Help me fix API-clone preamp problem

[SSLtech]

[quote author="dasbin"]Aha! Now HERE's a funny thing... the Output pin is shorting to ground (did a continuity check on tons of traces)![/quote]

-"Shorting"???

I bet you're just reading the primary resistance of the tx. Lift one wire and see if it goes away.

Keith

 

[dasbin]

Argh.

Dammit, you're right. 7.0 Ohms. And here I thought I had it all figured out.

Guess I'll try pulling the 2520 now.

 

[dasbin]

This is driving me insane! I just replaced the 2520 with an OPA627 I had laying around; it does the exact same thing still! The only difference is that with the OPA627 installed, when the Phase Reverse switch is hit, maximum gain seems to work... but if you give it too loud of a signal, it'll still cut out totally until you turn it down and then give it an equally loud signal again.

What the heck could this be? I mean, as someone else said, there's only a limited number of things that can happen here; I've pretty much run through them all.

 

[soundguy]

daaaaaaamn, I was betting it was the opamp!

There's audio going through the switches, maybe you have an intermittant thing with the switches...

ack

dave

 

[babyhead]

Have you tried bypassing the switch? It may be a bad solder joint or a worn out contact in the switch. You should be able to do this by shorting the center lugs to the corresponding outside lugs connected to the PCB with the switch on. Is this switch on the in or on the out? Maybe something in the switch is shorting to the chassis (while out of circuit)? Maybe this helps.

-Mark

 

[dasbin]

Eh... it's not the switch either. It's actually functioning correctly... I re-checked all the leads on it with the new knowlege that 7.0 ohms of resistance does not equal "shorted"

Still, things get better when the phase is reversed. The 5vAC doesn't show up on the output pin at max gain, there's no POP noise, and more gain is possible.
The phase switch is placed right between the opamp output and the output trafo. It basically just swaps signal with ground going into the trafo. So this points to some problem at ground, I think. Here is my "reasonable beliefs" list:

a) Something funny is going on with the ground, and
b) It probably has something to do with a capacitor, due to the nature of the problem

This leads me right back to square one: the DC-blocking cap. But I already replaced it. It's not the problem, even though when it's not there, it's fixed.
Ugh.

 

[JLM Audio]

Switching the phase switch should make no difference if the output transformer and associated wiring is ok. So sounds like something is wrong here. So I would disconnect the 2520 output from the transformer and phase switch and run it straight into a unbalanced input and see how it goes. Also is there still 200 ohms in series with the gain pot at max gain as the pop you describe sounds very like when an opamp is forced to try to make more gain than the opamp and wiring can stability do and goes into oscillation usually with a DC offset and a POP. You must have the cap and resistor in series with the pot or you will have a good chance of a DC offset at the output of the 2520 which will not last long driving into 7 ohms from the primary of the output transformer. Also how is the secondary of the 2503 output transformer wired to the XLR. Also have you tested that R3 is 20k? Otherwise draw all your mods on to the circuit above and post it to help fault finding or a good photo of the wiring can usually help.

Joe