[ACMP investiupgradifications] All things PREAMP

[maxwall]

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.

 

[mitsos]

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?)

 

[tarnationsauce]

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.

(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_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_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_bass.gif

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

[edit] Almost forgot the Output / Power schematic.

 

[mitsos]

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.

 

[okgb]

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

 

[tarnationsauce]

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.

 

[maxwall]

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.

 

[tarnationsauce]

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

 

[b3groover]

This whole thing makes those posts by Alan Hyatt on the homerecording.org forum a year or so ago a little more prophetic, eh?

 

[okgb]

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 ?

 

[mitsos]

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...

 

[audioforge]

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

 

[mitsos]

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?

 

[Mylithra]

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.

 

[maxwall]

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 ?

 

[tarnationsauce]

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

 

[okgb]

better that the phase reverse is on the output ,
wonder if those 1000pf to ground are Needed ?

 

[maxwall]

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.

 

[okgb]

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

 

[audioforge]

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
.