A low-pass gate couples amplitude and brightness on one control voltage for an organic, plucked decay

A low-pass gate (LPG) is a West Coast circuit: a voltage-controlled amplifier with an integrated low-pass filter, so a single control voltage raises loudness and cutoff together — at low CV the sound is soft and dull, at high CV both open (louder and brighter) simultaneously, and both close and darken together as CV falls. This coupling mirrors acoustic instruments, where a hard hit is both loud and bright and a soft one quiet and dull, giving a distinctive organic quality a separate VCF-plus-VCA does not easily reproduce (that would require carefully matching two envelopes). Most LPGs are built around a vactrol — a sealed LED shining on a light-dependent resistor (a photoresistive opto-isolator used as a voltage-controlled potentiometer). Because the LDR responds sluggishly and nonlinearly, even a short trigger yields a smooth, natural decay: this is why a Lopass Gate and a plain resonant low-pass filter sound different even at the same nominal cutoff. The Buchla Lopass Gate is the signature example and is central to the West Coast voice; it has been widely emulated.

Examples

A sine or complex oscillator into an LPG’s audio input, pinged with a short trigger, yields a pluck: loud and bright at the attack, then loudness and brightness falling away together into a marimba/bell-like decay. Reproducing this with a separate VCA and VCF would require matching two envelopes; a plain VCA gives a flat, un-darkening gate.

Assessment

Explain why an LPG sounds more natural for percussion than a VCF and VCA in series with separate envelopes, and name the two parameters it controls with one CV. Explain why a vactrol-based LPG and a resonant low-pass filter differ even at the same nominal cutoff, identifying the physical property of the vactrol responsible.