guys pretty specific instructions...

Toroidal power transformers have their windings spread out over the surface of the core, with primaries and secondaries on top of one another. This construction makes for low leakage inductance (passes high frequencies well) and high capacitance (passes high frequencies well) between windings. If you are interested in rejecting high frequencies from the power line, then the Faraday (copper foil) shield is necessary between windings to prevent capacitive coupling of HF garbage from the power line (RF, dimmer hash, etc.) from primary to secondary and vice versa.  Adding the shield makes your power transformer an isolation transformer at the same time, in a similar way to how "balanced power" tranformers work. I believe it is an excellent thing to have in all audio equipment power transformers, but it is seldom done because it is an additional expense to add to the cost of the power transformer and most manufacturers of audio gear really don't know how important it is and they are also under competitive cost pressures to cheap out.

There are some construction techniques and tricks that we use to get maximum performance from our Faraday shielded power transformers, but we are now beginning to enter into trade secret territory, so I'm afraid that I don't have more any construction details that I am prepared to explain.

If you are interested in a quote on your next power transformer for audio design, contact me off list.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 01, 2009, 07:36:58 AM
I personally appreciate the time your giving to this preamp fiasco. I hoping for the best.
Although something tells me this is not a typical DIY project. Its more of a engineers
puzzle than a simple circuit fix. But educating, nonetheless.
Thanks for the kind words. Since I upgrade and modify pro audio gear for a living, and end up fixing many problems that were originally designed into the gear, my approach is hopefully very scientific.  The transistor swap problem, for example is one that I want to see for myself.  It is true that substituting different transistors may have stopped the apparent oscillation, but I want to see the PCB layout, and figure out what is REALLY going on.  The power dissipation in these Q4, Q5 transistors isn't very high (unless it is oscillating), so a 2 Amp rated transistor isn't really necessary here. Q5 is just a current sink load for Q4 and it is set to 8.5 mA.  Q4 should only dissipate about 125 mW when it's working right.  All that to say that the large, metal can BC441 and BC461 are perhaps overkill to resolve this oscillation problem.  I have spent considerable time researching beefy transistors that have similar capacitance, gain-bandwith product and Hfe as the BC441/BC461, but will fit easily into the existing PCB.  I am not inclined to go for the TO-126 package as a solution (mechanical nightmare) when there are many appropriate TO-92 and TO-92L (tall package for audio driver) transistors that may work perfectly, and be a very close mimic to the Neve BC441/BC461 parts, which are hard to get and unnecessarily expensive.  I am seeking the very best, if you will, engineered solution.

Further investigation of the original Neve 338 plug-in amplifier as detailed in the AMS Neve '81 owners manual available as a download shows the reason for Neve using BC461-6 and BC441-6 TO-39 packaged transistors with heatsinks.
Pin 4 of the Neve BA338 Plug-In amplifier is labeled "boost (39 Ohm for 300 Ohm O/P)."  Strapping a 39 Ohm resistor betwen this pin and Ground increases the current sink Q5 collector current from 8.25 mA in the "non-boosted" position to 40 mA in the "boost" configuration.  This will increase Q4 dissipation from about 100mW to 480 mW which will be quite toasty without a heatsink and a big part.  The -6 suffix was the highest gain selection available on the BC441/BC461 transistors.  The TNC '81 clone does not use the "boost" position, so a hefty TO-92 transistor should work just fine.
I have now selected what I believe to be an optimal replacement solution to the Q4/Q5 transistor oscillation problem.
The replacements I have selected have a TO-92 package with CBE pinout-- same as the Chinese supplied BC557/BC547's. They are, however. rated at 800 mA current instead of 100 mA.  They are cheap and readily available.  The junction capacitances are more in line with the BC441/461 parts, and they are available in a selected gain range that should mimic the performance of the BC441/461 transistors.  Unfortunately, I must order them to try them out, so I won't have answers until next week.
Until then I am not going to recommend them by part number until I confirm that they work well. because it may just cause more confusion than already exists as to the best transistors to use as replacements.

I will confirm that a PN2907A looks like an excellent replacement for the BC461-6.  Gain is similar, they have 800mA rated collector current, and they will fit perfectly in the existing PCB with only one change.  Since they are EBC and the original factory transistors are CBE pinout, one must install a PN2907A with the Flat of the transistor body rotated 180 degrees from the original transistors.

While investigating the transistor problem, I discovered another mistake in the same circuit that came about from just stealing the Neve schematic to make a clone and not stealing it very carefully. Unfortunately those to whom the word R&D means "Rob and Duplicate" often have no clue how any of the circuitry works or what it does.  I haven't decided  what to do about this yet, but my uncomfortable tingles are beginning to go off.

Fellow DIY group members,  Let me detour for a moment to share some thoughts with you all regarding my efforts to help the owners of these TNC preamps to get their units working and usable, and at the same time provide myself with additional income to pay my bills.  I have been contacted by various engineering and manufacturing professionals who make superb quality (but not inexpensive) products.  They have warned me to be careful about the investigative work that I have been doing that has included detailed explanations and solutions that you DIY folks can learn from and make improvements to your gear (which is good).  The truth is, that there are folks out there, both Chinese and domestic, who are making money selling clone preamps that have been "developed" by simply copying the Neve circuitry.  Most of these folks have absolutely no clue what is really going on in the circuits.  They substitute parts that are hard to find or unusual because they are hard to get and they have no understanding of why Neve chose that part in the first place.  The power transformers, audio transformers and inductors in most of these clones are sub-par to be charitible, and the manufacturers of the the magnetic components don't even know how to fix their own parts, because they don't understand the circuit in which they are used, and don't understand how to make their own product better.
It has been said by others that the TNC folks have expended thus far only a minimal effort to address the problems with these units themselves, but have just watched others in the various Forums like this one tackle the various problems and attempt to solve them without any real effort on their part.  The present group buy preamps were apparently sold as-is.  TNC has made it clear to me that the work I am doing to the preamps is not under their authority but is strictly between me and owners of the preamps.  I have worked out a system for bringing in the preamps to modify them and add the new power transformer and other upgrades, and also making available a kit of upgrade parts, for the DIY types who are capable and wanting to do the upgrade and transformer installation work themselves. I think that plan will work well for both me and the preamp owners.

I started this project based on the idea that there were these preamps out there that had hum and buzz problems that could be fixed if I provided a good power transformer for them.   As I have learned more about these ACMP preamps, I have come to realize that there are a lot more design problems in these preamps than just the power transformer.  I can fix every one of the problems.  I have already designed a new power tranformer.  I have already fixed the Power supply PCB and the wrong capacitor values and voltages that were chosen.  I have now figured out what I believe to be the best replacement transistors, and located another problem in the EQ boards that will require changing another part. I can design new 5% tolerance inductors that are consistent and free from high Q resonance, and I can design meter driver circuitry and clip lights that work correctly with real audio waveforms.

The moral and economic dilemma I find myself in is that I design and engineer first-class audio gear to keep a roof over my head. I certainly do not need to make a lot of money on fixing the problems in these preamps, but if I cannot make it pay something, then I cannot continue to provide really good engineering services.  The difficulty I am facing right now is that at the same time I help you, the purchasers of the preamps, to make your gear work correctly, I am simultaneously giving away free engineering services to the manufacturer(s) who should rightly be paying for that engineering work.  If I solve the problems for you in a manner that "shows all my cards", the manufacturers get all their boxes re-designed and the problems fixed without them having to pay for it.  I have been warned that I have been engineering fixes for all the problems of the TNC preamps for free.  The truth is that most of the manufacturers of the really cheap Neve clone preamps truly don't even know that their circuitry has problems, let alone how to fix them.
As I write up detailed fixes for my DIY friends, I am at the same time writing it up for these manufacturers who have already demonstrated by their product line that they have no qualms at all about stealing intellectual property from others to make a buck for themselves. It is because there is some rumblings of another "Group Buy" in the works that I am especially concerned about fixing all the problems in the future boxes for free.

The truth is that, if the "Rob and Duplicate" folks get all their engineering for free, it really makes it hard for the legitimate audio equipment manufacturers to stay in business.  Their products cost more, sound better, and many of them have invested heavily in test equipment and engineering costs to make their products the best they can be.

I'm searching for a balance between helping the individual owners, which I really want to do, and inadvertantly assisting the unscrupulous, which is neither in my best interest nor in the best interest of legitimate manufacturers of the really good audio gear.  In reality, it isn't even in the best interest of every one of us who wants to have great audio gear available, because the folks who care enough to really design and build the best audio equipment will go out of business.

I apologize for such a long rambling post, but I wanted to share my concerns with you all, and invite you to comment.



 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on March 01, 2009, 08:01:51 AM
I'm searching for a balance between helping the individual owners which I want to do, and assisting the unscrupulous which is neither in my best interest or in the best interest of legitimate manufacturers of the really good audio gear..

You could always send an invoice to the mfg. ;)

BTW Mr. Robin Hood,

Thanks for all the knowledge and information you have provided up to now. I'm looking forward to see where you go with all of this.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 01, 2009, 08:42:13 AM
TNC  [ or Alan Hyatt ] should pay you and then get the jump on another run before
, as will happen ,  the chinese push whatever " improvements "
they get into the next round  , and there is no doubt that will happen
, even if it was tnc saying we need better transformers , they make for
grp A , Improve for grp B , take those and sell to Grp C , of course
getting away with as little as possible .

You can amortize the numbers and wait till you have a enough
to sell the upgrade packages like a grp buy at once , and then
piece meal with overstock [ or keeping some stock after ]
as it's adaptability to other design develops
[ I would hope this doesn't turn out more expensive  ]
 based on percentage of numbers out there .

Of course too , the people doing the whispering have some interest for themselves as well ,
but for years now these chinese sales have been a run of X amount , then move on to the next item ,
 no support , no looking back .
Some of your things may be too expensive for them and could be a low stocking item ,
 but i think using their model consider it a one off run , that may have some ongoing life after ,
 certainly if all goes well and people are happy with the value / service it will enhance  ,
 further establish your good reputation and may be allot of good promo

As a separate product , you could make whole , external power supplies ,
or look for other other products to adapt  [ & retro fit ]
the things you are working now for greater numbers .
[ i don't think the chinese care about old things , it's like the new car
 model , where they gotta keep pumping out new product for new profit
The world needs decently priced api 500 supplies .

Another point is , that as some are not in this as a business [ recording ]
they wouldn't likely buy the better product as it would not pay itself off
and therefore would go to the cheaper item as the only option , maybe
hoping to improve performance themselves but this isn't business lost
for the high end market .
On another hand as others end replacing too many components , they may
get the idea buying cheap is not worth it , or not to expect [ accept  ] much .

Supporting local business and not buying Chinese where possible ,
couldn't hurt our own economies , sometimes cheap is not worth it ,
as long as you can afford it .

But clearly , someone is stirring things up again , mostly we avoid that here ,
but as it is a grp , let us know how we can help .


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 01, 2009, 11:28:59 AM
Steve

Those are indeed some difficult thoughts and questions as to how to resolve this dilemma: fix the buyers defective preamps, but at the expense of providing free engineering to those who have stolen the designs in the first place.  I myself am no businessman, lawyer, electronics or audio professional, so my postings will certainly be naive and ignorant.

It seems we have a number of players in this game, each with his own (and many times conflicting) interests at heart: the manufacturer (Chinese), the distributor (TNC), the purchaser (including us DIYers), the re-engineering/repair people (at this time Steve), and the designers and manufacturers of high-end audio equipment (including Steve) who's product designs are being stolen and who are subsequently losing income from their own work.

First, I would have to say that those of us who bought these knew (or should have known) that it was a crap shoot, and should have had no real expectations that the product would work correctly or be warranted. There is also the ethical/moral dilemma of buying goods built from another's hard work (at no profit to that person), but I guess that by buying we all showed where we stood on that. So there should be no expectation that these will be fixed at anyone's expense but our own.

The fly in the ointment here is that TNC, the distributor, through some ignorant mistake, I'm sure, has printed a one year warranty on the users manual. I don't know what this implies legally. I am reasonably sure, having participated in and followed these group buys over several years, that the organizer of these is an honest, but business-unwise naive man who wants to do the right thing. He has done this at little or no financial profit (and possibly a loss) to himself, and I don't think there is any possibility of his honoring the warranty, for financial reasons. In this latest group buy he has picked up a partner, well known in the recording community. I have no knowledge of his interest or capabilities in all this. Anyway, I don't think there will be any help or responsibility from this sector.

I would divide the fixes for these into two categories - those that merely call for the replacement of parts with existing substitutes (transistors, capacitors, etc.) or minor circuit corrections, and those that require the manufacture of more expensive components, such as inductors.

Obviously the cheap modifications could be picked up on by manufacturers at little expense, and incorporated into future production runs should they be interested. Manufacturing the more expensive inductors might be cost prohibitive for them, but you can't be sure. They are some pretty wily people. Any commercial mods that are made, whether unpublished, potted or otherwise hidden, will probably get into their hands.

So to me the answer boils down to two possibilities:

1) Do the fixes, unpublished or not, with the real possibility that you will shortly see cheap Chinese equipment incorporating the fixes, to the detriment of the domestic producers. This would probably include even just supplying the new power transformer (though I have no real idea if it would be possible or cost-effective for the Chinese to reverse engineer and produce these, nor how much of a fix this alone is going to be.)

2) Let this project drop, and leave those of us who played this game to reap the rewards (another of life's learning experiences.)


I'm sure there are alternative answers I've not thought of. Have at it.  :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 01, 2009, 12:15:14 PM
A long & tangled web for sure with many unknown truths
It would be better however to keep this thread technical
as is the " Lab "
And we don't need this turning into H.R. or PSW

could start another thread in the Brewery to discuss
& debate mutual assistence . [ and hopefully determine
how to keep this positive for everyone ]

also a few of the more clever here still have not recievced
their pres yet  [ Clint ?  ] which will also feed more
knowledge into this problem [ & solution ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 01, 2009, 04:54:14 PM
Steve

Those are indeed some difficult thoughts and questions as to how to resolve this dilemma: fix the buyers defective preamps, but at the expense of providing free engineering to those who have stolen the designs in the first place.
. . .
So to me the answer boils down to two possibilities:

1) Do the fixes, unpublished or not, with the real possibility that you will shortly see cheap Chinese equipment incorporating the fixes, to the detriment of the domestic producers. This would probably include even just supplying the new power transformer (though I have no real idea if it would be possible or cost-effective for the Chinese to reverse engineer and produce these, nor how much of a fix this alone is going to be.)

2) Let this project drop, and leave those of us who played this game to reap the rewards (another of life's learning experiences.)


Although I am new to this forum, I am delighted with the mutual respect that is displayed to every participant here. I love that one doesn't have to plow thru tons of inanity to get to the answers that everyone so freely shares here.
In that spirit, since I have brought up this off topic subject (for this venue, at least), I will do my best to clarify my intentions and close off the discussion in this place.  Having gotten myself involved in what I thought were strictly technical discussions in other forums, and then coming to realize that there was way too much personality issues that cluttered up the ability to simply get to the truth, I confess that I really like the signal to noise ratio here and I don't want to have a part in creating static.

It was not my intention to stir up any kind of hornet's nest or to even make accusations. The world we live in is what it is.  I do not believe that the TNC partners had anything but the best intentions to bring decent technology to many people who simply are not in a position to spend thousands on mic preamps. I believe that they (naively, perhaps) had no idea of the magnitude of the problems that the preamps would manifest, and they appear to have held back essentially no funds with which to effect warranty repairs.

Crazydoc has, as always, spoken with a great deal of clarity in his post. I indeed agee with him that the buyers of these preamps are on their own to spend more money to really make them perform, if not spectacularly, at least decently.
It is my desire to make my services and expertise available to those who want to fix/mod/upgrade their preamps accordingly.

 I have tried to think of ways to provide the more subtle fixes privately,  to just those whose preamps I actually mod/upgrade or to those to whom I sell the upgrade kit of parts with directions of what to do.  The reality is, that in this day of internet communication, once you publish even a set of directions with an explicit copyright notice and all rights reserved legal clause, I suspect that I would be able to Google the preamp mod directions about 30 seconds after it popped out of my printer :>).  It is my opinion that trying to make the upgrade changes a trade secret is doomed from the get-go, so I have resigned myself to publishing the entire list of mods that I will be making available on the preamps so there will be no surprises.  The one thing I have going for me is that I am the sole supplier (for now) of the upgraded power transformers, so every one who wants one will get one from me.  The fast, soft recovery rectifiers that I am retrofitting to the power supply boards for those who want the cleanest power supply are no longer being made and I have pretty much the only remaining stock.

I have gone way too far already to drop the project, and I still feel that I can make such a substantial improvement in any owner's preamp that it will be well worth the upgrade cost.  There are enough of these units out there right now that if even 10 or 20 percent of the owners choose to upgrade, I could still make enough money to cover my initial development costs and make a little on each preamp that I fix.

I love to teach, as you can probably tell from my long-winded, but hopefully instructive posts.  I spend a lot of time writing my posts because I strive to be technically accurate, and still be able to make the concepts clear to those who aren't uber technical.  I have always felt that if one really understands his subject well, then he should be able to simplify its explanation and still maintain technical accuracy. "The Art of Electronics" by Horowitz and Hill (my favorite electronics book) is an excellent example of that abilty to simplify and clarify concepts.  There are many who read and participate in the DIY board who are eager to learn more about audio electronics.  There is so much that goes into good sound that isn't published or taught anywhere that I know of, and I am willing to share much of what I have learned over the years with the DIY group.  I trust that my continued participation here will be helpful to everyone.

So I will procede full speed ahead to finalize the list of upgrades to all three ACMP preamps.  I will post again when I have some new measurements or insights and I will keep it technical. The next thing for me to test is the effect that the new power tranformer has on the hum levels.  Those who are interested in the fixes/mods/upgrades that I will be offering can contact me off list to make arrangements.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on March 01, 2009, 06:25:49 PM
Steve, I agree with everything you've said. You have already given away a lot of free work already, and if this is your line of work then you should think about yourself first.

As you suggested, instructions posted by yourself with a mod kit would spread like wildfire on the internet. Perhaps you could offer a send-and-return mod solution for those who are unable to study the circuits themselves.

You can guarantee that your work will be incorporated into future Chinese designs. Whether or not this is a good thing is up for debate I suppose.

I'm still waiting to get my racks and will investigate when they arrive, but it'll be great if Peter (Clintrubber) can get his hands on his as he seems to have a fairly solid understanding of sandstate devices.

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on March 02, 2009, 07:52:05 AM
Perhaps we could do a Group Buy for Steve's engineering? If even 1/4 or 1/2 of us get together and help it should be a fair sum of money. Although I have no idea what kind of money a job like this is worth.
 am most definitely willing to participate in something like that. With enough people it will be pretty cheap for individuals.
 
The only caveat being that any aspect of his engineering that's released will most likely become someone else's "R&D".

Plus there will be money made for the specific parts, kits, and/or labor. Everyone wins! :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 02, 2009, 08:19:52 AM
Right Steve can have accountable " numbers " and maybe a better
idea of what to expect , how much to order .
BUT once it's out there  , it's out there  ,
so the finacial model has to be based on the initial run ,
gravey after that .

Although grp buys do have ecomnomy of scale , that's up to steve

I have sent my 81 inductors to Cinemag and expecting replacements
to be under the 20.00 mark in small quantities [ or 1's & 2's]
once they're set up anyone will be able to order at anytime

And , no they weren't bought  " AS IS " that wouldv'e hurt sales
i didn't expect a neve preamp  but i did expect it to work
but the changing story is typical of the vague details from the start
again  , no problem to open a thread in the brewery , but might as well
move forward and fix these enough to be usable



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 04, 2009, 05:26:19 PM
So I will procede full speed ahead to finalize the list of upgrades to all three ACMP preamps.  I will post again when I have some new measurements or insights and I will keep it technical. The next thing for me to test is the effect that the new power tranformer has on the hum levels.  Those who are interested in the fixes/mods/upgrades that I will be offering can contact me off list to make arrangements.

I spent yesterday finalizing the capacitors for the power supplies, and the phantom mods, and ordering same.
For you technical types: I will be using three different series of premium quality, low impedance, 105C electrolytic capacitors, depending on value, voltage, case size and function in the circuit.  I will be using Panasonic FM series for the smaller caps, Panasonic FC series for a couple of the 63 Volt caps since the FM series stops at 50V.  The big filter caps are Nichicon HE series which has in addition to the high temperature rating and low impedance, exceptionally long life.  I have ordered caps for all three types of preamp.  The new input filter caps have conservative voltage ratings and will be loafing instead of panting as they do their job.  I will be reducing the 24 Volt main filter cap values from 4400uF to 2000uF total for the 73 and 84 power supplies, and from 4700uF to 2200uF on the 81.  As I explained in an earlier post, that reduces the peak charging currents in the caps, which reduces magnetically induced noise from the power transformer.

In my first modified power supply, an 84, I removed all 12 of the 1N4004 rectifiers, and replaced them with fast soft recovery rectifiers.  The standard recovery 1N4004 rectifiers generate little bursts of RF when they turn off 60 times per second.  The good rectifiers eliminate that problem completely.  I plan to add an RC snubbing network to the PCB that will damp the tranformer ringing when the rectifiers turn off as well.

I used 9 of the original 1N4004 rectifiers that I removed from the power supply board as protection diodes for the regulators. They were easily tacked on in all the right places on the solder side of the PCB.

The phantom supply from the factory uses a 51 Volt zener diode as a reference.  This means that if the Zener is right on the money, the phantom regulator has 50.4 Volts output.  Diode tolerance is 5%, so it can be as low as 47.9V and as high as 53 Volts.  My modified phantom circuit works best with 52 Volts, so my plan is to replace the single zener with 2 series zeners, a 20V and a 33V.  I have hundreds of each, so I can select a pair that yields 52.6 Volts, which will give 52 Volts from the regulator.  The kits will have the selected pair of Zeners.

It is really important that the 6K81 phantom feed resistors be well matched.  When I match them by hand I usually select them to be within a couple of Ohms of each other. Almost everybody I know uses 1/4 watt or RN55 sized 6K81 resistors for Phantom.  I did the power calculation for the resistors and it turns out that the worst case power dissipation (shorted) is  0.397 watts!  With the most power-hungry microphone I know, (Scheops 12V), the power that must be dissipated by the resistors is just over 1/4 watt.  It is never a good idea to let metal film resistors run that toasty, so I will be using 1/2 watt sized resistors when I replace the phantom feed resistors in the preamps.
I have two choices here, buy a bunch of cheap 1% resistors and select them myself into matched pairs, or spend more on the parts and buy 0.1% tolerance resistors that I don't have to select. Because of the extra labor for selecting the pairs, the matched pair of resistors will end up being $1.50 in the upgrade kit of parts.  For the same money, I can purchase PRP GP1/2 25PPM 6.81K 0.1% resistors and not bother with all the matching of lesser parts.  These PRP resistors have copper leads, non-ferrous end caps and all the things that make a resistor sound good.  Their close cousins the PR9372 Audio metal film resistors are probably the best sounding metal film resistors available, better than Roederstein and many others.  The phantom resistors are in series with each other and provide a parallel load to the microphone across the input tranformer primary. Its effect as a load is about 20 dB less than the much lower input impedance of the transformer. It may seem really silly to put such a high quality resistor in such a low cost preamp, but I think in this case it ends up giving higher quality for about the same money as a selected matched pair of cheap resistors.  At this point, I am about 80% convinced that using the PRP resistors is the better way.

I have been offering the fast soft recovery rectifiers as an extra cost option ($1 per rectifier. $12 for a 73/84 and $16 for the 81) to those who have been sending in their preamps. (The Diotech-USA rectifiers that I use have been discontinued, and are unobtainium except for my remaining stock).  I  have to supply 9 or 13 1N4005 regulator protection rectifiers anyway at about $0.25 per rectifier, so it may make sense for almost everyone to get the good ones and use the ones they take out to do the regulator protection.

Thoughts on these two choices of components for the Preamp mods?




Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 04, 2009, 06:00:24 PM
Thoughts on these two choices of components for the Preamp mods?

Not specifically on those, but I must admit something else came to mind: reading all this good & valuable info
(thanks !) I started to realize (or at least I got the impression) that this goes well beyond tackling 'the Chinese flaws'.

I guess that most people want to go all the way once upgrading these beasts, but as a what-does-what it might be
interesting/good to roughly sort the upgrades on a scale ranging from say
'absolutely essential' to
'soundwise lifting it somewhat towards the original' to
(entering the realm of diminishing returns here) 'nice to have for technical reasons' to
'maximum overengineering but hey why not while at it ?' 

Please note this is not meant as criticism, but as a discussion item on how far to go with the technically refreshing on a 'character-preamp'. In all respect I somewhat felt that some of them might be on the path of making these circuits fully blameless (in D.Self-speak). Since these are 'character preamps' I figure we probably shouldn't make them too technically perfect, I assume at least part of the charm comes from ingredients that are technically probably pretty bad but still sound good. I mean, for a totally transparent preamp I know quicker ways to buy or solder one  ;)

Best regards (& again, please don't feel offended or anything since I'm not after criticising your good work in any way,
it's much respected),

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on March 04, 2009, 06:55:06 PM
Steve, many thanks, as others have said. 

Why not do something like we did for the Poor Man's 660, which was adopted by Analag and apparently is a real success.  He was concerned about his design becoming a Chinese product.  So I/we suggested in the thread that he withhold the schematic and include it only with the paid PCBs when shipped.  This has worked great.  No one has spilled the beans or published the schematic.  DIY and troubleshooting discussions haven't revealed enough for a clone maker to rob and duplicate.  On the other hand, no one can ever safely keep things under a lid when selling anything these days.  But at least this method keeps the design off the web. 

In your case, you could offer something like three services:
1) send in the box to you for repair, with perhaps various repair options.
2) purchase a full bag of PSU and parts for a full repair, DIY, with full documentation.
3) purchase a partial bag of parts for those who want to DIY or source some parts themselves, or use external PSU, but still with full documentation for all the fixes.
4) full documentation only, for a nominal small fee, or free, to members only.

You get paid.  You get to share all your fixes.  Keeps your fixes off the web.  You offer a cheap way to share all your fixes for those who want to tread lightly on purchasing parts (option 3) and consider option 4 (or as described below, perhaps free for members).

Finally, to complete the full spirit of DIY, you could also allow full documentation for members here via email, who can't afford or don't want to buy parts.  This would be important because it would reduce/eliminate the need for troubleshooting questions like, "what's the fix Steve said for x?" Therefore, troubleshooting discussion would be limited to implementation, and not design or drawings.  This keeps the noise down and makes it hard for interlopers to understand the whole picture.

Just my 2 cents.  It's a way to charge some coin for your time and engineering and your parts, while still also sharing in the DIY spirit.  And keeping it all somewhat discreet.  :)

EDIT: On the other hand, I can imagine Chance getting a copy of your fixes, and rightly so, since he owns some of these, and then including that list next time he goes to China for a new group order.  Once it's released to the Chinese factory, it's gone.  So, maybe my idea is silly.  Maybe it's silly to try and control this information...  :-[

I personally would like to get my hands on your soft recover rectifier diodes...  The PRP resistors seem fine.  I really appreciate the attention to the PSU, it's often overlooked.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on March 04, 2009, 07:14:04 PM
I wouldn't trust anyone outside of this forum to keep any documented fixes to themselves to be honest. The 660 is a great example of how that has worked here though. One look at some of the other forums discussing the topic of these preamps, and I'd feel pretty doubtful.

I reckon like you suggest, Steve could get a decent bit of work from servicing these units himself. It would be quite sad to see his efforts stolen for future designs at someone else's benefit.

I reckon a lot of people with next-to-no understanding of electronics might be annoyed at the suggestion of not giving instructions for free, but they have to understand the effort involved and the years of work required to be able to do this; something which someone has worked for, and now provides their main income.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 04, 2009, 07:44:20 PM

Not specifically on those, but I must admit something else came to mind: reading all this good & valuable info
(thanks !) I started to realize (or at least I got the impression) that this goes well beyond tackling 'the Chinese flaws'.

I guess that most people want to go all the way once upgrading these beasts, but as a what-does-what it might be
interesting/good to roughly sort the upgrades on a scale ranging from say
'absolutely essential' to
'soundwise lifting it somewhat towards the original' to
(entering the realm of diminishing returns here) 'nice to have for technical reasons' to
'maximum overengineering but hey why not while at it ?' 

Please note this is not meant as criticism, but as a discussion item on how far to go with the technically refreshing on a 'character-preamp'. In all respect I somewhat felt that some of them might be on the path of making these circuits fully blameless (in D.Self-speak). Since these are 'character preamps' I figure we probably shouldn't make them too technically perfect, I assume at least part of the charm comes from ingredients that are technically probably pretty bad but still sound good. I mean, for a totally transparent preamp I know quicker ways to buy or solder one  ;)


You bring up a very important point. I understand completely that it is not desirable to re-engineer or modify these units to "fix" all their circuitry shortcomings.  To do so would eliminate the very coloration that they were purchased to impart to their owner's recordings. 

Let me comment on the mods that I plan for these "character" preamps. The fixes that I have thus far proposed make little or no change to the desirable colorations, but should comprehensively fix the problems that are just problematic and compromise the intended sound quality.

1. Neve modules don't come with power supplies.  Since most of the actual audio circuitry in these units is copied directly from the Neve schematics, there aren't too many mistakes there.  Replacing the power supply tranformer lowers the stray magnetic field that was inducing unacceptable hum levels into the EQ circuits (fix).

2. The caps in the power supply had wrong voltages and or values and no protection diodes. These should have been there from the start, and the over voltage on the filter caps was likely to cause early failure of the power supply (fix).

3. Phantom supply -- Chinese added 1K resistor in series with the two 6K8 feed resistors.  Neve has only the 6.8K resistors.
Lack of balance in the phantom resistors can cause low-frequency THD and saturation of the microphone output tranformers. The 1K resistor can cause the mic to starve for power due to excessive voltage drop across the resistor.
Grounding the off position for the phantom switch discharges the phantom power, otherwise the cap stays charged and discharges when you plug in the next microphone.  Off isn't really off on these preamps. (mostly fix -- otherwise preventing the preamp from degrading the microphone's performance)

3. Gain switch -- speaks for itself -- not a problem in the Neve, but they didn't copy that part correctly.

4. New transistors in the 81: the need is for transistors that more closely match the characteristics of the original. The transistors that I have on order are very close in electrical characteristics to the Neve parts, but they fit in the Chinese circuit board correctly. (fix to eliminate oscillation and to more closely match Neve)

5. This is a new one,  The Neve circuit uses an AA144 Germanium diode as a Baker clamp for Q4 in the BA338 amplifiers.
This is the same Q4 that must be replaced in the ACMP preamps.  The Chinese, having no clue what an AA144 diode is, nor what it does in the circuit, substituted a 1N4148 silicon diode, which cannot not clamp Q4 because its forward voltage drop is about 0.7 Volts instead of about 0.2 Volts on the Germanium diode. The Baker clamp prevents Q4 from saturating when the amplifier overloads (clips) in the positive direction. If Q4 saturates, it "sticks" to the rail and takes a long time to recover from clipping (sounds terrible).  The Baker clamp helps the amplifier to overload gracefully and come out of clipping immediately. The Chinese circuit won't work like the Neve.  I plan to replace that diode with a Schottky diode that has a similar low forward voltage drop to the original AA144 which is pretty much unobtanium.  The diode only works when you clip the amp, but now it will act the same as the original circuit. (fix)  BTW  the substitution of that diode may be partly responsible for the oscillation problem -- I don't know for sure yet though.

6. Polarity checking and correction (fix).

7. Premium rectifiers in the power supply are probably overkill, but they just make the power supply quieter. I can't see how it hurts.

8.  When the time comes to shield/rework the inductors, my goal would be to measure some original Neve parts and duplicate them in terms of Value and DCR (which affects the Q of the Equalizer circuitry).  The shielding, if it proves necessary after the power supply mods, just gets rid of hum -- can't see that as a change in the "character".

From your point of discussion -- absolutely no offense taken.  I am in complete agreement with your caution not to "fix" out the color.  In many of the mods I do to vintage gear, the goal is to fix the nasty, noisy stuff, without changing the character that made that piece of gear prized in the first place.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on March 04, 2009, 10:53:04 PM
I don't want to reopen the business discussion here, but I wanted to propose a solution to this scenario,
which seems to sit at the heart of many folks' fear about publishing details of circuit fixes for the ACMPs:

...I can imagine Chance getting a copy of your fixes, and rightly so, since he owns some of these, and then
including that list next time he goes to China for a new group order.

The simple answer to that is to impose a licensing fee for TnC. I don't believe the TnC guys would forward
this group's solutions to the factory if they understood that they hadn't been given permission to do so. That said,
if Steve and TnC could negotiate a licensing fee, so much the better -- the product is improved, and Steve gets
compensated for his considerable time and expertise.

That said, even if the factory does get an illicit copy of Steve's circuit fixes, can they source all the right parts?
Can they reproduce the high-end power transformer? Not as far as we've seen. Case in point: having Neve schematics
in hand sure didn't generate a functional Neve preamp.

I'm if favor keeping the details off the public websites, as was suggested above, but it seems to me that the best
protection is not secrecy, but rather a negotiated license.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 05, 2009, 07:36:57 AM
From your point of discussion -- absolutely no offense taken.  I am in complete agreement with your caution not to "fix" out the color.  In many of the mods I do to vintage gear, the goal is to fix the nasty, noisy stuff, without changing the character that made that piece of gear prized in the first place.

Hello Steve,

Thanks for taking my words as they were intended and thanks as well for the added info, indeed all fixes without touching the character one would say.

The three cap types you mentioned remain as the thing I'm wondering about, unless these were meant for the supply only, and not for the AC-coupling ones, for which one might or not might want to use those (often dreaded) tants, at least already 'for authenticity'.

Thanks & regards,

  Peter

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 05, 2009, 07:44:29 AM
I reckon a lot of people with next-to-no understanding of electronics might be annoyed at the suggestion of not giving instructions for free, but they have to understand the effort involved and the years of work required to be able to do this; something which someone has worked for, and now provides their main income.

Hi,

I'm not sure this would still work. I mean, I don't want to give ill-meaning people an idea, but isn't the relevant info already out there ? (= here in this thread)

Various people* already contributed and then Steve joined and gave it an additional & completer 'scientific touch' and the result is a collection of experimental & common sense & 'experienced' info that would someone who's at least a bit skilled in the art enable to transform his/her preamps into 'cured' versions.

The only thing that I still could see working is to close this thread in some way & arrange access for 'legitimate people' (...) only. Not sure if we would & could go that path....

Bye,

  Peter 

*: the lucky b&st*rds that got their preamps already  ;)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 05, 2009, 08:13:57 AM
not quite so lucky peter
but hope you get your's soon
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 05, 2009, 10:31:08 AM
Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings, I've decided to mount a NC SPST microswitch, pushbutton or toggle, next to the gain knob on the face of the pre (hopefully I can find the space there.) This will connect the two sides of the trace I've cut at position 6 of the gain switch, leaving this gain setting intact. When rotating the switch through it's settings, I'll toggle this switch to break the connection, thus preventing the oscillation. Of course, it only matters when the switch passes from position 6 to 7, but I'll just have to get into the habit to doing every time I change gain. Not a good solution functionally, but I'll get that setting back.

It would be nice to mechanically couple this action, but I can't come up with a simple, cheap way to do it.

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make? I don't feel like going to all that trouble to find out.

For reference, see my post # 440   http://www.groupdiy.com/index.php?topic=27791.440
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tarnationsauce on March 05, 2009, 12:52:14 PM
Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings, I've decided to mount a NC SPST microswitch, pushbutton or toggle, next to the gain knob on the face of the pre (hopefully I can find the space there.) This will connect the two sides of the trace I've cut at position 6 of the gain switch, leaving this gain setting intact. When rotating the switch through it's settings, I'll toggle this switch to break the connection, thus preventing the oscillation. Of course, it only matters when the switch passes from position 6 to 7, but I'll just have to get into the habit to doing every time I change gain. Not a good solution functionally, but I'll get that setting back.
You can simply hit the mic/line button. ;) The line input does not use the 2nd gain stage.

It would be nice to mechanically couple this action, but I can't come up with a simple, cheap way to do it.

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make? I don't feel like going to all that trouble to find out.

For reference, see my post # 440   http://www.groupdiy.com/index.php?topic=27791.440
Firstly, I don't think you would want that witch be break-before-make. I am pretty sure there is DC running through the switch and if you used break-before-make you would get a pop at every position.
I took apart my switch. There is a possibility of modifying it to add an extra position. You would have to grind a bit of that wheel off.
(http://img.photobucket.com/albums/v646/red_3000/how_to/S1052286.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 05, 2009, 04:25:41 PM
The three cap types you mentioned remain as the thing I'm wondering about, unless these were meant for the supply only, and not for the AC-coupling ones, for which one might or not might want to use those (often dreaded) tants, at least already 'for authenticity'.

All but one of the caps in the Bill of Materials for the "kit'o'parts" are used to re-populate the power supply PCB with different caps. As a matter of fact, every electrolytic on the power supply board gets replaced for one reason or another.  Collectively, along with the replacement low-noise power transformer, they provide a robust, reliable power supply with lower ripple, noise, and significantly more "reserve" due to bigger output caps on the regulators than the original supply.

