Hot opamps, high neg current draw

[rtroast]

I have two A12s, neither of which appears to be working. The first, assembled last year seemed to work at first and was actually used on a session. The second has had problems from the start, exhibiting an oscillation on power-up that draws enough current to eventually shut down the power supply if left on too long. I took both of them to a friend knowledgeable in audio electronics to take a look at. Here were his observations:

The voltages at the op amp terminals are correct. Placing a 500ohm resistor across each power rail results in seemingly normal behavior from the power supply and onboard regulation. When an op-amp IC (BB OPA134) is plugged in the boards appear to behave normally. From this we conclude that the board is wired correctly?

However, using any combination of “2520â€

[tpryan]

What DC voltage do you measure at pin 6 of the suspect op-amps?

[rtroast]

I'll check in the next day or two (rest of rack is setup for tracking) - thx

[rtroast]

OK, checked out both boards. It appears that one of them is working after all (0v at pin6 and normal current draw) and it looks like all the opamps work. I have one question about something we noticed but first, the clearly non-working board...

The voltage measured -5.5v at pin6. Current draw approached a 1/2 amp and things got hot.

(On the working board)
We did see current draw increase but it seemed to vary when we touched certain components. Eventually, we discovered that shielding the board (with a big piece of brass in this case), brought the current draw down to normal. So we concluded there was an oscillation occurring that was stopped by holding the brass plate over the board. On subsequent tests though the current draw never rose so we assume that everything is working. Is this normal?

Thanks for the help. I'm kind of "glad" that the voltage on the second board was off, maybe one step closer to fixing it.

[tpryan]

So all of the op-amps work fine in one board, but on the other you get -5.5V at pin 6 no matter what op-amp you use?

[rtroast]

Well, they seem to. We're not actually listening to them, just applying voltage to the boards and measuring pin6. I believe we tried all the opamps in the "bad" board as well and got the same -voltage result. I can repeat the tests though over the weekend. I could also probably try out the good board for real as I can open up the chassis after we finish tracking today.

[rtroast]

I was finally(!) able to re-test my two A12 modules. As suspected, one is fully working. Voltages were checked with three opamps (1 SCA, 2 Lieber Red Dot). I confirmed operation by recording through it. Here are the results on the second board:

Voltages without opamps installed – 15.75v between pins 4-3, –15.75v between pins 4-5.

With an opamp installed voltage on the negative rail drops to between –7.3v (SCA) and –5.4v (Red Dots) and drawing 300-400ma respectively.

As indicated earlier, when an op-amp IC (BB OPA134) is plugged in the voltages return to normal. Placing a 470ohm resistor across each power rail produces similar voltage and current measurements.

I’ve compared the two boards and they appear to be wired identically. I’m happy to further try and diagnose the problem on this end if you have a suggestion on what to look at, or can send the board back in for repair.

[tpryan]

We'll need to see it. Send an e-mail to support@seventhcircleaudio.com for return instructions.