A lowpass gate is essentially a low pass VCF configured to behave like a VCA. The lowpass gate accepts an audio input and a control signal, in the manner of a VCA. When there is no control signal present, the filter's cutoff frequency is in the subsonic range, well below the audio frequencies; therefore, no audio passes the filter. Applying a control voltage causes the cutoff frequency to rise significantly, into the upper end of the audio range, so that most of the audio at the input now passes. If the output of an envelope generator is presented to the control input, the lowpass gate will shape the note envelope as a VCA would, but with some characteristic differences.
The concept of the lowpass gate originated with the Buchla 200 series modular synthesizer series, which offered a lowpass gate as a module in the series. The Buchla design used a vactrol to process the control voltage input; the vactrol had certain non-linearities which made the lowpass gate a good module for imitating some tuned percussion sounds. By feeding in short bursts of noise, or just impulse spikes, the lowpass gate could be made to produce sounds that resembled hand drums, congas, steel drums, or marimba. The original Buchla module is highly sought after and has inspired a number of imitators over the years.
Component Kit does NOT include the Vactrol
LPG Datasheet and Build Guide