  The remaining Panasonic FC 220uF/63V electrolytic replaces the 100uF/63V electrolytic in the Phantom power local filter on the mic preamp card.  In addition to reducing the 1K to 200 Ohms, and making the 6K81 feed resistors matched, this completes the beefing up of the phantom mic powering circuitry.  This cap is a DC filter that is in common mode with the microphone signal, so it really has close to Zero effect on the sound quality except for final filtering of ripple and noise from the microphone's power.

  I understand that the original Neve preamps used Tantalum caps for signal coupling from stage to stage. Tantalum caps are a mixed bag.  When used with DC bias as in the Neve circuitry which runs on a single 24V rail, they work OK.  My understanding is that there is a built-in diode in a tantalum cap that causes excess distortion if it is not biased on.  Tantalum coupling caps in audio circuits that run on Bipolar supplies which have little or no DC bias  aren't that pretty sounding.  I have no plans to offer tantalum replacements at the present time for the audio coupling caps.

I am thinking at the present time of handling parts kit orders for DIY types and/or people who live too far away to ship me their preamps for a reasonable cost, in the following way. I am thinking of charging some amount for the instruction booklet, which must be purchased with every order.  If you have multiple preamps and you are ordering multiple kits, you only have to buy one instruction/documentation booklet --  (sorry, please no group buys for multiple owners.) The price of the booklet will cover both the publication costs and some compensation for my IP.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 05, 2009, 05:32:46 PM
Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings . . .

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make?

Whether or not a switch is "shorting" or "non-shorting" in rotary switch lingo -- aka "make-before-break" or "break- before-make" is determined by the width of the rotor that moves through the contacts that are at fixed intervals of rotation.  A fat tab on the rotor stays connected to the previous contact position as it passes on to the next. (shorting) and a skinny tab on the rotor leaves the previous contact completely before making contact to the next (non-shorting).
To make a rotary switch non-shorting on only one position would require that the contacts, and not the rotor be different widths.  It's possible but I don't know of a switch manufacturer who has ever designed such a switch.
That's why the Neve has the "OFF" position in the middle of the range.

I think the best solution on the present run of ACMP preamps is to make one position an "OFF" position, just like the Neve.  You end up with one less gain click, but one can spread out the remaining resistor values in a manner, yet to be determined, to prevent a big gain gap and distribute the loss that way.  I hope to figure this out very soon.  For my mods, that will have to do, because the labor involved in adding a switch would be astronomical if you are not DIY.

I have taken the plunge -- the upgrade parts kits will contain two PRP GP1/2 6.81k 25PPM 0.1% Phantom feed resistors. I will be using the same resistors in the next revision of my Jensen Twin Servo preamps. They will arrive next week, along with my remaining capacitors.
The Q4, Q5 resistors arrived today, so we will soon know if they are a good fix to the '81 EQ oscillation problem.

More Later . . .
Title: ACMP disaster cleanup
Post by: electrochronic on March 05, 2009, 10:41:59 PM
Man, o man

This is some of the best reading I have come across next to Kevins ( KHStudio ) pultec inductor thread. Just great reading and knowledge here. You cant get this in any book I have ever read.

Steve,
I have read thru everyones postings and I have never seen a more sensible solution. As long as it remains a option to DIY fix and at reasonable cost , I fully support this. The information leak is a complex one and I have no easy answers. But something tells me that you already know how to deal with this and it should work out just fine.

I have several 81's and I looked at them with contempt every day before you came here and started helping out. Its nice to know some one actually cares about audio like myself and many other members around here. My thanks goes out .

Wish I had more Engineering chops , but that comes with experience and not just reading books on electronic circuit behavior which happens to be my sole resource next to this forum.

I have built Neve preamps already with second hand components and my own etched pcb boards more successfully and I cannot figure out how something like this can so easily have been screwd up. just annoyes me how this stuff can be produced with no oversight , accountability or care.

why build a worldclass circuit to have it sound like crap = pointless.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 05, 2009, 10:44:45 PM
Thanks for your replies regarding the gain switch.

Tarnation - I know you can bypass the audible noise from the switch by using the line position. However, the first preamp stage will still oscillate and produce the signal when the switch goes through this position - it's just not being passed through to subsequent stages. I doubt if this is doing damage to anything, but it's still there and I have to remember to be in line mode when switching through it.

It's like, if a tree falls in a forest and there's nobody there to hear it, does it still make a sound? In my world it does.

Steve - I know the gain switch problem is relatively unimportant compared to most of the others, but something in my mind desires order. My kids are somewhat OCD and I think I've inherited this from them. Unless the entire resistor ladder is reworked, there will not be equal gain steps between switch positions if one position is off (not too hard to do, I know, and not really a big deal anyway.  :)) But I would like to keep position 6 particularly (which I lose when I cut the trace) because this passes the cleanest signal, directly from the mic transformer to the input of preamp stage 2. No 1st stage preamp noise, and no resistors in the path (which probably don't produce any audible signal degradation anyway.)

A switch between the cut traces is still the best solution for me. The problem lies in remembering to open and close this switch at the appropriate time. I'm now thinking of a simple SPST toggle switch whose toggle lies against the gain switch knob when it is closed (to activate position 6), making it difficult to rotate the knob in this position, or at least jogging the memory to throw it in the other direction (opening the trace) before changing the gain.

This is just for my neurotic DIY purposes - I'm not suggesting it as an option for any commercial kit or mod.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 06, 2009, 08:56:15 AM
The three cap types you mentioned remain as the thing I'm wondering about, unless these were meant for the supply only, and not for the AC-coupling ones, for which one might or not might want to use those (often dreaded) tants, at least already 'for authenticity'.

All but one of the caps in the Bill of Materials for the "kit'o'parts" are used to re-populate the power supply PCB with different caps. As a matter of fact, every electrolytic on the power supply board gets replaced for one reason or another.  Collectively, along with the replacement low-noise power transformer, they provide a robust, reliable power supply with lower ripple, noise, and significantly more "reserve" due to bigger output caps on the regulators than the original supply.

  The remaining Panasonic FC 220uF/63V electrolytic replaces the 100uF/63V electrolytic in the Phantom power local filter on the mic preamp card.  In addition to reducing the 1K to 200 Ohms, and making the 6K81 feed resistors matched, this completes the beefing up of the phantom mic powering circuitry.  This cap is a DC filter that is in common mode with the microphone signal, so it really has close to Zero effect on the sound quality except for final filtering of ripple and noise from the microphone's power.

...

I have no plans to offer tantalum replacements at the present time for the audio coupling caps.

Hello Steve,

Thanks again, it confirms my assumption of cap-replacements only 'outside' the signal path (yeah, uhh, just like DC-servo caps... well, you'll understand how it's meant  ;) )
I'm with you, after the replacement of supply-caps the tants might be a later step, better first use & enjoy it as is & then later trying signal-path alternatives... or simply continue making music with a 'now fixed' preamp.

Have a good weekend,

  Peter 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on March 06, 2009, 09:42:55 AM
Hi Steve,

Thanks for all the effort you've put in so far, and for keeping this more or less open source.

I have two burning questions..
1. Have you tried any of the PSU mods (trafo and/or cap/diode changes) in the circuit yet? Any observations?

2. Can I use MUR860 diodes (supposedly fast/soft recovery) instead of the unobtainable ones you are using (have a small stash of them and no immediate use)..

thanks again!


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 07, 2009, 11:51:26 AM
Tarnation - I know you can bypass the audible noise from the switch by using the line position. However, the first preamp stage will still oscillate and produce the signal when the switch goes through this position - it's just not being passed through to subsequent stages. I doubt if this is doing damage to anything, but it's still there and I have to remember to be in line mode when switching through it....

 Unless the entire resistor ladder is reworked, there will not be equal gain steps between switch positions if one position is off (not too hard to do, I know, and not really a big deal anyway.  :)) But I would like to keep position 6 particularly (which I lose when I cut the trace) because this passes the cleanest signal, directly from the mic transformer to the input of preamp stage 2. No 1st stage preamp noise, and no resistors in the path (which probably don't produce any audible signal degradation anyway.) ...

This is just for my neurotic DIY purposes - I'm not suggesting it as an option for any commercial kit or mod.

I haven't tackled the gain switch yet, but two points come to mind --

1. If having the mic/line switch doesn't fix the connection between input and output, then some parts are going to eventually get damaged. IMHO this oscillation is very harmful and should be completely eliminated under all circumstances.  I hope to eliminate any possibility of a speaker and/or ear destroying pop from this preamp no matter how I fiddle with the switches and knobs.
I always go for goof-proof in my designs.

2. Perhaps it would be possible to have postition 7 be the position with the straight thru connection, and then spread out the other steps with resistor changes.  (Using good resistors here won't hurt if you have to change them anyway).

Sorry to take so long to get answers, but I at the same time I am working on these mods, I must be working on client's gear.



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 07, 2009, 12:37:02 PM
I have two burning questions..
1. Have you tried any of the PSU mods (trafo and/or cap/diode changes) in the circuit yet? Any observations?

 2. Can I use MUR860 diodes (supposedly fast/soft recovery) instead of the unobtainable ones you are using (have a small stash of them and no immediate use)..

Answer to 1:
I have my first 84 Power Supply PCB completely modded now, except that I may still remove the 0.1uF ceramic caps, especially the ones on the phantom supply.  The only marks on them are "104" which stands for 0.1uF, but the voltage rating and the ceramic formulation is a mystery, but they are really unlikely to be anything better than the cheapest. Their presence in the power supply is of dubious value. Some regulators really don't want too low an esr on their Input and Output terminals.  The Low Z electrolytics I am using really negate the need for them.  If they are 50 volt parts, then they are not a good thing for the phantom supply. I am also still considering a couple of minor tweaks to the Phantom Supply regarding startup current limiting to protect the pass transistor which has to work harder to charge up the bigger output cap.

 I also have an unmodified 73 PCB as well.

To measure the difference between the original circuit and the modified phantom supply circuit I simply capacitively coupled the output of the supply into my AA5001 THD analyzer reading Level dBm with true RMS rectifier, unweighted, unfiltered.  The original circuit measured about -58dBu with distinct 120 Hz ripple and really low frequency bouncing as the power line amplitude changed.  Always jumping, jumping.  With the bigger filters, and the current regulator diode in place of the 2K zener feed resistor, the output had NO discernable ripple and measured -89dB.  Interestingly the low frequency "jumping" perturbations were completely gone as well. Needless to say, I was pretty happy with the results.  That's a 30 dB reduction in hum and noise for about $5 worth of parts.
I hope to publish scope photos soon.

You might ask, why work so hard on the Phantom power supply?  The problem as I see it with ANY discernable ripple and noise on phantom is that most microphones have really crappy regulation – some none at all.  Their circuitry is single ended and they usually have crappy CMRR.  This low frequency bounce looks like it may be audible or at least intermodulate with audio if it gets into the microphone circuitry.  I may be wrong about all this, but I figure if the phantom is really clean, it cannot hurt..

Still no overall report yet,  I am still documenting the improved regulator performance before I reassemble the supply into the preamp and see where we are at.  I have wired up the power transformer to the voltage selector switch and Molexed the secondaries.  It fits in the same location as the original using different hardware 1/4-20 x 1.5" Button-head capscrew with a low-profile Nylock nut. It barely fits heightwise, but it fits.  I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). The lid clears the screw, but not by much, and in keeping with "goof-proof" design philosophy, I would rather add an ounce of prevention to prevent a pound of problems that need a cure.  It's easy to press down on the lid inadvertantly if you place something, even your elbow, on top of the preamp.
 
Answer to 2:  The MUR860 rectifiers are fine.  The only down side is mechanical, in that it is very difficult to cleanly replace an axial leaded rectifier with a TO-220. The metal tab is "hot" and connected to the cathode, so don't let the tabs touch each other.

More Later



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on March 07, 2009, 03:28:42 PM
...I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). ....


Uh, like, just use a nylon bolt for the power transformer.  Done.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 07, 2009, 03:58:51 PM
...I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). ....


Uh, like, just use a nylon bolt for the power transformer.  Done.

This is where the mechanical engineer side of me kicks in. Because nylon is non-conductive, it would indeed solve the accidental shorted-turn-by-the-lid-touching-the-threaded-end-of-the-screw problem.  I would not, however, trust mounting a 2 pound chunk of steel and copper to the very modest tensile strength of a single nylon screw. By the time you get the screw tight enough to compress the two rubber washers enough to securely captivate the transformer, the bolt is under considerable tensile strain.  The nut would have to be at least standard thickness, if not heavy instead of the thin nylock that I propose.   As long as the SS screw is blunt and has no sharp edges where the threads were rolled that could potentially cut through it, one or two pieces of tape on the cover will do the job nicely.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 07, 2009, 06:30:44 PM
I always go for goof-proof in my designs.

Aye, there's the rub. But I have an idea (always dangerous, those ideas.) I'll see if I get a chance to try it out this weekend.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on March 08, 2009, 10:57:50 AM
Steve, thanks for the reply on testing the new PSU... looks promising! Can't wait for the complete write up!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 08, 2009, 10:14:30 PM
Aye, there's the rub. But I have an idea (always dangerous, those ideas.) I'll see if I get a chance to try it out this weekend.
Well, here's the idea. And it worked. It switches through positions 5, 6, and 7 without a noise.

I mounted a small SPST switch http://www.radioshack.com/product/index.jsp?productId=3190382 on the gain switch, drilled and tapped a hole in the shaft, and mounted an arm on it (made from a plastic Lysol bottle in my recycle bin) that closes the microswitch when it gets to position 6. The microswitch connects the cut traces of position 6, after the rotary switch breaks contact with either adjacent position.

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod1.jpg)

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod2.jpg)

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod3.jpg)

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod4.jpg)

Here's the switch assembly wired up, and in the preamp.

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod5.jpg)

(http://home.att.net/~crazydoc/ACMP/gain-switch-mod6.jpg)

Not very elegant - just a prototype. The final version should have a mechanism for adjusting the distance from the shaft axis to the microswitch lever, as this is a critical distance for timing the closing and opening of the switch. (And put the protective cover back on the switch.) It's a lot of trouble to go to for very little gain (pun intended), but it was an interesting exercise.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 09, 2009, 02:39:57 PM
Well, here's the idea. And it worked. It switches through positions 5, 6, and 7 without a noise.

I mounted a small SPST switch  on the gain switch, drilled and tapped a hole in the shaft, and mounted an arm on it (made from a plastic Lysol bottle in my recycle bin) that closes the microswitch when it gets to position 6. The microswitch connects the cut traces of position 6, after the rotary switch breaks contact with either adjacent position. ...
Not very elegant - just a prototype. The final version should have a mechanism for adjusting the distance from the shaft axis to the microswitch lever, as this is a critical distance for timing the closing and opening of the switch. (And put the protective cover back on the switch.) It's a lot of trouble to go to for very little gain (pun intended), but it was an interesting exercise.

That's a really nifty solution.  You have a lot of mechanical ingenuity, which is really evident in this clever solution.

I began to investigate the gain switch mods today by realizing that the recently published schematic of the ACMP 73 doesn't match reality -- many of the resistor values are wrong.
I typically use a spreadsheet to work out resistor values when tightening up voltage dividers for meters and things like the gain switch.  I have already tidied up a  set of optimum values for the 3 dB steps in the Line in  attenuatuator. The values in the gain switch, however, made no sense at all.  Their calculated impedance and loss did not match the  measured mic gain steps of 6 dB +/- about .5 dB.  It appears that there is a complex loading effect of the 2nd stage amplifier's input impedance on the gain switch resistor values.

Not really being familiar with how the Neve input circuitry was realized, I printed out and studied up on the Neve circuitry.  The adaptation of the 11 position switch with 6 dB steps from the 22 position switch with 5 dB steps and 2 off positions was perhaps not done as cleverly as one could have.  My initial concept of a fix would be to create an off position between steps when in mic and perhaps spread the whole microphone gain range of 60 dB into nine 6.67 dB steps instead of ten 6 dB steps. If that proves unworkable, then I am pretty sure that I can at least spread out the high gain steps.  The weird loading on the attenuator network makes the calculation of these values difficult, and in addition, it is important to not change the total load seen by the transformers, or the sound will change as well as the gain.  A lot of stuff has to be figured out simultaneously to do these networks correctly.  I did notice in the particular '81 unit that I am working on, that some resistors had their coating damaged (wrinkled up) by whatever solvent they used to clean the PCB.  These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

More Later



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 09, 2009, 03:27:40 PM
I did notice in the particular '81 unit that I am working on, that some resistors had their coating damaged (wrinkled up) by whatever solvent they used to clean the PCB.  These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

I noticed the same thing - a few resistors have their coating almost dissolved off - and the adjacent ones seem to be fine. Would a glop of clear nail polish protect them without replacing the resistors?

And thanks for all your input in this.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on March 09, 2009, 06:21:19 PM
These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

More Later
And the plot thickens.....

Thanks for all the hard work, Steve and everyone else.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 09, 2009, 10:04:54 PM
I noticed the same thing - a few resistors have their coating almost dissolved off - and the adjacent ones seem to be fine. Would a glop of clear nail polish protect them without replacing the resistors?
And thanks for all your input in this.

I recommend replacement in the strongest possible terms. Metal film resistors are dependent on their coating to keep moisture from attacking the deposited metal film which is very, very thin.  The slightest bit of moisture with a voltage across the resistor will corrode the film and cause it to open up/change value/be ruined.  For more information see http://www.irctt.com/tanfilm/index.aspx

There are two reasons that I have no use for the old black Holco resistors. First of all, they were longer than every other brand and didn't fit in the circuit board worth a darn.  Secondly, the black laquer coating they used formed a huge miniscus to the lead and if you bent the lead too close to the body (like trying to fit it into a PCB) the coating would crack, breaking the moisture barrier, and leading to failure.  One of the nice things about tantalum nitride resistors, is that they have a natural oxide coating that passivates the resistance material and prevents moisture from harming it.  Like the bridges in Michigan that are made out of a special steel that forms a rusty coating that seals out moisture and prevents the distructive type of rust without periodic (perpetual) need for paint.  Metal film resistors are extraordinarily reliable when their coating is intact, but if not, all bets are off.  The film will be attacked in minutes.  The coating on some of these particular resistors looks like paint that's been hit by paint remover and is wrinkled and ready to be scraped off.  As far as nail polish is concerned, the ship has already sailed.  Metal films are cheap.  Make a list of the bad ones and let's get them replaced.  Heck, it may be just the excuse to tidy up the tolerances and values.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 10, 2009, 03:37:49 PM
This post describes the latest updates in developing the modifications/fixes for the ACMP preamps.

1. Gain Switch mods:
I modeled the input stage used in the 73 and the 84 in LT Spice to be able to determine the input impedance of the first and second stage.  It is not very high, and it changes depending on the gain.  That means that the second stage, which has +28dB of fixed gain in Mic mode and +18 dB of fixed gain in Line mode has a different input Z depending on whether it is in mic or line mode.  I added much more complete formulas to determine the proper resistors for the gain switch incorporating the input Z.  The schematic for the ACMP 81 has another mistake.  The load on the line transformer is shown as 6K8 but it is actually 2K2 as in the other schematics and the Neve.

2. Phantom Mods:
The TNC directions instruct you to turn off the phantom power berfore patching a mic.  Good advice, but without the 48V switch grounding the filter cap thru the feed resistor, the phantom stays up until externally discharged. It turns out that the ground is not carried through from the phantom power supply to the preamp card.  They use a 2 conductor patch cord, but only the +52 is wired.  The 48V ground ends at the preamp card.  The original 100uF/63V phantom filter cap (soon to be 220uF/63V) is grounded to the chassis by the adjacent screw/pad. I am considering changing that connection and using the 2-conductor cable to return the bottom of the filter. I'm not crazy about using the chassis as a ground wire.

3. New power supply transformer/power supply mods:
I determined the required RC network to properly damp the resonance of the new power transformer.  The fast, soft recovery rectifiers that I have added to the power supply do not generate bursts of RF oscillation when they turn off. However, because the transformer is loaded (draws current) only at the peaks of the AC waveform and is unloaded during the transition from positive peak to negative peak, when another rectifier turns on and current is drawn from the transformer winding again. The current from the transformer is drawn in short bursts, and then abruptly stopped.
The stored energy in the transformer causes the ringing.  If you add a series RC network across one of the secondaries, the ringing can be almost eliminated, removing one more source of unwanted spurious garbage from the power supply and simultaneously preventing it from negatively affecting the audio.  The scope photos are instructive:

(http://i691.photobucket.com/albums/vv274/StevenAHogan/48Voltwinding10kHzundamped.jpg)
The above photo is the 48 Volt winding of the new transformer when excited by a 10 kHz square wave into the primary. In normal operation the winding is passing sine waves, not square waves, but when the rectifiers turn off the same ringing is excited due to the sudden discontinuation of the current flow.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/48Voltwinding10KHzdamped.jpg)

After determining the proper RC network and wiring it across the secondary, the result is pretty dramatic.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/48Voltwinding1kHzundamped.jpg)

It's a lot harder to see the HF ringing at lower frequencies -- This is 1KHz undamped.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/48Voltwinding1kHzdamped.jpg)

You must look at lower frequencies to check your network, however.  Sometimes things look great at 10kHz but lower frequencies have a big peak that was not seen at the higher frequencies. This network works well at all frequencies and is the one that I will add to the Power supply PCB.  Interestingly, you really don't have to damp each of the windings individually.  By damping the 48 Volt winding, in this case, all the other windings behave as well.  The transformer just has to have some way to dump the stored energy and one winding is sufficient to do the job.  It makes it a lot less hassle, since the 73 and 84 have three windings and the 81 has four.  Damping the 48V winding does it for all of them.  I found a nice place for the RC network on the PCB to make it a clean mod. BTW all power transformers need this network. Some power supply manufacturers add capacitors across the diodes and/or capacitors across the secondary with no series resistor.  Although this kills the HF ringing, it causes a very large lower frequency resonance to occur.  On a commercial power supply I recently modded, there was a 0.33uF cap across the secondary and the result was a 20dB peak at 15kHz.  Needless to say, I prefer adding the series resistor.  The caps across the rectifiers, by the way are in series with each other and put a similar capacitive load on the transformer secondary, which makes for an unnecessary peak.    That's why I use the good rectifiers which require no snubbing,  and then I properly damp the power transformer, and all the spurious stuff goes away.

More Later. . .

 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 10, 2009, 08:35:03 PM
Steve,

In previous posts you make mention of the AA144 Ge Diode and its relative scarcity, and  been doing some followup on that diode part and sadly that part is either very hard to get or very expensive to get for unknown reasons other than they are well out of production and contain gold.


The AA144 / Sub is BAT86 Schottky Diode






Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 10, 2009, 08:52:10 PM
Hi,

Good question you bring up. I guess the clamp-function is not too depending on
the super-specific properties of that original type, while at it we obviously want to insert a proper one.

Note 1N4148, 1N914 are Si-types

I saw FX-builders-sites have decent prices, 1N277 & 1N34A for EU0,30 each ( www.musikding.de )
while www.banzaieffects.com has a large range of possibly suited types I won't be repeating here.

I'm with you on not wanting to pay those prices for a simple Ge-diode, but since each pre doesn't need too many
and while the lid is off then better make it a replace-&-forget thing, despite it can in total be done a few $ cheaper.
$4.75USD is a sick price though...

Bye,

  Peter

I'm lazy, how many does each ACMP81 preamp need ?


Bye,

  Peter

 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 10, 2009, 09:18:32 PM
Hi,

Those markups indeed silly & sad.

[OT & just kidding]But hey, I bet those most expensive discrete semiconductor devices live in the nifty boxes of the brand that shall remain nameless but here's a thread about it: http://www.groupdiy.com/index.php?topic=6481.0 (http://www.groupdiy.com/index.php?topic=6481.0)... I bet the Ge-types on that box come out at several hundreds a piece  ;) [/OT & just kidding]

And wasn't someone selling BC184-somethings recently on eBay, at ehh, $9.99 a piece ?!?

Quote
but on the other hand ,  I was made a sucker to buy the ACMP 81.
Hmm, sorry it feels like that, seriously. We knew free lunches are rare & we knew some DIY-ing would most likely
be in order. It now sure looks more 'repair' than most people (incl. me) would have expected, but ain't it still fun ?
What do others think ?

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 10, 2009, 10:21:57 PM
yeah it is , doesn't fit normally in multiple rack mount
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 10, 2009, 10:42:29 PM
Is there another Germaniuim diode suitable for sub-ing the AA144 like a 1N100A, 1N270, 1N276 1N277 , and maybe  these  1N34, 1N55  All DO-7 packages. Silicon diodes as indicated have a higher foward voltage drop unlike germanium diodes which is the desired behavior on this diode.

I have a friend that has repaired literally thousands of Neve modules and the first thing he checks on a broken one is that diode, which is often cracked and broken.  From the information that I have gathered so far, it is a weak sister in the design, that breaks often.  Since we have agreed to "open source" on these mods -- (please buy the parts kits when they come out), I will offer that I would not use a Germanium diode at all since these are fragile.  I plan to use a BAT86 small signal Schottky diode in that location.  It's forward voltage drop is not quite as low as germanium, but it is about half that of a regular silicon diode and most importantly, lower than the Vbe of the transistor that needs the Baker clamp.  That Schottky diode is readily available, relatively rugged, and has a couple of features that make it work really well in this application.  It is very fast, has no reverse recovery issues like regular silicon diodes, and its junction capacitance is more linear than regular diodes, so it contributes less distortion.  This is a clone, so I don't feel obligated to use exactly the same parts, as long as they function in the same way and don't mess with the "character".  The substitution of a Schottky diode for the Germanium in this application should work just fine.

The clamp is used on the Q4 in the 81 that needs replaced, so one needs 4 pcs, one for each BA338 amplifier clone.

While we are on the subject of "sticker shock" I will warn you that the current limiting diode that gives the more than 30 dB reduction in ripple and noise in the phantom power supply regulator costs about $3.00. -- about $0.10 per dB noise reduction.  Each preamp power supply only requires one. I am not aware of any other solution that works so well, with such a simple change, in that application.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Mylithra on March 11, 2009, 03:33:49 AM
Wow , did'nt realize that problem with the rack space. When does the bad news end with these.

Hopefully once Steve is done with these :)
We all really appreciate all the work you've done on this.

Yeah.. they dont quite fit if you leave no space. As they are now, I dont see how you could use them stacked on top of anything else. The inductors pick up power supplies from other pieces of gear 3 sometimes 4 spaces away. My rack is otherwise quiet. 
 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 11, 2009, 08:23:38 AM
if no better alternative , a 3.00 diode that you only have to buy once
doesn't seem too bad [ and not if balenced with other reasonable parts ]
i have 4 pres so 12.00 i can handle ,getting to 4 x psu xfmrs is sticker shock
in my economics world i have to ponder whether i should do them a couple at a time

btw good grp citizenship of you contributing to other threads ,thanks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on March 11, 2009, 12:34:44 PM
Yeah, when does the bad news on these Chance pres end?  Indeed.

I'm even wondering if I should sell mine.  I expected to replace the PSU automatically, and maybe some caps and transistors -- way back when I ordered them, just from looking at the photo.  But now, not only is it the switch and the inductor, and who knows what's next, maybe more resistors and rewiring this and that... I'd rather start from scratch with quality parts.

The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on March 11, 2009, 01:31:07 PM
It's true.

Chinese products as a whole really suck. These pres bring that suckiness to a whole new level in terms of QC.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on March 11, 2009, 01:56:23 PM
Yeah, when does the bad news on these Chance pres end?  Indeed.

I'm even wondering if I should sell mine.  I expected to replace the PSU automatically, and maybe some caps and transistors -- way back when I ordered them, just from looking at the photo.  But now, not only is it the switch and the inductor, and who knows what's next, maybe more resistors and rewiring this and that... I'd rather start from scratch with quality parts.

The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.
What do you have, Tommy?  I may know of a buyer or two.  Thanks.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on March 11, 2009, 03:13:12 PM
hi guys!

I've been reading this long, long, long story since the begining and hoping there's a new GB for that kind of stuff!

but now!!! dude...these things... :-X
I'm really curious about what you paid fot these.

Quote
I'd rather start from scratch with quality parts

more expensive but painless, I think.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 11, 2009, 05:50:06 PM

The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.

Actually, I believe that the problem with the affected resistors is with the resistors, not the cleaning process.  On the same board with the wrinkly finish resistors are all the rest of the resistors that are just fine. I think the coating on only some of the resistors was defective. Whatever residue is left of any solvents they used to clean the board is long gone, and I seriously doubt that any health hazard exists.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on March 11, 2009, 08:08:13 PM
Maybe those solvents are what caused the rust-colored spots on the chassis under the PS PCB in one of my 81s.  I also have a splattering of some sort of brownish residue on the inside of the top lid in that one.  Haven't opened the other one yet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: aomahana on March 11, 2009, 09:30:45 PM
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on March 11, 2009, 10:16:28 PM
I think the coating on only some of the resistors was defective. Whatever residue is left of any solvents they used to clean the board is long gone, and I seriously doubt that any health hazard exists.

Thanks for the reassuring words.  However, could it be that the solvent wasn't carefully applied and hit some resistors and not others?  Doesn't that fit the pattern of the quick and most uncareful production that we see before us in every other aspect?

I keep hearing about the toxic cleaning solvents companies use on their PCBs and in fact it is a health concern, it doesn't go away from the PCB.  The computer industry has been under pressure on these for some time and yet have not changed from chemical wash to water wash PCBs.  Last I heard.  There's also flame retardants, which are very bad.

It makes you wonder in this case because the production seems to have been so totally out of control. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 12, 2009, 12:30:48 AM
Maybe those solvents are what caused the rust-colored spots on the chassis under the PS PCB in one of my 81s.  I also have a splattering of some sort of brownish residue on the inside of the top lid in that one.  Haven't opened the other one yet.
It is much more likely that the brown spots came from the plater who plated the steel prior to painting.  The board would have been cleaned prior to assembly in the chassis.  I noticed that only certain values of resistor are affected, with perfectly intact resistors all around.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 12, 2009, 12:48:01 AM
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.

I am planning to make kits of parts available with the new power transformer with mounting hardware, the fast soft-recovery rectifiers, replacement power supply and Phantom caps, a matched set of zeners to trim the Phantom to 52 Volts, a current limiting diode to improve the Phantom supply, new resistors for the 317 regulators, matched 0.1% PRP Phantom feed resistors,  Mounting hardware for the transformer, etc etc. New transistors and schottky diodes for the '81 and whatever else I find out is needed. I have many of the parts already, but it is slow going getting to the bottom of some of the problems and confidently solving them in reliable way.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on March 12, 2009, 02:18:26 AM
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.

I am planning to make kits of parts available with the new power transformer with mounting hardware, the fast soft-recovery rectifiers, replacement power supply and Phantom caps, a matched set of zeners to trim the Phantom to 52 Volts, a current limiting diode to improve the Phantom supply, new resistors for the 317 regulators, matched 0.1% PRP Phantom feed resistors,  Mounting hardware for the transformer, etc etc. New transistors and schottky diodes for the '81 and whatever else I find out is needed. I have many of the parts already, but it is slow going getting to the bottom of some of the problems and confidently solving them in reliable way.



Steve, thank you for all the hardwork you've put into this.  I will be first in line when the kit is released.  You should be compensated for your solutions.

I have no faith in TnC standing behind these things at all.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 12, 2009, 11:58:17 AM
Here are some pics of the two offending resistors on my 81's preamp board. The other boards and resistors seem to be in pretty good shape, though some of the resistors have a dull rather than shiny surface. It looks like the coating was partially dissolved (and maybe dried with pressurized air?), as some of it is stuck to an adjacent resistor.

Steve - are these the same damaged resistors in your boards? (They have different values - 39 and 120k ohms.)

(http://home.att.net/~crazydoc/ACMP/ACMP-resistor-crud.jpg)

(http://home.att.net/~crazydoc/ACMP/ACMP-resistor-crud-detail.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: b3groover on March 12, 2009, 11:24:07 PM
Wow, those look terrible.  I haven't inspected my ACMP73s close enough to determine if they have a similar issue.  I should probably get on that.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 12, 2009, 11:56:19 PM
Here are some pics of the two offending resistors on my 81's preamp board. The other boards and resistors seem to be in pretty good shape, though some of the resistors have a dull rather than shiny surface. It looks like the coating was partially dissolved (and maybe dried with pressurized air?), as some of it is stuck to an adjacent resistor.

Steve - are these the same damaged resistors in your boards? (They have different values - 39 and 120k ohms.)

The Bad Resistors on the 81 I have here are the same two as yours, 1R17, 1R35 plus 1R3 (120K) is wrinkled and should be replaced as well IMHO.

The PRP Phantom feed resistors arrived today. The lowest measured 6806 Ohms and the highest measured 6812 Ohms, with the big part of the bell curve about 75% of them landing beween 6807 and 6809.  I can't help myself, I sorted them into 1 Ohm groups 6806, 6807, 6808, etc. from which I will pull the pairs for the preamps.  I predict no problems due to lack of matching in the modified mic preamps.

I have noodled on an easy, but reliable way to add the ground wire to the 48V switch.  Remember that I am trying to make all these mods able to be done in as short a time as possible to keep the labor charge down.  The Phantom filter cap on the Front-end PCB is grounded by means of the screw located just behind the mic/line switch.  I am thinking of connecting a short green wire with a serrated solder lug on one end to go under that screw in order to:
1. Make sure that the cap is securely grounded. I don't trust the PCB to standoff interface, but the teeth in the solder lug will dig into the copper PCB and the screw and make sure that the screw makes a good connection.
2. Allow the Green wire to be disconnected when the switch is unplugged from the preamp PCB without adding a connector.  This seems to me to be a simple, but effective way to add the ground contact to the Phantom switch.
The other end of the wire connects to the normally closed contacts of the switch in such a way that when the switch is on the feed resistor is connected to 52V and the filter cap charges up and the mic is powered.  When the button is in the off position that same resistor is connected to ground and the cap is discharged thru the resistor.
Should work OK I think.
   
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 13, 2009, 09:53:31 PM
Just a couple more resistor pics. I opened up my 71 again. These are on two of the inductor boards. 5R15 is another 120k resistor and R20 (at the DI input) is 100 ohms, per the schematic. The preamp board has a couple lightly pitted resistors, but minimally noticeable.

In one picture you can see spatter drops on the next resistor (and the  pc board), so it looks to me like the coating was softened, then subjected to high pressure air. I don't understand why just these few were vulnerable, and the majority were not.

(http://home.att.net/~crazydoc/ACMP/ACMP-resistor-crud-detail2.jpg)

(http://home.att.net/~crazydoc/ACMP/ACMP-resistor-crud-detail3.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 17, 2009, 05:54:23 AM
Another piece of info ...

Some time ago I posted my method and measurements relating to the main noise
component of my ACMP81s in 2 units with Zmix transistor mod, one with stock
onboard toroid psu and other with replacement onboard psu and toroid.

Where I left it last was that the Carnhill inductor replacement for the low-mid
stock inductor showed a big improvement (>15dBu) to the mains-harmonic components
of the noise floor (line input, unity gain, connected to Motu interface in/out (bal +4dBu)

Well - I've just added the OEP mu-metal shield (thanks Roddy for the fab idea!)
to the Carnhill inductor replacement and retested (in the my rack with nearby units powered on).

Results are really good.

Great, in fact. Essentially low-mid eq noise  has completely gone (Eq on, Low-Mid at max +ve gain, 220Hz)
<-98dBu all individual spectral components, overall -85dBu RMS
Practically the same as with no low-mid eq engaged .  ;D ;D ;D

Now the major noiser is the mid-hi inductor. So that's being replaced shortly. :D

For me, the mains noise issue is over -  game, set and match.   8)

Total upgrade cost for this unit upgrade - toroid, psu, transistors, inductors and shields - 100usd.

I'll mod and test the unit with stock transformer and psu shortly.
I expect to see the noise gone also.

Pics :

(http://www.users.on.net/~alexc/res/Carnhill+OEP1.jpg)
(http://www.users.on.net/~alexc/res/Carnhill+OEP2.jpg)
(http://www.users.on.net/~alexc/res/ACMP81_Carnhill+OEP2.jpg)
(http://www.users.on.net/~alexc/res/ACMP81_Carnhill+OEP3.jpg)

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 17, 2009, 08:29:11 AM
Nice Alex , thanks

Cinemag has a couple of my inductors now , and
felt they'd be less than 20.00 each , i'll make sure
cans come with those



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on March 17, 2009, 09:18:07 AM
Alex & Greg,

This is great! Thanks!

Alex, Looking forward to seeing the results after swapping out the JLM with the original.

Greg, do you know if these inductors will be a direct order item from Cinemag or might this end up being a BM organized purchase?

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 17, 2009, 10:33:07 AM
Tom from Cinemag was checking the inductors i sent from my 81
and mentioned that the values were close to the schematics ,
although core material is unknown .

