Korg-35 clone VCF

This filter is believed to be functionally identical, or at least very similar, to filters built around the Korg-35 IC. It was used in the first series of Korg MS-20 and in the MS-10. Later MS-20s used an OTA-based filter, that sound different.
The Korg-35 is a very simple design, built with only three transistors and one op-amp as active components. This is not the design to use if you want low distortion and low control voltage bleedthrough. But the filter has a rather unique sound, which makes it a worthwile complement to other filter designs. The core circuit for this filter was designed by Jürgen Haible, based on information from K. Tanaka.
Like the MS-20 (but unlike the MS-10) my filter has two cascaded filter sections. On the MS-20, the first section is highpass and the second is lowpass. On my filter, both sections are switchable to either highpass or lowpass. Each section has a slope of 12 dB per octave. With a toggle switch, I can gang the cutoff and envelope controls. This way, I can get a 24 dB/oct response if I set both filters to either lowpass or highpass. The peak and FM controls are still separate, so you could achieve some special responses and modulate the filters separately. The latter might be interesting in conjunction with a high resonance.
Unlike the Korg synths, my filter tracks the keyboard (1 V/oct). The tracking acurracy isn't very high but it is possible to play melodies while the filter oscillates. There is no temperature compensation though. The tracking can also be set to half or a fourth (2 or 4 volts per octave). This is useful if you want higher notes to be a little less bright, which is often the case with acoustic instruments. To prevent the filter from thumping when you play the keyboard, I have added a lag circuit to the keyboard tracking input.
I have added separate knobs and inputs for envelope and general FM. Both knobs are bipolar, so the modulation can be positive or inverted, depending on which direction the knob is turned. The envelope input has one thumbwheel per filter to select source.
Another addition is a signal indicator. It drives a bicolor LED, that lights green when there is enough signal and red when there is too much signal.
This filter has high distortion when the input signal is strong. This can be used to good effect. Therefore I have added a drive switch. In the normal mode there is only slight distortion when the filter is fed with a 10 V p-p signal and the input signal knob is set to "8" (unity gain).
But with the drive switch in "high" position the input gain is increased ten times. Now you can use the input level knobs to control the amount of distortion. The output gain is reduced in this mode, to prevent overloading of the following circuits.

This is how the Korg-35 VCF module looks from the back.