Low-Pass Gate (LPG)

A Buchla-style gate that opens with a fast attack and a slow, springy release — the source of the plucky, percussive "bongo" tail that defines West Coast synthesis. It dims volume and tone together, so quieter notes also sound darker, just like a real acoustic instrument decaying.

How it works

A control signal charges a simulated vactrol — an opto-isolator whose photocell snaps open quickly but releases slowly and exponentially. That asymmetric envelope is the gate's memory: it both lifts a lowpass filter's cutoff and drives a VCA whose gain follows a squared curve, so low control values barely open the gate while high values throw it wide. The result is a single pluck that fades in tone and level at once. Attack governs how fast that photocell opens, so the same gate can either strike or slowly swell, while Color sets how far the filter opens for darker or brighter voicings.

Controls

  • Level — the base amount the gate is held open; it sums with the CV input. Leave it at zero to play purely from triggers, or raise it to bias the gate ajar.
  • Response — the length of the release tail, from a snappy 2 ms click up to a long 2 s bongo decay.
  • Attack — the vactrol's rise time, the other half of the envelope. Fully down it's a sharp 0.5 ms strike; wind it up toward 500 ms and each note swells in like a bowed or pad attack instead of a pluck.
  • Color — how far the gate opens the lowpass at full travel, from a dark, wooden, muted voicing up to fully bright. Turn it down for the classic marimba/bongo timbre; it has no effect in VCA mode.
  • Mode — chooses the signal path: Both (lowpass then VCA, the full LPG), VCA (amplitude gate only), or LPF (dynamic filter with no volume gating).

Inputs & outputs

  • In L / In R — the audio to be gated and filtered. Mono? Just use L — one input is gated once and sent to both outputs. Patch a stereo pair and each side gets its own filter path, while a single shared vactrol opens and closes both together, so the pluck stays one gesture across the image.
  • CV — the gate/control input: pings the gate open and sums with Level. Patch a trigger or envelope here — each pulse plucks a note whose decay is set by Response.
  • Level / Response / Attack / Color CV — voltage control over each matching knob; patch an LFO or envelope to sweep the gate's bias, tail length, rise time, and brightness independently of the gate trigger.
  • Out L / Out R — the gated, filtered signal. With stereo in but only L cabled, the channels fold to mono automatically so nothing goes missing.

Tips

  • For the classic pluck, set Level to zero and fire short triggers into CV; dial Response to taste the tail length.
  • The squared VCA curve means small control values stay nearly silent — push Level or CV harder if the gate feels too closed.
  • Use LPF mode to ride the filter as a dynamic tone control without touching volume, or VCA mode for a clean, plucky amplitude gate.
  • Raise Attack alongside a long Response to turn the gate into a slow swell — bowed strings or evolving pads instead of plucks.