He recomends taking care of the power transformer first
[ replacing with as steve mentioned lower gauss higher va ]
& sheilding the inductors next [ doesn't need to be Mu metal
many homemade solutions will work ]

That said  he is sending me some prototypes to check in circuit
to be sure of their role in the noise problem
if that is confirmed [ i was satisfied with Alex's work ]
he will offer them at 20.00 or less with a small min of 25
[ Tom wants to make sure no one is needlessly spending money ,
but is happy to help ]

As the world turns ..................



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 17, 2009, 07:15:15 PM
Clever DIY teamwork work

Roddy, Alexc 

Great idea with the Mu Can , did a transformer sacrifice its shell to do this or can those
Mu cans be purchased empty , good to know for future projects and testing.

Alexc,
What PSU transformer did you use to replace the China version , same taps or just basic taps without Phantom supply ?

Greg,

How did those 2N2907 & 2N2222 work out in the Q4,Q5 positions ? Did you use T0-92 plastic or T0-5 metal cans ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 17, 2009, 07:47:07 PM
the xisiters got rid of the main buzz / hum when engaging the eq in switch ,
then the hum from the inductor bands became obvious ,

see above post for update
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 17, 2009, 09:02:45 PM
> did a transformer sacrifice its shell to do this or can those

Ha! No - you can get them separately. 15aud each for me.

>What PSU transformer did you use to replace the China version , same taps or just basic taps without Phantom supply ?

I used a chinese toroid from Altronics Australia. It's a pretty cheap-and-nasty one (22aud), but works OK.
At the 'crappier' end of the quality scale

Different taps than stock acmp81. It provides 120+120VAC primary and 24-0-24VAC secondary at 30VA.

I use it with the JLM 5Rail Power Station (around 50aud diy), which takes 24-0-24VAC and provides (all adjustable)
+48VDC phantom(up to max 300mA), +24VDC(up to max 1A), +/- 15VDC (up to max 1A)
plus an unused -V adjustable rail.

I use this setup in most of my DIY builds and find it to be reliable and trouble free.

I am going to leave them both with stock PSU and toroid and just increase the voltage of a couple of caps for safety.
And look at the gain switch pop thing as well as phantom bleed off.

That'll do it for me.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 17, 2009, 09:18:22 PM
Hey Greg

Regarding the Cinemag replacement - Great stuff.

At that price and with Cinemags usual undoubted quality, these things will improve dramatically.

"Team DIY & Tom of Cinemag. Taking care of the audio *little* guy..."
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on March 19, 2009, 08:29:13 AM
just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 19, 2009, 12:47:52 PM
just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe
Yes, I think you need to switch them all. Since a fuse, in general, limits the power to a circuit (to protect it), and power is a function of voltage times current, if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.) The T designation makes it a time delay (slow blow) fuse, to allow an initial inrush of current as the device powers up.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 19, 2009, 03:37:35 PM
My latest mod ,
Now i know you're going to say , hey wait Greg
this isn't going to affect the sound one bit , I know
but it amuses me and makes me feel better
[ do you think it's an insult to the knobs ? ]

btw either it's metric or they skimped and the shaft size is smaller
than normal and the old knobs won't fit 1/4 shafts ............useless

(http://i83.photobucket.com/albums/j319/okgb/DSC03587.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on March 19, 2009, 06:39:35 PM
My latest mod ,
Now i know you're going to say , hey wait Greg
this isn't going to affect the sound one bit , I know
but it amuses me and makes me feel better
[ do you think it's an insult to the knobs ? ]

btw either it's metric or they skimped and the shaft size is smaller
than normal and the old knobs won't fit 1/4 shafts ............useless

(http://i83.photobucket.com/albums/j319/okgb/DSC03587.jpg)

Hey Greg.  Can't really tell from the pix what you did.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 19, 2009, 07:15:32 PM
It is only insulting if is red   :D

Black is OK.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 19, 2009, 10:27:33 PM
It's subtle , you have to look , lol





hint felt marker over those offending letters in the circle
make it better
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on March 20, 2009, 08:14:13 AM
Quote
read the stamp on the end of the fuse ( metal cap), does it say AGC250 or 250 anything ? if not then you need a fuse with this designation.

are you sure? the manual says 250mA for 115V and 100mA for 230V.

best,
flox
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 20, 2009, 10:46:12 AM
just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe
Yes, I think you need to switch them all. Since a fuse, in general, limits the power to a circuit (to protect it), and power is a function of voltage times current, if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.) The T designation makes it a time delay (slow blow) fuse, to allow an initial inrush of current as the device powers up.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 20, 2009, 11:41:39 AM
Just a further note on the QC on this group buy stuff. While just now checking the value on the fuse in the ACM900 mic power supply (T160mA), I noticed the fuse holder wasn't inserted flush to the receptacle. When I took it out, the cause was immediately apparent - the prongs that hold the fuse were bent over to one side, and the fuse had been wedged in there to the side.

(http://home.att.net/~crazydoc/ACMP/fuse-holder-ACM900PS.jpg)

It's getting more apparent that we got what we paid for, but it's good we can correct most of it.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on March 20, 2009, 01:32:48 PM
Quote
if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.)
sounds quite logical now that i think about it  ::)

i'll have a look which fuses are used in the power supplies for my acm-1200 and acm-310 and will then go fuse-shopping :)

thanks,
flo
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 21, 2009, 05:07:20 AM
Just a further note on the QC on this group buy stuff. While just now checking the value on the fuse in the ACM900 mic power supply (T160mA), I noticed the fuse holder wasn't inserted flush to the receptacle. When I took it out, the cause was immediately apparent - the prongs that hold the fuse were bent over to one side, and the fuse had been wedged in there to the side.

It's getting more apparent that we got what we paid for, but it's good we can correct most of it.

I just added checking the fuse to my list of things to check for when upgrading the ACMP preamps with the new power transformer, etc.  The list does seem to get longer and longer.
Just an update .. my Schottky Baker clamp diodes for the '81s arrived today, and I placed the order for the Molex terminals and housings for the new transformer connections to the power supply PCB.  I will be using premium Molex terminals and housings to make the best connections to the male pins on the PCB.  I will be using the polyester housings instead of nylon, and I will be using the Molex Phosphor Bronze (springier and higher conductivity than the cheaper brass) Trifurcon terminals which have three points of contact to the square header pins instead of only one.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/6838.gif)
Molex Trifurcon Terminal

It seems appropriate to use premium connectors on the premium transformer.
BTW, final pricing is now in on the transformer which has, in addition to its low stray field design, a magnetic shield and a Faraday electrostatic shield.  The replacement '73/'84 tranformer will be $75 and the '81 version, which has an additional 24V secondary will be $80.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Purusha on March 21, 2009, 05:21:52 AM
Are these transformers OK for 220V (EURO) also?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on March 21, 2009, 01:10:15 PM
Are these transformers OK for 220V (EURO) also?
My replacement transformers are rated for 50/60 Hz with dual primaries that will connect to the same voltage selector switch as the original transformers. Although the nominal voltages on my transformer are 120/240Vac they are designed to operate correctly in the (modified) ACMP power supply PCB's from 105/210Vac (-12.5%) to 132/264Vac (+10%).  Lower than 105/210Vac may cause the regulators in the power supply to lose regulation and pass on ripple, but most power companies nowadays run Voltages high for minimum power loss in the distribution lines. Power at my wall is almost always exactly 120Vac. Of course, YMMV where you live.

If anyone has power less than 220Vac on a regular basis, you should let me know now, so I can make adjustments in the design before the transformers are wound.  To make the transformers maintain regulation at lower voltages, I make the secondary voltages higher.  That means that under normal operating conditions of 120/240Vac, the regulators have to dissipate more heat to throw away the higher unregulated voltage. It also means that replacement of the original input filter caps with higher voltage rated caps is a must.   It's a balancing act, and I generally find that if the regulators keep working at 105/210Vac, that covers almost any realistic low-line condition.  I predict that other studio gear will already be losing it at that low a line voltage.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Purusha on March 21, 2009, 02:04:24 PM
Thanks!  :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on March 21, 2009, 04:39:53 PM
Thanks for the update, Steve.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 22, 2009, 07:59:08 PM
This is the probably final version of my gain switch mod. The trace at position 6 is cut, and the SPST switch reconnects the trace after the gain switch connection to the adjacent positions is broken. (The extra hole in the arm was a mistake - the screw didn't have enough excursion to contact the switch, and it was easier to drill another hole than find a longer screw.)

Refer to http://www.groupdiy.com/index.php?topic=27791.520 post#539.

(http://home.att.net/~crazydoc/ACMP/gain-switch-2mod1.jpg)

(http://home.att.net/~crazydoc/ACMP/gain-switch-2mod2.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 23, 2009, 03:08:04 AM
Crazydoc,

Have you thought of using a low voltage photo optical sensor switch with flag arm paired with a miniature relay, used in many laser printers. Takes the mechanical idea to the next level. no parts come into physical contact.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 23, 2009, 04:16:59 PM
Crazydoc,

Have you thought of using a low voltage photo optical sensor switch with flag arm paired with a miniature relay, used in many laser printers. Takes the mechanical idea to the next level. no parts come into physical contact.
That sounds like a really good idea - I'll look into it. Thanks.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on March 26, 2009, 04:53:03 PM
Are we still moving forward, Mr Hogan?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 26, 2009, 06:07:01 PM
Would love to hear the results of the upgraded
pwr transformer by itself
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on March 26, 2009, 07:17:11 PM
Seriously. Seems like we hit a brick wall here.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on March 26, 2009, 07:26:09 PM
Seriously. Seems like we hit a brick wall here.

In what way?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on March 26, 2009, 08:04:02 PM
The flow of info.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on March 26, 2009, 08:18:21 PM
I think we've got plenty. Steve's done his work and will follow up with any offerings of service I'm sure.

Other than that, we seem to know most of the issues, and people can report back when they tackle them. I'll be sure to do so when I get mine soon.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 26, 2009, 09:54:27 PM
Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy

...

And I propose likewise for the ACM-mics:

http://www.groupdiy.com/index.php?topic=334580 (http://www.groupdiy.com/index.php?topic=334580)


...

Cheers,

  Peter


Looks like we lost the TnC/ACM-mic thread !   :o

Removal of older stuff only in The BlackMarket as I understood, or could it be related to the latest software move ? Does that link needs to be upgraded ?

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 26, 2009, 10:41:13 PM
I would think steve has other gigs

clint i thought i saw the mic thread a page or two down  , not that far

otherwise i'm waiting to recieve the inductors and check them in circuit
with orgi pwr xfmr [ Alex reports good results shielding them as well ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on March 27, 2009, 02:26:21 AM
Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy

...

And I propose likewise for the ACM-mics:

http://www.groupdiy.com/index.php?topic=334580 (http://www.groupdiy.com/index.php?topic=334580)


...

Cheers,

  Peter


Looks like we lost the TnC/ACM-mic thread !   :o

Removal of older stuff only in The BlackMarket as I understood, or could it be related to the latest software move ? Does that link needs to be upgraded ?

Bye,

  Peter
This one? http://www.groupdiy.com/index.php?topic=27793.80
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 27, 2009, 07:27:02 PM
Thanks guys !

(I've updated the link to the Mic-thread on the first page of this thread)

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 28, 2009, 02:05:07 AM
Just tested the acmp81 (+transistor mods) with onboard stock toroid and PSU
and both inductors replaced with Carhills+OEP shields. No other changes.
Tested in my rack with other units nearby and powered on.

The mid-hi and mid-lo EQ bands are now both noise free.

At IP and OP gain set for unity, EQ In, Mid- Hi (1.5KHz) and Mid-Lo (220Hz) IN and with max +ve gain,
mains harmonics are <90dBu (50Hz) and <105dBu (100Hz and on).

Overall noise is around -87dBu which is basically my converters.

The whole unit is now what I consider a very good performer in terms of unity gain noise floor.

Basically the EQ is now not adding any extra mains noise.


Another thing of interest : +24VDC current draw is 150mA.

The 2n3055 is biased very low - around 40mA.
Compared to around 80mA that  I normally use.
So I think these need to be rebiased. I'll do that and post the actual value I find.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on March 28, 2009, 09:36:03 AM
Just tested the acmp81 (+transistor mods) with onboard stock toroid and PSU
and both inductors replaced with Carhills+OEP shields. No other changes.
Tested in my rack with other units nearby and powered on.

The mid-hi and mid-lo EQ bands are now both noise free.

At IP and OP gain set for unity, EQ In, Mid- Hi (1.5KHz) and Mid-Lo (220Hz) IN and with max +ve gain,
mains harmonics are <90dBu (50Hz) and <105dBu (100Hz and on).

Overall noise is around -87dBu which is basically my converters.

The whole unit is now what I consider a very good performer in terms of unity gain noise floor.

Basically the EQ is now not adding any extra mains noise.


Alexc,

Sounds like the smoking gun. It's really very good to have a confirmed solution for the biggest and most frustrating problem with the 81's (at least from my perspective).

I (for one) fully appreciate your troubleshooting and documentation methodology that you have followed and shared.

Most excellent.

Thanks for sharing!

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on March 28, 2009, 12:07:31 PM
I going this route because it is cheaper to start
and certainly won't hurt [ maybe the pocket book ]
BUT
even Cinemag recomended changing the pwr xfmr first

that said i would think the cinemag inductors to be an improvement
in the very least over anything chinese and that's my bet .
of course all the psu mods also won't hurt either
[ perhaps more so in long term reliability ]

I'd think replacing the i/o xfmrs to be an improvement as well ,
but would it be worth it ? it's not one of the problems right now

Alex , does your unit radiate noise into units place above or below it in the rack ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 28, 2009, 06:54:47 PM
Thanks - I hope it helps others. Certainly helped my units!
Difference compared to stock is like day and night.

I haven't so far noticed any excessive noise radiated by the acmp-81 into other units.
I'll do some more checks and post back.

Replacing components may well help reliability - power traffo, psu, audio traffos and so on.

There are several PSU caps that are low voltage rated (24V, 35V rated) and also the one on the output
amp board (25V rated on 24V supply). And the phantom power pops pretty bad too when switched.

I guess everyone will decide how far they want to go in improving these units. 
I wanted to find the minimum changes to get the audio working right.

The Carnhill inductors are nothing fancy, so I expect the benefits of changing should also translate to any
reasonable quality, shielded part.

The great news is that the acmp-81's sound pretty good! Definitely worth the effort.

For clean guitars, this would be my goto solid state preamp for sure.


 

 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 28, 2009, 08:40:42 PM
Alexc,

What mains voltage are you running your tests with 115Vac , 220Vac, or 240Vac ?

this may make a difference because less amps on the secondary if above 115Vac mains
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 28, 2009, 09:41:04 PM
I'm running at 240VAC.

If you mean that because I'm running the 230VAC rated transformer primary
at 240VAC, then yes that would result in a somewhat higher unloaded secondary voltage

That may or may not result in a lower current draw on the transformer secondary.
I'm not sure. I don't think so. I'd have to check more detail on how regulators operate.

In any case, it would be a fairly small amount which I don't think would translate into
much difference in the radiated secondary field.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 28, 2009, 10:46:53 PM
Bias current in the 3055 turns out to be 55mA. A little low but whatever - it works.

That is - input a signal and turn up gain until the onset of clipping at output.
Adjust the 3055 bias pot such that the signal clips symmetrically.

55mA is the current at that point on my unit.
Max 3055 current with wild clipping is 80mA. Current with no signal is 40mA.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on March 29, 2009, 04:17:31 PM
I going this route because it is cheaper to start
and certainly won't hurt [ maybe the pocket book ]
BUT
even Cinemag recomended changing the pwr xfmr first

that said i would think the cinemag inductors to be an improvement
in the very least over anything chinese and that's my bet .
of course all the psu mods also won't hurt either
[ perhaps more so in long term reliability ]

I'd think replacing the i/o xfmrs to be an improvement as well ,
but would it be worth it ? it's not one of the problems right now

Alex , does your unit radiate noise into units place above or below it in the rack ?

Hey Greg,

I do plan to upgrade my 4 81's with Steve's kit down the road.

I'm looking forward to hearing about the cinemag inductors though when they come through. I'm thinking that replacing those will be the most notable improvement and I will then be able to use the EQ's. I'll put in an order as soon as they are available either directly or via a Black Market thread if it goes that route.

Cheers,
j
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on March 29, 2009, 07:52:16 PM

In any case, it would be a fairly small amount which I don't think would translate into
much difference in the radiated secondary field.

If there is even the slightest bit of relevence to this I can't be sure. However, there seems to be a exception in the test data between Steve Hogans results and anyone troubleshooting in the international community as yourself (240vac). nothing has been concluded by these two operating conditions and any discrepacies resulting from it. I don't recall Steve Hogan running his test results with a 240Vac primary or even mentioning it. I'm not totally sure if this will skew the resulting test data any. It seems if 115Vac is being used the secondary field on the transformer would be stronger and yield different noise figures. If the current was greater on the secondary do to a 115Vac mains powering then it would seem likely the secondary field would also be reactively different in some significant way. Is it possible ?

I have to keep in mind that these transformers were constructed in a country where 115Vac is non-existent and it is diffiucult to know what amount of quality control , if any, was performed with a lower mains voltage.
Currently , it is easy to assume very little quality control went into these power transformers even with 240Vac as the mains voltage. So anything goes at this juncture.

Alexc,
can you draw up a simplistic illustration on how your measuring the 55ma of current on the 3055 power reg. I'd like to familiarize the connections , are you inserting test leads into the output path (emitter) of the 3055 and monitoring current draw while adjusting the trim pot ? or measuring current drop across the 3055 (emitter,base) or (Emitter , collector) etc ...?

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on March 30, 2009, 06:51:21 AM
I think the voltage and current pulled by the circuit from the secondary is the same regardless of
whether the primary is operated at 115VAC or 230VAC.

So the radiated field from the secondary should be very similar.
The primary current, being higher, may well radiate significantly more.

However, without explicitly testing, I can only speculate.

To measure the current in the 2N3055 driver transistor you have to insert an ammeter in series
with the collector. I could it because I had already cut the wiring in my psu mod unit.
Lifting the wire to pin 3 (which is at _24V) on the primary of the output transformer is probably the
easiest way. I'll try that and let you know with pics.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on March 31, 2009, 04:18:40 PM
Hi,

Don't know if these have been added to the to-do/attention-points/known-bugs lists yet ?

* [safety] when in 230V-zone, be sure to replace 250mA fuse for 100mA fuse
    (after you've set the voltage-selector to 230V & before switching it all on for the first time)

* [cosmetics/ACMP81] silkscreening 'bell' symbol incomplete for high band: it's just '-  -' (is this indeed a common thing ?)

* [cosmetics/ACMP81]  green LEDs in switches quite a lot brighter than the rest


Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 01, 2009, 12:25:11 PM
To measure the current in the 2N3055 driver transistor you have to insert an ammeter in series
with the collector. I could it because I had already cut the wiring in my psu mod unit.
Lifting the wire to pin 3 (which is at _24V) on the primary of the output transformer is probably the
easiest way. I'll try that and let you know with pics.

FWIW: an easier alternative which will get you about the same number will be measuring the voltage across the 47 Ohms power resistor & using Ohms law.
The current through the resistor will be a little bit different from the current through the 2N3055 but not by much, and since this is a roughly-so-many-mA's ballpark adjustment...

Regards,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 02, 2009, 10:14:55 AM
Hi,


All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

As they were meant to recreate those famous boxes of back then, what's your opinion ? Goal met ? And to which extend?

That's obviously most easy to answer for those that have access to the real deal, but people might have been comparing them
also already to the various micpre-only builds with proven signal-transformers (either originals or 'valid copies') as well.

BTW
I realize the '81 might be considered the most 'uncomparable' since it originally didn't exist with the class-A output topology,
but OK, switching off the EQ for now makes it look like a preamp-only '73 or '84.   


Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 11:53:23 AM
Are we still moving forward, Mr Hogan?

I just spent over an hour composing a reply to this question. However, I lost the whole post because I got logged out when I was working due to my only signing on for 60 minutes.  I should learn my lesson and never compose directly on the Board software, because if you lose it you get to start all over again since I haven't figured out a way to retrieve it from cyberspace. So here it is from scratch again:

Sorry for the long delay since my last post.  I have had to fit the R&D on the preamps around getting work done for my regular clients at the same time.  The new Molex terminals and housings for the new transformers shipped to me on Monday.  The new transformer cores have been wound, annealed, coated and are ready to wind.  The Magnet wire that had to be ordered arrived Monday.  I needed to make some final measurements to the prototype transformer in circuit in order to make final adjustments to the design before we start winding the production run.

Yesterday I tested the new transformer in an 84 with a rebuilt/recapped power supply board.  I ran the AC line voltage up and down to confirm the exact voltage where the regulators drop out of regulation and to confirm correct voltages at nominal 120/240 line voltage and at maximum high line voltage of 132/264 Vac.

As a result of my experiments on the '84, I decided to adjust the phantom winding up about 1 Volt, and to adjust the Bipolar 12 Volt windings down a bit, since they didn't lose regulation until 90 Volts, which is overkill.  The 24 Volt winding lost it at 100 Vac, which is fine for 60 Hz since at 50 Hz it will go higher, but still under my design goal of 105/210V minimum.

I then tried the prototype transformer in a modified '81 power supply driving an otherwise unmodified '81.
The changes to the phantom and bipolar 12 windings were the same.  The 81 has two 24V windings, however.  One winding powers only the output stage card.  The other winding/regulator runs all the rest of the 81 circuitry. The heavily loaded regulator gave up at 107 Volts, so I knew that I would have to raise the voltage on that winding.  I was concerned, however, that the extra current demand might be caused by oscillating Q4,Q5 transistors.  The transistors were quite warm, so I decided to shotgun replace all of them along with the Baker clamp diodes with my recommended replacements.

After replacement, the current demand seemed about the same, so I adjusted the voltage up on that winding.   I then spent several hours generating the CAD drawings for production of the two transformers.

I should now explain that I will be modifying the colors of the transfomer lead wires somewhat.  I hope nobody will have a problem with that, but the obscure primary color coding of the Chinese transformer makes no sense at all to me, and I am the one who has to wire up the new transformer to the selector switch.

Part numbers on the replacement transformers are TSS-Mp73/84  and TSS-Mp81.

New color code =
Primary 1 = Black, White
Primary 2 = Black/Wht, White/Blk
Phantom Secondary = Blue, Blue
24 Volt Secondary = Brown, Brown
Bipolar 12V Secondary = Orange, Yellow(CT), Orange
Additional 24V Secondary on TSS-Mp81 only = Violet, Violet
Faraday Shield = Green/Yel

The leads will break out of the transformer in such a way that everything dresses in nicely, and the windings line up with their respective connectors on the PS PCB.

I will now write a second post with all the rest of the stuff that I lost.


 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: jdbakker on April 02, 2009, 01:21:53 PM
I just spent over an hour composing a reply to this question. However, I lost the whole post because I got logged out when I was working due to my only signing on for 60 minutes.  I should learn my lesson and never compose directly on the Board software, because if you lose it you get to start all over again since I haven't figured out a way to retrieve it from cyberspace.

Yeah, it's no fun at all when that happens. I've gotten in the habit of hitting Select All - Copy (Ctrl-A / Ctrl-C) before posting or previewing messages. At first I was afraid that I'd forget to do it at exactly the wrong time, but I've found that I'm now doing it without even thinking about it.

JDB.
[very interested in the fixes, and eagerly awaiting the final DIY transformer-plus-parts package, although I'm wary of the shipping cost of such (relatively) heavy items to Europe]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 01:23:33 PM
Power Transformer stray field at 240V vs 115V

. . . there seems to be a exception in the test data between Steve Hogans results and anyone troubleshooting in the international community as yourself (240vac). nothing has been concluded by these two operating conditions and any discrepacies resulting from it. I don't recall Steve Hogan running his test results with a 240Vac primary or even mentioning it. I'm not totally sure if this will skew the resulting test data any. It seems if 115Vac is being used the secondary field on the transformer would be stronger and yield different noise figures. If the current was greater on the secondary do to a 115Vac mains powering then it would seem likely the secondary field would also be reactively different in some significant way. Is it possible ?



In terms of stray, unwanted magnetic field being radiated from a power transformer, one of the most important variables is flux density in the core, with lower flux density resulting in less tendency for the flux to leave the core since it is far from saturation. For 115 (120V) use, the primary windings are connected in parallel, and for 230 (240V) use they are connected in series.  Although there is twice the voltage, there is also twice the number of turns, so the flux density will be exactly the same (at the same frequency).

Another issue that effects flux density is the frequency of the Mains.  At 50 Hz the flux density of any given transformer will be 20% higher than that same winding driven at 60 Hz.  This means that there will be more stray field at 50 Hz than at 60 Hz.  When I design transformers for 50/60 Hz, I design it for 50 Hz and the 60 Hz operation will be better.  The TSS-Mp73/84 and TSS-Mp81 power transformers are designed with a 50 Hz flux density of 10.54 kGauss, which is quite low.  It is even less at 60 Hz.  Another thing that effects stray field is the consistency and geometry of the primary winding.

There is a difference between 50 and 60 Hz in the way the filter caps in the power supply charge.  There will be more PP ripple at 50 Hz than 60 Hz due to more discharge time between waveform peaks.  For that reason, if you want the regulator to maintaing regulation down to 105/210 at 50 Hz, the 60 Hz drop out Voltage must be lower.

I forgot to mention that in addition to the magnetic shield band built into the new replacement transformer, I am also going to experiment with the effect of a copper band around the perimeter of the toroidal transformer.  In theory the thick copper band makes a shorted turn that intercepts only the stray flux and lowers it by loading it with a short.  I will report later on whether or not it makes any difference.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 02, 2009, 04:29:36 PM
And how was the hum & noise from the frequency bands with inductors ?

tia regards Greg
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 04:39:13 PM
And how was the hum & noise from the frequency bands with inductors ?

I didn't measure that yesterday.  The last time I measured that I believe I was getting about a 20 dB reduction over the stock transformer.   I will be investigating the need for cans very shortly, and that will include a careful study of that.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 04:43:19 PM
I will now write a second post with all the rest of the stuff that I lost.

Q4, Q5 oscillations, 81 EQ board, clipping indicator, and audio coupling cap issues

I pulled all the '81 EQ cards and shotgun replaced Q4 and Q5 and D3 with my recommended replacements in my primary test-bed '81 (Thanks Crazy-doc).

I didn't check for oscillations before replacement, since I was on deadline to finalize the transformer winding voltages, so I will have to observe the problem on one of the other '81s here for repair/upgrade, since I have no desire to replace the new parts with the old. I have a couple of questions from those who have already tackled this problem:

1. Is the oscillation problem consistent and repeatable?
2. Is it a low frequency (motorboating-type) oscillation or a high frequency oscillation?
3. Anyone put a scope on it so I'm sure that I am identifying and solving the problem thoroughly and completely?

I have the following observations about the EQ cards:

1. Due to the long, daisy-chained power supply and signal busses on the 81 preamp, and the presence of a 47 Ohm series resistor isolating each EQ PCB from the 24V Buss, I believe it would be a good thing to up the value of the 100uF/35V PS decoupling cap to 470uF/35V.  Since I purchased lots of them for the Power supply mods, I figure, what's four more?. This will give the individual EQ sections a 5 times bigger reservoir of electrons from which to draw locally, and may reduce any power supply interaction between sections.

2. There is no high-frequency, or for that matter, low frequency local power supply bypassing on the +/-12V rails for the TL072 opamps on the EQ PCB's.  There is an overabundance of 0.1uF caps on the power supply boards, where they do almost no good, due to the large electrolytics ensuring regulator stability.  The 0.1uF caps on the power supply are totally ineffective in bypassing the opamps in the circuitry due to the inductance of the many inches of trace/wires that are between the power supply and the opamps.  I tacked 0.1uF power supply decoupling caps to the bottom of each EQ card close to the opamps.  There are a couple of close-by pads that can be used for that purpose.

3.  As I mentioned before, the clip indicators employ a half-wave rectifier so that they only correctly indicate on Sine waves.  Asymmetrical waveforms (music or speech) may or may not indicate correctly depending on the shape of the waveform, since the peak light only sees the positive half of the waveform.
The peak circuitry is present in the clone preamps, but not on the Neve circuitry.  The input Z of the clip indicator circuit is 100K on the EQ cards, which should not cause any issues.  On the preamp card, however, the input Z is 10K due to 1R62 being 10K instead of 100K.  This somewhat low impedance acts as an additional load to the 22uF preamp output cap (1C13) which also is loaded by 51K (1R26) and either the input Z of the output amplifier (5K) or the much higher input Z of the "Filter" first EQ section, depending on the position of the EQ in/out switch.  For that reason, I suggest that IC13 be replace with a 100uF cap.  You can use one of the 100uF caps removed from Power supply decoupling duty that got replaced in item 1 above.  I would also recommend changing 1R62 to 100K like all the rest of the clip lights.

Whether or not to increase the size of the coupling capacitors in certain locations of the ACMP preamps is a dilemma.
In these preamps, there are coupling caps between each EQ stage that roll off the low frequencies a little bit more as the signal passes through each section. The result of these successive rolloffs is significant low and low mid phase shift.  A 20 Hz square wave will have huge tilt.  If I were modifying these for one of my regular clients who wanted "better audio", I would probably significantly increase the size of the coupling caps over what Neve (and the clone) used.  The resulting sonics would be a subjectively less compressed sound, and a smoother, more extended bottom, and better low-mid resolution.  In that way, it would affect the Neve-original "color", which may not be wanted since these preamps are "color" preamps. As a matter of fact, almost every British-designed piece of gear that I have ever seen uses coupling capacitors that are way smaller than one should use if you care about maintaining waveform fidelity to frequencies lower than 100 Hz.

  So unless I receive instructions from someone wanting more extended bottom end on their particular preamp, I am planning to restrict my capacitor changes to those capacitors that have become low-frequency "bottlenecks" due to the ACMP clone implementation of the Neve-like circuitry.  As an example, the very low impedance 5K load of the class A output stage was never in the 81.  Neither was the 10K load from the clipping indicator circuitry.  As a result, this 22uF (1C13) will be too small to deliver the same low frequency response that was in the Neve. This one in my opinion should really be changed because it compromizes the sound of the preamp when the EQ is bypassed.

For those of you who have sent me your preamps already and are patiently waiting for me to return them, I hope you will understand that I want to do as complete a fix as possible the first time, so I am working hard to have a complete list of fixes so you are happy.  Thanks for your patience.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on April 02, 2009, 09:10:07 PM
Thanks for the update, Steve.

How many of these coupling caps are there in a 73?  Would it be possible to mod this is such a way to switch between the different cap sizes thereby having a pre with two different sounds?  Maybe I'm way off here....

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 09:49:10 PM

How many of these coupling caps are there in a 73?  Would it be possible to mod this is such a way to switch between the different cap sizes thereby having a pre with two different sounds?  Maybe I'm way off here....


There are quite a few caps that could be upsized in the 73 if you wanted to improve the bottom end and midrange.
A quick check revealed the same 22uF cap driving the 5K load of the output line amp and a 10K input resistor to the clip light circuitry.  Changing the 10K to 100K and the 22uF to 100uF is a good idea.

With regard to making the cap size switchable ... anything is possible -- take crazy-doc's gain switch modifications.  However, I cannot see that switching them would be very practical.  To be honest, I've never had a customer request that I put the small caps back in after I made them bigger. Of course, YMMV.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 02, 2009, 10:14:41 PM

I didn't check for oscillations before replacement, since I was on deadline to finalize the transformer winding voltages, so I will have to observe the problem on one of the other '81s here for repair/upgrade, since I have no desire to replace the new parts with the old. I have a couple of questions from those who have already tackled this problem:

1. Is the oscillation problem consistent and repeatable?
2. Is it a low frequency (motorboating-type) oscillation or a high frequency oscillation?
3. Anyone put a scope on it so I'm sure that I am identifying and solving the problem thoroughly and completely?


Steve

Here's a post I made on the big GB thread at HR a couple of months ago:

http://homerecording.com/bbs/showpost.php?p=3084168&postcount=7286

The oscillations were about 70MHz. There must have been some type of intermodulation/beating for this to be made audible of course. Swapping out the transistors as described resolved the problem. I think the residual noise that I describe afterwards with the eq sections engaged, was, in retrospect, the inductor resonances we're having so much trouble with.

I'll be very interested to hear of your degree of success in eliminating these resonances with the new power transformer, so that shielding might not be necessary.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 02, 2009, 10:50:15 PM
mine had a higher freq broadband hum/buzz as soon as the eq switch was in
regardless of eq settings , once the q4/5 were changed that went away and
the more mains related [ or so sounding ] hum became apparent .
Alex's testing as mentioned in the thread showed this eliminated by removing
the inductor totally or replacing it
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 11:23:09 PM
I forgot to mention that in addition to the magnetic shield band built into the new replacement transformer, I am also going to experiment with the effect of a copper band around the perimeter of the toroidal transformer.  In theory the thick copper band makes a shorted turn that intercepts only the stray flux and lowers it by loading it with a short.  I will report later on whether or not it makes any difference.
Toroidal power transformer stray field not reduced by copper band around circumference

I'm sure many of you have seen conventional EI power transformers with a big copper strap around the outside of the transformer.  That copper band outside of the core acts as a shorted turn which intercepts stray flux and reduces it by converting it to heat generated by the current flowing in the low-resistance turn.

I was thinking of trying the same thing on the toroidal power transformer in order to further reduce its flux emissions, but I realize that a copper band around the perimeter of the toroid is at right angles to the direction that a shorted turn must be oriented to pick up the unwanted flux.  So the copper band is effective with EI cores, but no help with toroids.  Like Edison, I will add that to my very large collection of ideas that I know don't work.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 02, 2009, 11:28:58 PM

The oscillations were about 70MHz. There must have been some type of intermodulation/beating for this to be made audible of course. Swapping out the transistors as described resolved the problem.
 

Somehow I thought you might have exactly what I was looking for.  I read your linked post, looked at your scope photo, and now will check out the oscillation and its cure here.

Thanks!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 03, 2009, 07:29:19 AM
>All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

Definitely! They do sound really, really good. I've never used a an N1081 and so can't compare with the
real thing. But soundwise, they are up there with any of the diys I've done.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 03, 2009, 12:31:22 PM
>All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

Definitely! They do sound really, really good. I've never used a an N1081 and so can't compare with the
real thing. But soundwise, they are up there with any of the diys I've done.

Hi Alex,

Nice, since i recall the main concerns initially went to the quality/sound of the signal transformers used.
... concerns which have been pretty much taken over by the other magnetics (power-TX & EQ-coils).

Can't imagine they'll be spot on or even pretty close, but decently close would be nice.
Once that's completed, I'll compare them to a BA283-project with the groupbuy-Carnhill-iron for an indication of that.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on April 03, 2009, 12:55:40 PM
Hi,

Does anyone know whether Max's RM8 inductors would be a drop-in?
http://www.groupdiy.com/index.php?topic=32475.0

Alex, Sorry, I don't recall now but did the Carnhil's fit or did you have to rework the connections and/or mounting?

Many Thanks,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 03, 2009, 04:21:04 PM
it looks like you can use the " white cup "  that the original inductor is in
from the chinese pre so that the pins will line up ,
whichever inductor you choose may or may not fit inside the cup , but
you may be able to cable tie it on and the cup may help for putting a shield around it

good luck , this is what i plan to do
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on April 03, 2009, 04:30:49 PM
it looks like you can use the " white cup "  that the original inductor is in
from the chinese pre so that the pins will line up ,
whichever inductor you choose may or may not fit inside the cup , but
you may be able to cable tie it on and the cup may help for putting a shield around it

good luck , this is what i plan to do

Cool. I haven't removed the stock inductors to inspect them at all yet.

Are you planning to use Max's or is there any news from Cinemag about a possible replacement?

Thanks,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 03, 2009, 05:28:38 PM
I'm waiting for Cinemag prototypes right and will report when
i install them .
I think  [ only my personal opinion ] they didn't want people
spending money if this would not solve the problem but
i can't see how it wouldn't be an improvement , the value
judgement however [ is it worth it ? ] is up to the individual

the carnhills have the advantage of already being inexisitence and used
for neve type circuits , the british pound is low but they are still a little more
than cinemag

Alex do you think your carnhills would fit in the white cup ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on April 03, 2009, 05:42:24 PM
I think they will most likely be an improvement. Apparently, removing the stock devices was an improvement though. :)

If they have mu-cans or something and the same or similar spec's then I'm sure they will be great.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 03, 2009, 06:09:40 PM
Max's RM8 will work electrically, but not as a drop in replacement.

You could use the stock inductor cup, but you need to watch the height -
to avoid ending up too tall for the 1RU case.

Mine with the OEP shield, no cup, just fit into the height.

Without trying the Cinemags, I can't know how they will work.
But if they are shielded, I can't see them *not* being a massive improvement on the stock.
Only one way to know for sure!

You can see the pics I posted for the OEP shield hack - need to insulate the inductor
with heatshrink to prevent any shorts to the can. I insulated the inside of the can with a piece of heatshrink and
then hot glued the inductor into the can.
I used short wires and then hot glue ;D to pcb.

There's enough pcb footprint space there, but I also insulated a couple of resistors there with hot glue
to ensure they don't short to the can.

Also, I ran grounding wires from the inductor core clips to can to star ground. May not be
necessary but I did it anyway.

