Compressor types: VCA, FET, opto, vari-mu, digital

Why the type matters

Every analog compressor uses a different element to reduce the gain, and each has its own timings, curves and distortion. Emulations reproduce their character. Choosing the right type for the source is half the job: an opto on a ballad vocal, a FET on a snare or an aggressive rap vocal, a VCA on a drum bus, a vari-mu on the mix bus.

VCA (Voltage Controlled Amplifier)

• How it works: a voltage-controlled amplifier, precise and fast, with adjustable times and predictable behavior.
• Character: clean, punchy, controlled; can be very fast. With slow attacks it “pushes” the transients.
• Models: dbx 160 (program-dependent attack and release, “OverEasy” soft knee in later versions), SSL G Bus Compressor (ratio 2, 4, 10:1; attack 0.1, 0.3, 1, 3, 10, 30 ms; release 0.1, 0.3, 0.6, 1.2 s and Auto), API 2500 (feed-forward/feedback, “Thrust” that filters the lows in the sidechain), Focusrite Red 3.
• On what: drum bus, mix bus (glue), kick, snare, bass, rhythm guitars.
• SSL mix bus start: 2:1, attack 10–30 ms, release Auto or 0.1–0.3 s, GR 1–3 dB.
• Drum bus start: 4:1, attack 10–30 ms, release 0.1 s or Auto, GR 3–4 dB.
• Plugins: Waves SSL G-Master Buss, UAD SSL G, Cytomic The Glue, Arturia Comp VCA-65 (dbx 165), Native Instruments VC 160, Fruity Compressor in Hard/Medium.

FET (Field Effect Transistor)

• How it works: a field-effect transistor used as a variable resistor, in a feedback configuration. Very fast.
• Reference model: UREI/UA 1176: attack from 20 to 800 µs, release from 50 ms to 1.1 s (program-dependent), ratio 4, 8, 12, 20:1. Fixed threshold: you compress by raising Input and recover with Output. The attack and release knobs are “backwards”: fully clockwise = faster. All-buttons (British mode): all ratio buttons pressed together, effective ratio between 12 and 20:1, much higher distortion, altered timings, an explosive sound.
• Character: aggressive, present, adds harmonics and “bite”; pulls out detail and ambience.
• On what: rap and rock vocals, snare, room mics, bass, guitars, parallel compression of drums.
• Starts: vocal: ratio 4:1, attack at 12 o'clock (about 300–400 µs), release at 2–3 o'clock (fast), GR 3–7 dB. Snare: 4:1, slow attack (knob at 9 o'clock), fast release, GR 4–6 dB. Parallel room: all-buttons, medium attack, fast release, GR 10–15 dB.
• Others: Empirical Labs Distressor (digital in its control, analog in its sound, emulations of various types), Purple Audio MC77.
• Plugins: UAD 1176, Waves CLA-76, Arturia Comp FET-76, IK, Native Instruments FET Compressor; in FL an approximation with Fruity Limiter in COMP, minimum attack, release 50–100 ms, plus light saturation.

Opto (optical)

• How it works: the signal lights a light source (electroluminescent panel or LED); a photoresistor reduces the gain in proportion to the light. The cell has inertia: slow, program-dependent timings.
• Reference model: Teletronix LA-2A: T4 cell, fixed attack of about 10 ms, two-phase release (about 50% in 60 ms, the rest in 1–15 s, longer after heavy compression); only two controls, Gain and Peak Reduction (plus a Compress/Limit switch). LA-3A (transistor version, faster and brighter), Tube-Tech CL 1B (tube opto with adjustable timings).
• Character: soft, musical, “invisible”; levels without attacking the transients.
• On what: vocals (especially as a second stage after a FET), electric bass, strings, pads, acoustic guitar, piano.
• Starts: vocal: Peak Reduction up to 2–5 dB on loud phrases; bass: 3–6 dB; mix bus rarely.
• Plugins: UAD LA-2A, Waves CLA-2A, Pro-C 2 in Opto style, Fruity Compressor in Vintage/R as an approximation.

Vari-mu (variable-gain tube)

• How it works: tubes whose amplification (mu) changes with the bias voltage: more signal = less gain. The ratio grows with the level: a very soft knee, compression that “tightens” on strong peaks.
• Models: Fairchild 660/670 (1950s–60s, six “time constants”: attack of about 0.2–0.4 ms, release from about 0.3 s up to 25 s in program-dependent mode — approximate values), Manley Variable Mu (1990s, the name “Variable Mu” is a Manley trademark; widely used in mastering), Gates STA-Level, Altec 436.
• Character: dense, warm, “glues”, rounds off transients; tube color even with little GR.
• On what: mix bus, mastering, vocal and string buses, drum bus when you want softness.
• Starts: mix bus: GR 1–2 dB, slow attack, medium-slow release; mastering: 0.5–1.5 dB.
• Plugins: UAD Fairchild and Manley Variable Mu, Waves PuigChild, Pulsar Mu, IK.

Other analog types (for completeness)

• Diode bridge (Neve 2254, 33609): thick, grainy color, great on buses and vocals.
• PWM (Pye, Crane Song): cleanliness and high speed.

Digital

• How it works: calculates the reduction in software: exactly adjustable timings, lookahead, peak/RMS detector, variable knee, filtered sidechain and mix, no noise and no distortion unless wanted.
• Character: transparent or, with modeling styles, similar to the analog types.
• Plugins: Fruity Compressor (threshold 0/−60 dB, ratio 0.4–30:1, attack 0–400 ms, release 1–4000 ms, Type Hard/Medium/Vintage/Soft with or without automatic release /R), Fruity Limiter (COMP), Maximus, FabFilter Pro-C 2 (styles Clean, Classic, Opto, Vocal, Mastering, Bus, Punch, Pumping), Waves Renaissance Compressor, TDR Kotelnikov (free, mastering).
• On what: everything, especially when precision is needed (peak control, sidechain, mastering).

Quick comparison

Classic combinations

• Vocal: FET (peaks, 3–5 dB) → opto (leveling, 2–3 dB). “1176 → LA-2A” is the most-cited vocal chain.
• Electric bass: opto (leveling) → light FET (peaks) or vice versa.
• Drums: VCA on the drum bus + all-buttons FET in parallel.
• Mix bus: VCA (SSL) or vari-mu, 1–2 dB.

By genre

• Trap/rap: aggressive FET on the vocal, precise digital on the 808.
• Pop: serial chains (FET + opto + digital) for a rock-steady vocal.
• Rock: FET on snare and room, VCA on the bus, opto on the bass.
• Jazz/acoustic: light opto and vari-mu.
• EDM: digital for sidechain and control, VCA for glue.

Common mistakes

• Thinking the “famous” model is always right: a 1176 on a slow pad is useless.
• Forgetting that emulations have color and saturation even at zero GR: check the input level (many are calibrated with 0 VU = −18 dBFS).
• Fairchild/vari-mu pushed beyond 3–4 dB on the mix bus: muddiness and loss of punch.
• Using the LA-2A to tame fast transients: it is too slow, you need a FET or a limiter first.

How to hear it

• Same vocal, same GR (5 dB), compare FET, opto, VCA and digital at equal volume: listen to consonants, breath and tail.
• On a drum loop: VCA attack 30 ms = punch; FET minimum attack = squashed and aggressive; opto = soft and without punch.