Weird oscillation

[PBE6]

Last week I built JC Maillet's Super Full-Wave Octaver:

http://www.lynx.bc.ca/~jc/superFullWave.html

Interesting on its own, I liked it better when I fed it into a homemade distortion circuit with hard clipping. Last night I decided to put them together, so although it's missing a few final filter stages it looks like this at the moment:



Funny thing is, when I turn the gain past about 2 o'clock it starts to sound like a tremolo!! I figure I screwed something up with the opamp gain section, possibly not enough resistance on the output? If anyone sees anything obvious in the schematic, please let me know.

(The tremolo effect is actually kinda fun, but I'd prefer to know how I screwed up before I go replicating nonsense..)

[GibsonGM]

Just a stab in the dark, but - is your power supply decoupled enough?  At the opamps?  Perhaps something is getting in the power line and creating the oscillation?  Caps, man...

[PBE6]

Possibly, but I'm using a 9V battery. If anything's leaking in it could be the diodes, but the last time that happened there was a serious buzz in bypass mode, even with a large cap across a large resistor.

[Keppy]

I would remove your low impedance connections to Vref and reference those to ground instead. C3 and C6 can go to ground instead of Vref, and so can D1 & D2 if you move them to the other side of C7. Your resistances aren't too low for opamp outputs as far as I can tell, as long as R5 is 2.2k, not 2.2r.

These changes theoretically shouldn't make a difference with C2 present to make Vref low impedance at audio frequencies, but it doesn't hurt to remove that variable. If that cap has high ESR or is otherwise compromised to raise the impedance of Vref, you could have problems given the low impedances you're referencing to that point.