With respect to changing audio transformers - there's no room in there. On the unit where I
replaced the psu with a JLM one, there is room to move things around and putting a Carnhill
output traffo (narrow form gapped), but the 2 inputs would need to be wired in -
the carnhills are too big to fit on the pcb. Some rearrangement of the output amp could free
up space on the left hand side to wire in Carnhill input traffos off the pcb. Not easy, though.

EQ aside, they do sound quite similar to my fabio n72s with Sowter iron. Definately the same
sort of 'crisp with a slight crunchiness' that makes discrete transistor + transformer different
from an IC based desk channel.

With EQ, there's massive bottom end available and as much high end as you could ever want.

I've been using these every night for 3 or 4 hours on guitars for the last week now
There's a ton of EQ possibilities there to explore.

I'll have these running thru a dual g1176 box shortly.
Thru the LA2A, I can't get over just how fab the combination is.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 04, 2009, 11:25:48 AM
EQ aside, they do sound quite similar to my fabio n72s with Sowter iron. Definately the same
sort of 'crisp with a slight crunchiness' that makes discrete transistor + transformer different
from an IC based desk channel.

Hi,

That's still with the stock transformers ?

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 04, 2009, 05:14:31 PM
yep
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 06, 2009, 04:27:42 AM
yep

Nice, thanks.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 06, 2009, 04:29:23 AM
[QC]I got a ACMP-81 that's missing the copper bolt for it's rightside
inductor (the lower-mid band).
I don't expect any influence on the coil-value, but sloppy assembly it is.

Missing bolts more common ? I saw a pic on the net of an ACMP-84 that had one
of the three missing as well, but still can't imagine this is anything deliberate.

Haven't removed PCBs yet, is the bolt essential for holding the coils in place ?
Didn't check yet if the coils are glued to the PCB as well, or maybe it's now
just the wires holding it in place...[/QC]

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 06, 2009, 07:37:07 AM
The bolt attaches the core to the holder (cup)
The core is also glued to the holder.
The winding leads are wrapped and soldered to holder pins
The holder pins are soldered to the pcb

The unbolted ones still look pretty strongly attached with the glue.
1 out of 4 had a bolt in mine. They unsolder pretty easy.

Other qc stuff - lighted switch not lighting, signal led not lighting

I found a  shorted solder on the Q switch led. Hairline solder short.
And one of the leds had a leg not soldered to the pcb

Glad to read you have finally received! Look forward to hearing your impressions.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 06, 2009, 08:17:17 AM
plenty of the pcbs and xlr connectors had screws loose in mine
don't think i have any bolts but would that affect inductance ?

Another thing i mentioned was that the soldering on my inductors looked
different  [ more shiney  ] as if done at a different time .

likely a good idea to eventualy take out every board and inspect it
i still don't really use any of mine yet , too bitter [ and sh*tty ]

go  peter go
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on April 06, 2009, 08:30:51 AM
about half and half with my inductor bolts/screws... Makes me wonder. But then so do a lot of things...

I haven't noticed any loose connections although I've had mine apart and back together again a couple times so far... Also no degraded/damaged resistors here.

I do like the sound of the pre's in the shop but also will not put them into service (and critically compare to other pre-amps) until I get the inductor resonance issue sorted out along with some of the PSU issues.

Cheers,
j
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 06, 2009, 05:27:21 PM
Thanks Alex for the coil-info. Weird actually, sounds like they could have saved on all bolts then.
But in case there's a possibility to make the bolt go through the PCB (traces permitting) it'll have amore sturdy connection of course
- depending on which inductors will finally end up in these boxes I'll change that.

Thanks Greg, sounds like we all better inspect before powering on, will do that before I'll fire up my other one.

But about the issues with the oscillating transistors & inductors, if I'm not mistaken simply hitting
the EQ-bypass button takes them both away from the signal-path (unless they spoil the rest of
the stages by means of the common supply).
That leaves us with a box that has a pre-amp function that's now minus two troubles, so more usable until coils & replacement-transistors are in.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 08, 2009, 01:56:19 AM
> don't think i have any bolts but would that affect inductance ?

I think they are brass and and should not influence inductance (not magnetic)

>But in case there's a possibility to make the bolt go through the PCB

You can drill a hole in PCB and use a longer brass bolt (screws thru the threaded inductor core)
and a washer/nut under the pcb. I wouldn't bother. But then again, I wouldn't keep those inductors either.

>But about the issues with the oscillating transistors & inductors, if I'm not mistaken simply hitting
>the EQ-bypass button takes them both away from the signal-path

You are not mistaken.
Works well and very quiet in 'out of box' condition (cringing with remembered autonomic repulsion reaction) with EQ switched out (gain switch pop excepting)


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: EEMO1 on April 10, 2009, 04:05:54 PM
 got my acmp73 all hooked up today for the first time.

 i think i got some extra noise (hiss?) on it... eq or not, no change. rotating the toroid, no change. i think it's usable though. i think my g9 on full boost is more noisy then these. bf76 plugin on protools gives more hiss aswell.

recorded 2,5 songs worth of vocals today with my g7, eq and no eq. mainly cut below 50hz and a little boost on 12khz.

 am I an idiot or do I need my ears checked?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 10, 2009, 05:38:42 PM
am I an idiot or do I need my ears checked?

Why?

There should be a small amount of hum at full mic gain with 220Hz at max, and 360Hz at max (high Q for '84). Then rotate the toroid for a null which is a compromise between (IIRC) 50Hz and 150Hz. Push the transformer towards the corner of the case too since there is a little play. The white noise at this setting is probably about as noticeabloe as the hum.

It should also get thicker and start to crunch as you overdrive the input section.

I've just spent a day working on the '84 preamps and have figured out what can be changed for benefit and also some things which cannot. I should be making some useful measurements tomorrow and will make a list of mods. What I'm going to do with that list though, I'm not sure yet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 10, 2009, 05:48:46 PM
out of the two 73's i got one works fine  [ the only out of 4 pres i got ]
and the other 73 has a light broadband buzz regardless of pre / eq settings

twisting the pwr xfmrs for me merely changed the timbre of the buzz
you may have gotten lucky  , not all were duds , but more 81s than 73s i think
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 10, 2009, 05:58:06 PM
Greg, I'd source another transformer and maybe speak to Steve Hogan. Otherwise, there is space to mount the transformer an inch or so to the right which you could try, although it sounds like your toroids are really duff.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 10, 2009, 07:09:02 PM
for my 81's i will try the inductors first , as i could stand to keep the costs
down , may look at outboard standard core xfmr , maybe the jlm psu option
it'll get there , just a question of how much
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 10, 2009, 07:21:50 PM
I'd just go with the inductors local to the transformer then. The 360Hz midrange setting seems the worst on this preamp. Check which bands are an issue. 'Scope the 24V rail and see if it's clean.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: EEMO1 on April 11, 2009, 03:50:34 AM

 thanks rodabod.

 so the basic pattern with these, shielding/changing inductors, new psu transformer...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 11, 2009, 05:34:13 AM
If the noise is there when the EQ is switched out, it won't be the inductors, I think.

The only things in-circuit with EQ out is the preamp stages (x2) and the output stage.

Also, if it is toroid noise, it would be more of a buzz (lo-freq) than hiss (mid-hi freq).

Can you take a real time analyser (RTA) screenshot? Maybe your DAW has one.
Then you can get an idea of how much noise is where.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: EEMO1 on April 11, 2009, 06:18:10 AM

 yeah,

 im going to do that next week.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on April 13, 2009, 08:59:15 PM
Last word on my acmp-81s - I had been comparing 2 units, each with transistor and inductor mods
one with stock onboard psu+toroid, one with replacement onboard psu+toroid.

After lots of testing, I put the other unit back to stock psu+toroid. So they are both identical now.

I found the stock toroid+psu was about 4dB quieter noise floor than with replacement.

Now they test virtually identical and are very quiet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 21, 2009, 03:28:53 PM
ACMP inductor brass bolts:

[QC]I got a ACMP-81 that's missing the copper bolt for it's rightside
inductor (the lower-mid band).
I don't expect any influence on the coil-value, but sloppy assembly it is.

Missing bolts more common ? I saw a pic on the net of an ACMP-84 that had one
of the three missing as well, but still can't imagine this is anything deliberate.

Haven't removed PCBs yet, is the bolt essential for holding the coils in place ?
Didn't check yet if the coils are glued to the PCB as well, or maybe it's now
just the wires holding it in place...[/QC]



The pot core inductors used on these preamps use gapped cores.  That means that the although the outside edges of the 2 core halves are touching, the inside legs do not.  Typically one leg is ground down to make a gap that sets the inductance of any coil placed inside the core.  All the coils I have seen so far are glued together.  Some have screws in them and some don't.  It doesn't really matter to the coil.  I suspect that the bolts were used to hold the whole assembly together while the glue dried, and some of the bolts were not removed.  The coils are light and the screws serve no real purpose once the glue dries, so don't go the the hardware store looking for brass screws.  Remove the screws if they are loose, or ignore them.

Sorry that my posts have been so sparse lately.  I am still actively working on the parts kits and mods.
I have 40 power transformers in production right now.  All 40 pcs have the primaries finished, the faraday shield installed, and today they are winding the 48V winding.  This is going slow, but I am working hard to make sure that every unit is spot on.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 21, 2009, 04:31:13 PM
What to do about ACMP clipping indicators?

Today I am working on the EQ cards on an unmodified '81.  I confirmed a 65 MHz oscillation that can be "tuned in" by manipulating the various EQ gain controls and the output gain. I am going to carefully change out the Q4, Q5 transistors, add the schottky diodes and confirm that the oscillation disappears.

I am wondering exactly how to approach the clip light circuitry in these preamps.

The clip indicator circuitry consists of a TL072 (or a TLO74 in the '81) that uses one half of the opamp as a precision half wave rectifier using the simplest possible configuration.  (1 opamp and 1 1N4148 diode).  The opamp follows the audio waveform during the portion of the audio waveform that is positive with repect to ground.  On negative halves of the waveform, the opamp saturates (output is pinned to the -12 volt rail), and stays there until the audio goes north of ground again. When the incoming signal goes more positive than ground, the opamp must slew all the way from -12 volts to begin to follow the positive half of the waveform again. The severe slewing from ground to -12V and back again happens every time the audio goes negative. As I have mentioned before in this forum, this rectifier circuit and the better inverting half-wave circuit employed in a couple of places in these preamps are half-wave so they completely ignore the level of the negative half of the waveform.   A full-wave rectifier is necessary to monitor audio levels since real audio isn't a sine wave.  The clip lights, therefore cannot be relied on to accurately monitor levels.

The second half of each TL072 is used as a comparator, so that opamp is either stuck at the positive rail or the negative rail, depending on whether or not positive audio level exceeds the clip light threshold.

There is no easy way to modify these circuits to either make them full-wave (ideal) or to prevent the opamp from saturating during negative waveform halves.   There is no power supply decoupling for these amps at all on the 81 EQ cards.  At the very least, some 0.1uF caps should be applied from each rail to ground close to the TL072 amps.

There are, as I see it, 2 options for dealing with the clip light circuitry:

Option 1:  Add two 0.1uF caps to the +/-12V rails on each card that has a clip light circuit within 1" of the opamps. These can be tacked onto the back (solder) side of the PCB.  Then live with the marginal performance of the clip circuitry.  Note that some of the clip light circuits have 10K input Z that presents an extra load to the preamp/eq circuitry and can have a modest effect on LF response.  Any perturbance at the opamp input due to the amplifier saturating every time there is a negative going waveform may cause some perturbance on the audio, not just when the signal clips, but all the time.  The extra loading from the clip circuit may necessitate making a coupling cap bigger in the mic preamp/EQ circuitry in order to prevent extra LF loss compared to the original Neve circuitry (that had no clip lights).

Option 2:  Disable the clip circuit altogether by removing the TL072 from its socket and hard wiring each clip LED off by lifting an appropriate resistor.  I would also disconnect the remaining clip circuitry completely from loading the preamp/EQ circuit.
Sometimes that can be done by lifting/removing a coupling cap or a resistor.  The procedure for disconnecting the clip circuitry must be carefully determined for each location.
I would personally choose option 2 if it were my preamp, but YMMV.  I can probably do either approach when modding the preamps.

I am hereby soliciting feedback on these 2 options.  I don't know whether it is possible or not, but it might be nice to modify just the circuit that drives the analog meter on some units, to make it full wave.  That way the meter wouldn't lie quite so much.  The factory meter driving rectifier circuit is an improved half-wave circuit that doesn't saturate to the rails, but it is still half-wave.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on April 21, 2009, 04:56:26 PM
What to do about ACMP clipping indicators?
There are, as I see it, 2 options for dealing with the clip light circuitry:

Option 1:  Add two 0.1uF caps to the +/-12V rails on each card that has a clip light circuit within 1" of the opamps. These can be tacked onto the back (solder) side of the PCB.  Then live with the marginal performance of the clip circuitry.  Note that some of the clip light circuits have 10K input Z that presents an extra load to the preamp/eq circuitry and can have a modest effect on LF response.  Any perturbance at the opamp input due to the amplifier saturating every time there is a negative going waveform may cause some perturbance on the audio, not just when the signal clips, but all the time.  The extra loading from the clip circuit may necessitate making a coupling cap bigger in the mic preamp/EQ circuitry in order to prevent extra LF loss compared to the original Neve circuitry (that had no clip lights).

Option 2:  Disable the clip circuit altogether by removing the TL072 from its socket and hard wiring each clip LED off by lifting an appropriate resistor.  I would also disconnect the remaining clip circuitry completely from loading the preamp/EQ circuit.
Sometimes that can be done by lifting/removing a coupling cap or a resistor.  The procedure for disconnecting the clip circuitry must be carefully determined for each location.
I would personally choose option 2 if it were my preamp, but YMMV.  I can probably do either approach when modding the preamps.

I am hereby soliciting feedback on these 2 options.  I don't know whether it is possible or not, but it might be nice to modify just the circuit that drives the analog meter on some units, to make it full wave.  That way the meter wouldn't lie quite so much.  The factory meter driving rectifier circuit is an improved half-wave circuit that doesn't saturate to the rails, but it is still half-wave.

Hi Steve.  Thanks for the update.  I'm going to say option 2 for the clip circuit and I would really appreciate your investigating the meter option as well.  Thanks for all the hard work!  I'm ready to place my order as soon as you give the word.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 21, 2009, 06:34:15 PM
ACMP inductor brass bolts:

[QC]Missing bolts more common ? I saw a pic on the net of an ACMP-84 that had one
of the three missing as well, but still can't imagine this is anything deliberate.[/QC]


The pot core inductors used on these preamps use gapped cores.  That means that the although the outside edges of the 2 core halves are touching, the inside legs do not.  Typically one leg is ground down to make a gap that sets the inductance of any coil placed inside the core.  All the coils I have seen so far are glued together.  Some have screws in them and some don't.  It doesn't really matter to the coil.

Thanks for the explanation, good to hear. 

Sorry that my posts have been so sparse lately.
Please don't apologize, everybody will understand. I mean, like us here most of us will have waited some ten months for the stuff to show up and that was bearable/anticipated, so a few weeks or more won't matter  ;)

Regards,

  Peter 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 21, 2009, 06:47:03 PM
 fyi  there was a miscomunication between Cinemag & I  ,
so the replacement inductors will be on their way to me
and i'll report back on the results when i have them in .
Although Cinemag still recomends looking at the pwr xfmr first
i can't see the inductors not being better , and in Alex's
case to see if that a consistent fix .

re the clip leds ,perhaps disconnect most if not all ,
what did the neves have ? if there's distortion that you don't
hear or see ..........................
sometimes in the heat of the battle but one might not be looking in that direction
don't think it's worth allot of work , just an added feature by someone trying
to sell something cheap ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 21, 2009, 07:51:05 PM
Removing Glypped Screws on ACMP Preamps

I know this may seem like a trivial aspect of fixing these preamps, but in order for me to keep the price of the modifications/repairs down, I must be able to disassemble and reassemble the various cards quickly.

The screws that hold down the various circuit boards to the chassis have been quite variable in their mechanical status.  On some of the units that I have in house, the screws were in so tightly that I could not get them loose with a properly fitting manual screwdriver.  I had to use a high torque drill to get them un-stuck. Why they used a Gorilla to install the screws is beyond me.  The biggest pain in the neck, however is the red goopy glyp that was used, not only to secure the screws on their perimeter, but the Chinese assembly personel glopped the goo on the top of the head so that the phillips recess is completely filled with goo.  If this were my own unit, I would take the hour (or two) it takes to dissolve the gunk with flux remover and Q-tips until the last trace of the gunk was removed from the slot so a screwdriver would fit correctly.  I can't spend that time on these screws.  I must be able to get the gunk out and the screws removed in a reasonable amount of time.  If they weren't so tight, I could just dig out some of the gunk with a sharp pick and unscrew the screw and soak it in solvent while I fix the PC boards.

Unfortunately on many of the screws, if you don't get them really clean, the screwdriver won't fit all the way and I run the risk of stripping out the slot.  It's also a pain that they used a collection of shim washers to adjust the PCB height to line up with the front panel holes.  I can buy replacement metric screws to replace the ones that get stripped in the removal, but I am hoping to figure out a really fast way to remove the screws.

Anybody out there have any ideas that I haven't thought of?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 21, 2009, 07:54:34 PM

Option 1:  Add two 0.1uF caps to the +/-12V rails on each card that has a clip light circuit within 1" of the opamps. These can be tacked onto the back (solder) side of the PCB.  Then live with the marginal performance of the clip circuitry.  Note that some of the clip light circuits have 10K input Z that presents an extra load to the preamp/eq circuitry and can have a modest effect on LF response.  Any perturbance at the opamp input due to the amplifier saturating every time there is a negative going waveform may cause some perturbance on the audio, not just when the signal clips, but all the time.  The extra loading from the clip circuit may necessitate making a coupling cap bigger in the mic preamp/EQ circuitry in order to prevent extra LF loss compared to the original Neve circuitry (that had no clip lights).
Thanks, Steve.

The audio perturbances you mention above - is this just the LF response you mention, or something else in addition? If so, could you please expound?

If it is only the LF response, I'd rather keep the "flashing lights", as useless as they may be, and use larger coupling caps in the audio circuit and decoupling caps near the op amps where feasible. (Of course, that's because I'd do it myself. I'm sure it would be much simpler to just disable the clipping circuits if modifying a large number of these. :))
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 21, 2009, 08:05:03 PM
Removing Glypped Screws on ACMP Preamps

I know this may seem like a trivial aspect of fixing these preamps, but in order for me to keep the price of the modifications/repairs down, I must be able to disassemble and reassemble the various cards quickly.

The screws that hold down the various circuit boards to the chassis have been quite variable in their mechanical status.  On some of the units that I have in house, the screws were in so tightly that I could not get them loose with a properly fitting manual screwdriver.  I had to use a high torque drill to get them un-stuck. Why they used a Gorilla to install the screws is beyond me.  The biggest pain in the neck, however is the red goopy glyp that was used, not only to secure the screws on their perimeter, but the Chinese assembly personel glopped the goo on the top of the head so that the phillips recess is completely filled with goo.  If this were my own unit, I would take the hour (or two) it takes to dissolve the gunk with flux remover and Q-tips until the last trace of the gunk was removed from the slot so a screwdriver would fit correctly.  I can't spend that time on these screws.  I must be able to get the gunk out and the screws removed in a reasonable amount of time.  If they weren't so tight, I could just dig out some of the gunk with a sharp pick and unscrew the screw and soak it in solvent while I fix the PC boards.

Unfortunately on many of the screws, if you don't get them really clean, the screwdriver won't fit all the way and I run the risk of stripping out the slot.  It's also a pain that they used a collection of shim washers to adjust the PCB height to line up with the front panel holes.  I can buy replacement metric screws to replace the ones that get stripped in the removal, but I am hoping to figure out a really fast way to remove the screws.

Anybody out there have any ideas that I haven't thought of?

I just used a 0.4mm jeweler's screwdriver (part of a cheap set I got at Harbor Freight Tools) to dig the stuff out, and then used a good fit Phillip's head to remove the screw. I guess I was lucky that mine were put in by chimpanzees - none of them were so tight I needed power tools for removal.

Gotta love those washers - I discovered them when I went to reinstall a board, and saw one lying on the bottom of the case. "Where the hell did that come from?"
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 21, 2009, 08:36:44 PM

Option 1:  Add two 0.1uF caps to the +/-12V rails on each card that has a clip light circuit within 1" of the opamps. These can be tacked onto the back (solder) side of the PCB.  Then live with the marginal performance of the clip circuitry.  Note that some of the clip light circuits have 10K input Z that presents an extra load to the preamp/eq circuitry and can have a modest effect on LF response.  Any perturbance at the opamp input due to the amplifier saturating every time there is a negative going waveform may cause some perturbance on the audio, not just when the signal clips, but all the time.  The extra loading from the clip circuit may necessitate making a coupling cap bigger in the mic preamp/EQ circuitry in order to prevent extra LF loss compared to the original Neve circuitry (that had no clip lights).


The audio perturbances you mention above - is this just the LF response you mention, or something else in addition? If so, could you please expound?

If it is only the LF response, I'd rather keep the "flashing lights", as useless as they may be, and use larger coupling caps in the audio circuit and decoupling caps near the op amps where feasible. (Of course, that's because I'd do it myself. I'm sure it would be much simpler to just disable the clipping circuits if modifying a large number of these. :))

Were it just the LF response, I would just put in bigger caps.  IMHO bigger coupling caps, especially in some critical areas, should be used to improve the sound of the preamp.  One certainly would not want caps that result in worse LF response than the Neve circuitry, since the caps that Neve used in their preamps are much smaller than I would use if I were designing for best sounding audio.  Again, since this is a color preamp, one needs to carefully consider the values of all these caps to mimic the original LF response including making the cap values larger to accommodate the extra loading of the clip circuits and non-original line driver input Z.

My concern with the clip light rectifier amplifier is that the + input is hung directly across the output of the amplifier stage being monitored.  The discrete transistor audio amplifier stages in the ACMP preamps don't have nearly as low an output Z as an opamp would, so perterbations in the load can add distortion easier.

When the rectifier amplifier saturates, (output stuck to the negative rail) the amplifier is no longer linear.  I have not studied the potential input Z changes that might happen as the amplifier alternately latches to the negative rail, and then gets un-stuck as the input signal goes postitive.  The amplifier is in hard slew limiting when it snaps negative and it is in hard slew limiting as it unlatches and heads in a positive direction to trace the positive going half of the waveform.  In the inverting version of these half-wave rectifiers, the opamp output is clamped 1 diode drop below ground, so it doesn't have to move 12 volts down and then 12 volts up.  It's a lot more gentle on the opamp to clamp it to a diode drop.  Unfortunately there isn't a way to apply the clamp in the existing circuit.

The way to check this out, I suppose, is to put the THD analyser on the output of an amplifier stage and check to see if the THD increases with the clip circuit added.  I would want to not only check the numbers, but very carefully look at the THD residual to look for artifacts. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on April 21, 2009, 10:39:54 PM
Just glancing quickly at this page...

In case you still need votes...  I vote for option 2 removing all clipping circuitry in the '81s. 

I have no interest in any LED lights on the thing whatsoever, except power. 

Simpler the better.  Cleaner the better.  Back to nature.  Back to neve.  :)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 22, 2009, 12:33:39 AM
the red goop , i just gave it a quick snap
non of mine too tight  [ opposite if anything ]
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 22, 2009, 04:02:51 AM
If you push these preamps hard enough, I think the first thing you notice is blocking distortion when the bias is pushed out of range. Things are already getting crunchy as you approach the LEDs lighting anyway. I wasn't entirely sure if the clip opamps going into saturation would be a big issue seeing as they sit on a separate supply. I didn't think the 10K load they present seemed significant.

Regarding upping coupling caps, I think it's a bit like working with a Marshall preamp... the distortion character is shaped by the coupling, and things can potentially get a bit ugly if you upset this.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 22, 2009, 02:53:16 PM
the red goop , i just gave it a quick snap
non of mine too tight  [ opposite if anything ]


The goop filling the screw recesses is sticky and soft.  Your comment about a quick snap makes me wonder if a little freeze spray might make it brittle enough to dislodge in one piece.  Maybe worth a try.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TB-AV on April 22, 2009, 04:16:51 PM
"Hi Steve.  Thanks for the update.  I'm going to say option 2 for the clip circuit and I would really appreciate your investigating the meter option as well.  Thanks for all the hard work!  I'm ready to place my order as soon as you give the word.

Paul "

+1 to all of above as it pertains to 73's

WRT the red goop. Mine only has it on the edge of screw heads and it has not been an issue.

-----

Resistors. PR9372.

Steve you mentioned how much you liked these resistors and others seem to concur. I was wondering what you or others think of replacing all or most of the resistors with these. Seeing that some of the resistors are dodgy at best and this would be diy rather than pay for time. This is also for the 73's. So the cost seems minimal.

It just seems like to me if I've got to get in there with a soldering iron for a few things, might as well upgrade as many important parts as is financially feasible.

Thanks again and looking forward to your rework kit.

Tom B.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on April 22, 2009, 07:26:42 PM
Me as well. I figure since some of us are going to be breaking these things open and doing the soldering ourselves, might as well change out everything you can. If it takes a few months to get through my 8 pres, then so be it.

It should be noted that I would MUCH rather have Steve do the work but the cost of shipping these things back to Cali would be prohibitive from here in sunny western NY. Steve seems like a frickin genious when it comes to this stuff and I have no doubt that when these things are spruced up with his kits, these will be amazing. I figure the upgrade kits will be close to the cost of each unit and I think factoring in shipping you could easily double the cost.

Harrumpf.

Can't wait to get some definitive pricing!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on April 22, 2009, 08:07:23 PM
fyi  there was a miscomunication between Cinemag & I  ,
so the replacement inductors will be on their way to me
and i'll report back on the results
Great! Looking forward.

Re the led's:
I would have ordered without but it wasn't an option :(
+1 on taking them out.. Maybe just lighting them up as power-on led's.. That way they at least serve some purpose..

Cheers,
J
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on April 22, 2009, 09:01:56 PM
Steve,

optio II is better overall, Ears are much better clip indicators than leds anyway.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Baltimore on April 22, 2009, 10:41:53 PM
I run my preamps with at least one clip light blinking most of the times that I use them...I say ditch 'em.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 10:48:00 AM
If you push these preamps hard enough, I think the first thing you notice is blocking distortion when the bias is pushed out of range. Things are already getting crunchy as you approach the LEDs lighting anyway. I wasn't entirely sure if the clip opamps going into saturation would be a big issue seeing as they sit on a separate supply. I didn't think the 10K load they present seemed significant.

Regarding upping coupling caps, I think it's a bit like working with a Marshall preamp... the distortion character is shaped by the coupling, and things can potentially get a bit ugly if you upset this.

Your comment about blocking distortion started me on an internet search of that phenomenon.  My initial research on the subject deals mostly with Vacuum tube circuits, especially guitar amps, which unlike mic preamps, are often intentionally overdriven.  I am not sure that the phenomenon of the tube grids drawing current and thereby upsetting the bias has a similar effect in these solid state circuits.  The really high resistor values in the vacuum tube circuits prevent a charged up coupling cap from quickly discharging after overload, and the solid state resistor values are much smaller.  There are no grids that suddenly go from MegOhm impedance to very low.  I am willing to be educated on this, so I am not pronouncing final verdict expertise here.

  It seems to me that if the time constants of the filter caps across the biasing resistors are significantly longer than the time constant of the lowest signal frequency being amplified, then the biasing shouldn't move enough to be a problem. It is possible to choose an asymmetrical test tone that could move the bias with continuous application, but real music doesn't do that, and in the case of the mic preamps the input transformers prevent ultra-low frequencies from ever being passed on to the amplifier circuits.

One of the biggest problems with single-ended powered circuits is that large caps in the biasing take a while to charge up.  This seems to me to be mostly an inconvenience rather than a problem.

Back to the ACMP preamps:
There are a couple of places where the loading of the individual amplifier circuits are greater than the loading that was originally in the Neve. The output amplifier in all the preamps has a 5K pot for an input.  The output coupling cap feeding that cap is only 22 uF.  When the EQ is switched in, there is a 100uF cap feeding that pot.  That's a x5 difference in LF response just at that location due to the difference in value.

I will do more homework, but I would guess that if there is some "crunchiness" going on, it may be in the output stage which has the output transformer in its feedback loop.  More research is needed.

Again, making major changes to the cap values will change the sound. Some may like it better.  Some may not.  My goal is to just check for major bottlenecks and deal with them.

By the way, for what it is worth, my rule-of-thumb for sizing audio coupling caps in solid state circuitry is 100uF for 10K loads, 220uF for 2 to 5k loads, and 470uF for 600 Ohm loads and bigger yet when the impedances get lower.
Sometimes those values get too big to be practical, so one has to adjust.

   The ACMP preamps use way smaller caps. Note the 470uF 1C2 in the first stage of the '81 preamp in series with the parallel combination of 1K8 1R5 and 91 Ohm 1R6 for a total series Z of 86.6 Ohms.  At 20 Hz a 470 uF cap is 16.9 Ohms.  At 2 Hz a 470uF cap is 169 Ohms. This cap by itself will result in significant LF rolloff and phase shift, which may be part of the color of the preamp.  For best frequency response and lowest waveform distortion 2200uF would not be too big in this location.  By the way, note that 120 Ohm resistor (1R28) is switched in only in mic position. During line-in operation there is only the 1K8 (1R19) in series with the 470uF 1C12.  The low frequency response is 10 x lower in line operation than in mic.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 11:07:57 AM
Resistors. PR9372.

Steve you mentioned how much you liked these resistors and others seem to concur. I was wondering what you or others think of replacing all or most of the resistors with these. Seeing that some of the resistors are dodgy at best and this would be diy rather than pay for time. This is also for the 73's. So the cost seems minimal.

It just seems like to me if I've got to get in there with a soldering iron for a few things, might as well upgrade as many important parts as is financially feasible.


The biggest problem with PR9372 is availability in the smaller 1/4 watt size from standard distributors like Michael Percy, etc.  I actually used the close cousin GP1/2 resistor for the phantom resistors due to wanting 0.1% tolerance and requiring 1/2 watt rating just for power dissipation reasons.  The GP1/2 use the copper leads and non-ferrous end caps like the 1/2 watt PR9372 resistors, but the Phantom resistors are in a location that has a secondary effect on distortion, so the extra fanciness of the PR9372 version was overkill.

Although there is certainly nothing wrong with changing the resistors to PR9372 quarter watt resistors, I personally wouldn't do it to my preamp remembering that these preamps are "color" preamps.  I would use them (and do) in certain locations in my 990 based Jensen Twin Servo preamp, which is designed for absolute minimum coloration. (I use a lot of Caddock and even some Kiwame resistors in that design).

I believe that a more practical choice for resistor replacements would be Roederstein MK-2 resistors. I wouldn't wholesale replace the resistors, though, just the ones that show signs of physical deterioration and the ones that must be changed to fix the gain switch pop, etc.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 23, 2009, 12:15:20 PM
Your comment about blocking distortion started me on an internet search of that phenomenon.  My initial research on the subject deals mostly with Vacuum tube circuits, especially guitar amps, which unlike mic preamps, are often intentionally overdriven. 


Hi Steve. I shouldn't have probably used that term which as you correctly described, is something experienced with valve amplifiers. The perceived effect I am describing though is very similar; drive very high levels and the preamp falls over and recovers. I'd need to see what levels I was driving to see if the bias was swinging enough out of range. It is not pleasant.

Quote
Back to the ACMP preamps:
There are a couple of places where the loading of the individual amplifier circuits are greater than the loading that was originally in the Neve. The output amplifier in all the preamps has a 5K pot for an input.  The output coupling cap feeding that cap is only 22 uF.  When the EQ is switched in, there is a 100uF cap feeding that pot.  That's a x5 difference in LF response just at that location due to the difference in value.

I'm guessing it's maybe the '81 you speak of which has the 100uF coupling cap after the eq? I have the '84, but the same differences exist effectively from a quick glance. The original had a 22u coupling cap too. Thing is though, the Neve designs weren't consistent either. In addition to this high-pass filtering, the HF feedback caps on the amp stages when in series cause a low-pass filter which cuts off within the audio range. I haven't checked if the Chinese did this too, but it wouldn't surprise me.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 01:39:37 PM
ACMP '81 Q4,Q5 update

Scope photo below shows 70 MHz oscillation from an unmodified AMCP '81 EQ.
(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMP81EQ70MHzOscillation.jpg)

I have now finalized my '81 EQ transistor mods for the BA-338 clone amplifiers.

I will be using Fairchild BC327-16 transistors to replace xQ4 (was BC557) and Fairchild BC337-16 for xQ5 (was BC547).
xD3 (was 1N4148 in the ACMP) will be replaced with a BAT-86 Schottky diode. This diode has the required low forward voltage drop to act as a Baker clamp similar to the Germanium diode used in the original Neve circuit.  The Germanium diodes are a weak spot in the Neve circuit -- the first place to look when an EQ goes south.  The Schottky diode is, in my opinion, a more reliable better choice.
 
Each EQ card will have the 100uF 24V power supply decoupling cap (C25) replaced by 470uF/35V to improve isolation between EQ sections and provide  more locally available current for each EQ card.  I may transplant some of the 100uF caps to LF bottleneck locations such as 1C13 in the mic preamp.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMP81EQmodifiedPCB6top.jpg)

The BC327-16 and BC337-16 are very close electrically to the original Neve BC461-6 and BC441-6 transistors.  They have graded Hfe to match the Neve original range, have plenty of collector current capability, and have exactly the same TO-92 package and CBE pinout as the factory transistors used in the ACMP preamps, making replacement a breeze.  It appears, as expected, that they also cause the 70 MHz oscillation to disappear as well.

(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMP81EQmodifiedPCB6bottom.jpg)

I have added a couple of 0.1uF caps to the backside of the card from the +12V rail to ground and from the -12V rail to ground to lower to provide local power supply decoupling for the TL072 and TL074 opamps used in the clipping indicators and meter drivers. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 02:23:21 PM

I shouldn't have probably used that term (Blocking Distortion) which as you correctly described, is something experienced with valve amplifiers. The perceived effect I am describing though is very similar; drive very high levels and the preamp falls over and recovers. I'd need to see what levels I was driving to see if the bias was swinging enough out of range. It is not pleasant.


As I was composing the post describing my xQ4, xQ5, and xD3 mods to the '81 EQ PCB's, it occurred to me that replacing the Baker clamp diode with a schottky type will help the overload characteristics.  The Baker clamp prevents Q4 from saturating, so it comes out of overload quickly.  The '84 amplifiers are not the same, so I'm not sure what might be causing your particular overload nasties.  Asymmetrical clipping may be happening, which can cause some shifting of the bias.  I haven't investigated the overload characteristics of these preamps.

Deane Jensen taught me many years ago that the overload behavior of any piece of audio gear makes a huge impact on its sound quality.  Some circuits overload very badly -- sometimes latching up to one rail for a time until it finally comes "unstuck" and finally returns to following the audio waveform.  Overload on such a device is very ugly, because the otherwise very short time that the overload occurs is extended in time to the point where it is always noticed. All clipping indicators employ some kind of time stretching circuitry to extend the LED flash long enough to allow it to be seen.

The reality is that occasional clipping is going to happen.  however, if the circuit overloads cleanly -- just clips with no hang and no burst of oscillation when it finally comes out of clipping, then occasional overload is almost undetectable.  When I have modified various European audio modules -- a Siemens EQ I worked on comes to mind, I have observed nasty slew limiting, and overload behavior that included bursts of oscillation on the trailing edge of any clipped waveform in the unmodified units.  My mods to the circuits clean up a lot of that uncontrolled behavior, and the results have been very noticeable.  Making any piece of gear clip cleanly and symmetrically is really important to how the unit sounds when being used, as opposed to being measured. 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 03:11:31 PM
Power Transformer Interest

The first production run of 40 power transformers is being made as I type this. There are two types of transformer:
TSS-Mp73/84 which is made to replace 73/84 preamp power transformers and
TSS-Mp81 which is made to replace 81 preamp power transformers.

The 81 transformer has an extra high-current 24 Volt winding and has a different lead break out than the 73/84 transformer.  The primaries, the faraday shield, the Phantom winding are identical on both models, and all 40 pieces have been done up to that point at this time.  Now I must decide how many '81 transformers and how many 73/84 transformers to make in this first run.  I obviously know how many units of each are required for those who have already sent units to me to get fixed, but I don't have a good count yet of how to divide up the 40 pcs based on those who plan to order kits.  The transformers will be the most expensive ($75 for the 73/84 and $80 for the 81 version) component in a kit of parts to modify your preamp in a DIY fashion.

If you would be so kind as contact me directly via email ASAP with your plans. I can be reached at stevehogan at soundsteward.com.  This will help me make the appropriate decision of how many of these first 40 pcs. should become '81's and how many should become '73/84's in order to cover the initial needs of interested parties.

Please don't respond to me on this board so we can keep the signal-to-noise ratio high. With this inquiry, I am dangerously close to having commercial content which I am trying to avoid, but I really need to figure out how many of each part to make and many of you have been waiting a long time already. 

Thanks! 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 03:29:48 PM
I run my preamps with at least one clip light blinking most of the times that I use them...I say ditch 'em.

