The Buchla Bongo: Lowpass Gates
A lowpass gate couples a filter and an amplifier so that quieter sounds are also duller, just like real acoustic instruments. Driven by a vactrol's asymmetric response, a single ping produces the organic, woody percussion that defines West Coast synthesis.
Why Quieter = Duller
Strike a drum softly and it doesn't just sound quieter; it sounds darker. High-frequency partials decay faster than low ones because they lose energy more quickly to the vibrating membrane and surrounding air. This is a fundamental property of acoustic sound: loudness and brightness are coupled.
A standard synthesizer VCA doesn't model this. It scales all frequencies equally, so a quiet note has exactly the same timbre as a loud one, just softer. The result sounds electronic in the worst sense: lifeless and disconnected from the physics of real instruments.
The Lowpass Gate
A lowpass gate solves this by combining a voltage-controlled filter and a voltage-controlled amplifier in a single circuit, driven by the same control signal. When the control voltage drops, the sound gets both quieter and darker simultaneously. The filter cutoff and gain track each other, creating a natural acoustic decay that a VCA alone cannot achieve.
Most lowpass gates offer three modes for comparison. VCA mode bypasses the filter; you hear a standard amplitude gate. VCF mode keeps the amplifier open and only sweeps the filter. Both mode couples the two together. This is the signature lowpass gate sound where quiet equals dull.
The Vactrol Secret
The classic lowpass gate, Don Buchla's Model 292, uses a vactrol: an LED shining on a light-dependent resistor (LDR), sealed in a small package. The LED responds instantly to voltage changes, but the LDR is physically slow. It heats up quickly when illuminated (fast attack) but cools down very gradually (slow, exponential decay). This gradual, curved decay is called exponential: each moment it loses a fixed percentage of what remains, rather than a fixed amount. A linear decay would lose the same amount each moment, like a clock ticking down. This asymmetry is what gives the lowpass gate its character.
A standard ADSR envelope has a predictable, linear-ish shape. The vactrol's response is different: a sharp onset followed by a long, gently curving tail that never quite reaches silence. This two-stage decay, a fast initial drop followed by a slow fade, creates percussion sounds that feel organic and alive, as though they're resonating in a physical space.
The Buchla Bongo
The iconic "Buchla bongo" is disarmingly simple: a triangle wave through a wavefolder, pinged through a lowpass gate in coupled mode. The wavefolder adds harmonic complexity that the LPG's filter can sculpt during the decay. The vactrol's asymmetric response does the rest: a sharp attack blooms into a woody, resonant tail. Organic percussion from pure electronics, with no samples and no noise generators.
Try it: hit Ping for the classic bongo, or play the keyboard to trigger the lowpass gate with sustained notes.