Acid Filter

A TB-303-style resonant low-pass for running external audio through that unmistakable acid squelch. Four-pole ladder character, a high-pass in the resonance path for the classic vocal "wow," and enough feedback to sing on its own. Patch it across a bassline, a drum loop, or anything that wants to drip.

How it works

The signal passes through a four-pole zero-delay-feedback ladder. As you raise resonance, the cutoff frequency is emphasized harder and harder; a one-pole high-pass sits in the feedback loop, which is the trick that gives the 303 its hollow, squelchy bite rather than a generic boom. The input is gently saturated, so pushing level and resonance together adds grit. At full resonance the loop self-oscillates into a pure tone. Stereo input is filtered by two fully independent filters — no cross-feed — so a stereo source keeps its width and the two resonant peaks evolve apart.

Controls

  • Cut — the cutoff frequency, swept exponentially from 50 Hz to 10 kHz. This is the knob you ride for the signature acid sweep.
  • Res — resonance, from clean to screaming. Near the top it self-oscillates; the higher you go, the more it whistles around the cutoff.
  • Mix — dry/wet blend. At 100% you hear the filter alone; back it off to fold the unfiltered signal back in for parallel-filter weight.
  • Squelch — sets where the high-pass inside the resonance loop sits, swept exponentially from 20 Hz to 2 kHz. Roll it low for a fat, round, Moog-like resonance; push it high for the thin, nasal honk that makes a 303 sound like a 303.
  • Drive — pushes the signal harder into the saturator that lives inside the feedback loop, from clean 1x up to a screaming 8x. Because the dirt sits in the resonance path, it interacts with Res for that unmistakable acid scream rather than a flat fuzz.

Inputs & outputs

  • inL / inR — the stereo audio to filter; patch one for mono, both for true stereo width.
  • Cut / Res / Mix / Squelch / Drive CV — voltage control over each knob; patch an envelope to cutcv for plucky per-note acid accents, or wobble any of them from an LFO for evolving squelch.
  • outL / outR — the filtered stereo signal.

Tips

  • The acid sound lives in motion: crank Res high and modulate cutcv with a fast decay envelope per note.
  • Wind Res all the way up with nothing patched to the audio inputs and it self-oscillates — a playable sine you can tune with Cut.
  • Hot input levels lean into the built-in saturation, adding dirt as you drive it harder.