Hi everyone.
I've been working on a project, but having some oscillation issues with this part of the circuit. The pot allows adjustment of the amount of crossover distortion in the (audio) signal. This forms a rudimentary noise gate, but more importantly a desired effect for signal processing. This is not intended for accurate reproduction, only the effect, for guitar. Debate among yourselves whether you feel this is a desirable effect, but lets just accept that it's what I want.
My problem is that while the circuit is stable when the pot is adjusted from low to about 50% of its resistance, it breaks out into oscillation at ~135kHz as the wiper approaches the output of the first op amp. This then lets the signal through relatively cleanly (as though the diodes were in the feedback loop, which they no longer
completely are), modulated by the oscillation frequency of course.
I have tried introducing some series resistance from the pot wiper to the non-inverting input of the second op amp, removing the zener diodes (which are intended to clamp the output swing of the first op amp to provide more graceful clipping in the case of overload and to avoid signal inversion that may happen when hitting the rails), and altering the circuit layout, without any success.
The op amp has 100n MLCC local bypassing, and no other part of the circuit suffers from oscillation.
The loop area of the negative feedback loop is larger than ideal due to user interface/mechanical constraints, and as yet I've only prototyped it on the breadboard and matrix/vero, I have not tried a proper PCB with groundplane. Changes to lead-dress or layout (I've built it twice on matrix board) have not altered the oscillation frequency though, so I've largely discounted this as the cause.
I have also not yet tried making the input op amp inverting, as I require a high input impedance and do not have a spare op amp and would like to avoid an extra buffer.
So, does anyone have any suggestions? Any help would be appreciated.
