Optical Audio Compressor

An analog audio compressor built around an LED driving a light-dependent resistor — a photo-coupled gain element sometimes called a vactrol, used in classic optical compressors like the LA-2A. The envelope detector reads the input signal's amplitude, sets the LED brightness accordingly, and the LED's glow drives the LDR's resistance, which in turn controls the gain stage.

What the circuit teaches, and what no textbook paragraph can teach as cleanly, is that compression is a feedback loop made visible in the time domain. The attack and release behavior you hear is literally the thermal and photonic response time of the LED and the LDR. The circuit's sound is its own physics. Tuning the gain staging and the attack/release behavior of the envelope detector was most of the work, and listening to the compressor's own transient response on different sources is how you develop the intuition that a Waves plugin UI can't teach you.