[ACMP investiupgradifications] All things PREAMP

clintrubber

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*** 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 ) 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


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
 

clintrubber

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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
 

okgb

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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 ?
 

clintrubber

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[quote author="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 .[/quote]
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).

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 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
 

okgb

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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
 

CHANCE

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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.
 

clintrubber

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[quote author="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.[/quote]
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
 

TomWaterman

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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
 

CHANCE

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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
 

clintrubber

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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
 

Jim50hertz

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[quote author="okgb"]the Orange , may say 70's but it wouldn't be in my first 10 choices[/quote]

Yes, like brown cordruroy curtains and flock wallpaper but hold on, is that not rust? or terracota? or perhaps burnt earth? :wink:
 

TomWaterman

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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
 

tommypiper

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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:
 

clintrubber

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[quote author="TomWaterman"]Peter, I read the news over at PSW.
-T[/quote]
Thanks Tom, I should take a look over there from time to time.


[quote author="tommypiper"]I have a long list of potential mods and fixes.[/quote]
Cool :thumb:

As it happens I heard of a nice place to post that list :wink:

Bye,

Peter
 

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