Problem with level in a NEUMANN V475-2A help needed

[nkefaloyannis]

Hi all, I just built a DIY summing mixer with a NEUMANN V475-2A card. What I found during calibration and check, is that although the 1st channel works as expected. i.e I feed the input with a 1kHz sine at balanced line level -18DBFS through a pair of 5.1k resistors as Re, and I 've installed a stereo 20k linear for Rg. I get 0db unison when the pot reaches 6.2k and I can get headroom from -12db at 0Ω, all the way to +12db just over 15k. I understand that is correct and according to Neumann. On the 2nd channel though when Rg reaches +3db I am starting to get clipping, and just over +3db the output starts to fade down, like something overloads the amp and it shuts down. It seems to me, like a degraded or burnt component somewhere on this channel, or maybe something is wrong with the bias of the amp, but as I don't have any knowledge on electronics- I am a sound engineer myself- I wouldn't know where to look or what to measure with a multimeter or even replace. I thought of asking the community, in case someone happens to know the insides of these cards and can direct me in some way.
Regards,
Nikos Kefaloyannis

 

[rock soderstrom]

Hi all, I just built a DIY summing mixer with a NEUMANN V475-2A card. What I found during calibration and check, is that although the 1st channel works as expected. i.e I feed the input with a 1kHz sine at balanced line level -18DBFS through a pair of 5.1k resistors as Re, and I 've installed a stereo 20k linear for Rg. I get 0db unison when the pot reaches 6.2k and I can get headroom from -12db at 0Ω, all the way to +12db just over 15k. I understand that is correct and according to Neumann. On the 2nd channel though when Rg reaches +3db I am starting to get clipping, and just over +3db the output starts to fade down, like something overloads the amp and it shuts down. It seems to me, like a degraded or burnt component somewhere on this channel, or maybe something is wrong with the bias of the amp, but as I don't have any knowledge on electronics- I am a sound engineer myself- I wouldn't know where to look or what to measure with a multimeter or even replace. I thought of asking the community, in case someone happens to know the insides of these cards and can direct me in some way.
Regards,
Nikos Kefaloyannis

Have the electrolytic capacitors already been replaced? It could be a dried out electrolytic capacitor.

Edit: The blue tantalum capacitors should also be checked.

 

[nkefaloyannis]

first of all, a big thank you to fellow member rock soderstrom for his input to my problem. This is what I have done so far to no avail, unfortunately. I replaced the 2 10uf electrolytics on the power lines of the problematic channel, no change.I replaced the 3 roederstein electrolytics plus the Philips one but nothing new happened. Then I started swapping parts between the channels. I swapped the whole MO-3 power board, nothing. I swapped the MO-1 board, no change. I swapped the MO-2 board, nothing. I swapped the tantalum cap, no change. I swapped the 2 WIMA caps, no change. My PSU is an 150-0-150 to 2x12 toroidal to a regulator with LM317/LM337 outputting 24.0V measured. I swapped it with a laboratory PSU in case there was an issue with current but nothing changed. I suspect that the limited headroom in the 2nd channel dictates limited voltage or current somewhere in the circuit but at this point I am really running out of ideas. Any help will be much appreciated.

 

[rock soderstrom]

first of all, a big thank you to fellow member rock soderstrom for his input to my problem. This is what I have done so far to no avail, unfortunately. I replaced the 2 10uf electrolytics on the power lines of the problematic channel, no change.I replaced the 3 roederstein electrolytics plus the Philips one but nothing new happened. Then I started swapping parts between the channels. I swapped the whole MO-3 power board, nothing. I swapped the MO-1 board, no change. I swapped the MO-2 board, nothing. I swapped the tantalum cap, no change. I swapped the 2 WIMA caps, no change. My PSU is an 150-0-150 to 2x12 toroidal to a regulator with LM317/LM337 outputting 24.0V measured. I swapped it with a laboratory PSU in case there was an issue with current but nothing changed. I suspect that the limited headroom in the 2nd channel dictates limited voltage or current somewhere in the circuit but at this point I am really running out of ideas. Any help will be much appreciated.

Well, now I'm slowly running out of ideas. Maybe it's the adjustable capacitor in front of the transformer or the 1k trimmer resistor in the NFB loop?

Both are neither on the daughter boards so swapping them would not have affected the result.

 

[Evertide]

The adjustable capacitor is adjustted correct ? Both input caps willt have the same valu , the ground will be in the middle...

 

[nkefaloyannis]

PROBLEM SOLVED!!!

Hi, finally I found the problem, and it had to do with the pot. Due to my stupidity, I implemented wrong the wiring instructions I had in the papers from NEUMANN. It called for connection of the pot in pins 12,13 and 18,19 plus shielding to go in pin 14 and 17 respectively. What I had done was to carry the shielding wire all the way across to pin 3 of the pot. As soon as I cut that in a glimpse of epiphany !! )-: it all sorted out. Now, why did this upset the gain in the 2nd channel and not in the 1st one is beyond my understanding and had me chasing dragons. Anyway, all good now and I have finished and boxed the build. I have included photos of it in this post. Many thanks to all fellow members for their input. Good for me to focus on the pot, otherwise I was about to start swapping the traffos!!.

Regards,

Nikos