Perhaps ironically, I brought a prototype amp home tonight that decided to misbehave.
Everything looked fine on the "preliminary checks" I outlined a few posts back (Flir fine, gains as expected, low gain not turning into full scale noise (that is, not oscillating wildly). No "fat sines" either (lower level oscillation). I didn't put it on the APx, mainly because I'm lazy. Also because it worked, and I decided to take the win and go home.
Except...as I'm listening, I decide to drop it into low gain. And the sound goes off. As in, nothing. Totally muted.
Hmm, that's weird, I thought, and switched it back to high gain. Still nothing. Then, a few seconds later, it clicked back on.
I tried it again, and it did the same thing.
Then I tried a different set of headphones. Both high and low gain work fine. Huh?
I still need to confirm exactly what's happening tomorrow, but I suspect it's going into oscillation, but only on some combinations of load impedance (headphone driver, plus parasitic C and L from the cable might just be hitting the wrong spot with one headphone, but not on another.)
So why was it muting? Because this one had the protection systems in place, so instead of lighting up the driver, it went into protection and lifted the relay. Good deal.
We'll see what it's really doing tomorrow. Probably needs a bit more compensation work.