Thanks for the feedback.  With all due respect, I would recommend backing off your levels, unless you are intentionally going for extra distortion. If any of the clip lights are coming on, you are probably well into clipping.

When I say that the clipping indicator LED's on the ACMP preamps lie, what I mean is that the preamp/EQ circuitry can be already clipping (especially in the negative direction) and the clip lights will not flash.  If they are flashing, you are already into clipping in a big way.  The simple half wave rectifer circuit is slow and not that accurate due to the 12V excursion the TL072 opamp output must make every half cycle.  Perhaps re-adjusting your recording chain gain structure will allow you to run the preamp at a lower level.  I would use external metering to set appropriate recording input levels and preamp output levels.

I realize that that would be an easier proposition if the preamps had a quieter noise floor, but that is coming soon.

My thinking on the clip lights has softened to the point that the benefits of having some kind of indicator is perhaps better than none at all.   I will give those who sent units to me the option and obviously the DIY folks can make that choice easily.   I can include the extra 0.1uF caps in their kit of parts or not, as they wish.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on April 23, 2009, 03:31:30 PM
This has been some very interesting reading.  Thanks Steve and everyone else who has contributed to the discussion.

 ;D

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 23, 2009, 06:53:07 PM
Steve

Of great interest to me (and probably many others) is the effect of the new power transformer on the inductor hum/resonance in the 81. After correcting the oscillation with the transistor swap, this problem poses the greatest challenge in using these units effectively.

Does the new transformer correct this problem, or is shielding or inductor replacement still going to be necessary to get these ship-shape?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 23, 2009, 11:59:11 PM

Of great interest to me (and probably many others) is the effect of the new power transformer on the inductor hum/resonance in the 81. After correcting the oscillation with the transistor swap, this problem poses the greatest challenge in using these units effectively.

Does the new transformer correct this problem, or is shielding or inductor replacement still going to be necessary to get these ship-shape?

When checking out the transistor mods by replacing first all the xD3 diodes, then the xQ4 and xQ5 transistors one '81 EQ PCB at a time, I also briefly looked at the inductor's tendency to pick up hum.  (I was doing the transistor swap on a "virgin" otherwise unmodified '81 unit with the stock power transformer and power supply).  Until I get more power transformers, I can't properly evaluate how low the noise drops compared with the stock transformer.

My comments below are preliminary and subject to revision. --Wow!  just had an earthquake here !! 2nd one today.


One thing is really clear, that the choice of mounting method for the stock inductors is unfortunate.  The very wide circle of pins and corresponding pcb layout leaves a lot of loop area to pick up trash.  The wire leads that connect the winding to the pins aren't twisted, and it seems that I can increase the hum a lot by waving my hand about the inductor or touching it.  Seems that this circuit in the '81 is operating at a very high impedance and is acting like a single coil guitar pickup. When I drop a mu-metal shield can over the whole inductor including the base, There is a significant reduction in hum, but it is steady only when the can is grounded (when the can is acting as both a magnetic and electrostatic shield).   It appears that there is more help by grounding the can than the fact that the can is mu-metal.  It appears that there is significant electrostatic pickup of garbage as well as magnetic.  I need to get a copper pipe cap to just try an electrostatic shield only. Of course, the lid on the preamp acts as an electrostatic shield to the outside world, so that may also have a significant effect.  The chassis and lid are also steel so they may effect the inductors by conducting magnetic lines of force from the power transformer.

 It may be that to really make the preamps bulletproof in terms of inductor pickup from both internal and external interference sources that a shield of some kind will be necessary.  Best results will be obtained when the inductor is completely contained within a mumetal can of some kind with a lid.  My first thought would be to remove the inductor completely from its base, install it into a grounded mu-metal transformer can and bring its twisted wire leads out one of the holes in the can lid. The existing mounting scheme with the white base is pretty but it doesn't work nearly as well as other methods when it comes to external pick-up of trash.

It appears to me that the glue holding the pot core into the white shells is epoxy, so it will be a real pain to break them apart without damaging the pot core. To encase the whole thing with the white base requires a much larger (more expensive) can than would otherwise be necessary to just hold the inductor.  I haven't removed the inductors and measured them  as a standalone component yet.  Maybe that is next.


Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 24, 2009, 09:44:28 PM

It appears to me that the glue holding the pot core into the white shells is epoxy, so it will be a real pain to break them apart without damaging the pot core. To encase the whole thing with the white base requires a much larger (more  expensive) can than would otherwise be necessary to just hold the inductor.  I haven't removed the inductors and measured them  as a standalone component yet.  Maybe that is next.


Today I removed samples of every inductor from an ACMP73, ACMP 84 and 2 ACMP-81 preamps. I also looked up the inductors made by Carnhill that most closely match the inductors in the TNC clones in order to figure out how difficult it would be to just make new, accurately made inductors in the smallest possible mu-metal cans to keep costs down.

I identified the exact core used by Carnhill to make their 9047 and 9048 inductors which look like a close match to the inductors in the '81.  I need to get the DCR's of the Carnhill inductors in order to know how they differ from the clone inductors.  Interestingly, these two Carnhill inductors are made from RM8 cores instead of Pot cores, so they have less self shielding than a similar pot core.  They are assembled with grounded clips instead of Glue, which allows their somewhat conductive cores to act as an electrostatic shield.

There were several interesting revelations as I measured, analyzed and compared 9 pcs of the factory supplied inductors. I measured DCR using my Fluke 8842A, and measured Inductance and Q using my GenRad 1659 Bridge.
Here are some observations: 

1.  All the Chinese inductors are made from the same size pot core.  Maximum inductance is 6.3H  and the lowest is 182 mH overall, leaving me to think that the gapping and/or the permeability/material of the cores may be different.

2.  Interestingly, the 6L1 inductor and circuit values on the PCB in the 73 and 84 are identical. Both inductors measured very similarly, however,
The frequencies on the 73 panel say 300, 160, 80, 50, OFF
The frequencies on the 84 panel say 360, 160, 70, 45, OFF

I haven't measured yet the center frequencies of these EQ's yet, maybe neither one is correct.

3.   The 5L1 inductor is identical in both the 73 and the 84.  Both the circuit values and the PCB's are identical.
This time the kiloHertz frequencies also match:  7.2, 4.8, 3.2, 1.6, 0.7, 0.36, OFF

4.   The 6L2 inductor in the '84 is unique to the '84 and does the high-cut filter.  I measured only one piece.

5.   The '81 4L1 and 5L1 inductors are unique to the '81. I measured 2 of each, and they were similar to each other.

6.   The ferrites use for these types of inductors are actually fairly conductive. If you look at catalog photos of the Carnhill inductors, you will see that they have used standard metal clips to hold the two halves of the cores together. These spring clips are generally grounded.  The cores on the Chinese inductors are not grounded, which may explain why the inductors are so prone to electrostatic pickup. Due to the difficulty of removing the pot cores from their white holder without cracking or otherwise breaking the pot cores, it is hard to add a grounding clip.

7.   I won't bother to rehearse the numbers here, but I measured two of each type of inductor, with the exception of the high cut filter in the 73 6L2.  There wasn't much variation between the samples, but they didn't quite read as advertised-- L5 in the 81's for example topped out at 4.7Hy when the schematic calls for 5Hy.

Thanks for emailing me with your power tranformer interest.  I have almost all 40 pcs allocated.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on April 24, 2009, 10:00:16 PM
I hopped off but I´ll to say if I didn´t  You would be the guy to consult with!

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 24, 2009, 10:03:34 PM
Hi Steve,

You didn't manage to get the specs for the 6L2 inductor in the '84, did you? Just saves me the hassle of pulling it out.

If it helps, I can provide response curves if you'd like them. Thing is, I was looking for copies of original reponses of either the '73 and '84 channels, and nobody responded, so they aren't really much use as yet. I think you'll find that none of the frequency bands are dead-on in reality. The bands are also relatively wide.

Cheers,

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 24, 2009, 10:12:03 PM
Here's a supposedly classic Neve eq curve:

(http://www.playgroundstudio.com/blog/wp-content/uploads/2008/10/neve-eq-q.jpg)

I'll try and mimic this and post results tomorrow.

Also saw this (http://www.fader8.com/audioblog/example%20eq%20phase%20plots.1073.png) which I may try to emulate too; that looks like a wide 700Hz cut to me.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 25, 2009, 08:03:52 AM
Ok, here's my '84 at Hi-Q, 3.2KHz, @+15dB and +8dB for a rough comparison. Q-factor looks slightly wider:

http://img14.imageshack.us/img14/5130/3k2eqcomparison.png

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 25, 2009, 12:51:23 PM
Very similar. Thanks for posting those. What does it look like with "Hi Q" out?

Here's a similar sweep that alexc posted a while back, but on the 81 at 1.5kHz. Only 8dB of boost at max (excluding 20 to 30Hz.)

http://home.att.net/~crazydoc/ACMP/alexc-freq-sweep-81.jpg
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 25, 2009, 01:43:05 PM
Very similar. Thanks for posting those. What does it look like with "Hi Q" out?

Quite similar again. It's not radical the Hi-Q switch. It did cross my mind making it really tight for hard notches, but since it does sound nice, i'll probably leave it.

Alex's curve does look wide, but at low boost, Q is wide.

See if you could replicate this:

(http://www.fader8.com/audioblog/example%20eq%20phase%20plots.1081.png)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 25, 2009, 08:50:19 PM
See if you could replicate this:

You'll have to explain their meaning to me. The bottom graph seems to represent phase shift/distortion, but what that means in realtime audio I have no clue (though I assume straight horizontal lines in each graph would be desirable, preferably at zero dB or degrees.)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 25, 2009, 09:17:21 PM
See if you could replicate this:

You'll have to explain their meaning to me. The bottom graph seems to represent phase shift/distortion, but what that means in realtime audio I have no clue (though I assume straight horizontal lines in each graph would be desirable, preferably at zero dB or degrees.)

The first graph (I hope) is just a frequency response graph for a -6dB cut @ 700Hz. The phase response graph shows us how the phase is shifted in degrees by the eq. This is partly what gives the eq its sound character and is why analogue eq's can often be preferable over digital as they have a noticeable "sound". On the other hand, we can design linear-phase eq's by using digital techniques which should in theory be more transparent.

A case of major phase shift in a sort-of eq is your typical 2-way loudspeaker cross-over. Often the tweeter will be wired out of phase with the woofer since during the cross-over frequency the phase shift will be such that the lows will be out off phase with the highs coming from the crossover.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 26, 2009, 04:39:19 PM
How do you generate a phase response graph?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 26, 2009, 06:28:18 PM
How do you generate a phase response graph?

Well, these days you'd hope it could simply be done "automatically" by hardware or software. We don't really need that here though. I was more interested in the response curves for starters.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 26, 2009, 08:52:35 PM

You'll have to explain their meaning to me. The bottom graph seems to represent phase shift/distortion, but what that means in realtime audio I have no clue (though I assume straight horizontal lines in each graph would be desirable, preferably at zero dB or degrees.)

Any real circuit or filter (one made from resistors, capacitors, or inductors) has a phase response that changes in exact correspondence to the change in magnitude.  This is absolutely predictable and repeatable due to the fact that the magnitude of a real filter is changed by the interaction of resistors, (which do not shift audio signals in time) and the reactive capacitors and inductors which have a 90 degree phase shift.  The phase graph shows how a steady state signal is displaced in time as it passes through a filter or other circuit. Digital filters can change magnitude only  (without affecting phase) by doing math on the instantaneous levels and changing the numbers to make the magnitude bigger or smaller.  In real filters the action of capacitors and inductors cause 90 degree phase shifts of the signal compared to resistors.

Unfortunately, Roddy's phase graph is in need of calibration in terms of the numbers, but the general shape is discernable.  During that portion of a frequency response curve which has flat magnitude, the phase plot will be at zero degrees and flat as well.  When the magnitude response begins to roll off, then the phase response will move negative degrees and when the magnitude response staightens out again (or zero's) the phase curve will again go straight.  When the magnitude response goes up, the phase response will go positive.  This is completely predictable because the magnitude response is changed by the time (phase) shifts between the components that bend the curve.

To play with this, download a free copy of LT Spice from the www.linear.com website.  You can use this easy-to-use spice simulator to play with various passive filters and trace their magnitude and phase plots easily.  You can even plug in the ACMP (Neve) values and predict within a hundredth of a dB exactly what is going on.

Although computer simulating the phase response of a filter is easy in Spice, making phase measurements is very difficult due to the need to very accurately determine where zero degrees (no time displacement) really is.  Roddy's phase graph appears to me to be in need of calibration in that respect.  It also is being affected by the phase response of his converters, I think, or there is a really severe HF rolloff on the DUT.

Phase basically describes the difference in time of any given frequency between that frequency entering the circuit and the time it leaves the circuit expressed in degrees of a sine wave of that same frequency.

One must be very careful to think about phase correctly when it come to describing its effect on audio waveforms.
Let's consider an audio waveform passing through a device in which all frequencies are delayed exactly the same amount of time.  The shape of the waveform would be undistorted because no frequencies would be delayed more than any other frequency and therefore all the spectral components that make up that waveform would arrive at the proper time to make the waveform exactly the same before the device and after the device.  The waveform shape stays the same, it just comes out of the device a bit later than it went in.  The time delay can be nanoseconds or weeks -- but as long as all frequencies are delayed exactly the same amount, the waveform remains undistorted.

If I choose a frequency-independent delay time that is, let's say, equal to 1 degree of a 1 kHz signal, that same amount of delay would represent 2 degrees of a 2 kHz signal, 10 degrees at 10kHz, 20 degrees at 20 kHz and 40 degrees at 40 kHz, etc, etc.  Such a frequency-independent delay would have Linear Phase. If we want to know how any audio device affects the shape of the audio waveform in a meaningful way, one must mathematically remove the phase degrees that are the result of frequency-independent delay and then plot deviation from linear phase.  Deviation from linear phase is the graph that matters, for example, when evaluating audio tranformers.  The traditional "phase" response at 20 kHz made by observation on an oscilloscope is meaningless without removing the frequency independent delay first.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 26, 2009, 09:37:02 PM

You didn't manage to get the specs for the 6L2 inductor in the '84, did you? Just saves me the hassle of pulling it out.


These are measurements of only one sample, so there was no averaging to help find the typical values:

         ACMP-84   
Sample:  Connections:     L (mH)   R (Ohm)    Q          Nominal Frequency:

6L2        Pin 1-2               51.6       40.3     7.66        18 kHz
6L2        Pin 1-3               74.7       49.0     9.05        14 kHz
6L2        Pin 1-4               95.86     56.2    10.08       10 kHz
6L2        Pin 1-5             139.27     68.9    11.86         8 kHz
6L2        Pin 1-6             181.74     79.3    13.28         6 kHz

Pin 1 is common on this inductor.

FWIW, The L/C filters on these ACMP preamps are actually passive and are just buffered by the amplifiers on the cards.

This is why the Q's are so low -- they are made to shape the "big picture" response, not notch out problems.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 26, 2009, 10:01:40 PM
Quote from: Steve Hogan
Any real circuit or filter (one made from resistors, capacitors, or inductors) has a phase response that changes in exact correspondence to the change in magnitude.
I don't understand this. Does this mean, if you split a signal (say a 1kHz sine wave) into two parts of equal magnitude, put one part through resistor R, and the other through resistor 2R, and then sum the signals, they will be out of phase, since they now differ in magnitude?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 27, 2009, 12:37:58 AM
Quote from: Steve Hogan
Any real circuit or filter (one made from resistors, capacitors, or inductors) has a phase response that changes in exact correspondence to the change in magnitude.
I don't understand this. Does this mean, if you split a signal (say a 1kHz sine wave) into two parts of equal magnitude, put one part through resistor R, and the other through resistor 2R, and then sum the signals, they will be out of phase, since they now differ in magnitude?

I'm not sure I understand the question, but in the case of resistive voltage dividers, pads, etc. there are no frequency dependent components, just resistors which pass all frequencies equally.  All frequencies are attenuated equally, not just one, so the phase vs frequency graph is a straight line at zero degrees.  When I wrote "Any real circuit or filter..." perhaps I should have written "Any real filter ..."  In other words, a filter circuit that treats some frequencies differently than others -- Either to boost or attenuate.  In the real world this takes frequency dependent components like capacitors and/or inductors, which do have phase characteristics that go along with the magnitude changes they produce.  After re-reading your question I wonder if you are connecting/confusing phase with absolute polarity.  That's one of the reasons it's a bad idea to label a polarity reversal switch "phase".

It took my attending my first formal electronics class for me to catch on to the fact that Ohms law doesn't work with capacitive and inductive reactance. 

Chapter 1, pages 29-42 of The Art of Electronics, by Horowitz and Hill, has a good explanation of all of this sort of stuff.  They spent a lot more time writing that chapter than I did composing my couple of paragraphs. Their explanations are very good and easy to understand, and if you don't have this book, I highly recommend it.  Horowitz and Hill should be on the bookshelf (if not in the bathroom) of every audio DIY addict.  I refer to it at least 3 times a week.   My copy is in tatters.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 27, 2009, 11:36:02 AM
Thanks for trying to explain it to me Steve - I'm sure you have better things to do. I wanted to confirm that purely resistive components have no effect on phase, as I had always believed. I guess it was the wording "Any real circuit or filter (one made from resistors, capacitors, or inductors)" where I inferred "resistors (or) capacitors or inductors" that confused me.

I'd always thought that phase was a function of frequency, inductance and capacitance, but not magnitude -I'll need to wrap my mind around that. But I guess, in my simple way of thinking, if you charge a capacitor with a higher voltage, it will charge faster, thus affecting its reactance for a given frequency, and thus the phase of a signal passing through it.

Anyway, thanks for the explanations, but please don't waste any more of your time on them. I've found this online text that I'll try to peruse - it seems pretty straightforward:

http://www.allaboutcircuits.com/
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 27, 2009, 06:30:29 PM
ACMP Gain Switch Mods -- Line-In first:

The ACMP preamps all share the same basic input circuitry.  The Line input transformer is a 4:1 stepdown transformer supplied by the Chinese with the same load that Neve used to terminate their Line In transformer: 2K2 in parallel with a 2.2nF cap. When I first saw the cap directly across the secondary of the transformer in the schematic, I was suspicious that I would encounter severe overshoot from the transformer due to the capacitive load resonating the transformer. As the square waves below show, I was correct.

Step-down line-input transformers will usually terminate best with just a resistive load.  This transformer is no exception.  It has excellent square wave response when terminated in just the resistive portion of the ACMP load.  With the 2K2 load resistor in parallel with the 3 dB per step Line attenuator, the Line input transformer sees a load ranging from 656 Ohms to 709 Ohms, depending on the gain setting of a '73 or '84.
The "Line In" attenuator network in the ACMP-81, on the other hand, has an extra 510 Ohm resistor (1R68) in series with the stepped attenuator which causes the load on the transformer to vary from 872 Ohms to 912 Ohms depending on the position of the gain control. I'm not sure why the extra loss is put into the '81, but it can be adjusted by changing the value of 1R68.

So here are the pics:

(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr1kHz900RL2n2.jpg)

1 kHz ACMP-81 Line Input Transformer Secondary with 900 Ohm Load in Parallel with 2.2nF cap (as delivered).


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr1kHz900RL.jpg)

1 kHz ACMP-81 Line Input Transformer Secondary with 900 Ohm Load only, 2.2nF cap removed.


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr10kHz900RL2n2.jpg)

10 kHz ACMP-81 Line Input Transformer Secondary with 900 Ohm Load in Parallel with 2.2nF cap (as delivered).


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr10kHz900RL.jpg)

10 kHz ACMP-81 Line Input Transformer Secondary with 900 Ohm Load only, 2.2nF cap removed.


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr1kHz700RL2n2.jpg)

1 kHz ACMP-73/84 Line Input Transformer Secondary with 700 Ohm Load in Parallel with 2.2nF cap (as delivered).


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr1kHz700RL.jpg)

1 kHz ACMP-73/84 Line Input Transformer Secondary with 700 Ohm Load only,  2.2nF cap removed.


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr10kHz700RL2n2.jpg)

10 kHz ACMP-73/84 Line Input Transformer Secondary with 700 Ohm Load in Parallel with 2.2nF cap (as delivered).


(http://i691.photobucket.com/albums/vv274/StevenAHogan/ACMPLineInputXfmr10kHz700RL.jpg)

10 kHz ACMP-73/84 Line Input Transformer Secondary with 700 Ohm Load only, 2.2nF cap removed.

So what do the square wave photos tell us?  The line input transformer will be far less harsh and have a much clearer midrange if the overshoot is eliminated.  I would recommend pulling out the 2.2nF cap (C2 in a '73 or '84; 1C21 in an '81) in a heartbeat.

In addition to determining that the 2.2nF cap is a bad idea, I also derived a revised set of 1% values for the Line input attenuator resistors that take the maximum error relative to 3 dB gain steps from 0.25 dB to less than 0.03 dB (calculated). There will be additional variations due to the 1% resistor tolerance which will degrade that a little bit, but with the revised values it will still be much more accurate than the original values.  The 0.25 dB improvement is modest, however, so changing the resistors is optional with the exception of the 39 Ohm resistor in the '81 which is afflicted by the wrinkly bad coating syndrome and should be replaced along with any other resistors showing signs of deteriorated blue coating. In the revised set of values, only the 200 Ohm resistor stays, and the rest are changed. If one doesn't trust the original resistors, there is no problem swapping them out -- it takes mostly time, because the resistors are relatively cheap.

Next is the Mic Transformer analysis, and the rework of that attenuator network to eliminate the pop and spread the gain steps out among 10 working positions and an  "OFF" instead of 11 working positions.  This one's a lot harder to figure out than the line in.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 28, 2009, 11:09:48 AM
Hi Steve, I found exactly the same results for the line input transformer. I've also done the same for the mic in (more tricky) and the line out.

You can get the characteristics "good" but ideally I was aiming to get closer to the Carnhill sound which has something slightly different going on in the low-end (in this circuit) from what I can see.

Just a thought; are you happy publishing all this information which the Chinese/US distributors will blatantly steal for future designs?

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on April 28, 2009, 12:36:39 PM
Steve,

You mentioned in a earlier post 'allocation of power trasnformers' , Is there a formal list for ordering one
of these power transformers. Or is the first run of 40 to be followed by many more ?

Also, with regards to phase changes due to magnitude and other influences,  How do these changes manifest themselves
audibly to the listening experience. I'm trying to connect the technical explanation with real world listening results.

Is this like flipping the phase reversal switch on a console channel strip where you can hear a change or is it much different ?

It would be nice to train the ear to recognize bad phasing , but this seems difficult.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 28, 2009, 01:40:35 PM
Steve,

You mentioned in a earlier post 'allocation of power trasnformers' , Is there a formal list for ordering one
of these power transformers. Or is the first run of 40 to be followed by many more ?

Also, with regards to phase changes due to magnitude and other influences,  How do these changes manifest themselves
audibly to the listening experience. I'm trying to connect the technical explanation with real world listening results.

Is this like flipping the phase reversal switch on a console channel strip where you can hear a change or is it much different ?

It would be nice to train the ear to recognize bad phasing , but this seems difficult.

Re Transformer availability:
At the present time I have been notified by prospective purchasers an interest in approximately 50 transformers, almost equally divided between 73/84 types and '81 types.
I plan to do another production run, but only after the first 40 are sold. After the amount of hours I have spent figuring all this out (hundreds more than I first anticipated),  I am hoping to modify as many units and/or sell as many kits as possible.  My cash flow will not permit me to make more transformers until I have sold the first 40.  Those who are having me do the installation of their tranformers and other mods will have first crack at the first 40 pcs, and those who want kits will be processed on a first come first served basis. Some clients have sent deposits on kits, and they will get first divs on the kits. Approximately 20 pcs are already spoken for with deposits.  Refer to my contact info below if you want to reserve a kit or send your unit in for modification on this first batch of tranformers.

Re Phase changes from filters:
Until the invention of digital audio with digital filters that could change magnitude without a corresponding change in phase, all equalizer circuits changed the phase in a way that corresponds to the magnitude (frequency response) changes.  We have generally come to hear these phase changes as a normal part of how an EQ changes the audio.  This is NOT the same as Polarity reversal (unfortunately labeled "phase" on some devices) which reverses the absolute polarity of the audio signal.  As I and others have discussed in depth elsewhere, reversal of absolute polarity is clearly audible due to the fact that real music is not made from sine waves, but often very asymmetrical waveforms, which sound wrong when the absolute polarity is reversed.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 28, 2009, 02:32:14 PM
Hi Steve, I found exactly the same results for the line input transformer. I've also done the same for the mic in (more tricky) and the line out.

You can get the characteristics "good" but ideally I was aiming to get closer to the Carnhill sound which has something slightly different going on in the low-end (in this circuit) from what I can see.

Just a thought; are you happy publishing all this information which the Chinese/US distributors will blatantly steal for future designs?

Roddy

To be honest, I was thrilled that the Line input transformer could be made to behave properly by simply removing the unfortunate parallel capacitor which resonated with the leakage inductance in the transformer to cause such a nasty overshoot.  I now consider the HF response on the cap-less line input transformer to be excellent.

This morning I isolated the mic input transformers and made exact measurements of their DCRs, their turns ratio, etc. This is a bit tricky because one must remove the 12K load resistor, the 1.2nF cap, and lift a resistor on the gain switch and turn the switch to the lifted position to free the secondary from all circuit loads.  The phantom feed resistors had to be removed as well, but they are going to be replaced anyway.  I did a preliminary look at the square wave response, and it had a nasty overshoot similar to the line in transformer with the factory supplied components, especially in the 2:1 (low gain) position. It looks like the square wave overshoot and ringing can be fixed just like the line input transformer, but it will likely require an RC network in addition to the resistive load provided by the gain switch and the rest of the circuitry.

I have spent many hours modeling the amplifiers in LTSpice to determine their actual input impedance in order to correctly adjust the Gain switch resistor values.  The shunt leg of the switched attenuator is bridged by the input impedances of the preamps, and the input impedances are surprisingly low.  The first stage input Z is only 3700 Ohms, and the second stage input Z is only 4938 Ohms in "Mic" and 8700 Ohms in "Line".  This means that all the gain resistors in the switched pads must be "fudged" to account for how the lower portion of the voltage divider is changed by being loaded by the preamp.

The circuit as delivered, loads the mic transformer differently when it is feeding the first stage compared to when it feeds the second stage.  This has the unfortunate effect of changing the frequency response of the transformer slightly and changing the loading on the microphone.  Because the load on the secondary is reflected to the primary of the transformer, the Input Z of the mic preamp, with the as-delivered factory gain switch values, is lower when the gain switch is set to higher gains than when it switched to lower gains.  This is quite independent of the change to the Input Z which occurs when the mic transformer primaries are switched from series to parallel with the rear panel switch.  Although the gain increases when the primaries are paralleled, (1:4 step up instead of 1:2) not all the gain is realized because the input impedance becomes much lower at the same time, so the microphone is loaded down more.

Since I must rework the resistor values in the gain switch anyway, I am considering changing them in a way that keeps the load on the transformer the same, so the Mic input Z of the preamp will stay the same at all front panel gain positions.  You should see the Excel spreadsheet that I use to figure all that out at the same time.

My game plan is to change the nominal 11 postions gain switch with 6 dB nominal steps, to 10 positions with 6.6 dB steps and an "off" position in the middle that prevents the pop. 

Regarding the Chinese figuring out what I am doing and doing it themselves? Even when given a schematic, the Chinese and others seem to not get it right, mainly because they have no idea why the circuit is the way it is.  We batted that all around a while back in this forum and I figure that ship has sailed. Those that buy the kits will be paying $25 for the instruction booklet that contributes (a little) for my hours of figuring this all out. I am open to advice on ways to protect my intellectual property investment, but I believe that the better folks understand what is going on inside their gear, the better they can use it, so I tend to be pretty free with information. In that way I probably take after my mentor Deane Jensen.  After all is said and done, we are striving to make tools that produce great music.

 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 28, 2009, 03:37:06 PM
I noticed the shift in input impedance of the resistor ladder too as it is switched. It is slightly unfortunate, but then again, the Neve values shifted too. Perhaps not quite as much though.

In case it interests you, here is a response graph of the stock TNC mic channel versus one which I adjusted the transformer networks for:

(http://img207.imageshack.us/img207/6451/micpretest.png)

Mic transformers can be a bit more tricky since some modern mics present as little as 50 Ohms which usually means more ringing. What I've done in that instance above is set the response to fall closely to the original transformers.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 28, 2009, 08:26:42 PM
I noticed the shift in input impedance of the resistor ladder too as it is switched. It is slightly unfortunate, but then again, the Neve values shifted too. Perhaps not quite as much though.

Mic transformers can be a bit more tricky since some modern mics present as little as 50 Ohms which usually means more ringing. What I've done in that instance above is set the response to fall closely to the original transformers.

It turns out that the higher the gain of the amplifier, the lower the input impedance. The first preamp stage has 30 dB of gain and has lower input impedance than the 28 dB second stage.  By backing off the first stage gain to 29 and increasing the second stage to 29 when operated in mic mode, it might be possible to better balance the load on the mic input secondary. Right now I am trying to figure out where to put the "off" position, to distribute the gain/attenuation in an optimum way.  From a user point of view making all the steps 6.67 dB apart will make the smallest change.  There is no position, however, with that gain structure that has the transformer directly connected to the second stage, as there was in the original popping topology.

In my twin servo preamp and other stand alone custom preamps I use a "Schoeps" switch which switches a pair of 60.4 Ohms in front of the Mic transformer primary. This builds out the low Z microphones like Schoeps and TLM 170's to about 150 Ohms so the tranformer response stays correct.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 29, 2009, 06:08:02 AM
This thread remains growing w.r.t. valuable info, very nice!

Then this:

Has the topic of transistor types been brought up yet ?

The BC184 NPNs are there, but for the BC214 (PNP) the BC557 is substituted, which per datasheet has poorer noise performance. Might not matter that much, but since it has been reported that engaging the EQ increases the noisefloor one might be better safe than sorry.

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 29, 2009, 07:55:09 AM
There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

I did look into this briefly, and since I did have a bag of NOS Motorolas, I switched these in place of the original ST models. Interestingly, when re-measuring, I noticed a drop in THD by about a factor of five.... I'm not sure why this would be the case, but I didn't set the bias before or after swapping, although I was only testing the unit at zero-level.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 29, 2009, 08:08:44 AM
There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

I did look into this briefly, and since I did have a bag of NOS Motorolas, I switched these in place of the original ST models. Interestingly, when re-measuring, I noticed a drop in THD by about a factor of five.... I'm not sure why this would be the case, but I didn't set the bias before or after swapping, although I was only testing the unit at zero-level.


I forgot which were 'the best' (...) but have some old RCAs that I probably use when the s'iron is going.
Factor of five, that's a lot ! From where to where ?
One would obviously expect THD to remain about the same when swapping devices & setting for same current again.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 29, 2009, 08:28:12 AM
Factor of five, that's a lot ! From where to where ?

Just checked my test reports; from "modded" to "modded plus 2N3055" I get a drop from 0.01% to 0.006%. That's a factor of two. I think I was previously comparing earlier figures (these preamps have now existed in various different stages, each with their own test results).

What makes the THD measurement a little more tricky is the PSU noise (not just from the unit itself but picked up from other racks) which interferes with the figures.

I'll maybe measure this again with another box. Interestingly, the PCB looked a little scorched where the heatsink is mounted before I even touched the circuits. It's either that, or somehow a dye is leaking from the heatsink onto the board (maybe from de-fluxer?)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 29, 2009, 08:37:49 AM
Interestingly, the PCB looked a little scorched where the heatsink is mounted before I even touched the circuits.

Haha, no stone remains unturned.

We might need to groupbuy a few of these, if only for the title, OK, actually only for the title:

http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1 (http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1)

 8)



W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I realize we might not necessarily want to set for minimum THD, to 'nice-up' things.


Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 29, 2009, 10:09:12 AM
We might need to groupbuy a few of these, if only for the title, OK, actually only for the title:

http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1 (http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1)

Hah!

Quote
W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I didn't actually. The thing is, the output sections on these preamps are like power amps. I'm only ever going to be driving around zero-level nominal, maybe +8dBu on peaks with my current soundcard. Certainly for me at least, the majority of the character comes from the preamp section, whether with the original transformers or with Carnhills.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on April 29, 2009, 10:20:36 AM
W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I didn't actually. The thing is, the output sections on these preamps are like power amps. I'm only ever going to be driving around zero-level nominal, maybe +8dBu on peaks with my current soundcard. Certainly for me at least, the majority of the character comes from the preamp section, whether with the original transformers or with Carnhills.


I've read various thoughts on this, it's the iron, it's the pre, it's the tants, it's the Marconi-knob, it's the last section etc, but who knows, I guess in the end it'll be the sum of all contributions.

My plan is to combine this amp with a variable post-output-TX-attenuator, so to drive the output-stage & then level down to usable levels again.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 29, 2009, 11:25:52 PM
Here's quite a bonus I found today while working on my 73. There is a metal ring that has been inserted on the preamp board during manufacture, coupling one of the input transformers with capacitors C1 and C2. In effect it makes this component group into a flux capacitor, bringing the current and voltage back into phase.

(http://home.att.net/~crazydoc/ACMP/ACMP73-surprise.jpg)

As an added bonus, when it is magnetically coupled with the 3 (yes, three) washers inserted between the 48V phantom switch and its standoff,  it allows me to travel the space-tone continuum with the touch of a dial, with a sound that is rich and intimate, spacious and articulate. It combines the eq boards through algebraic phasing with a high-power transformer drive to create an indispensable acoustical window.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wmtunate on April 30, 2009, 12:19:07 AM
I wish I could confirm your findings, but I haven't yet had the balls to get my '81s up to 88mph.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 30, 2009, 08:14:23 AM
Some of the BEST QC ,

i didn't get that with mine , can you do up a bom for
that flux cap ?


anyone [ fwiw ] hear from chance recently about
the supposed warrenty on pres & mics ?

Well i mean curious if he's actually pretending to
or in the totally walk away phase
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on April 30, 2009, 09:05:06 AM
Warrantee. You some kind of funny boy?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Baltimore on April 30, 2009, 10:56:35 AM


anyone [ fwiw ] hear from chance recently about
the supposed warrenty on pres & mics ?


Well, I recieved 4 84s instead of 81s and I've been trying to get him to send me the right pres since January. He can't even pull that off...I realize that this is somewhat of a blessing in disguise, as my '84s are at least somewhat useable out of the box.  But i would love that extra eq on the 81s. Anyone wanna trade?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on April 30, 2009, 05:03:41 PM
Here's quite a bonus I found today while working on my 73. There is a metal ring that has been inserted on the preamp board during manufacture, coupling one of the input transformers with capacitors C1 and C2. ...

Wow, a washer. Thats a nice surprise  >:( , possible cap explosion waiting to happen.
and people have the nerve to tell me I'm not politically correct in bashing China quality on a case by case basis.
looks like it was planted by a rebellious employee.

Well call me blind and stupid.  I must confess that I thought crazydoc had started to live up to his namesake with his futuristic waxing poetic about preamps.  Funny bit, but I didn't get the point until electrochronic explained what I should be looking for in crazydoc's picture.  That piece of metal does not look like something that will add to long-term or even short-term satisfaction.

To this day I still haven't figured out what was going on with the front panel knob photos a while back in this thread.

Because the mounting tabs for the input transformers are not flush with their pin side bottoms, the factory has used washers to fill the spaces between the PCB and the tabs before hand-soldering in the transformers.  Obviously one of the washers got away from the assembler in this case and because it got stuck under the caps, didn't just fall out when the board was turned over.  Looks like the inspector (ha) missed it like I did.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on April 30, 2009, 05:58:04 PM
Sorry about that, Steve.  ;D

My poetry was ripped off and modified from the CAD mic site:

http://www.cadmics.com/M179.php

I've long admired their hyperbolic mic descriptions.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Spendor on April 30, 2009, 06:52:29 PM
Hi there - just received my ACMP84. Fired up OK and prelim test seems OK  - quiet and clean. Has the jump on the gain of course so very keen for a solution there. I went looking for decaying resistors (looked OK) and found 5R11 on the mid EQ board missing. Could someone confirm whether this is universal or just missing on my board? if i do need to fix it what is the value? Thanks!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on April 30, 2009, 07:07:32 PM
the knobs were a guise for the real improvement
felt marker over the logo
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on April 30, 2009, 08:28:54 PM
found 5R11 on the mid EQ board missing.

From what I can see, 5R11 shouldn't be necessary. It shunts one of the inductors and was omitted.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: TB-AV on May 01, 2009, 12:10:30 PM
"I believe that a more practical choice for resistor replacements would be Roederstein MK-2 resistors. I wouldn't wholesale replace the resistors, though, just the ones that show signs of physical deterioration and the ones that must be changed to fix the gain switch pop, etc."

Thanks for the guidance on the resistor replacement. Anyone know a good source for these resistors. I can't seem to find them.

Tom B.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 01, 2009, 12:24:06 PM
Has the topic of transistor types been brought up yet ?

The BC184 NPNs are there, but for the BC214 (PNP) the BC557 is substituted, which per datasheet has poorer noise performance. Might not matter that much, but since it has been reported that engaging the EQ increases the noisefloor one might be better safe than sorry.

