This is my latest take at the Steiner-Parker VCF, in which I tried to improve the design in order to correct some flaws of the original design : scratchy resonance potentiometer, untamed auto-oscillation.
From Timo Rozendal:
Changes from original design: - no switch but separate buffered input for every mode (LP,HP,BP), - lowpass input is normalled to highpass and bandpass, so you can e.g. transition smoothly between high and lowpass with the knobs - 'tweak' resistors are replaced by trimpots - added decoupling capacitors for ic's - added a flexible footprint for the matched transistors, I tested it with THAT300, AS394 (Letvian LM394 clone), 2n3904 and BC547 (I don't hink the original 2SC1583 will fit) - provided panel pcb, so no wiring required
The inputs have enough gain to overdrive a normal signal in the filter. The resonance is still a bit untamed and chaotic and not so useful for melodies (plus it won't self resonate at low frequencies, a known trait of this filtertype)