Ladder Filter

A Moog-style 4-pole transistor-ladder low-pass — the classic creamy, fat synth filter. It rolls off at a steep 24 dB/oct, overdrives with warm saturation, and at high resonance sings a clean, musical self-oscillation you can play as a sine voice.

How it works

Four one-pole low-pass stages are cascaded inside a single feedback loop. Turning up resonance feeds more of the output back into the input, emphasising the cutoff frequency; push it far enough and the loop sustains itself, oscillating at the cutoff pitch. A transistor-style tanh saturation sits on both the feedback path and the driven input, so the resonance stays smooth and bounded instead of blowing up, and the drive control adds genuine analog grit. The output is tapped after the four poles, which keeps even the self-oscillation a clean sine.

Controls

  • Cutoff — the corner frequency where the low-pass starts rolling off; tracks 1V/oct, so it can be played like a pitch. Default sits around 4 kHz.
  • Res — emphasis at the cutoff. Low for gentle tone-shaping, high for a vocal peak, all the way for self-oscillation.
  • Drive — input gain into the ladder. Clean at 0; as you push it the filter saturates harder and thickens, with makeup gain keeping the level roughly steady.

Inputs & outputs

  • In L — the left (or mono) audio (or CV) signal to filter.
  • In R — the right signal. Patch it and the right channel gets its own ladder, running through the same Cutoff, Res and Drive settings so both sides track identically. Leave it empty and the left input is filtered in mono and sent to both outputs.
  • 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 (centred is off).
  • Res — resonance modulation; sums straight into the resonance amount.
  • LP L / LP R — the 24 dB/oct low-passed output. If you patch both inputs but only take LP L, the two channels are folded together so the right side isn't lost.

Tips

  • Crank Res to the top with nothing patched to In L and play V/Oct — the ladder becomes a pure, fat sine oscillator.
  • Patch an envelope to FM for classic plucky filter sweeps; a touch of Drive adds the bite that makes bass and lead lines cut through.
  • Stack Drive with high Res for squelchy, overdriven acid tones.