Compressors

A compressor doesn't add gain, distortion, or delay — it changes the relationship between a signal's loud and quiet parts, automatically turning the gain down above a threshold and back up as the signal falls away. This chapter covers the two circuit families that define the category — optical (LED/LDR) and VCA (OTA-based) — and the four controls, threshold, ratio, attack, and release, that every compressor design is built around.

Every other circuit in this book changes a signal’s tone or adds distortion. A compressor does neither — it changes the relationship between a signal’s loudest and quietest moments, automatically turning gain down once the signal crosses a threshold and back up as it falls away again. The result reads as more sustain, a more consistent level, and — depending on how aggressively it’s set — a “squashed” or “punchy” character, but the mechanism underneath is gain reduction responding to level, not any kind of clipping or filtering covered elsewhere in this book.

The four controls every compressor design shares

Threshold sets the level above which gain reduction starts; below it, the signal passes at unity gain untouched. Ratio sets how much reduction is applied once above that threshold — a 4:1 ratio means every 4dB the input exceeds threshold becomes only 1dB of output increase, and pushing ratio toward 10:1 or higher starts to behave like a hard limiter rather than a musical compressor. Attack sets how quickly the compressor responds once the signal crosses threshold — a fast attack (under a millisecond) catches a pick attack’s transient immediately, while a slower attack (tens of milliseconds) lets that initial transient through uncompressed before gain reduction kicks in, which is part of why some compressors sound “punchier” than others at the same ratio. Release sets how quickly gain returns to normal once the signal drops back below threshold — a fast release can produce an audible “pumping” as the gain visibly recovers between notes, while a slow release smooths that recovery into something closer to a gentle, continuous leveling.

Optical compressors: an LED and a light-dependent resistor doing the gain reduction

An optical compressor — the mechanism behind the classic LA-2A-style sound — uses an LED whose brightness tracks the signal level, shining onto a light-dependent resistor (LDR) whose resistance changes with that brightness. Wiring the LDR into a gain stage’s feedback path turns changing light into changing gain: louder signal, brighter LED, lower LDR resistance, more gain reduction. The LED and LDR’s own physical response time sets the compressor’s attack and release characteristics directly — commonly a few milliseconds of attack and tens to low hundreds of milliseconds of release — which is exactly why optical compression reads as smooth and “invisible” rather than aggressive: the light-based gain control simply can’t react instantaneously, and that inherent lag is the character, not a limitation being fought against.

VCA compressors: an OTA doing the same job, much faster

A voltage-controlled-amplifier (VCA) compressor — the mechanism behind the MXR Dyna Comp and most classic pedal compressors — uses an operational transconductance amplifier (an OTA, commonly a CA3080 or similar) whose gain is set directly by a bias current rather than by light and a photoresistor. An envelope detector (a diode rectifier tracking the signal’s amplitude) drives that bias current, so gain reduction tracks the signal far more immediately than an optical design’s LED/LDR lag allows — attack times under a millisecond are achievable, and the resulting character is faster, more aggressive, and distinctly “squishy” rather than smooth, the sound behind chicken-pickin’ country compression and rhythm-guitar squash alike. The classic single-knob “Sustain” control on a Dyna Comp-style pedal is doing double duty as a simplified threshold control — one knob standing in for what a studio compressor would split across several.

Choosing between optical and VCA is choosing a character, not a spec sheet

Neither topology is objectively better — they’re different tools for different jobs. Optical compression’s slower, LED/LDR-limited response reads as smooth and musical, well suited to sustain and clean leveling where the compression itself should stay out of the way. VCA compression’s faster envelope response reads as punchier and more present, better suited to rhythm work and the deliberately audible “squish” some styles specifically want. A JFET-based compressor sits in between — fast like a VCA design but with its own distinct character — and a limiter is simply a compressor pushed to a very high ratio, prioritizing a hard ceiling over musicality.

Common mistake: troubleshooting a compressor like a gain stage

Because a compressor’s gain is deliberately signal-dependent rather than fixed, checking it with the same fixed-bias-point expectations used for an overdrive or fuzz stage (see Boost and Buffer for the fixed-gain case) leads to false alarms — a gain-control node that moves around while playing isn’t a fault, it’s the compressor doing its job. The genuinely useful checks are different: confirm the OTA or gain-stage IC has proper supply voltage, confirm the envelope detector’s output actually tracks input level (near 0V at idle, rising with signal), and confirm the gain-control node isn’t stuck at one extreme — pinned at either end means no compression is happening (stuck low) or maximum compression is stuck on regardless of input (stuck high), and either points at the envelope detector’s rectifier diode or capacitor rather than the gain stage itself. Pedal Troubleshooting covers this same low-output/no-effect fault pattern for pedal circuits generally.