Bye,

  Peter

I realized this is only relevant for the '81-preamps.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 07, 2009, 09:37:42 PM
"I believe that a more practical choice for resistor replacements would be Roederstein MK-2 resistors. I wouldn't wholesale replace the resistors, though, just the ones that show signs of physical deterioration and the ones that must be changed to fix the gain switch pop, etc."

Thanks for the guidance on the resistor replacement. Anyone know a good source for these resistors. I can't seem to find them.

Tom B.


TAW Electronics in Burbank, CA stocks many, but not all Roederstein MK-2 Values. What they don't have in Roederstein they have in RGM which are decent imports, better than the ones on the Clone amps now.  They are also factory authorized Wima Distributors and have an excellent selection of FKP-2 polypropylene foil and film caps.

GAIN SWITCH MODIFICATION UPDATE

Well, I just finished working out the new resistor values for the gain switch. My proposed cure does involve cutting the offending trace between section 1 and section 2 on switch position 6.  That position will become the "OFF" position in the reworked attenuator.

The rework will involve replacement of almost every resistor in the attenuator.   A few are unchanged, one is moved, and the rest are all new values.  The original gain switch had 11 positions with 6 dB nominal gain steps.  My new version has 10 active gain steps and an OFF position which prevents the disastrous "pop" resulting from the connection of output to input of the first gain stage during one of the switch transitions.

My new proposed Nominal Gain steps from max gain are:
-0dB
-6.67
-13.33
-20.00
-26.67
OFF
-33.33
-40.0
-46.67
-53.33
-60.0

In the first five steps the input transformer is loaded directly by the first 30 dB gain stage.  The output of the first stage is attenuated by the first stage output pad and fed to the second stage with 28 dB gain.

In the "OFF" position the 240 Ohm resistor (1R51 or 8R23) at the bottom of the pad is moved to replace the original 12K resistor (1R45 or 8R17) This terminates the input of the second gain stage with 240 Ohms in the "OFF" position.
 
The last 5 gain steps load the mic input transformer directly with a series of 5 separate voltage dividers. The lower leg (resistor) of each divider is loaded by the rather low 4938 Ohm input Z of the 2nd stage.  This results in a lot of spreadsheet calculations to select pad values that simultaneously give the correct voltage loss and still hit the target load Z for the transformer secondary.  In the factory version of this preamp, (and also the Neve versions, for that matter), the values of the pad resistors result in the terminating load on the input transformer making a big jump, from about 2700 Ohms in the high gain steps, to about 4500 Ohms in the low gain steps.  This change in load will cause an almost x2 change in the input impedance of the preamp, loading down microphones much more in high gain than low gain settings.  It also makes it near impossible to properly select a proper damping network to tame the overshoot and ringing of the input transformer at all gain settings.  The lightly loaded low gain settings will have more extended HF response than high gain settings.

All this goes away with the values I have worked out for the gain switch.

The new load on the transformer averages 3633 Ohms at all gain steps, both low and high gain, with a minimum load of 3589 at -53.33 step and a maximum of 3662 at the -40 step.  Those two are sort of outliers, since almost all the other steps are very, very close to the average.  Although this load value is still lower than optimum for the mic input tranformer, there is nothing that can be done since the input impedance of the first gain stage is only 3700 Ohms, there is no way to lighten the load.   The consistent loading on the transformer will enable a good RC network value to be selected that will work for all gain settings.  Overall I consider it a good fix.  It does require changing out a bucketload of resistors, however.
I don't have all the required resistor values in stock, so I will test the new setup as soon as the new resistors arrive.
More Later . . .
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on May 08, 2009, 10:30:14 AM
Thanks, Steve for your continued work on this project.  Very impressive!  I'm getting anxious to get started and can't wait to hear the new pre.  Do you have an approximate time when kits will start shipping?  Thanks again.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 08, 2009, 11:32:52 AM

Has the topic of transistor types been brought up yet ?

The BC184 NPNs are there, but for the BC214 (PNP) the BC557 is substituted, which per datasheet has poorer noise performance. Might not matter that much, but since it has been reported that engaging the EQ increases the noisefloor one might be better safe than sorry.

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

After studying the data sheets on the BC214 and the BC557 et al, it appears that the BC557 is what Horowitz and Hill call a "jelly-bean" part.  General purpose transistor with wide specs.  The BC214 is at end of life status but still available. It has a 2 dB Max noise figure where the BC557 noise figure is 2 dB typical and 10 dB max.  Good design requires that one use the "max" figure when designing for worst case performance in production.  Depending on the BC 557 to be low noise is folly, even though in most cases, there won't be a problem.  That's what "typical" means.

The BC214 hFE@2mA is 140 to 400.
BC557 comes in graded hFE of A=110-220, B=200-450, C=420-800
In the same family as the BC557 is the BC560  which is graded for low noise -- NF = 1.2 dB typical, 2 dB max
What's more important is the frequency range of the specification extends from 30 Hz to 15,000 Hz (the MHz in the data sheet must be a typo -- this is not a microwave transistor).  The low frequency specified means that the 1/f noise should be pretty good.

To sum up, this means that the BC560, IMHO, would have been a better choice than the BC557.  The BC560 is available in both B and C grades.  The B grade is a close gain match to the (Neve) BC214, but the BC560 has better specs and would be a superior replacement for the BC214.  These transistors are cheap, but there are an awful lot of them in the '81 EQ PCBs.  I honestly don't know whether or not changing out those transistors will affect the level of the hiss (white noise) of the EQ or not, since the EQ operates at nominal line level and the buffer amplifiers are all essentially unity gain compound voltage followers.  Some of the BC214's were used as input amplifiers and others were used as current sources, in which the noise may have even less relevence.
Careful control of the hFE may be important to prevent instability, which makes the pre-graded gains of the BC560 attractive.

I count 12 pcs of BC557 being used in place of the BC214 in the ACMP 81.

I suppose that I can add it to the list of potential mods.  For the DIY person ordering parts kits, the cost will be negligible, maybe $2 or $3.  But the labor time keeps rising for those who are having me do the mods, and that causes me concern.

I haven't done any work on the output stage yet -- still working my way thru the block diagram, so I have no input at this time on 2N3055 transistor types and their effect on performance.



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 08, 2009, 11:45:15 AM
Thanks, Steve for your continued work on this project.  Very impressive!  I'm getting anxious to get started and can't wait to hear the new pre.  Do you have an approximate time when kits will start shipping?  Thanks again.

Paul ;D

If I can quit finding and fixing problems with these preamps so I can finalize the mods, I will be ready to start shipping kits and fixing units.  When I started this whole thing, my friend told me that there were these group buy preamps that had hum and maybe you could provide a new power transformer . . .  The list of changes has gotten really long now.  I will have tranformers next week, and I hope to have everything finalized by the end of the month, unless the inductors are still a problem.

BTW, I intend to replace the 22uF output caps in both preamp amplifiers with 100uF.  The loading gets down to less than 4K on the first stage and less than 5K on the second stage when driving the output direct (EQ out).  22uF is just way too small to preserve the LF phase response with such low impedance loads and it becomes a LF bottleneck.  Normally I would recommend 220uF for loads in that range, but I am wanting to preserve the Neve-like British character which includes caps that are too small.  100uF is a good compromise.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on May 08, 2009, 01:00:59 PM
Steve, thanks.  The more info the better for those of us doing DIY fixes.  Anything that comes to mind will be helpful for some of us.  The clients sending you units could have a range or a couple options whether to replace everything, or just the critical fixes, etc. Thanks for sharing your findings.

Since you're going over them with a fine toothed comb...  (BTW, I definitely understand this expression now, after my daughter's friends got head lice at school, a common problem...yikes  :o   :D)  Any comments about the values and quality of the electrolytics?  And likewise for the film/whatever caps in the EQ circuits? 

The caps are doubtless mediocre quality... how important do you think those are for imparting sonic personality?  In my own Neve 1272 experiments, I found changing all the electrolytics significantly changed the character.  (I also replaced all the tants with electrolytics, probly the biggest influence.)

And... it's interested to see you writing that the Neve circuits have too-small caps which restrict the low end.  As they are famous for a robust low end.  All part of the sonic balance I suppose?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 08, 2009, 03:12:10 PM
i bought two 73's & two 81's
my plan  [ so far ]
is to make the 73's as neve as possible [ barring spending
as much money as making one ]
and make the 81 as good as possible
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 08, 2009, 04:45:35 PM
. . .
Any comments about the values and quality of the electrolytics?  And likewise for the film/whatever caps in the EQ circuits? 

The caps are doubtless mediocre quality... how important do you think those are for imparting sonic personality?  In my own Neve 1272 experiments, I found changing all the electrolytics significantly changed the character.  (I also replaced all the tants with electrolytics, probably the biggest influence.)

And... it's interested to see you writing that the Neve circuits have too-small caps which restrict the low end.  As they are famous for a robust low end.  All part of the sonic balance I suppose?

Let me qualify my comments below with the following general observation:
The units we are improving are very low cost.  These preamps are not High-end, high-cost items.  That being said, most of the parts are of higher quality than one would expect for the money spent.  It makes no sense, IMHO to replace every resistor with PRP9372 resistors, or Caddock or Vishay bulk metal film, or replace the decent mylar caps with ill-fitting polypropylene (many of the caps are superb Wima FKP-2 polypropylene film/foil 2.5%) The build quality of the switches and other mechanical parts limit how far one should go and how much one should spend to be reasonable and not be "polishing a turd" to diamond brilliance.

I believe that these preamps with appropriate, reasonable modifications can perform at a very satisfactory level.  They will end up costing more than twice what you paid, but even twice what you paid is a good deal for the performance they are capable of when "hot rodded".
 
So let's start with the easy question first.  It seems that the film caps are fine. They are of sufficient quality to not require replacement except for a DIY type who wants to really push the limits on his own dime and time.

The supplied resistors appear to be metal film instead of carbon film, and the mods that I propose change values, and therefore there will be an automatic quality upgrade for many of the critical resistors -- like those in the input attenuators.

Electrolytic caps.  The supplied Rubycons are 105C and decent.  Not top of the line, but not junk.  All electrolytic caps have non-linear capacitive reactance.  All things being equal, the larger the capacitor value, the lower the capacitive reactance (impedance) at a given frequency, and the less influence (distortion) it will create in a circuit.

At 20 Hz a 470uF cap is 16.9 Ohms, a 100uF cap is 79.6 Ohms, a 47uF cap is 169 Ohms, a 22uF cap is 361 Ohms, and a 10uF cap is 796 Ohms.

The load on the first stage of the mic preamp is about 3100 Ohms.  When fed by a 22uF cap the non-linearity at 20 Hz is 361/3461 or  roughly -20 dB.  A 100uF cap is 80/3180 or about -32dB.  The lower the capacitive reactance of a coupling cap is compared to the resistive impedances in the circuit, the less damage to the signal will be done by whatever non-lineariities come from the crappy electrolytic cap.  Tantalum caps are extremely non-linear if not operated with a DC bias, so they should never be used as coupling caps on circuits with bi-polar supplies unless bias circuitry is added.  At that point, a non-polar electrolytic is much simpler and sounds much better.

I understand that the original Neve preamps used Tantalum caps. These have a limited practical size/voltage value to keep the cost down to reasonable.  22uF/35V is a big tantalum.  A 220uF/35V electrolytic is pretty average and low cost.  It is my opinion that making the coupling caps bigger has much more influence in making a piece of audio gear sound good, than the particular brand of electrolytic.  If you make them big enough, their impedance becomes so low that they only represent a very small portion of the signal so they add little distortion.  In circuits without DC bias, I use non-polar electrolytics (usually Panasonic SU series) caps because they have less distortion than unbiased polar caps.

In the ACMP preamps at hand, most of the caps are OK.  I would just replace the ones that are bottlenecking the LF with bigger ones.  The LF response of any piece of audio gear can be tested easily with low frequency square waves.
100 Hz should have very little tilt, 20 Hz maybe 10%.  Many audio devices are way worse. 

I am curious, when you replaced the tantalum caps with electrolytics, did you keep the same values?  What differences did you hear?
 

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on May 09, 2009, 01:53:12 PM
I am curious, when you replaced the tantalum caps with electrolytics, did you keep the same values?  What differences did you hear?

Thanks for the thorough explanations, Steve. 

On the BA283 card, I swapped these tants out and replaced with electrolytics: increasing C1 and C8 from 10uf to 22uf, and C14 and C15 I kept at 22uf on one card and increased them to 33uf on another, depending what I had.  Both sounded clearer, more open, particularly the top end.  I used the polarized cheaper Black Gates on one card and Panasonic FCs on other.  I also increased off-board caps C5 from 680 to 4700uf and C3 from 470 to 1000uf.  Most of the remaining electrolytics I kept at original values or upped depending what I had.  I find they sound wonderful.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 09, 2009, 02:41:49 PM
ACMP73 gain switch position 6 mod:

Crazydoc,

Have you thought of using a low voltage photo optical sensor switch with flag arm paired with a miniature relay, used in many laser printers. Takes the mechanical idea to the next level. no parts come into physical contact.

Here are the results of my latest turd-polishing (more like grinding than polishing in my case.)

(http://home.att.net/~crazydoc/ACMP/optoswitch-1.jpg)

(http://home.att.net/~crazydoc/ACMP/optoswitch-2.jpg)

(http://home.att.net/~crazydoc/ACMP/optoswitch-3.jpg)

Parts were a slot interrupter, reed relay, resistor, scrap of drywall corner bead, scrap of perfboard, a few machine screw and nuts, and a piece of cardboard - less than $4. And I borrowed about 30ma of current from the +12v supply.

Again, the hard part was drilling and tapping the switch shaft. This time I broke the bit off inside the shaft, and had to drill from the side to get it out.

(http://home.att.net/~crazydoc/ACMP/switchshaft.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 10, 2009, 11:39:19 AM
Let me qualify my comments below with the following general observation:
...The build quality of the switches and other mechanical parts limit how far one should go and how much one should spend to be reasonable and not be "polishing a turd" to diamond brilliance...

Steve

We all greatly appreciate the time and trouble you've gone to to improve these, and the helpful and instructive explanations you've provided. It's been a great learning experience.

However, as you've alluded, I think you may be starting to get into the area of diminishing returns on improvements for time spent, particularly from a fiscal standpoint. I hate to see you investing much more time and effort into these. With your current improvements we should have fairly robust, long lasting units with pleasing sonic characteristics.

The question still remains as to the influence of the new power transformer on the inductor hum. After the eq transistor oscillation, this to me was the most offensive characteristic of these preamps, making the eq section unusable at low signal levels. I guess you haven't gotten to the point of testing this yet, but I look forward (with hope) to hearing your findings.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 10, 2009, 12:16:05 PM
ACMP73 gain switch position 6 mod:

Here are the results of my latest turd-polishing (more like grinding than polishing in my case.)

Parts were a slot interrupter, reed relay, resistor, scrap of drywall corner bead, scrap of perfboard, a few machine screw and nuts, and a piece of cardboard - less than $4. And I borrowed about 30ma of current from the +12v supply.

Again, the hard part was drilling and tapping the switch shaft. This time I broke the bit off inside the shaft, and had to drill from the side to get it out.


Peter,

Your ingenuity continues to impress me. I salute your solution. Too bad it's not very easily implemented in quantity.

Steve
 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 10, 2009, 01:19:18 PM
Steve

We all greatly appreciate the time and trouble you've gone to to improve these, and the helpful and instructive explanations you've provided. It's been a great learning experience.

However, as you've alluded, I think you may be starting to get into the area of diminishing returns on improvements for time spent, particularly from a fiscal standpoint. I hate to see you investing much more time and effort into these. With your current improvements we should have fairly robust, long lasting units with pleasing sonic characteristics.

The question still remains as to the influence of the new power transformer on the inductor hum. After the eq transistor oscillation, this to me was the most offensive characteristic of these preamps, making the eq section unusable at low signal levels. I guess you haven't gotten to the point of testing this yet, but I look forward (with hope) to hearing your findings.

One of my regular clients has compared me to a bulldog in the way I am tenacious in finding the best practical solutions to optimizing audio gear. Even when the hum is fixed, the white noise level may prove to be higher than necessary with a few well-placed better transistors and reworked, lower impedance pads.  I am seriously considering adding the 12 BC560 transistors to the '81 list of mods.  In for a penny in for a pound. I already have to remove and replace the cards anyway.

I have spent some time with LTSpice and some measurements looking at the possibility of raising the input impedance of the first stage, and by lowering the gain 1 dB on that stage (105 Ohms instead of 91) the input Z goes from 3700 to 3950 Ohms.  The gain can be adjusted up on the 2nd stage by lowering the 120 Ohm gain set resistor to 105 as well.  Overall gain stays the same, but it is distributed 29dB+ 29dB instead of 30 dB + 28 dB.

It is important to realize that these gain resistors (and thus the amplifier gains) are switched in the original Neve front end, so the ACMP clones are simplifying the switching and the gain structure.  Unless one starts all over again with the 22 position gain switch of a real Neve, which is not a practical option here, I believe that by carefully reworking and balancing the gains of the first and second stage, I may be able to get better performance and still maintain the character of the original.   The three transistor Gain stage used in the preamp stage has multiple nested feedback loops, both AC and DC coupled.  I have modeled the frequency response curves as I have played with various capacitor values and I can see how they interact in a synergistic way.  At the present time, I have finalized making the output caps 120uF/25V which is the biggest cap that will fit in the original space.  This appears to have only positive benefits.  Changing some other cap values appears to not be such a good idea, due to the complex interaction of the feedback loops making bumps in the frequency and phase response, especially at the low end, where the capacitive reactance comes into play.

By the way, I keep finding errors in the schematics. A careful comparison of the 73 and 84 schematics show differences that are not real in the output section of the '84 preamp. They both use the same card and values. The '84 schematic doesn't accurately reflect how the clip light circuit is fed thru 2C30.  Note the duplication of component designations on the '84 schematic. I haven't figured that schematic out, completely, but it's wrong with regards to the preamp card.  I'm up to at least a dozen errors so far, including the fact that the schematic shows 0.1 uF bypass caps on the TL072 opamp on the preamp cards, and they are not there in any of the preamp cards.

For those who wish to disable that particular clip light, just remove the coupling cap 1C24 on the 81 and 2C30 on the 73/84 preamp card.  This simply prevents the audio from reaching the clip light circuit which happily monitors nothing.
For those who wish to keep the clip light, add two 0.1uF caps to the back of the card in the appropriate locations (I will have pics later), Replace the 10K resistor (1R62 in the 81, 2R11 in the 73/84) with 100K to lighten the load on the second stage output, which already has 51K and 5K in parallel.
 
Back to the low first preamp stage input Z.  In addition to the overall gain changing the input z, it appears that the biggest change is caused by the value of one of the internal feedback loop resistors.  15K in the first stage (1R14) and 18K in the second stage (2R14 in 73/84; 1R27 in '81).  This resistor feeds an out of phase signal into the input of the amplifier, thus providing negative feedback to the amplifier. Changing this resistor doesn't change the overall gain, just an internal feedback path.  Because it partially "cancels out" the signal by adding an opposite polarity replica to the incoming signal, it simultaneously lowers the input Z by a lot.  The difference between 18K and 15K is over 1K difference in input Z between the first and second stage with the same gain!  I'm still trying to find the best balance of values here.

In my reworked low-gain mic pads I have attempted to scale the resistor values down to the minimum necessary to simultaneously load the  mic tranformer with the right load and output the right voltage to the second preamp stage.  This means less noise generated by the pads.  It might even make sense to use a lower noise transistor in place of the first BC184C in order to make the preamps as low noise as possible without changing their sonic character.  Something like an MPSA18 might work even better than the BC184C in that first transistor position.

And last but not least, in answer to your original query, I believe I will have some production transformers later this week, so I can give better answers to what needs to be done to the inductors with regards to shielding, etc. soon.

More Later . . .
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 10, 2009, 02:59:24 PM

Peter,

Your ingenuity continues to impress me. I salute your solution. Too bad it's not very easily implemented in quantity.

Steve
 

Thanks, Steve. This would be pretty easy to implement, if it weren't for having to couple to the switch rotation. Too bad there's not a glue I could use with confidence to fix the head of a machine screw or other extender to the end of the switch shaft.



Back to the low first preamp stage input Z.  In addition to the overall gain changing the input z, it appears that the biggest change is caused by the value of one of the internal feedback loop resistors.  15K in the first stage (1R14) and 18K in the second stage (2R14 in 73/84; 1R27 in '81).  This resistor feeds an out of phase signal into the input of the amplifier, thus providing negative feedback to the amplifier. Changing this resistor doesn't change the overall gain, just an internal feedback path.  Because it partially "cancels out" the signal by adding an opposite polarity replica to the incoming signal, it simultaneously lowers the input Z by a lot.  The difference between 18K and 15K is over 1K difference in input Z between the first and second stage with the same gain!  I'm still trying to find the best balance of values here.

In my reworked low-gain mic pads I have attempted to scale the resistor values down to the minimum necessary to simultaneously load the  mic tranformer with the right load and output the right voltage to the second preamp stage.  This means less noise generated by the pads.  It might even make sense to use a lower noise transistor in place of the first BC184C in order to make the preamps as low noise as possible without changing their sonic character.  Something like an MPSA18 might work even better than the BC184C in that first transistor position.


I have not been happy with the noise in the preamp at high gain. I bought 100 each of the MPSA18, BC550 and more BC184C's, thinking I could hand select lowest noise transistors for the first preamp stage. I tried 3 or 4 of each, without any notable difference when listening, so I gave up on it. Do you think this or any other mod might be worth pursuing to lower the noise?
 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Steve Hogan on May 10, 2009, 05:49:15 PM

I have not been happy with the noise in the preamp at high gain. I bought 100 each of the MPSA18, BC550 and more BC184C's, thinking I could hand select lowest noise transistors for the first preamp stage. I tried 3 or 4 of each, without any notable difference when listening, so I gave up on it. Do you think this or any other mod might be worth pursuing to lower the noise?


You are good! You may have saved me some trouble if you already tried the MPSA18 -- that's my favorite low-noise, high gain transistor and I have used it successfully to seriously improve German mic preamps especially.  It is unlikely that more selection from the units you had would give you lower noise.

Microphone preamp noise. . .  that's a big subject.  If you don't want to read technical, I will summarize that the Neve gain structure and consequently the clone ACMP preamps are not optimized for lowest noise, and there really isn't a lot that can be done about it. In the following tech discussion I will try to be accurate but not mathematical, so forgive me if I simplify a bit to try to give the general idea.

The goal is to amplify a 50 to 200 Ohm microphone with as little added noise as possible. I will use 150 Ohms in the following discussion.

There is an inherent noise generated by any given resistance that is a function of its value and absolute temperature.
Low-noise (like wirewound) resistors generate a noise very close to theoretical. Noisy resistors have "excess noise" which adds to the theoretical.

E.I.N. (Equivalent Input Noise) is usually how mic preamps are specified because of their adjustable gain. It allows one to compare different preamps with different gains by giving a number that represents the equivalent noise at the input of the preamp that will increase by the gain of the preamp.

One must always specify the bandwidth of the noise or one cannot make meaningful comparisons. A preamp that is 3 dB down (signal wise) at 20 kHz might measure better than my 990 Jensen Twin Servo mic preamp which has just under 200 kHz bandwidth.  There is way more noise power between 20 kHz and 200 kHz than in the entire 0-20kHz.
to make meaningful audio band noise measurements an accurate brickwall 20 kHz low-pass filter should be used or the measurements are bogus.  The 990 will measure way worse, and yet be much quieter to the ear without the 20 kHz filter.
 
If a preamp has -132 dBu 20 kHz equivalent input noise, then the measured noise level at the output of the preamp is the equivalent input noise + the gain of the preamp.  If I have 60 dB of gain I would measure -132 dBu E.I.N.+ 60 dB gain = -72 dBu measured noise at the output of the preamp.  In reality one measures it backwards.  Measure the noise and subtract the gain to get the E.I.N. By the way, -132 dBu E.I.N. is spectacularly good -- almost theoretical from a 150 Ohm source.  When measuring noise in a mic preamp one must always resistively terminate the input with the same impedance as the microphone.  I use a 150 Ohm resistor between pin 2 and 3 to measure noise.  Shorting the input is cheating and will give an unrealistically low number.  Unterminated inputs can give you very audible hiss -- as much as 10 or 20 dB worse noise than a terminated input because the resulting high impedance at the amplifier input is far away from the impedance "sweet spot" that gives the amplifier its lowest NF.

Every amplifier has a Noise Figure (NF). This is how much noise the amplifier adds to the signal it is amplifying compared to the signal's own noise theoretically made bigger with no added noise.  The Noise Figure numbers are in the transistor data sheets we discussed a few posts back.  For example, let's say that the equivalent input noise of a 150 Ohm microphone is -132dBu.  If we amplify the microphone 20 dB with a noiseless theoretical amplifier, the signal from the microphone increases 20 dB and the noise goes up 20 dB as well.  No noise is added by the amplifier circuitry, so the signal to noise ratio remains unchanged.  However, let's say we use a mic preamp that has noise figure of 3dB (that's really good, by the way).  The signal goes up 20 dB but the noise goes up 20 dB+3 dB = 23 dB due to the Noise Figure of the amplifier. The signal to noise ratio got worse by 3 dB in this preamp.

Increasing Noise Figure in mic preamps comes from two main sources, Voltage losses and electronic noise.  The DC resistances in the microphone transformer are losses.  The signal doesn't go up as much as it would if there were no losses, but the impedance does go up, which makes more noise.

All amplifiers, whether discrete or IC opamps, have a noise figure that is related to the impedance that the amplifier is working with.  There are two kinds of noise generated by active circuits.  Voltage noise is the noise the circuit makes at very low impedances (shorted input). Current noise is the result of noisy currents being pulled through high value resistances and is the noise that predominates in high impedance circuits.  "Low noise" opamps have generally low voltage noise, so they may be very noisy in a high impedance circuit.  You would not want to use an AD797 super low noise opamp, for example, to DI interface to a high impedance passive guitar pickup, because it would be horribly noisy.

A 990 or an AD797 or other sub 1 nanovolt/rootHz voltage noise amplifier will have a spectacularly low noise figure when used in low impedance circuits, but have terrible noise when used in high impedance circuits.  Every transistor and every amplifier circuit has an impedance "sweet spot" where the transistor or amplifier contributes the least amount of noise (has the lowest NF).  The trick to making a low noise mic preamp is to make that 150 Ohm microphone look like the "sweet spot" lowest noise impedance to the amplifer.  To do that, one must either design an amplifier with a 150 Ohm "sweet spot", or use a transformer to step up the 50 to 200 Ohm microphone to the higher impedance "sweet spot" of the amplifier at hand.

The low ratio transformer used (1:2) or (1:4 in high gain mode) makes a 150 Ohm microphone look like 600 Ohms or (2400 Ohms) to the preamp input.  An optimum amplifier for a 1:2 ratio transformer would need to be very low voltage noise -- like a 990 or AD797 in order to have a decent noise figure.  The Neve transistor amplifier will have lowest NF at a much higher impedance.  A higher ratio transformer would have been a better noise match to the amplifier, all other things being equal.  (They weren't equal, though, because the bandwidth was better on the low ratio transformer so noise was traded away for other sonic benefits).

The heavy loading on the transformer secondary also is bad for noise.  For best signal to noise ratio, a microphone should be lightly loaded with a bridging impedance.  150 Ohm mics should be loaded with about 1500 Ohms. This is because we want maximum voltage output from the microphone not maximum power transfer which one obtains from a terminated source.  There is a 3 dB noise improvement between terminating a source (150 Ohm source/150 Ohm Load) and bridging the load.  Terminating the source drops both the noise and the signal 6dB, unterminated, the signal goes up 6 dB but the noise only increases 3 dB.  Rule of thumb, don't terminate microphones if you are trying for lowest noise.

The heavily terminated transformer loads down the microphone so the result is less signal at the secondary of the tranformer than the impedance would dictate if there were no losses.  Noise goes up with the impedance which includes resistive losses, but the signal doesn't go up as much, so there is a loss of signal to noise ratio and an increase in NF for the preamp (which includes the transformer).  By the way, Jensen transformer data sheets have the Noise Figure of the tranformer included.  This is a measure of the the extra noise that results due to resistive losses in the transformer, compared to a theoretical lossless transformer with Zero Ohms resistance in its windings.  Because of the resistances in the windings of a real transformr, the output impedance of the transformer goes up faster than the signal compared to a theoretically perfect transformer.

Transformerless designs btw are really bad because the noise generated by the feedback resistor network is part of the noise contribution.  That means that for the feedback network to not contribute significant noise, it must be much smaller than 150 Ohms.  The amplifier must be capable of driving a sub 150 Ohm feedback network.   Real transformerless mic preamps usually only have low equivalent input noise at maximum gain, because that's the only gain setting where the feedback resistor network has low impedance. Their E.I.N. gets really bad at low gains, but in that situation the microphone output is really hot, so it doesn't matter so much.

Because of the transformer's resistive losses, and because we do not want to load down the microphone, it will advantage the noise to make the transformer secondary load as high as possible.  The Jensen JT-16-A transformer (which was originally developed as a Neve replacement) is a 1:2 ratio transformer and its optimum load is 6.19K. That's one of the reasons to work on raising the input Z of the first stage amplifier in the ACMP preamps.

Because the first stage has a fixed gain of 30 dB, whatever noise it adds to the signal will be there, and the signal to noise ratio will decline permanently as it goes through the signal path.  That's why it's usually a lower noise approach to make the amplifiers variable gain instead of fixed gain.  The Neve preamps don't lend themselves to that.

I know that Avedis has done remarkable work in his "Neve" colored preamp.  He has reworked the gain structure significantly to fix both the microphone loading problem, the transformer loading problem, and the noise problem.
None of the clones do that.  They just copied what Neve did, both the good and the bad.

Well this post is already ridiculously long.  Hope it is helpful to clarify why designing really good audio gear is more complicated than it may first appear.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 10, 2009, 06:31:39 PM

Has the topic of transistor types been brought up yet ?

The BC184 NPNs are there, but for the BC214 (PNP) the BC557 is substituted, which per datasheet has poorer noise performance. Might not matter that much, but since it has been reported that engaging the EQ increases the noisefloor one might be better safe than sorry.

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

After studying the data sheets on the BC214 and the BC557 et al, it appears that the BC557 is what Horowitz and Hill call a "jelly-bean" part.


Hi Steve,

Thanks for your response.  I fully agree with the assumption that these BJTs shouldn't be that relevant anymore since they're after the 'real' preamp, but since people have reported (white) noise increase by engaging the EQ, I got the thought that it might be possible to improve upon that a bit by swapping the BC557-PNPs for better ones.

But who knows, maybe on original Neve units it might be the same behaviour, despite the original types. It'd surprise me though, since I just like you would like to keep living under the assumption that in an amplifier the first pre-amp stage makes lots of gain & sets noise figure and the next stages must then mess things up very badly if they want to noticably spoil that noise-performance.


For circuits where noise might be an issue and BC547/557 etc are specified I always switch to using BC550/560, they're dirt cheap & while it may not be noticable in the end still better be safe than sorry.

Or put another way, and while being too lazy now to get the datasheets out, I can't easily imagine a situation where '550/'560 would do worse in any respect than '546/'547/...'557/'558 (given we don't need for instance the higher voltage rating of the '546 etc)

Said yet another way, I figure/assume '550/'560 are simply the low-noise selection from the "jelly-bean" production, just like '109 was for '107/'108.


So yep, using the BC560 (with the correct hFE-range) for these preamps has crossed my mind, but I'm 'afraid' other reasons have mingled  ;)   Likewise for the 'power-devices' in the amp-stages: most likely BD139/140 (with proper hFE range) could have been used as well but 'cos the original types could be had we went for those.

Likewise for the BC184 but the Chinese people have already used those. Obviously '550 would work fine there as well etc.

As far as I understood the 're-creations from for instance Joe Malone & Great River don't use the 'original types' but without any noticable effect (IIRC the latter uses MPSA-something).
But since most DIY-choices will often be based on a combination of (1) technical facts & (2) some rational or non-rational preferences despite knowing otherwise  ;) & also a bit on (3) 'just because', one might end up with using '214 i.s.o. a better type.
I realize of course that the three mentioned reasons above are less suited for an upgrade-kit, there the facts are to be weighted the most.

Quote from: Steve Hogan
I count 12 pcs of BC557 being used in place of the BC214 in the ACMP 81.

I suppose that I can add it to the list of potential mods.  For the DIY person ordering parts kits, the cost will be negligible, maybe $2 or $3.  But the labor time keeps rising for those who are having me do the mods, and that causes me concern.

Yep, 12. Only changing them here 'because we can' and those boards need to get out anyway. Can imagine it's less attractive for 'ordered mods'. It could perhaps be economized by figuring out which swaps are relevant and which not, but that would require some more checking, but then on a single unit (or by sim). 

Quote
I haven't done any work on the output stage yet -- still working my way thru the block diagram, so I have no input at this time on 2N3055 transistor types and their effect on performance.

This choice might probably be one that mainly goes per (2) & (3) of the tastes of rationale mentioned above. But on a good day, (2) might be based on listening tests (let's all promise we do these then under condition of an adjusted bias-trimmer  8) - otherwise a proper comparison will become pretty wild).

Finding reasons for a '3055-choice based on (1) seem difficult, anybody knows a few ? 

Best regards,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 11, 2009, 02:40:46 PM
Steve

Thanks for the great post on noise, though I'm going to have to chew on it for awhile (even without the math.)

Though it doesn't look like much can be done for the noise as the circuit stands, do you think it would be possible to build an alternative low noise first stage of the preamp to substitute for the original? It looks like it would be easy to break into the traces at C1 and C3 to insert a different preamp stage.

I've been thinking of doing this for a while now, possibly using an IC op amp such as the TI OPA211.

http://focus.ti.com/lit/ds/symlink/opa211a.pdf

I'm sure there are impedance/noise considerations I'm not aware of, as you outlined in your post above.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Rader Ranch on May 14, 2009, 07:52:44 PM

Well, I recieved 4 84s instead of 81s and I've been trying to get him to send me the right pres since January. He can't even pull that off...I realize that this is somewhat of a blessing in disguise, as my '84s are at least somewhat useable out of the box.  But i would love that extra eq on the 81s. Anyone wanna trade?

Don't know if the messages here are working for me, but I'd be into this if you haven't had a taker yet. I only have 2 81's though, not 4.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Tasa on May 16, 2009, 05:02:25 PM
Great job all!!!(http://)

It has been hard work on these KungFu ChNeves!
I just hope that the end will result in something usable.
I feel that all is going that way.... ;)

BIG THANX to all of you....so many smart people working on this project.
I am a bit embarrassed cause my electronics knowledge is not zero, lets say 1.5 (out of 10), compared to all of you.
I will share my experiance in hope we can all get this to where it should be.
I am sure that I will learn so much from all of you.

I have 2 81's (bought from one of the members), on first look and some testing....one is with bad (wrinkled) resistors, like some of you have and some more, bent IC (6U2) pins on bass pcb, big pop on switch, a lot of noise, hiss and hum with or without EQ (toroid rotation does not help at all). Some Q switches and phase are hard to press out (from what I can see, too much paint in holes and also hole angle might be a problem).This one will stay off until fixed.
The other one has all good resistors, a very small pop (its more like small click than pop) on the gain switch and it has very little noise, hiss and hum. No problems with Q and phase switch. Not bad at all compared to bad one.
What is wierd about "good" one is that level is too hot on Mic.
Bass(NS Stick-active) - Radial J48 MK2 DI(pad in) - MIC IN(+48V ON) - out to line in RME FF400(unity gain)....
81 Input Gain on 6 o'clock, output gain on 8 o'clock and it clips and distorts with or w/o EQ!! My mistake? Impedance mismatch?? or an 81 issue???
When set not to distort (lower In and Out gain)....sounds "nice" as it can be for now at that level...EQ is usable on this one with some minor problems...
...same connection but through Focusrite Red Pre...all works good, nice levels.

I did test Line in....and levels are way low than they should...
I will test more and give some more details with photos of what looks like "very good" QC.

Maybe I missed it, but can anybody tell me info about current consumption per winding for ACMP81?

For those looking for:
AA144
http://www.banzaieffects.com/AA144-pr-16438.html
BAX13
http://www.cricklewoodelectronics.com/Cricklewood/product.php?printable=Y&productid=17260&cat=275&page=6&js=n

Any US source?

Thanx for all help.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 16, 2009, 05:26:12 PM
What is wierd about "good" one is that level is too hot on Mic.
Bass(NS Stick-active) - Radial J48 MK2 DI(pad in) - MIC IN(+48V ON) - out to line in RME FF400(unity gain)....
81 Input Gain on 6 o'clock, output gain on 8 o'clock and it clips and distorts with or w/o EQ!! My mistake? Impedance mismatch?? or an 81 issue???
When set not to distort (lower In and Out gain)....sounds "nice" as it can be for now at that level...EQ is usable on this one with some minor problems...
...same connection but through Focusrite Red Pre...all works good, nice level

Remember that oscillation will reduce headroom, although there may be further issues too.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 20, 2009, 12:32:17 PM
Maybe I missed it, but can anybody tell me info about current consumption per winding for ACMP81?

