Compressor Effects

An optical, or "opto," compressor uses a small light source shining onto a light-dependent resistor to control gain reduction: the circuit brightens the light in response to loud signal, and the resistor's response to that changing light is inherently a little slow and non-linear, rather than instantaneous. That lag is what gives opto compression its characteristically smooth, natural-sounding response rather than an abrupt clamp, which is why it's favored for rhythm guitar and vocal parts where an audibly "working" compressor would be distracting.

FET (Field Effect Transistor) compressors use a transistor as a voltage-controlled resistor, which can react to level changes almost instantly compared to an optical circuit's lag. That speed lets a FET compressor grab a transient the moment it happens, which is why these designs are known for a punchier, more aggressive-sounding compression that adds attack and presence rather than the opto style's smoother, more forgiving squeeze.

A VCA (Voltage Controlled Amplifier) compressor uses a dedicated amplifier chip whose gain is set directly by a control voltage, rather than relying on an optical or transistor circuit's inherent response curve to shape the sound. That direct control makes VCA compression more precise and transparent, closer to a pure volume adjustment than a coloring effect, which is why it's the design most modern pedals and studio bus compressors default to when the goal is control rather than character.

Tube compressors use vacuum tubes somewhere in the gain-reduction or makeup-gain path, and tubes naturally distort in a soft, even-harmonic-heavy way as they're driven harder, unlike the cleaner limiting of a solid-state circuit. That means a tube compressor is coloring the tone as it compresses, adding a low-level harmonic saturation on top of the level control, which is why these designs are prized for sounding musically "fat" rather than strictly transparent.

Clean guitar has nowhere to hide dynamic inconsistency the way a distorted tone does, since overdrive naturally compresses the signal on its own. On an uncompressed clean tone, a fingerpicked bass note can sit noticeably louder or quieter than the treble notes around it, so a compressor's main job here is balancing that note-to-note evenness and adding a touch of sustain that a clean signal wouldn't otherwise have.

On distorted or overdriven electric tones, compression is less about evening out volume, since the gain stage already does some of that, and more about extending the sustain of lead lines and tightening the attack of rhythm playing so palm-muted chugs feel consistent from one hit to the next. Placed carefully, it can also raise a signal's average level closer to feedback threshold without raising its peaks as much, which is part of how sustained, singing lead tones are achieved at stage volume.

Compression is one of the easiest effects to overuse without noticing, because unlike distortion or modulation, over-compression doesn't announce itself with an obviously different sound; it just flattens the life out of your dynamics until every note feels the same. Listening specifically for pumping (an audible swell as gain reduction releases) or breathing (a rise in background noise between notes) is how you catch a setting that's gone too far, and bypassing the pedal every so often to compare is the only reliable way to notice compression that's crept in gradually.

Compressing before an overdrive or distortion pedal evens out your picking dynamics before the gain stage amplifies them, which keeps the drive tone consistent regardless of how hard you pick. Compressing after gain instead squeezes the already-distorted signal itself, smoothing out the sustain and note decay of the drive tone rather than the input dynamics, so the two placements solve genuinely different problems rather than one simply being more correct.