On my freshly-built Dual SVF, everything seems to work correctly except the self-oscillation. It’s only audible at extremely high frequencies - my tuner says about F7 - and making the self-oscillation tone go lower than this will cause it to lose volume until it’s entirely silent. Playing a note a few semitones lower will cause it to rather slowly fade, like turning a volume knob, but playing a note more than an octave or so lower will cause the volume to fade very quickly, like it’s being closed by a very fast envelope. The self-oscillation is played at the expected lower frequency until it goes silent(well, roughly - I can’t really tune it with this issue going on, but playing a note two octaves lower will give a note two octaves lower). Playing a low note again will not re-trigger the self oscillation until I play a high note again. This behavior is identical in both filters, and low notes through the regular oscillators sound fine.
The weird thing is that everything else seems to work. Both filters in all three filter modes work as expected, and turning up the resonance of a low note with a low cutoff frequency will produce the expected change. Sometimes it even sounds like the self-oscillation is back, but it’ll be gone when I turn the oscillators off.
So far I have:
Verified the filter settings
Verified the self-oscillation tuning settings
Re-flowed every joint
Turned the trimpots through both extremes, listening and playing notes through while tuning
Swapping one 4053 chip with a spare of a different brand
I’m reasonably certain that the parts are all correct - I sourced them myself, but I verified and separated them before assembling, counting them out and taping them into different places on a labeled sheet of paper, and verified placement before moving on the the next set of components. I’ve also cleaned the flux off the board and there are no obvious shorts anywhere. I’ve used this control board with other filters I’ve built and it’s worked fine. Tomorrow I’m going to re-flow the joints again with a fresh tip. I don’t know a whole lot about electronics, and would really appreciate it if anyone had any ideas. Thanks!
(the two 8-pin ICs with illegible printing are both TL072Ps like the others)
EDIT: I couldn’t sleep, so I re-soldered every joint and nothing changed. I can confirm that the self-oscillation is somewhat audible along the entire frequency range as long as at least one oscillator is playing at full volume, which seems really weird. The lowest frequency it remains audible at without an oscillator playing is F7, not F8 as I originally said. To clarify, it becomes inaudible when the self-oscillation frequency goes lower for any reason, including changing the cutoff frequency or adjusting the trimpots, not when the input MIDI note is lower.