Here's what's on the schematic. I've never measured them, so don't know how real the numbers are.

15v (x2 - actually 30v ct) - 100ma
26v (x2)  - 200 ma each
45v - 20ma

http://img.photobucket.com/albums/v646/red_3000/how_to/ACMP81_lout-power.gif
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on May 21, 2009, 04:26:47 PM
Anyone heard for Steve lately?  I sent him an email several days ago and have not heard back.  Assuming he's just busy.  Thanks.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on May 21, 2009, 06:27:26 PM
i know steve and others have given lots of input in the last few months, before i start to have a look at the last 10 thread-pages: has anyone already posted kind of a summary for the easy-to-do fixes (bc441/461 change,...) for the acmp-81?

are there any known issues with the acmp-84?

best regards,
flo
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 21, 2009, 06:55:50 PM
i know steve and others have given lots of input in the last few months, before i start to have a look at the last 10 thread-pages: has anyone already posted kind of a summary for the easy-to-do fixes (bc441/461 change,...) for the acmp-81?

Hi,

Imho:
Be a man, just read them  ;) Lot's of good info, it'll be hard to avoid learning various interesting stuff about these preamps.
Following a recipe you're asking for would not do that, you'll just be replacing which is not what DIY could be.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: floxe on May 21, 2009, 07:06:23 PM
Quote
Following a recipe you're asking for would not do that, you'll just be replacing which is not what DIY could be.

hehe - when i bought the acmp's i didn't do it for DIY-purposes, i expected to get at least useable units. anyways - i don't have much time to "repair" the units but will try to get through the information given here.

thanks,
flo
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 21, 2009, 07:20:58 PM
Hi,

I understand. Despite everybody ordering these should have known that there would be at least some work to do (let's be honest, no free lunches); the amount of 'repairs' is sure more than anticipated. Hey, like always, it's time or money. So if in a hurry or not interested in DIY and/or learning (free choice, note I'm not blaming you, just go away here  8) - kidding) the obvious alternatives are sending your work to Steve or anybody else who's qualified or buy the real deal or something else that comes close. Yep, that's less time, but a lot more money...

Bye & enjoy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on May 21, 2009, 08:16:48 PM
In the beginning I had planed on doing some mods to these.....just not so many.  Oh well, time to get my feet wet.  LOL

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 22, 2009, 01:06:42 AM
i know steve and others have given lots of input in the last few months, before i start to have a look at the last 10 thread-pages: has anyone already posted kind of a summary for the easy-to-do fixes (bc441/461 change,...) for the acmp-81?

are there any known issues with the acmp-84?

best regards,
flo

Here's a very brief overview:

73 and 84:
- New (non-resonant) eq inductors, or mumetal cans/shields for the current ones
- Rework the gain switch to remove the oscillation blast between positions 6 and 7

81:
- New eq inductors as above
- Gain switch mod as above
- Replace the Q4 and Q5 transistors on boards 2, 3, 4, and 5 which are prone to oscillate, making the entire eq section unusable.

These fixes will probably get them to the level we expected when ordering them. A better/less-noisy PSU transformer would be helpful too.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 25, 2009, 06:58:35 AM
I have modded mine fairly extensively now. Took a few days. I don't think the inductor swap is maybe necessary for the '73 and '84 unless you have a particularly radiant power transformer. Mine are quiet.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 25, 2009, 07:50:37 AM

Do tell roddy , how are your's now  , what'd you do ?

i have two 73's one usable out of the box [ although i don't trust it ]
and the other with a slight broadband buzz [ haven't looked at it ]

Have to say this whole thing has soured me for any enthusiasm
to get it done ,
Still waiting on the Cinemag prototypes , they of course have allot
of business so told them it wasn't a panic



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 25, 2009, 08:35:22 AM
I modded mine in several stages. Firstly damping the transformers and testing. I then followed this up with a re-cap with tantalums and poly films (some with different values), some PSU mods (these weren't 100% necessary), some resistor value changes, swapped the output transistor, changed RF filtering, and finally changed audio transformers for Carnhills and re-routed the cabling a little.

The difference is noticeable, although the stock units do some pretty fine in my opinion, maybe just slightly rougher in the top-end. From what I have seen, the sound is in the circuit, as is so often the case. It's not just a combination of distortion from tantalum caps and specific transformers.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on May 25, 2009, 04:40:33 PM
Hey Roddy,
did you recap the whole thing? :o
I've used my 84s a couple of times. They are usable, but I too noticed them being a bit "rougher in the top" but haven't had time to do anything about it. Waiting for Steve to put out that list he was talking about so I can do everything at once.

Anyway, about this top end thing.. I've noticed some people (Tommypiper maybe) saying they like to replace the tants in Neve circuits with 'lytics... Is this a possible solution, or what solved this for you? 

How much of a difference would you say there was between the original trafos and carnhills?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on May 25, 2009, 06:10:24 PM
Here's a very brief overview:

Since people might read just sections, let's repeat this one as well
(it might actually be the most important one for gear of this level of (un-)reliability):

- be sure to update your fuse-ratings if you're in non-110V-mains area.

So for 230V-use switch to a lower value, start from say half the original mA rating. 

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 25, 2009, 06:35:20 PM
Hey Roddy,
did you recap the whole thing? :o

I only replaced what I thought was necessary. Maybe around 30 capacitors. Some are fine as they are in my opinion.

Quote
Anyway, about this top end thing.. I've noticed some people (Tommypiper maybe) saying they like to replace the tants in Neve circuits with 'lytics... Is this a possible solution, or what solved this for you?
 

Yep, that's possible. I used tantalums though. There are ok electrolytics there already, although some values are off.

Quote
How much of a difference would you say there was between the original trafos and carnhills?

Between the Chinese transformers and the Carnhills? Not massive, but they are measurably inferior, however this can be partly rectified by using suitable damping networks. The mic model and output model do have a high end resonance as stock. I could post pictures from my phone camera of 'scope readings if people were interested.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 27, 2009, 10:48:16 AM
Steve Hogan -

There is some discussion of the ACMP81 power supply here:
http://homerecording.com/bbs/showthread.php?t=275506&page=3

Any comments?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mpinphilly on May 31, 2009, 12:54:08 PM
Hello all:

I've been following this thread without joining--really appreciative of all the investigations you all are doing on these preamps--but I wanted now to ask a question...

I know that judging the usefulness or even quality of gear is tough to impossible, given that the application of a given tool is what matters, anything can be the "right" tool for a give source/song.  That said, I'm a seriously unknowledgeable person when it comes to the insides of gear and so I'm trying to get a sense of how "worth it" it is to send my two 81s and two 73s into Steve Hogan for his upgrades.  Steve has been super gracious and helpful in emails I've sent to him--explaining all the things he can do to upgrade these pres--but I still don't have an overall sense of whether or not to invest more dough in these or sell them off and put the entire cost of the the four towards one higher end pre.

I suppose my question is this simple: once Steve Hogan does his upgrades and I've invested around 500 bucks or so total in each of the pres, is that still a fairly sweet price for each of these channels?  I.e., once Steve rips out the crappy parts and adds better ones, is it likely that these pres will be nice additions to a project studio?  To give context for my pre selections, I have the Sytek 4-channel thing (with burr-brown on channels 3&4), a JoeMeek channel strip, and two channels of racked PM-1000s.  I was hoping the ACMPs would be a nice change of pace for vocals and drums, but I could get just one channel of a higher-end pre to use mainly on vocals..

Anyway--hope this is an appropriate question for this particular forum.  I know this is about investigating these pres, but I figured maybe there were other folks lurking here who, like me, always assumed we'd have someone mod these pres, but aren't clear how good they'd be post-mods.

thanks a bunch!
mp
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on May 31, 2009, 02:12:29 PM
if your's are useable as is right now ,
that's the answer try it an see

if not as some have mention
the sound is in the circuit as well as quality of parts
i don't know what compares in that price
maybe you want to pick two to start with
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: chris carter on May 31, 2009, 04:14:08 PM
Mpinphilly,

You aren't alone.  Like you, I've been following this thread for over a year  :o  I got two 81s and am coming to the conclusion that there will not be a TNC solution. 

As far as I know, nobody knows what these will sound like when they are fully modded by Steve.  I don't think Steve even knows yet because I don't think he's done the full mod to any yet.  The CL 7602 mkii is the closest comparison as it has much in common with the TNC preamps.  One would set you back a little over $700 new with a legitimate warranty.  For me, I really want the four bands, so I think I'd rather sink the money into these 81s I have.  It has been discussed, however, to offer more than one mod price point.

Speaking of which, Steve hasn't been around here lately....since the 10th I believe.... I'm hoping he's been in the lab finalizing the mods...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on May 31, 2009, 04:48:10 PM
I've been following this thread without joining--really appreciative of all the investigations you all are doing on these preamps--but I wanted now to ask a question...

Have you tried rotating the transformersinside the '73 preamps? They should be fairly usable as-is.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on May 31, 2009, 05:18:50 PM
Mpinphilly,

You aren't alone.  Like you, I've been following this thread for over a year  :o  I got two 81s and am coming to the conclusion that there will not be a TNC solution. 

As far as I know, nobody knows what these will sound like when they are fully modded by Steve.  I don't think Steve even knows yet because I don't think he's done the full mod to any yet.  The CL 7602 mkii is the closest comparison as it has much in common with the TNC preamps.  One would set you back a little over $700 new with a legitimate warranty.  For me, I really want the four bands, so I think I'd rather sink the money into these 81s I have.  It has been discussed, however, to offer more than one mod price point.

Speaking of which, Steve hasn't been around here lately....since the 10th I believe.... I'm hoping he's been in the lab finalizing the mods...
I got an email from Steve a few day ago.  He's been pretty busy but hoped to post this weekend.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on May 31, 2009, 05:29:59 PM
Hello all:

I've been following this thread without joining--really appreciative of all the investigations you all are doing on these preamps--but I wanted now to ask a question...

I know that judging the usefulness or even quality of gear is tough to impossible, given that the application of a given tool is what matters, anything can be the "right" tool for a give source/song.  That said, I'm a seriously unknowledgeable person when it comes to the insides of gear and so I'm trying to get a sense of how "worth it" it is to send my two 81s and two 73s into Steve Hogan for his upgrades.  Steve has been super gracious and helpful in emails I've sent to him--explaining all the things he can do to upgrade these pres--but I still don't have an overall sense of whether or not to invest more dough in these or sell them off and put the entire cost of the the four towards one higher end pre.

I suppose my question is this simple: once Steve Hogan does his upgrades and I've invested around 500 bucks or so total in each of the pres, is that still a fairly sweet price for each of these channels?  I.e., once Steve rips out the crappy parts and adds better ones, is it likely that these pres will be nice additions to a project studio?  To give context for my pre selections, I have the Sytek 4-channel thing (with burr-brown on channels 3&4), a JoeMeek channel strip, and two channels of racked PM-1000s.  I was hoping the ACMPs would be a nice change of pace for vocals and drums, but I could get just one channel of a higher-end pre to use mainly on vocals..

Anyway--hope this is an appropriate question for this particular forum.  I know this is about investigating these pres, but I figured maybe there were other folks lurking here who, like me, always assumed we'd have someone mod these pres, but aren't clear how good they'd be post-mods.

thanks a bunch!
mp
I'm doing a "Steve Hogan" mod to a 73 myself and have just hooked my two brothers up with a 73 each.  Their two units are being shipped to Steve for mods.  They will have a little more into their units than I but we're hoping that we'll end up with a great sounding pre.  It's really a crap shoot.  My 73 doesn't sound bad now, (I rotated the tranny) but it's still noisy.  Too much so for vocals and that's what I really wanted this pre for. 

One thing you need to consider though, once you're finished with the mods, what would this unit be worth if you had to sell it.  The TNC gear has gotten a bit of a black eye and to figure you'll have $500 - $600 in the unit, it may be a hard sell to get that out of it.  Just a thought.

Paul ;D

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on May 31, 2009, 06:29:00 PM
As far as I know, nobody knows what these will sound like when they are fully modded by Steve. 

I'm sure somebody will correct me if I'm wrong (and please do) but my take on all this is that any mods being done will not affect the "sound" of these pres to any appreciable degree.

To my mind, the changes to these can be divided into two categories:

1) Making them usable. This would entail getting rid of the inductor hum, the gain switch blast, and, on the 81, the noise derived form the transistor oscillation on the EQ boards. Whether these are best done by power transformer upgrade, inductor upgrade, shielding, transistor replacement or other, remains to be seen.

2) Making them last. This would entail upgrading some parts, mostly in the power supplies - in particular capacitor capacitance and voltage ratings.

As far as improving or changing the sound or coloration of the preamps, some of Steve's suggestions will probably have some minor beneficial effects, such as changing the values of coupling caps, reworking the gain switch resistors and removing the 2.2nF cap from the line input transformer (and possibly the cap on the mic input also.). But I think in general, the sound will be about the same before and after reworking.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mpinphilly on May 31, 2009, 10:20:32 PM
Thanks everyone for responding here.  I was assuming the closest thing to these pres would the CL Labs stuff, which I've never heard but heard great things about...

For me the eye-opening point is the resale of these things.  Given my set up and how rarely I track more than a couple of tracks at a time, it may be a good idea to not chase more cash after these.  I have utter faith in Steve Hogan and it's pretty amazing someone with his credentials is offering these mods...  So that's one of the things that actually makes it harder to let these go--someone like Steve Hogan might be modding them! 

I also appreciate the suggestion to rotate the transformers.  Also, it's good to hear that these may not sound any different post-mod, save for being less noisy, but mainly will be more reliable... 

Part of the problem is that my ethos here is so utterly non-DIY or Lab-esque.  I really want to understand electronics, but I find it to be rather abstract...  I just need to get one of those electronic kits that kids get and start there.  So, for example, I know what a transformer even is, which right now I only know is something that gear has inside it (or doesn't).

thanks again!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rattleyour on June 01, 2009, 01:47:35 AM
I facilitated the purchase of six 73s, but as I'm living abroad right now I haven't heard them yet. Only two actually belong to me in the long run.

Aside from one DOA unit, my studio partner, who is himself a professional tech and accomplished AE, has been using three of the 73s quite happily alongside a host of vintage and boutique pres. I believe those three 73s (maybe four if he fixed the DOA) are actually in service in one of San Francisco's nicer studios.

The other two went to another AE friend of mine, and he's been using and loving them as well.

So ultimately I'll have to decide for myself, but I find it encouraging that two of my AE friends, whose ears I trust, are happy with their stock 73s.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 07, 2009, 07:54:10 PM
Any news out there?

For anyone considering doing their own shielding, this looks like a suitable material, if used in layers, as it is thin.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: chris carter on June 07, 2009, 08:58:32 PM
I e-mailed Steve last week, but no response.  I hope he's still working on these because right now he's my only hope of turning these 81s into more than doorstops  :D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on June 07, 2009, 09:20:57 PM
I expected a post from him last weekend, (see my post above).  I hope all is well.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Jeff Chitouras on June 08, 2009, 11:13:51 AM
I received an email response from Steve this past weekend. He has been up to his eyeballs in finishing other work and expects to now be able to focus on the ACMP pre-amps he has received for upgrading.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on June 08, 2009, 12:55:57 PM
I was under the impression that Steve still has to check out the inductors.  This sounds like he's going to start modding units.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electric on June 12, 2009, 03:24:39 PM
Where are people in the USA sourcing their BC441 and BC461's?

Also - do they need to be BC441-6/BC461-6?

I See Newark.com has BC441/BC461 (without the "-6"). Are these sufficient?

http://www.newark.com/magnatec/bc461/_/dp/25M7693
http://www.newark.com/magnatec/bc441/_/dp/96K6848?_requestid=62823

Regards,
Electric
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on June 19, 2009, 12:00:41 PM
Anyone heard from Mr. Hogan in a while?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: kazper on June 19, 2009, 01:12:16 PM
Where are people in the USA sourcing their BC441 and BC461's?

Also - do they need to be BC441-6/BC461-6?

I See Newark.com has BC441/BC461 (without the "-6"). Are these sufficient?

http://www.newark.com/magnatec/bc461/_/dp/25M7693
http://www.newark.com/magnatec/bc441/_/dp/96K6848?_requestid=62823

Regards,
Electric

They will work, just lower gain rating for the lot.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 19, 2009, 07:26:12 PM
Quote
Anyone heard from Mr. Hogan in a while?

Nope. I hope the bad karma that seems to surround these pres hasn't got him.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on June 19, 2009, 08:24:30 PM
Anyone heard from Mr. Hogan in a while?
Not in several weeks.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 20, 2009, 01:10:08 AM
Cinemag inductors are now on their way to me
will let you know
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wscrane on June 20, 2009, 12:31:20 PM
I just got some 81s in my shop which have a severe hum problem with the EQ in, and was referred here.

As far as I can tell its the same as alexc described, and apparently fixed.  Alexc maybe you could confirm this ---  With the EQ out, the largest component is 180Hz  at -89dBV.  With the EQ in (and set to flat) the largest component is 120 Hz at -47dBV.  In both cases the output gain was 100%. 

Also, could you name your supplier for the Carnhills and mumetal cans?  Thanks.

Here's something I tried yesterday that didn't work, but its a data point...   One thing that bothered me about the circuit is that the output of each filter board has two grounds. Take for example the low presence board. One is 5R32 to the 24V ground.  The other is 5R36 to the 12V ground.  A difference in potential between these grounds will appear as signal. And I believe they arrive from separate long wires that are at opposite ends of a ribbon cable which forms a large loop right by the power transformer.  So... I lifted all the clip lite stuff off the signal path.  This includes 3R36, 4R32, 5R35, 6R35, and the two wire cable from board 6 to 7.  Funny thing is -- the hum got worse.  The 120Hz went to -39.5dBV.  So I think it is correct that the clip lite circuit is adding hum, but it turned out to be mostly antiphase and cancelling the main hum.     


   
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 20, 2009, 03:46:35 PM
I just got some 81s in my shop which have a severe hum problem with the EQ in, and was referred here.

As far as I can tell its the same as alexc described, and apparently fixed.  Alexc maybe you could confirm this ---  With the EQ out, the largest component is 180Hz  at -89dBV.  With the EQ in (and set to flat) the largest component is 120 Hz at -47dBV.  In both cases the output gain was 100%. 

Also, could you name your supplier for the Carnhills and mumetal cans?  Thanks.

Here's something I tried yesterday that didn't work, but its a data point...   One thing that bothered me about the circuit is that the output of each filter board has two grounds. Take for example the low presence board. One is 5R32 to the 24V ground.  The other is 5R36 to the 12V ground.  A difference in potential between these grounds will appear as signal. And I believe they arrive from separate long wires that are at opposite ends of a ribbon cable which forms a large loop right by the power transformer.  So... I lifted all the clip lite stuff off the signal path.  This includes 3R36, 4R32, 5R35, 6R35, and the two wire cable from board 6 to 7.  Funny thing is -- the hum got worse.  The 120Hz went to -39.5dBV.  So I think it is correct that the clip lite circuit is adding hum, but it turned out to be mostly antiphase and cancelling the main hum.     
 
Hi WS - welcome to the club (of which membership is unsought.  ;D  >:( )

The absolute first thing to do is replace the Q4 and Q5 transistors on boards 3 thru 6. These oscillate and cause the major EQ noise component. Various substitutes have worked - I used 2N2222 and 2N2907 because I had some on hand - read thru the thread to see alternatives.

After that you can fiddle with the inductor noise. Also, rotating the toroid has help some folks, to a minor degree.

Alternative grounding schemes have been tried with little to no success.

There's a wealth if info here, especially from Steve Hogan, who seems to have dropped out of sight.

Schematics are at http://recordinghacks.com/tnc-audio-preamps/



Any suggestions/fixes from your end would be welcome.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on June 20, 2009, 08:37:24 PM
wscrane,

As crazydoc says.

First replace the transistors - that will improve most of the problems. Maybe you can
live with it at this point. Remaining noise is in the low band, low frequencies, typically
at mains frequency and several harmonics.

If you can - replace the inductors with shielded ones. I used Carnhills from Audio Maintenance
and OEP shields from RS.

There's a fair bit of figuring the 'pin out' of the replacement inductor to the footprint
of the old one and mucking about with insulating and wiring and canning the Carnhills too.
I just wired and hot glued into place. This is probably not a very pro way to do it, so
maybe not suitable for someone else's unit. I put pics up earlier.

Cinemags are hopefully the best bet for rreplacements.

For sure, these 2 upgrades, almost exclusively, got rid of the excessive mains related noise, bringing it
to the level you would expect for a unit like this - which is pretty good!

Then, there is the loud pop on the gain switch. To workaround that, you can re-wire a resistor
and lose a switch position or some other more elaborate way to avoid it. I tend to remember to turn the gain all the way down if I'm changing the switch position and live with it.

---

My modded pair have been seeing use most nights as my lounge room guitar pre's. No probs to date
and they sound great. Teamed up with a dual g1176, and that's as good as I'm ever likely to hear!
I then send that combo to a Finalizer for recording and a tube hifi amp and speakers for monitoring.
The immediacy and depth of the sound is quite amazing.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wscrane on June 21, 2009, 12:59:11 PM
First replace the transistors - that will improve most of the problems. Maybe you can
live with it at this point. Remaining noise is in the low band, low frequencies, typically
at mains frequency and several harmonics.

Alexc, Thanks for the info.  I'm dubious about the transistor swap.  Could you be more specific about your particular experience?  Namely in what freq band is are the old transistors causing trouble and what kind of dB reduction did the swap accomplish?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 21, 2009, 04:41:28 PM
the transistors cause an oscilation which
shows up as a broadband buzz / hum
definitely helps  , then you can see that the
hum left is more mains centered and you can use the eq if you
avoided those frequencies
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 21, 2009, 04:45:42 PM
The fundamental oscillation is at about 70MHz. Put your scope on it.

(http://home.att.net/~crazydoc/ACMP/ACMP81-oscillation.jpg)

I'd guess eliminating this improves the noise by 20 to 30 dB, and makes the preamp usable with the EQ engaged.

Alexc may have some data to back this up.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on June 21, 2009, 07:47:42 PM
Once again, as crazydoc and okgb say...

The acmp-81 transistor change fixes the broadband noise present when EQ is engaged.
Until that is fixed, the EQ is entirely and completely unusable.

Anyway - I wrote extensively about this earlier in thread. Plenty to wade thru!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wscrane on June 22, 2009, 11:55:04 AM
In the ACMP-81s I have here, the largest hum component is 120Hz but harmonics go out to the tens of kHz.  With the EQ in and set flat and the output gain at 100%, the main component is about  -47dBV at 120Hz.  At 1200Hz its down to -70dBV.  To me, it looks like normal diode shutoff spikes. My analyzer shows roughly the same thing tarnationsauce shared in reply #170 labelled ACMP-81 (stock transistors.)  From Alexc's numbers I thought it was the same thing he was reporting.  Is this the problem we're calling "RF induced broad band buzz”, or is it not?

crazydoc, if I may offer an opinion about your scope trace.  You can measure RF in any circuit, especially ones with coils.  That doesn’t necessarily mean its oscillating.  If you’re in the US, 70MHz is broadcast TV channel 4.  But let’s suppose you’re right, that there is an oscillation, and that it is somehow being generated by the eight transistors you are pulling.  By what possible mechanism could low level RF at a single frequency have a broad-band audio effect?  And why would it look exactly like diode shutoff spikes?

alexc, I read the whole thread, and many of the links to other boards.  Maybe I'm dense but its mostly just led to questions, some of which I'm trying to get answers to right now.   
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 22, 2009, 12:55:12 PM
crazydoc, if I may offer an opinion about your scope trace.  You can measure RF in any circuit, especially ones with coils.  That doesn’t necessarily mean its oscillating.  If you’re in the US, 70MHz is broadcast TV channel 4.  But let’s suppose you’re right, that there is an oscillation, and that it is somehow being generated by the eight transistors you are pulling.  By what possible mechanism could low level RF at a single frequency have a broad-band audio effect?  And why would it look exactly like diode shutoff spikes?
ws

I don't know the answers to your questions. I do know that replacing those transistors eliminated the 70MHz signal on the EQ boards, and the major EQ noise component at the output. At full gain all that was left was the usual electronic hiss, and some hum from the inductors at certain settings, which was subsequently eliminated by shielding the toroid and the inductors.

These units are obviously infused by some evil spirits. All I can say is replace the transistors (not a time consuming or expensive proposition) and see for yourself.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Knarleybass on June 22, 2009, 03:14:48 PM
is there an "idiots guide" to replacing those transistors out there?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on June 22, 2009, 04:15:49 PM
I think that what we were counting on from Steve Hogan.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on June 22, 2009, 07:31:24 PM
Hi wscrane

I don't know about diode spikes.

What I did see, using the spectrum analyzer was :

EQ Out  essentially flat spectrum, some components around -90dBu at 50Hz, 100Hz, 150Hz
EQ In + All bands flat shows a lot of spectral components at around -55dBu approx from 50Hz to around 5KHz.

The 8 transistors in question are known to be the wrong type and were getting hot.
Using  the modification from member Zmix categorically removed this broad spectrum noise resulting in an
EQ In + all bands flat to be the same as EQ Out.

There could be a great many reasons for the behaviour and I can't offer an explaination beyond
"wrong part = bad. right part = good"
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 23, 2009, 11:44:43 AM
is there an "idiots guide" to replacing those transistors out there?
You can start with this thread over at hr.com:
http://homerecording.com/bbs/showthread.php?t=275506

You'll have to register to see some of the pics.


wscrane -

regarding moving wires around, see the last several pages of that thread. I guess it works for some, and not for others. I still believe to most substantial improvement is by changing transistors, and no other mod will make a significant difference until this is done, on the majority of ACMP 81's.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: wscrane on June 24, 2009, 12:44:39 PM
alexc,

Please excuse me for asking this again, but I still can't figure out if we are talking about the same problem... 

Tarnationsauce posted the output specturm of an ACMP-81 in reply #170.  It could be described as a hum or a buzz.  It is composed of a number of narrow peaks which are strictly at integer multiples of the line frequency and which trend lower in amplitude as the frequency goes up. The largest component is at twice the line frequency.  Is this the "broad spectrum noise" you are talking about?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 04:13:44 PM
Hello everybody,
i will have two amcp 81  in a few days.

I understood all about the modification for the transistor shield ... for the 70hz ossilation.

But for clip sound between the position 7 and 8 of the input gain i would have some explain about the resistor remplacement and link.

Here, http://www.groupdiy.com/index.php?topic=27791.msg381731#msg381731 (http://www.groupdiy.com/index.php?topic=27791.msg381731#msg381731)

if someone could explain me ?
thank you.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on June 24, 2009, 04:23:08 PM
Somehow the designers/manfacturers of this version forgot how to switch from gain stage to another
Original has off positions between those

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 04:26:56 PM
thank you MATTI,

But i would to do the same solution as 0dbfs but don't understand really how to do.his pictures don't help me.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on June 24, 2009, 04:43:34 PM
O sh*t I didn´t find the pictures ( from my "archives" ) of that but I remember seeing the solution here, you just lose a gain position doing it...
but you gain no pops

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 04:48:05 PM
fine, bcause the other method are to difficult fo me.

So i don't understand how to do it he talked about link between 43 and 42 and on the pitcures 43 and 44.

Perhaps will be more explicit when i will have the board in my hand.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: MATTI on June 24, 2009, 04:55:53 PM
I don´t have these preamps as I sold them before hitting Europe, so no specific help from me, sorry.

Matti
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on June 24, 2009, 05:08:41 PM
fine, bcause the other method are to difficult fo me.

So i don't understand how to do it he talked about link between 43 and 42 and on the pitcures 43 and 44.

Perhaps will be more explicit when i will have the board in my hand.

stereokillah (aka: mono-maker),

Take a look at the board and draw the changes on a copy of the schematic so you can follow it.

Here it is in ascii:

--------- SNIP ----------
      SP     SP
       |       |
      R44   R43
       |       |
       -------
       |     
      SP     SP
--------- SNIP ----------

I changed the value of R43 from 620 Ohms to 1k5 Ohms.
SP = Solder Pad
The lines are soldered connections.
Note that the original R43 connection to it's solder pad is NC (not-connected) on one side.

This turns that input gain position to an OFF position and prevents the momentary positive feedback path.

The reason for the pop at that position is a positive-feedback loop due to the switch being a MBB (make-before-break) type of switch. ie; when the second gainstage is switched into the chain and during the moment that both position 6&7 are active, the output of stage2 feeds input of stage1. Oops!

Check out the schematic and follow the signal flow for that state and you should see the loop.

Cheers,
jonathan
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 05:53:37 PM
Thank you very much for this help 0dbfs (aka:None saturation.)

and sorry but realy need to know if i weel understood ;D

HERE THE SWITCH
--------- SNIP ----------
      SP     SP
       |       |
      R44   R43
       |       |
       ------- and here the llink between 43 and 44
       |     
      SP     SP
--------- SNIP ---------
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on June 24, 2009, 06:17:45 PM
Thank you very much for this help 0dbfs (aka:None saturation.)

and sorry but realy need to know if i weel understood ;D

HERE THE SWITCH
--------- SNIP ----------
      SP     SP
       |       |
      R44   R43
       |       |
       ------- and here the llink between 43 and 44
       |     
      SP     SP
--------- SNIP ---------

Exactly!

And the bottom (away from the switch) R43 SP connection (originally connected) is now lifted in the mod with no connection.

NOTE: I think I just bent the R43 resistor leg over, soldered it to R44, and used heat-shrink around the R43 leg to prevent connection with the original R43 solder-pad. A little tight but workable.

Hope that clears it up.

Cheers,
j
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 06:36:48 PM
yes so much clear,

thank you for your time to  explain.
 
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on June 24, 2009, 09:18:45 PM
>Tarnationsauce posted the output specturm of an ACMP-81 in reply #170.  It could be described as a hum or a buzz. 
>It is composed of a number of narrow peaks which are strictly at integer multiples of the line frequency and which trend
>lower in amplitude as the frequency goes up. The largest component is at twice the line frequency.   of
>Is this the "broad spectrum noise" you are talking about?

Yep - around 30dB-ish of noise covering lowfreq to around 10K with EQ In (all bands flat) compared to EQ Out.

Change transistors and there will then be little difference between them.

Then you start looking at each of the EQ bands and you will encounter the next issue. Inductor noise
in both mid bands, but primarily bad in the mid-low band, particularly when boosting at harmonics
of main freq. In my opinion, there is no fix for this apart from shielding inductors or replacing with
shielded inductors.

Good luck.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 24, 2009, 09:47:01 PM
yes thank you Alexc i've readen all your post the 40 pages in fact.

but you would to say , even after changed the transistor the stay but  less after if i change the inductor to carnhill inductor the noise is reduced but stay again more than 80 db ?

I think to change the output transfo to carnhill 1148 but i don't the sepcification of the chineese one.

i'm a older newbie i lurn.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on June 24, 2009, 10:35:36 PM
Hey mono-maker,

My understanding from alexc's work is that using the carnhill mu-metal trafo-can you can basically eliminate the mains based inductor hum without replacing the original inductors themselves.

That means that at a minimum one would need three mod's to make these channelstrips useable with the full intended functionality:

- transistor swap.
- mu-metal can for the two low-mid eq bands.
- gain stage mod to prevent the pop if desired.

From there Steve has provided kits and/or services to upgrade other parts of the units which provide better signal path and longevity improvements to the unit.

Alexc, please correct me if I am wrong about the carnhill cans and original inductors.

Btw, I have carnhill cans ordered and on the way and will be able to confirm alexc's findings soon. All indications seem to point in the positive direction for that as far as I can tell right now. I don't expect to add any new information to the current pool of knowledge.

Cheers
J
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 24, 2009, 10:47:10 PM
I hate to beat a dead horse here, but for those with continued doubts about the EQ fix - FIRST CHANGE THE TRANSISTORS!

This fixes that horrible noise you hear when you switch on the EQ pushbutton switch, even though all the filters are switched off. I haven't heard of any other real fix for this problem.

You're not going to even hear the inductor noise (which is what you'll hear when you switch each EQ filter on and through its frequency settings) until this is done.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 24, 2009, 10:54:35 PM
BTW, Steve Hogan is alive and well - just buried in other work to keep his head above water. He says he will post this week with pics of the new transformer in place.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: alexc on June 24, 2009, 11:13:43 PM
>but you would to say , even after changed the transistor the stay but  less after if i change the inductor
>to carnhill inductor the noise is reduced but stay again more than 80 db

Stereokillah - Firstly, changing transistors removes the first 30dB or so of noise.
Then, when you engage mid-lo and max boost at 50Hz-300Hz, you will see an extra 22dB or so appear at mains-harmonics.
Replacing inductors reduced this by about 12dB or so.
Adding shields to the replacement inductors reduced it another 8db or so.
Similarly in the mid-hi band but to about 1/2 as bad overall to begin with.

These numbers are repeatable to around 3-5dB in general, depending on proximity of unit to other powered on units.
I note that your '80dB' may or may not be the same as my '80dB'.


>My understanding from alexc's work is that using the carnhill mu-metal trafo-can you can basically eliminate the
>mains based inductor hum without replacing the original inductors themselves.

0dBfs  - what you say is correct, except I use Carnhill inductors + OEP mu-metal transformer shielding cans
            and I would suggest 'reduce' rather than 'eliminate' when using stock inductors + shields.

I very strongly recommend replacing inductors with a shielded, higher quality part to get
the best performance with respect to the system mains-harmonics noise.

If that is not possible, then try shielding stock ones.

Crazydoc - I don't understand. Change the transistors, you say? Will that help? ;D

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 24, 2009, 11:25:16 PM

 Hey Alex [or anyone ] have the pin out handy for the 81 inductors ?
the cinemags arrived today , and when i get up tomorrow i'll look at
installing them  [ using the old cup or not  ]
they seem to have model numbers on them , that's promising
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 25, 2009, 10:07:10 AM
ok fine and soory i would to say -80 db

i 've oredered  BC327-16 and BC337-16 plus diodes bat86 like Steve Hogan cause i don't find BC441 and BC461 with -6 mentioned
will oredered carnhill .  vtb9048  .  vtb9047 and output transfo 1048
So i can't find too the shielded from oep on farnell web site.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 25, 2009, 10:21:16 AM
i find this can perhaps will be fine

http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720 (http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 25, 2009, 11:44:00 AM
i find this can perhaps will be fine

http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720 (http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720)
I see two possible problems. One is it is probably too large to fit on the board. The second is that it is made of fer-blanc électrolytique - electrolytic tinplate. This may not be the equivalent of mumetal in its RFI/EMI rejection.

This can from Farnell was suggested earlier in the thread, which is probably of more appropriate size and material:
http://uk.farnell.com/oep-oxford-electrical-products/a262can/screening-can-transformer/dp/1172345?crosssellid=1172345&crosssell=true
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 25, 2009, 12:26:53 PM
thank you Crazydoc, all RFI/EMI rejection in farnell france was in aluminium. i thought it was bad for this solution.
For sure  this are smaller
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 27, 2009, 10:09:28 PM
Cinemag inductors installed ,
seem inconclusive at the moment to stop the Mains related hum ,
so far without measurment gear just a mic in hand
i used cable ties for the moment and extended the leads

with all gain at zero switching eq in does increase hiss ,
the inductor bands seem to be a little noiser closer
to the lowest freq settings depending on gain  ,
[ but in a broadband way not hum ]
will try sheilding them next , this is just me with a 57 mic talking
into the unit . [ and in a room where you have to twist the mic
like a gtr pickup to get minmum noise  ]

I used the 2n2907 & 2222A transistors but they do seem to
get fairly warm , anybody been running those for a while  ?

(http://i83.photobucket.com/albums/j319/okgb/induc1001.jpg)
(http://i83.photobucket.com/albums/j319/okgb/induc1003.jpg)
(http://i83.photobucket.com/albums/j319/okgb/induc1005.jpg)
(http://i83.photobucket.com/albums/j319/okgb/induc1006.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 27, 2009, 10:21:43 PM
what is the method to change  the inductor to the carnhill .  vtb9048  .  vtb9047
is it the same link on the place, sorry i don't know how to say that in english.
i need to do some modification ?
Resistor or other

Thank you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 27, 2009, 10:34:41 PM
 take the old inductors out , make sure you match
the taps to the inductance of both
[ the chinese seems to start on the pin marked 2
on top of the white cup , use this as your lowest
starting tap of new inductor going clockwise looking
from the top ]

Anyone want to see the Cinemags check back
to the previous page . they don't have the mu metal
cases , anyone know a good / cheap source ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 28, 2009, 09:20:06 AM
Thank you okgb,
it was the night i did'nt see the number on the chinese inductor .
with you help i could put it easily.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on June 29, 2009, 07:02:21 AM
with all gain at zero switching eq in does increase hiss ,

Hi,

'Hiss' as in 'noise' or 'hum' ?

Apart from 'the really wrong ones' you already addressed, IIRC there were some transistors that could be changed to lower-noise, see somewhere earlier (...) in this thread. This shouldn't matter much with most gain already pre-EQ, but just in case...

