Overdrive/fuzz squeeling and buzzing.

[iainpunk]

Hey everyone, need some help

Ive build the following schematic on my breadboard, its a pedal that does both a grainy overdrive and a gritty fuzz.

Both out a and b go to a spdt switch to switch between overdrive and fuzz.

Th problem comes when i turn up the volume on the fuzz side, when above noon (Which is about unity volume) it starts to squeal and/or buzz really hard, like an oscillating fuzz. This however, is a bug and not a feature. I tried everything on the debugging list i could think of.

Any suggestions?
Thanks in advance, iain

[iainpunk]

Nevermind, the switch has capacitive properties, large enough to let signal back in to the 2nd stage, which makes it oscillate.

Im goibg to buffer the middle putput with a gain of -1 so its in phase with the output at the end so i can blend between them

[thermionix]

Unrelated, but you might consider an input cap too.

Nevermind, the switch has capacitive properties, large enough to let signal back in to the 2nd stage, which makes it oscillate.

I'm not totally following you, but if you've selected Output B, the Q3/Q4 portion of the circuit is still connected.  Maybe it's not a switch capacitance issue, but I'm probably misunderstanding.

[iainpunk]

Well, leaving the "2nd stage" connected or not would in theory (and practice)(if the 'out b' were buffered) not make any difference, since jfets have only a small capacitive impedance on their inputs, and about 20Mohm of resistance. This doesn't really change the signal.

And yeah, i have an input cap, but forgot to draw it, since its on the other side of the bypass switching.