Polivoks Filter

A 12 dB/oct OTA filter modelled on the Soviet Formanta Polivoks — growly, low-end heavy and brutal. It piles on resonance eagerly, screams into a full self-oscillating sine, and the Drive knob shoves the input into the filter until it grits out. Reach for it when you want a bassline or lead with teeth: it sits somewhere between an acid-Roland squelch and the MS-20's bite.

How it works

A zero-delay-feedback state-variable core gives the clean, stable 2-pole response; two OTA-style tanh stages give it the dirt. The Drive stage overdrives the audio before the filter, and the resonance feedback runs through a saturating cell — so pushing Drive and Res together stacks grit on grit. Past about 90% the resonance crosses into self-oscillation: a bounded, near-sine tone that tracks the cutoff, so the filter doubles as a sine voice.

Controls

  • Cutoff — corner frequency, exponential from roughly 20 Hz to Nyquist

(knob centre lands near 4 kHz). Tracks 1 V/oct.

  • Res — resonance, 0–100%. Eager: a touch adds growl, the top end tips

into rich self-oscillation.

  • Drive — overdrives the input into the filter, 0–100%. Clean at the

bottom, a saturated dirtfest at the top — especially with resonance up.

  • Mode — switches the output response: LP (lowpass) or BP (bandpass).
  • FM — attenuverter for the fm input (scales and inverts exp FM depth).

Inputs & outputs

  • In L — audio to filter. On its own it's treated as mono: one filter does

the work and the same signal comes out of both outputs.

  • In R — the right side of a stereo pair. Patch it and the right channel

gets its own OTA cells, so each side grits out on its own instead of the two clipping into each other. Cutoff, Res, Drive and Mode are shared, so both sides stay locked to the same corner.

  • V/Oct — 1 V/oct pitch into the cutoff (play the filter like a key-tracked

voice, great with self-oscillation).

  • FM — exponential cutoff modulation, scaled by the FM attenuverter.
  • Res — CV summed into resonance.
  • Out L / Out R — the filtered signal (LP or BP, per Mode). Feed stereo in

but cable only Out L and the two sides fold back to mono, so nothing is lost.

Tips

  • Crank Res to self-oscillation and patch a sequence into V/Oct — the filter

becomes a growly sine lead with no oscillator at all.

  • Stack Drive with high Res on a bass for that brutal Polivoks snarl;

back Drive off for a cleaner squelch.