The heart of the compressor circuit, shown in Figure 1, is IC2, an NSL-32 resistive optocoupler. The value of the resistor inside the device depends on the amount of light produced by its built-in LED that shines on it. This way, the resistor value can be varied from about 500 kΩ (off) down to 500 Ω (on). This type of photoresistive element is also called a vactrol. They are widely used in audio applications, such as in compressors, filters, and envelope generators, where their slow response time and “soft” behavior are valued for their musical qualities.

Vactrol IC2 is the magic part in this circuit that allows the dynamic compression of an audio signal.
Figure 1: Vactrol IC2 is the magic part in this circuit that allows the dynamic compression of an audio signal.

IC2’s resistor together with R1 form a light-dependent attenuator. When the signal becomes too strong, the LED inside IC2 will light up. This will lower the value of IC2’s resistor, attenuating the input signal. The stronger the input signal, the higher the attenuation will be.

The dynamically attenuate...