API 325 & Cinemag CMMI-7C...

That sounds like something is oscilating.

I hooked up my Langevin input transformer with one sec. out of phase and got the same sound. I was feeding two positive signals into a push-pull amp, and when I turned the trim pot to the center, it started oscilating, which gave me that weird high end sound.

Scope the output if you can. Maybe it wants a mic input instead of a sig gen, as a mic is floating and balanced and all that good stuff. But the xfmr should take care of that, no?

I got those results with a mic, I usually test with a mic first. Hopefully I'll have some time later today to see whats up.

dave

ok, had some time tonight to play-

I flipped the polarity on the wiring of the transformer and got simlar results, so that rules that out. Hooked up another transformer and got similar results, that rules out the transformer itself. I still dont have parts in to properly terminate the secondary, but I did hook up the scope-

What does oscillation look like on a scope?

When I put in 1K at about -20dB with output of the 325 at 0dB, with the probe after the output transformer, if I zoom way in, the sine wave basically looks like three or four sine waves smooshed together and then there looks like there is a much much higher freq series of sine waves sitting on top of the main signal. I should have taken a photo. The picture definitely makes sense to the 3-D ish worbly high end kind of sound that I was hearing.

Is my description sort of understandable?

dave

That sounds like (or dosen't sound like because you can't hear it) ultra-sonic osc.
Try bypassing the hell out of the feedback resistor, I'm talkin 0.1 uf, on the opamp, just to narroew it down.

I went so far as to replace the old 47pf (same results) and then bypassed with a .1uf. With the .1uf cap, the overhash went away, but Im still not seeing a clean sine wave, still looking at 4 of them tightly bunched together. Cant do any real listening tests as you can imagine what the thing sounds like with a .1u bypass cap, heh.

It would be wonderful if this was just caused by overshoot from the input transformer which the opamp is just amplifying the hell out of. Hopefully Ill be able to make that detmination when the parts arrive.

dave