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A Vactrol is an electronic component used in modular synthesizers and other audio equipment that combines an LED (Light Emitting Diode) with a photoresistor or a photodiode in a single package. When the LED is illuminated, the resistance of the photoresistor or photodiode changes, allowing for voltage-controlled adjustments of the connected circuit.

In modular synthesizers, Vactrols are often used for creating voltage-controlled elements such as low-pass gates (LPGs), filters, VCAs (Voltage Controlled Amplifiers), or any other module where a smooth, natural-sounding response is desired. The unique characteristic of a Vactrol is that it has an inherently slow and smooth response time, which mimics the behavior of some acoustic instruments and imparts a more organic, “musical” quality to the sound.

The Vactrol’s slower response time is due to the time it takes for the LED to light up and the photoresistor to react to the change in light. This characteristic makes Vactrol-based modules particularly suitable for creating percussive and plucked sounds, as well as imparting a distinct character to filter sweeps and amplitude changes.

Vactrols have become a popular component in the design of certain modular synthesizers, especially in the “West Coast” synthesis approach popularized by Don Buchla and Serge Tcherepnin. While some modern modules may use digital or other analog techniques to emulate the behavior of Vactrols, many synthesists still appreciate the unique, natural-sounding response of true Vactrol-based designs.