Sallen-Key Filter

An aggressive 2-pole (12 dB/oct) resonant filter voiced after the Korg MS-20. Where a clean filter gives you a polite peak, this one screams: as you push the resonance the peak overdrives and distorts, and at the very top it tips into a singing, self-oscillating tone. Reach for it when you want a filter with attitude — squelchy acid lines, snarling leads, and resonance that bites back.

How it works

A stable zero-delay-feedback state-variable core supplies the 12 dB/oct slope and stays perfectly in tune, while the resonance is a separate feedback loop wrapped around it. That feedback runs through a diode-style clipper, so the louder the resonance rings, the harder it saturates — the peak gains harmonics and grit instead of staying clean, and the same clipper keeps it from ever blowing up. Add the drive stage in front and you can push the whole thing into musical overdrive.

Controls

  • Cutoff — the corner frequency, roughly 20 Hz to 20 kHz. Tracks 1V/oct, so you can play it like a pitch and let it sing.
  • Res — resonance. Low settings give a focused peak; from about two-thirds up it starts to distort and scream; at the top it self-oscillates into a bounded tone.
  • Drive — input gain into the filter, with soft saturation. Clean at zero, thick and overdriven as you turn it up — and it feeds the resonance clipper harder, so it makes the scream nastier too.
  • FM — attenuverter for the FM input: scales and inverts how much that CV moves the cutoff. Centred is off.

Inputs & outputs

  • In — the audio (or CV) signal to filter.
  • V/Oct — 1V/oct pitch control of the cutoff, for keyboard- or sequencer-tracked filtering.
  • FM — cutoff modulation; depth and polarity set by the FM attenuverter.
  • Res — resonance modulation; 0–10 V sweeps from no extra resonance up to full.
  • LP / HP — the low-pass (the signature voice) and high-pass versions of the input, available at the same time.

Tips

  • Crank Res to the top with nothing patched to In and play V/Oct — the filter self-oscillates as a raw, slightly dirty sine-ish oscillator.
  • Patch a snappy envelope to FM and ride Res high for classic squelchy, screaming acid sweeps.
  • Push Drive and Res together for an overdriven, vocal honk; back the drive off for a cleaner peak.