A12 noise problem

[bobschwenkler]

I have a pair of A12s that I built about a year and a half ago with Avedis 1122s (is that the right model number?).

They both have exhibited a quite intermittent noise problem. It starts out as a low level noise, and becomes louder. As it becomes louder it may start to get fluttery sounding. Sometimes it will ramp up to quite a loud level over a few (10-20) seconds, and then all of a sudden pop back down to a quieter level.

I have four more modules (x2 J99, x2 N72) that have shown no similar issues at all.

I've got no experience with troubleshooting electronic parts failures, but my intuition tells me it is something with the onboard regulator caps.

I have tried different op amps (with no noise problems), but the problem has been too intermittent to be sure at all that it is the op amps.

[tpryan]

Make sure all the ground connections are solid, especially J7, pin 4. Also, try wiggling the power supply connector while the noise is happening. If it goes away, you may have poor crimp connections or a dirty connector.

[bobschwenkler]

I'll check those things, although I took a lot of care to get my crimp connections done well with the proper tool.

The strange part is that it is limited to these two modules of the same type (I did the wiring for them all at the same time).

Does this sound like it could be a capacitor? Any other suggestions?

[tpryan]

Does the noise stop if you change gain settings? Does it seem more likely to happen at high gain or low gain settings?

[bobschwenkler]

It has been too intermittent to know if it occurs at any specific setting; I usually have things set at medium/high gain. It does not stop when I change gain. I think. I'm not totally sure about that either though... I'll keep those in mind next time the problem occurs.

I could go through and re-solder some of my ground joints... but I can't see anything that looks bad to my eye.

The strange part to me is just that I have the same thing on two channels. The chances of me messing up the exact same thing twice to cause the exact same problem seem pretty low, which is a big reason why I suspect a component.

I've tried leaving them on for hours while I'm just sitting at the computer, but of course it never happens then. Only when I'm recording.

[dkatz42]

Any chance it's something outside of the pres, like a mic or cable that you sometimes plug into either pre?

[bobschwenkler]

No,

Different cables, mics, etc. At first I thought it was one of my mics, and then it happened to another mic and I thought "crap, two mics have gone bad" then it happened again and I realized it must be the mic pres. I'll look over them again and maybe re-solder some joints and just check it out more thoroughly when it happens again.