spVCF

Steiner-Parker VCF

The Steiner-Parker VCF is a well known topology for a semi-state-variable filter.

The unique feature of an spVCF is that you have one output for three inputs, you can inject your signal for lowpass, bandpass, or highpass filtering. The special feature on the Karltron spVCF is another trio of outputs, which are just passively filtered to help you keep the lower or higher ends of the signal.

Use Low in and Med out for a milky resonant filter, or High in and Low out for edgy sound sculpting. It's self-resonant with the squelchy Steiner style because resonance goes back into the bandpass lane.

FM is an attenuverter for envelope inputs to go bipolar. The cutoff knob is exponential and V/O tracking provided with a trimpot (no tempco). The Drive and Res knobs control internal VCAs to give a clean level adjustment and a huge range of possible sound tones. If you've used a Steiner VCF before you know it is sensitive and sounds different based on the input level, and can easily be made to be smooth, self-resonate or squeal.

https://karltron.com/spVCF/