A low-pass gate (LPG) is a module that combines the functionality of a voltage-controlled low-pass filter (LPF) and a voltage-controlled amplifier (VCA) into a single circuit. Low-pass gates are known for their characteristic natural and organic sound, often associated with the “West Coast” or “Buchla” style of synthesis.
The primary function of an LPG is to control both the amplitude and frequency content of a signal simultaneously, creating a more unified and expressive response to the control voltage input, typically an envelope or control voltage source. As the control voltage increases, both the amplitude and the high-frequency content of the input signal increase, resulting in a more dynamic and organic sound.
A low-pass gate module often has the following components:
- Signal input: Where the audio signal (e.g., from an oscillator or noise source) is fed into the low-pass gate.
- Control voltage input: Where a control voltage (CV) signal, such as an envelope or LFO, is used to modulate both the filter and the VCA simultaneously.
- Mode switch or control: Some low-pass gates offer a switch or knob to change the behavior of the module, such as a “VCA only” mode, “LPF only” mode, or “both” mode for the combined response.
Low-pass gates are often built using a special type of component called a “vactrol” (a combination of a light-emitting diode and a photoresistor) or other opto-isolator components. These components introduce a natural, exponential response to the control voltage, creating the characteristic smooth and organic sound associated with low-pass gates.