he 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.

The filter core is a diode latter similar to the Yusynth VCF. Significant modifications in the I/O and resonance system. To get a single Drive knob to run all three inputs, we need some internal VCAs, and the cleanest way to do that in 2022 is to use an SSI2164 IC, which did not exist when the Steiner filters were first invented. This chip is by far the most expensive item in the Karltron lineup, almost $10 for the IC alone. I can’t stock a ton of them and kits may become backordered. It’s worth it to have a clean audio source with well calibrated levels on three channels simultaneously. The fourth channel of the SSI2164 is used by the resonance circuit. We’re using the datasheet circuits, with 20 kiloohm interfaces around it.

With the trimmer at the face you can adjust the cutoff to track volts per octave to make a semi-sinusoidal VCO when resonance is high, or just to keep the filter tracking with your melody completely.