VCF-1 self oscillation

gimble88 · Joined: May 21, 2022 Posts: 18 Location: Oregon

I built a T.H. VCF-1 and really like it. I have a question about self resonance. The filter does all the things I expected including pretty nice V/O tacking and it pings really nicely.

But, if the resonance is turned up enough for the filter to oscillate it produces a rail to rail (minus a diode drop or so) trapezoid wave. As I turn up the resonance it starts with a sine wave but in 1/2 a second keeps growing until it's clipping at the rails. I try to adjust the resonance to retain the sine wave but it either grows to the clipping point or decays to silence - I can't find an in between point.

I have seen filters demonstrated which produce a sine wave when self oscillating. Is the VCF-1 capable of that or is there a mod I might do which would help here? I can see a couple of ways to reduce the amount of resonance fed back but I also notice the beginning of self oscillation begins at different positions of the rez control depending on the cutoff freq.

I'm not new to electronics circuit design and building but new to synth modules so this is my first filter other than my LPG.

Thanks,
Spence