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on June 29, 2009, 09:38:31 AM
What i am experiencing is a system type noise
more hiss and less hum unless i have the lo mid on the 220 hz
although there is an almost ground like hum that lessens if
i cut at the 220 and this goes away totally with eq switched out

this is consistent with either the Chinese inductors or the cinemag
which likely indicates shielding needed , if not something done about
the power xfmr  , It may also be unrealistic amount of gain but i guess
they won't get used for Narration .

Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: 0dbfs on June 29, 2009, 09:57:54 AM
I have found that shielding the original chineve inductors with the oep mu-metal cans reduces the mains hum to a useable level.

I have also experienced the hiss with the eq engaged. No specific fix for that yet that I am aware of although rotating and nulling the toroid affects the quality of that hiss.

I still think that locating the toroid outside the unit will make it work best.

FWIW, I have an api 3124 where the 4th preamp has some mains induced hum. That 4th pre is located right next to the toroid.

Cheers,
J
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on June 29, 2009, 03:37:05 PM
Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .

I have them in mine, and have not noticed any heat issues (nor were the original transistors overheating.) I haven't specificaly checked their temperatures, though I'll do so this next week if I get a chance.

I too have noticed increased hiss when the eq is switched in at high gain. I figured this was just due to the design, but am now wondering if the 2N2222 and 2N2907 could be contributing? Maybe I'll try different substitutions to see if there's any audible difference.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 29, 2009, 04:23:34 PM
sorry have read all the 40page but my english comprehension is poor.
and don't fine or don't remember me solution for the pop when you make the eq ON.
on my side transistor change is very good , i've change c11 and c16 like said  on the thread and change the iec ground to make it on the screw hole in the right side of the rack.
and i win 1to 2 db.
now the signal is about -64 db when eq is on all gain  of max (eq in and out) i'm waiting for changed the inductor and sielded it.

thank for all your help.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on June 29, 2009, 04:28:44 PM
other thing, when i touch the switch (without the knobs) of the mid low frequency and the hi low a big signal appears.
but it's not the case o n the other switch.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on July 05, 2009, 09:00:14 PM
Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .

I checked mine today (though I can't find my non-contact thermometer.) They do run on the warm side, though I can hold my finger on them for 10 seconds or so without major discomfort.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: chris carter on July 10, 2009, 04:51:05 PM
Anyone hear from Steve Hogan??
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on July 10, 2009, 05:31:30 PM
Anyone hear from Steve Hogan??
I got an email from him today.  He's been very busy and promises to post soon.

Paul ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on July 13, 2009, 10:04:45 AM
He's been saying that for two months.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on July 14, 2009, 05:09:12 AM
i ve receive my second 81 bought from a member here.
i confirm my first report.
i have no Humm from the inductors.
220V 50Hz here.
what i ve done change the 8 bad transistors with 2N5321 and 2N5323.
mod the pop mic input with the "0dbfs" tip.
add a 600R on a switch at the back of the output TX.
remove the input capacitor on the line TX.
eveything work good.
BUT this time i use a square wave to test the Traffos.
I ve done it for the line TX and remove the input capacitor to tame the overshoot.
but oh my god the ouput TX is AWFULL.with overshoot and crosstalk...
remove the 10n :it s worse. solder it again..
then I engage the EQ on (all others EQ switches on OFF) crosstalk nearly disappear. and small gain increase ...
Square wave is good when mesured on the 2n3055 so only the output tx is the culprit.
So I swap the output tx for a 30 Ohm/600 schlumberger traffo i have (bigger than a utc
A10 ,shielded ,pinouts within "glass pearl"..) put only a 600r across the output:
perfect square from 30 hz to 30Khz.
i will try with a carhnill output (but they re mounted on my 1084s clones)asap.
so my advice will be:
if you live in europe swap the inductors don t seems necessary.
try 2n5321 5323 instead of bc441 61 if you don t have some..
remove the input line tx capacitor. throw away the chineese output tx..
or maybe it s only this one i have who s bad....

ps i would like to read rodabod comments on the chineese output tx since i remember he change to carhnill ones..
audiforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on July 14, 2009, 11:30:05 AM
Audioforge

Did you get the buzz in the mid and upper mid bands of EQ ? This is a different noise that is
usually found after the transistor replacement. Then again your on 220V power so you may not
have the same problem as those running on 115V

To confirm your findings, I'll probably need to use a step up power trafo from 115V to 220V and switch my 81 mains power supply switch (red switch on back of 81) from 115V to 220V on back ( swap out fuses as well - 100ma type for 220V mains) and test to see if the buzz disappears.

add a 600R on a switch at the back of the output TX.
remove the input capacitor on the line TX.
can you give reference to the PCB pad locations from above quote ( example r43, 1c21 ) so its easy to follow your modifications. Would like to try them out.

those 2N5320/21 NPN and 2N5322/23 PNP parts are somewhat pricey, but this category of transistor is higher current rated , hence the price. Here is another sub 2N5681/82 NPN and 2N5679/80 PNP both 1amp rated/100-120V/hfe 40-150 (TO-39)
and another  TIP110/112 NPN and TIP115/117 PNP (TO-220 case, observe BCE leg orientation) 2amp rated/60-100V/hfe 500-1000 with high power dissipation around 50W, this will last forever compared to the 10W T0-39 types and can be remotely wired with short leads from the PCB and heatsinked to the case with proper insulator. In fact it shoud'nt need a heatsink at all since it can handle much higher power.

The 2N2907A, 2N2222A ( T0-18 metal case) power dissipation capability is just marginally rated over the BC547,557 (TO-92 plastic case) parts pending replacement in the ACMP 81. This is why they still run hot. I think around 600-800mw as compared to the 10w rating of the BC441/461 and 2n5321/5322 (T0-39 metal case) variety .
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on July 14, 2009, 11:57:41 AM
hello electrochronic, audioforge,

i 'm in europe too.

i've changed the transistor and hum lost some db 
changed C11 and C16 (10uF/35V) to 100uF/35V hum lost 1or 2 dbu
put shielded sheet on the tx psu (bought on black market) and  hum lost around 5db
changed the two inductor for carnill shielded with oep case and when i give mid low and mid high i have a very litlle changement aournd 50hz and mid hash

only have some noise when i give some gain on the last  low band 330 to 33.

So i don't have information about the 600r  across the output where i need put it please.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on July 14, 2009, 12:00:22 PM
i would like to read rodabod comments on the chineese output tx since i remember he change to carhnill ones..
audiforge

They are actually ok. Primary inductance is good and matches well with the Carnhill equivalent. It does need to be terminated correctly though; you can adjust by measurement or ear. The original Carnhill has some interaction in the low-end which depends on how you terminate it.

Audioforge, PM me your email address.... I will give you some details that could be handy.

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on July 14, 2009, 01:59:59 PM
electrochronic
yes ,inductors hum being relative to the power traffo EMI it s seems that 220v 50hz tame this trouble. it was my conclusion after testing the first unit you sent me (thanks again franck)(and i beleave i was the first to have one in europe at this time).

i do my test for the EQ with sine wavefrom a generator  sent to the line input .choose a frequency eg 220 hz or 500 hz etc at 0db. according to the frequency choosen for  the mid eq  ; and see the sine wave at the output with probes and oscillo. i see very good sine wave form whatever eq gain.
i still wait for a technical procedure to reproduce your test conditions showing hum on the inductors....
franck and sterokillah.
a output TX need to be terminated to do measurement.in real world too.
what i usually do on my diy's with 600r output TX is very simple.
take a two pole switch .solder a 600 or 620r on a pole then a wire lead to one of the output tx pinout. a second wire from the other switch pole to the other  output pinout of the output TX.
that s all..
this way i can let the the output traffo unterminated if i go to 600r input gear or terminate it when i go to other gear without 600r input tx.in this case i can use the switch on or off and let my ears choose what i like the best....
stereokillah :i didn t change any capacitors..i need to try later...
franck :
my local dealer don t have bc461 but 2n5321/23 .around 1.1 euros vat included each.sourcing the original bc transistors in small quantity with the shipping will cost me at least the same and the spec are very similar to the original with less noise and more consistant hfe...
they seems to be happy in the 81 boards ;no oscillation nor overheating....
rodabod.
i was thinking at first glance that the mic and line TX from the chineese units will be quite bad but it appears there quite good (at least the line tx) when testing them with a square wave and removing the input capacitor(line tx; i don t do it this time for the mic tx).my thought for the output tx was :well it s just a output tx that look quite similar to the original one..square wave show me i was wrong.
i will do a test on thursday with the first unit i ve mod (with a  chineese tx to compare the 2 chineeses between them) and will connect one of my carhnill output tx to compare how they hold on with square wave...
btw i sent you a pm
will post my report as tests will be done...
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on July 16, 2009, 07:22:20 AM
new tests report
i ve test my first tnc81 (the one iv e bought from franck) vs the second one.
again with SQUARE wave send to line in.
the output chineese tx ,600 ohm terminated, of the first unit don t show crosstalk
and have less overshoot than the second unit which is very bad.
removing the 2 1000pf capacitors on the output board tame a little the overshoot but not in a significant way.
engage the EQ boost a few the output level and degrade the square wave but it s due to the design.
first conclusion :chineeses output TX quality could be very variable. one just "acceptable" and one frankly bad .
i ve try different r+c combinations but if it show better responses with sine wave ;square wave (which translate the best a build in tx quality) clearly show thoses chineeses tx are not up to the task.
final test : i swap the worse output tx to a 1066 carnhill repro i have in my 84 clone.
with his 10n+R ,as always 600r terminated. GOOD square wave from 100hz to 25kz,
very little overshoot ,no crosstalk. engage the EQ do the same square wave degradation and little boost (i presume due to fact of the circuit resistance and capa added).
So my final? conclusion IS:
if you want an acceptable tnc81 (unless you live in an 110v 60hz area and have inductor hum)  the best is to BUY a 1066 output tx. the carnhill repro are good and quite cheap from Colin. what you have at the 2035 output will be what you have 'except from 20hz to 100 hz not perfect" at the output TX.
audioforge
btw my 2 carhnill output tx "sing". do someone know how to tame this. it drive me nuts.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on July 27, 2009, 07:12:15 PM
hello,
someone should know where i can buy push button to change EQ on off.
this switch 4 pole is broken in one my units

i find it here but it's out of stock.

http://www.rapidonline.com/Electronic-Components/Switches/Latching-PCB-Switches/Illuminated-push-button-switches/70729

thank you
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on July 29, 2009, 06:56:44 AM
I have noticed one final issue with my '84 preamps, and unfortunately I think it stems from the power transformer.

The problem mainly affects low output ribbons and some dynamics. I believe the issue is a generally noisy earth (hum). When the unit is powered up and set with high gain but with no microphone connected, it is very quiet. However, connecting a microphone suddenly increases hum level.

This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side. This makes me think that the transformer is radiating EMI into the case and forming a circuit (a turn) which is leading to earth currents.

I am considering just swapping the toroid for a good quality one. I did consider breaking the ground connection around the case by using washers, but I though this might be a crude solution. My other idea was to lift the microphone input pin1 away from chassis earth and derive it from the line output pin1 so that it takes its earth from the next (clean) piece of kit. Again, probably not the most elegant solution, especially if the line output / line input following has pin1 lifted at one end!

Roddy
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on July 29, 2009, 06:58:24 PM
"This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side"
hi rod .have you try out some isolation sheet above the psu tx screw. maybe it touch the top of the rack case....
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on July 30, 2009, 06:10:27 AM
That was the first thing that crossed my mind since it would be very dangerous. But no, it seems to just be the electrical connection between the top and bottom halves of the case; the toroid bolt only touches one side.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on July 30, 2009, 09:51:25 AM
That was the first thing that crossed my mind since it would be very dangerous.
Why would that be dangerous? (The toroid screw touching the top of the case.)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: jdbakker on July 30, 2009, 10:08:11 AM
That was the first thing that crossed my mind since it would be very dangerous.
Why would that be dangerous? (The toroid screw touching the top of the case.)

Dr. Egon Spengler: There's something very important I forgot to tell you.
Dr. Peter Venkman: What?
Dr. Egon Spengler: Don't cross the streams.
Dr. Peter Venkman: Why?
Dr. Egon Spengler: It would be bad.
Dr. Peter Venkman: I'm fuzzy on the whole good/bad thing. What do you mean, "bad"?
Dr. Egon Spengler: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.
Dr Ray Stantz: Total protonic reversal.
Dr. Peter Venkman: Right. That's bad. Okay. All right. Important safety tip. Thanks, Egon.


If the bolt through the toroid touches the top of the case, it (plus the bottom, side and top of the case) forms an extra, shorted winding on the transformer. Best case: a blown fuse, worst case: studio fire.

JD 'oh, the embarrassment' B.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on July 30, 2009, 11:08:22 AM
"I see," said the blind man to his deaf dog as he picked up his hammer and saw.

Thanks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on July 30, 2009, 11:19:54 AM

 Btw Burdij's posting on the W.M.  links his site
which has xfmr shields that one could use for
the inductors of these pres .

i never go to H.R. but anyone know what the tnc boys are up to
these days , do they ever say anything ?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: chris carter on July 30, 2009, 02:04:41 PM

 Btw Burdij's posting on the W.M.  links his site
which has xfmr shields that one could use for
the inductors of these pres .

i never go to H.R. but anyone know what the tnc boys are up to
these days , do they ever say anything ?

This is the ONLY forum where TNC communicates with the masses:
http://recforums.prosoundweb.com/index.php/m/0/16069/2592/15306/#msg_16069
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on August 04, 2009, 07:07:20 PM
I have noticed one final issue with my '84 preamps, and unfortunately I think it stems from the power transformer.

The problem mainly affects low output ribbons and some dynamics. I believe the issue is a generally noisy earth (hum). When the unit is powered up and set with high gain but with no microphone connected, it is very quiet. However, connecting a microphone suddenly increases hum level.

This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side. This makes me think that the transformer is radiating EMI into the case and forming a circuit (a turn) which is leading to earth currents.

Right, that's another evening of work on these. I've just swapped out one of the mains transformers for a good quality Czech model which I have used before. Total silence hum-wise with the case top off. Hums badly with the case top back on. Balls.

As I said before, the hum disappears with the mic input unterminated. I don't think this is a case of having a dirty earth in the mic interfering. I found that terminating the primary with a short causes the same response, so I'm guessing the terminated coil is acting as a generator due to EMI. I was surprised the better quality transformer did not improve the issue. I'm also surprised the Carnhill's metal can isn't helping significantly here either, unless it is maybe being picked up by the wiring from the input XLR. I will terminate the trafo locally to test this rather than at the XLR.

More work to be done I reckon. I still have the option of using plastic washers as a fix, but it's a crude fix, and not safe in some respects.

Here's a photo if anyone's interested:

(http://img7.imageshack.us/img7/133/tncphoto.jpg)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on August 04, 2009, 08:14:40 PM
Sorry for the dumb question, but are all the boards isolated from the case? I hadn't thought to check mine and I'm far too from home atm. thanks for keeping us posted.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on August 05, 2009, 04:25:08 AM
Roddy
looking at your pic. see you swap the output TX for a carhnill.
i just did it for my 2 81 yesterday and it really improve the units in a significant way.
back to your trouble ,i see you swap the mic input tx too but keep the chineese one on the board. have you try unsolder it to isolate all his connections from the circuit?
your noise generator could be here..?...
keep us in touch for further infos
strange issue  ???
audioforge
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on August 05, 2009, 07:22:04 AM
Sorry for the dumb question, but are all the boards isolated from the case?

No, they are not, but it doesn't seem to be causing problems.

i see you swap the mic input tx too but keep the chineese one on the board. have you try unsolder it to isolate all his connections from the circuit? your noise generator could be here..?...

Yes, that's the first thing I did before fitting the new ones; lifted and grounded the old ones where necessary.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: audioforge on August 05, 2009, 07:48:56 AM
humm. to resume your troubles.
it only happen with the mic input within high gain and the cover top screwed?
not with the line input?
difficult to trace the signal with an oscillo and the cover on....
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on August 05, 2009, 08:40:05 AM
it only happen with the mic input within high gain and the cover top screwed?

Yes.

Quote
not with the line input? difficult to trace the signal with an oscillo and the cover on....

Line input is fine, although there is much less gain. It is silent at max. gain.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: sintech on September 07, 2009, 12:01:17 PM
Finally got around to looking into an 81 hiss issue (only when the EQ is engaged) 

The eq passes audio and works fine, but always with a sea of white noise.

Was able to quickly trace this to the Acmp's bass card, and swapping these cards between two 81's confirms that the fault also swaps units. A visual inspection side by side, things look pretty good.

As the fault is 100% on the card, I've re-capped all the lytics, swapped all the diodes, re-changed Q4 and Q5 (these were modded) changed both 184's Q2, Q3.

Q6 is part of the clip LED circuit and I don't have spare Q1, BC214 to hand.

If it's not the BC214, it has to be a resistor or non-polarized cap or the switch.

Any suggestions or pointers would be really welcome. :)

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: sintech on September 07, 2009, 01:56:42 PM
Ok, just for the record, the schematic states Q1 BC214, but it's actually a BC557 in place, I probed this and could hear hiss on the emitter.

BC560 is the nearest I have in my box! and it's pin compatible!

and did the trick!  :) no white noise backdrop. Yay! DIY rocks
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on September 11, 2009, 09:23:56 AM
Ok, just for the record, the schematic states Q1 BC214, but it's actually a BC557 in place, I probed this and could hear hiss on the emitter.

BC560 is the nearest I have in my box! and it's pin compatible!

and did the trick!  :) no white noise backdrop. Yay! DIY rocks

Was known  ( http://www.groupdiy.com/index.php?topic=27791.msg409012#msg409012 (http://www.groupdiy.com/index.php?topic=27791.msg409012#msg409012) ),
but it doesn't hurt repeating.

Cheers,

  Peter

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on September 11, 2009, 04:14:53 PM
I missed it , so good to see
it would also be good to make a master page
of fixes & mods

FWIW steve doesn't specificaly mention that one transister but globaly
[ still not a bad idea  ] all of them
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on September 11, 2009, 04:37:02 PM
it would also be good to make a master page
of fixes & mods
I second that! Not that Im lazy or anything   ;D
but I do have 2 84s that are (sort of) usable atm but could use some love (read: improvement). where o where to start?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on September 11, 2009, 08:03:43 PM
Move toroid to reduce hum to minimum level. Pay attention to hum harmionics and find a null.

Sound-wise, you can replace some caps and the transformers. Change the Zoebel networks too. I have figured out values for the original transformers; PM me with an email address if you want to see some values.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: electrochronic on September 12, 2009, 11:59:57 AM
Good call Greg

A master page or sticky for just a hit list of official and proven fixes for the preamps would be real time saver
this thread is pretty large to sort thru right now. too much detail to remember over 40+ posts.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on September 12, 2009, 12:08:22 PM
It'll sure work for a few weeks, but note there have been previous summaries & re-summaries in this thread.
(don't want to be negative, just realistic)
There might pop up 'side-threads' as well, just like for the GSSL
("don't want to read it all, please tell me exactly what I need to do in this new thread"), but
I'll make sure any of such attempts will be redirected  ;) - I guess it's still better to have one big thread with all info in it
than various parallel ones.

Bye,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on September 12, 2009, 01:22:35 PM
Someone would need to be responsable for updating it
i could do that but
last time i complied something here is was above the 20,000
charactor limit and could be not be saved

but here one file and simply mark the date and the thread page that it is current to
would at least give people a start on prep .

nobody cleans there own posts where they are trying to be clever & heard
whether it is relevent or helpful so there is allot of digging reloading
comparing that burns one out somewhat having to watch people listen to
themselves talk [ just like me right now ]

cutting out all the me too , reasking the same questions from people who didn't look
in the first place ..............cut the clutter ,

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on September 14, 2009, 01:07:34 AM
I could host the master HOWTO on recordinghacks.com; it would fit well with the schematics and circuit-board photos ( e.g. http://recordinghacks.com/tnc-audio-preamps/ ).

I was sort of waiting for word from Steve Hogan on the final fixes. Every time I ping him he says he'll be back here within the week, but it hasn't happened yet.

Anyway, I'm happy to host the page and even keep it updated. I could write it too, but probably won't get to it for a while. Anyone else want to pitch in to put the first draft together?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on September 28, 2009, 08:45:09 PM
Any word on the kit that Steve was working on?

I'd love to hear where he's at with this?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on September 29, 2009, 04:31:22 PM
Any word on the kit that Steve was working on?

I'd love to hear where he's at with this?
I haven't heard from him in quite a while.

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on September 29, 2009, 08:42:17 PM
Seems like this is off the plate for him. :(
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: tommypiper on September 29, 2009, 09:53:50 PM
Any word on the kit that Steve was working on?

I'd love to hear where he's at with this?
I haven't heard from him in quite a while.

 ;D

That's very disappointing and bizarre.  Wasn't he taking orders for his kits and power transformers?  He wrote that he had ordered the power transformers and other parts... I don't think he took any payments yet, but didn't he have a reserved list that guys were signing up for?  I know he put me on the transformer list...  Steve come back, man!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Rockit88 on September 30, 2009, 02:08:24 PM
I sent my ACMP84 along with a sizable deposit to Steve Hogan last March.  The repairs/mods were promised to be completed by the end of April. 

Here it is nearly October.  He hasn't made the slightest effort to contact me with updates on the work.  He has replied to several of my emails with the same answer--"I'm really busy...I'll post updates next week...blah, blah , blah," then months of silence.  So much for the integrity of people you meet on  internet forums.  I don't think he is a crook but it seems this has gotten away from him.

I'm a little disgusted and dismayed.  I think these things must be cursed.  It is going on YEARS since I threw in with the Chance group buy and I still don't have a working unit.  Actually, I don't even have a non-working unit, as I shipped mine off to SH, plus I am out my deposit money and shipping costs.  Fortunately I have several othe good pres and I can't really miss what I never had. 

I am a fairly patient person but I have just about given up.  I suppose I will have to figure out how to resolve this.

Does anyone else have similar stories?   Or perhaps a little insight on what is going on with Mr. Hogan. Thanks for listening.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on September 30, 2009, 04:42:59 PM
Just out of curiosity, how much was he going to charge to fix the 84?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on September 30, 2009, 08:26:51 PM
I too sent him an 81, at the end of February, for him to use as his "guinea pig," along with a deposit (and of course I paid shipping costs.)

I've had some correspondence with him since then, and it is my impression that he lost a major client due to the economic downturn, and has had to take on better paying (and probably more meaningful business-wise in the audio community) work to keep his business viable. He seems like an honest, well-meaning guy, and I still have confidence that he will come through with our fixes (if they are possible) or return our units and our money, but it seems that he too has fallen under the karmic black cloud that hovers over these group buy preamps.

I still really appreciate the meaningful information he has posted here (in what now seems the remote past), as it has been helpful in my own troubleshooting and fixes of these units.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on September 30, 2009, 08:32:51 PM
I sent my ACMP84 along with a sizable deposit to Steve Hogan last March.  The repairs/mods were promised to be completed by the end of April.  

Here it is nearly October.  He hasn't made the slightest effort to contact me with updates on the work.  He has replied to several of my emails with the same answer--"I'm really busy...I'll post updates next week...blah, blah , blah," then months of silence.  So much for the integrity of people you meet on  internet forums.  I don't think he is a crook but it seems this has gotten away from him.

I'm a little disgusted and dismayed.  I think these things must be cursed.  It is going on YEARS since I threw in with the Chance group buy and I still don't have a working unit.  Actually, I don't even have a non-working unit, as I shipped mine off to SH, plus I am out my deposit money and shipping costs.  Fortunately I have several othe good pres and I can't really miss what I never had.  

I am a fairly patient person but I have just about given up.  I suppose I will have to figure out how to resolve this.

Does anyone else have similar stories?   Or perhaps a little insight on what is going on with Mr. Hogan. Thanks for listening.
I bought a 73 in the GB and watched the forums for mods.  When Steve Hogan came along, it sounded like a great solution!  I contacted him to get on the list and then sent him a deposit to get in on the first run of transformers.

A few months after that I put together a deal to buy two units for my two brothers.  They had them shipped directly to Steve and they also sent him deposits.  He quoted them $300 to $375, (estimate), to mod one of the 73's.  I've gotten replies to my emails, (same as doc), but that's about it.  One of my brothers was going to send out another email but I haven't heard anything yet.

Paul
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: rodabod on September 30, 2009, 08:44:28 PM
Steve's probably very busy. I get very, very busy sometimes and forget to get back to people (especially by email). It's easy. Steve, if you are reading, just tell people that you are busy with stuff; it makes things easier in the long-run.

If you have a '73 or an '84 then iI'd suggest modding yourself. The details are here. Otherwise, collate the fixes here, and get a tech. The same can be said for an '81, but it is slightly more tricky.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on September 30, 2009, 10:24:34 PM
It's looking more and more like I will just do the fixes myself.

I'm just waiting for someone to put together a comprehensive and detailed "how to" list. I'm not real experienced with this stuff but can generally get it done with the right guidance.

I was really looking forward to Steve's Kit. He is easily one of the most thorough and complete techs I've read. I'm really disappointed that it didn't work out. It really does seem like these things are cursed.

At any rate, I'd love to see a comprehensive list of fixes for these mothers at some point. Luckily for me, I only have 84s and 73s.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: recordinghacks on October 02, 2009, 03:20:31 PM
I just spent an entertaining 20 minutes on the phone with Steve Hogan.

He hasn't disappeared to Brazil with our preamps, and doesn't appear to have been cursed by them although he admitted (with a laugh) that sounds like an interesting theory.

He apologized for having dropped out of this community temporarily; he has been overbooked with some other projects that promised faster returns on investment. (I'm sure all other self-employed folks can sympathize.)

Anyway, at the risk of misquoting something, I'll try to give a brief status update from Steve. If anything I write below seems incorrect, confusing, upsetting, or in any way raises your blood pressure, please contact Steve directly for clarification.

The R&D on the ACMP pre's is nearly done. The remaining areas of investigation are the gain staging and the inductors.

The gain staging is tricky, in part because of the loud click between stages, but generally because the design deviates from the original and isn't very well-done.

The inductors seem to be vulnerable to EMI, and probably require shielding and/or grounding for best performance.

I think Steve will have some good news about the audio transformers, but I'll let him cover that.

Steve's plan is to upgrade the units he has on hand first, this month. He plans to ship them in October.

While modifying them, he'll document the upgrade process for the book that will be enclosed with the DIY kit.

I have to say that hearing Steve talk about these mods made me very eager to hear the new-and-improved preamps. The turnaround time on the mod process has been disappointing, but I think the ultimate result will be great.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on October 02, 2009, 03:48:35 PM
awesome news, Matt!

I just can't wait to hear these mothers after the mods.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: Paul G on October 02, 2009, 06:39:58 PM
Thanks for posting this, Matt.  Looks like my brothers will get their units before I get my kit.   sigh.......LOL

 ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 05, 2009, 10:23:06 AM
It's looking more and more like I will just do the fixes myself.

I'm just waiting for someone to put together a comprehensive and detailed "how to" list. I'm not real experienced with this stuff but can generally get it done with the right guidance.
...
At any rate, I'd love to see a comprehensive list of fixes for these mothers at some point. Luckily for me, I only have 84s and 73s.

Partly in jest, partly serious: please don't increase the length of this thread by such remarks; the less of those, the easier it is to browse this thread.
Yes, it's long, but the info is all there folks, and we've had a few previous requests already for a summary.

In addition to that, the essence has already been summarized recently. Please don't come here for instant gratification, this is DIY after all.
In other words: technical questions are appreciated over here, and you'll find you get lots of help. But please don't ask for cutting corners to save time, your 24hours/day are as valuable as they are for anyone. Please note I'm not targeting you specifically. Enjoy the modding, it shouldn't be too hard.

All the best,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 13, 2009, 05:46:31 AM
Hi guys,

just bought a ACMP81 and want to mod it

so after gone thru this giant thread...
here's the plan:

- shielded inductors and power tx
- Gain switch mod (resistor) for avoiding the "pop"
- Replace the Q4 and Q5 transistors on all boards
according to Steve Hogan posts, I will be using BC327-16 transistors to replace xQ4 (ACMP=BC557)
and BC337-16 for xQ5 (ACMP=BC547) as they are direct drop in (same pin out).


Now, the questions.

- worth changing 2N3055? if yes wich brand? MULTICOMP or MAGNATEC (more expensive)
- for those who have changed the output tx for carnhill, the sound change in a significant way? worth the investment?

for the transistor:

I found 2 type, bc32716 and bc327-16. are they the same? first one is available in Europe, second one only in US (but shipping 18€ :o)

TRANSISTOR PNP TO-92
http://fr.farnell.com/fairchild-semiconductor/bc32716/transistor-pnp-boitier-to-92/dp/1017667
Bipolar Transistor
http://fr.farnell.com/fairchild-semiconductor/bc327-16/transistor/dp/1368081

same for the bc337

TRANSISTOR NPN TO-92
http://fr.farnell.com/fairchild-semiconductor/bc33716/transistor-npn-to-92/dp/1017669
Bipolar Transistor
http://fr.farnell.com/fairchild-semiconductor/bc337-16/bipolar-transistor/dp/1368082

thanks for your help
cheers.
Jr.
 



Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on October 13, 2009, 07:52:25 AM
 
 With little editing , i have complied [ halfway so far ]
relevent ideas [ & speculations ] of this thread
[ simply cut & pasting leaving out me too type posts ]

recordinghacks will host it , let you know when it's done
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 13, 2009, 02:57:53 PM
yep, after gone thru the thread again (great move Okgb, could be painless)
seems that the Carnhill tx does improve the sound, the output tx is not so good as the input...17 pound @ audiomaintenance, it could not hurt.

I'm still in the doubt for the transistors.
it look to me that the original N$ve are 2A and bc327-16/bc327-16 are 0.8A...
there also 2N5320 and 2N5322 but expensive, to-39 and I need to adapt BCE,same for BC441/461.

any idea on a straight sub to-92 package?anyone?
Jr.

Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: stereokillah on October 13, 2009, 06:22:47 PM
don't remenber exactly but i had other than bc461/441 one menber give the good value on tis thread and it' is to-92
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 14, 2009, 03:39:06 AM
Hi Mr stereo!

I recall that in the thread you said you've ordered bc327-16 and bc337-16 and that's what Steve Hogan said it should be perfect replacement.
but they are hard to find in europe!
I compared the datasheet of bc327-16 and bc32716, I can't see any big difference so I think I will go for them.

no trouble or overheating in you unit?
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 14, 2009, 07:16:47 AM
Quote
Further investigation of the original Neve 338 plug-in amplifier as detailed in the AMS Neve '81 owners manual available as a download shows the reason for Neve using BC461-6 and BC441-6 TO-39 packaged transistors with heatsinks.
Pin 4 of the Neve BA338 Plug-In amplifier is labeled "boost (39 Ohm for 300 Ohm O/P)."  Strapping a 39 Ohm resistor betwen this pin and Ground increases the current sink Q5 collector current from 8.25 mA in the "non-boosted" position to 40 mA in the "boost" configuration.  This will increase Q4 dissipation from about 100mW to 480 mW which will be quite toasty without a heatsink and a big part.  The -6 suffix was the highest gain selection available on the BC441/BC461 transistors.  The TNC '81 clone does not use the "boost" position, so a hefty TO-92 transistor should work just fine.

ok I will go with bc32716 and bc33716,
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 14, 2009, 08:44:43 AM
I compared the datasheet of bc327-16 and bc32716, I can't see any big difference so I think I will go for them.

The same devices; as you will have suspected BC327 is the actual type-number. The '-16' addition (also '-10', '-40' are around IIRC)
indicates the hFE-range.

Regards,

  Peter
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 14, 2009, 09:08:38 AM
great, thanks peter!!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 14, 2009, 01:26:50 PM
I just received my acmp81...and...hummm...yeah, gone with the wind!!  ::)
but not so much buzzzz.

I've got some shield from hotbaby, will try this asap...also transistor swap.

last question about output tx.
I guess wiring the same?

cheers.
Jr.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: redddog on October 14, 2009, 03:54:40 PM
keep us posted.

(is it ok to say that, Peter?)

 :P
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 14, 2009, 04:01:28 PM
You're so funny...  8)
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 14, 2009, 04:23:32 PM
Quote
keep us posted.

ooohhh you can count on that!  ;D
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on October 15, 2009, 10:12:11 PM
OK, finally got around to doing some mods on this thing.. Namely the PSU mods and some others suggested by other lab members. Accidentally shorted my phantom supply while testing it (OOPS). I tested the NPN in my DMM and it gave me 1000 hFE (pretty nice, but the datasheet says 250 MAX) so I figured it was done.  Replaced it with a BC550 and was back in the game. So DONT SHORT YOUR PHANTOM SUPPLY! 
One thing I was looking forward to was replacing the 2N3055 with a NOS moto and rebiasing properly.  But I'm having trouble with my generator and scope. If I hook them up directly, I can see the signal nicely, and when I hook up the generator to the preamp while it's hooked up to my DAW, I can see/hear the signal there, but when I hook up the scope to the preamp I cannot see the signal in my scope. Weird?  Set to mic in, and low impedance.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 16, 2009, 04:41:53 AM
Quote
the PSU mods

what do you plan to do, change the value of caps or change the psu?
I want to know cause I'm not sure what is the best thing to do!

I've started to test the unit...pretty windy!
so I've shielded the inductor and the power tx with Hotbaby's shield and something buged me.

when I take a look at some 81 pictures, inductors are on board 3 & 4.
I've got 3 inductors on board 2,3,4!! is that normal?

anyway, still some noise (only on mic, line is totaly quiet) and less bzzzzzzz when eq is on.
it's funny because when I engage the eq, the buzz is pretty loud and after a moment it's starting to fade to a not so anoying level (but still not good)

I've ordered the tansistors, they'll be here in couple of days. we'll see then!

cheers.
Jr.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: okgb on October 16, 2009, 08:48:12 AM
As requested earlier can we keep the posts to
relevant  Q & A only ?
[ it took me hours to complie half of this thread ]

3rd inductor  ? i 'd guess that's gonna pick additional noise
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 16, 2009, 10:01:05 AM
yep, I only made shield for 2 inductors, need one more...
but I can't understand why I have a different board than every pics a I saw!
made I've got a wrong unit with 2x board 3...
I'll try to take a pict this week end!

thanks again Okgb!
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on October 16, 2009, 11:36:17 AM
fanatic:

PSU mods were posted by Steve Hogan way back.. like January or a bit later.  I swapped the rectifiers for UF400x (only fast ones I had around besides MUR860 which would require serious bending and I didn't feel like doing it). Swapped the 2x2200uF filter caps for 2x1000uF  63V, swapped the little 10uF for a 100uF and upped the post regulator cap to 470uF as it was the largest that would fit. All nice panasonic caps. This was in the 24V area.  
Then I swapped the first cap in the phantom circuit (I would have swapped the rest but I had no more that would fit, and didn't change the rest). I also changed the little ceramic bypass caps to 0.1uF Wima MKS or something.

I started to get hum again and had to turn the toroid again, now it's gone.

I also get some weird buzz with the lid off, put the lid on and still the same, but it mostly went away when I screwed the top lid screws.

Questions:
1. I sometimes (not always) get buzz when I put my hand close to the preamp board (with the lid closed). Anything come to mind as a cause?

2. Has anyone tried swapping the preamp transistors for MPS-A18s or similar?  Should be a bit cleaner than the BC184c...mine has entirely too much "haze."

fanatic, about the PSU board I think it might be possible to design a new board that fits the same holes using a standard off-the-shelf toroid (2x25 for example). A standard toroid won't be as nice quality as Stave Hogan's custom but it would be much cheaper, and better quality than the crap that's in there now...
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: diyfanatic on October 16, 2009, 03:02:42 PM
hey Mitos, thanks for the psu tips!

Farnell is very quick, transistors ordered yesterday afternoon, received today!!  :o

I'll try take some pictures of my 3 inductors unit and to do the swap this week end if I have the time...

I feel my unit is like the guy with 3 nipples!  ;D

cheers.
Jr.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: crazydoc on October 19, 2009, 12:28:32 PM

2. Has anyone tried swapping the preamp transistors for MPS-A18s or similar?  Should be a bit cleaner than the BC184c...mine has entirely too much "haze."


Here's from my post#742 back in May:

Quote
I have not been happy with the noise in the preamp at high gain. I bought 100 each of the MPSA18, BC550 and more BC184C's, thinking I could hand select lowest noise transistors for the first preamp stage. I tried 3 or 4 of each, without any notable difference when listening, so I gave up on it. Do you think this or any other mod might be worth pursuing to lower the noise?

I gave up on these preamps for awhile (waiting for Steve Hogan's mods - still not forthcoming.) I now have one 81 and one 73 with all my own mods that certainly work adequately - no hum, buzz or gain switch blast, but there is still more "transistor hiss" than I would like.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: mitsos on October 19, 2009, 12:38:04 PM
OK, I missed that.. Though I did search. I was thinking of swapping ALL the 184C for MPSA18.  I too don't like the hiss on my 84, but it's only at really high gains, so maybe I'm nitpicking.
Title: Re: [ACMP investiupgradifications] All things PREAMP
Post by: clintrubber on October 19, 2009, 02:26:13 PM
[W.r.t. noise & ACMP81]Maybe somebody with RealRuperts can chime in here ? I don