Compressor Types Explained: OPTO vs FET vs VCA, and How to Choose by Source
Every compressor type does the same job. When the input crosses the Threshold, the part above it is reduced according to the Ratio. What differs is how the gain-control element moves.
That movement changes how compression comes in and how it lets go. Set two different types to a 10ms attack and a 100ms release, and they still won't sound the same.
OPTO (Optical)
An optical compressor pairs a light source with a light-sensitive resistor. The resistor takes time to react to changes in light, so the whole thing moves slowly.
Compression comes in gently, which tends to leave transients intact. Release time depends on the signal level, and the gain recovers more slowly right after heavy compression. You often can't tell it's working, yet the level evens out. It suits vocals with uneven dynamics, sustained bass, acoustic guitar, strings and pads.
It's too slow to reliably catch fast peaks. On busy rhythmic parts, the gain may not recover before the next note, and the sound stays pushed back.
FET (Transistor)
A FET compressor uses a field-effect transistor as a variable resistor. It reacts very quickly, so even very short attack times work.
It grabs peaks fast, and pushed harder it makes a sound jump forward. It also adds a little harmonic color. People reach for it on vocals that need to sit up front, snare and kick, punchy bass, and drum buses.
A fast attack can clamp the transient and take the punch out of each hit. There isn't much distance between energetic and squashed, so compare against bypass at matched loudness.
VCA (Voltage-Controlled Amplifier)
A VCA compressor sets its gain with a control voltage. It follows Threshold, Attack and the other controls closely, so the result is easy to predict.
It adds very little color and leaves the source mostly as it was. If you want some color, add saturation after it. It can hold peaks firmly at high ratios or glue a bus together at low ones. It works well on the stereo bus, and on vocals or bass that need precise dynamic control.
The three types at a glance
| OPTO | FET | VCA | |
|---|---|---|---|
| Response | Slow | Very fast | Follows settings (wide range) |
| Impression | Smooth, natural | Energetic, dense | Clean, accurate |
| Color | Mild | Noticeable | Minimal |
| Best on | Vocals, acoustic guitar, sustained bass | Drums, punchy vocals and bass | Buses, anything needing precise control |
Choosing by source
Treat the numbers below as starting points. Watch the gain reduction (GR) meter and adjust based on how much compression you're actually getting.
- On vocals, use OPTO or FET with about 3 to 6dB of GR on the peaks. OPTO with a slower attack smooths the voice out. FET pushes it forward.
- On bass, OPTO or VCA keeps the low end steady, and FET adds punch. Start around 3 to 6dB of GR. A slower attack keeps more of the punch. For a step-by-step walkthrough, see Are Bass Compressor Settings Difficult? Why Beginners Struggle & Easy Setups.
- On individual drums, FET is the usual pick. A slower attack lets the transient through and keeps the snap. A faster one tightens the sound and makes it denser.
- On the drum bus or stereo bus, apply a VCA lightly, around 1 to 3dB of GR. Listen for pumping.
- On acoustic guitar, a light touch of OPTO evens out the strumming without drawing attention.
How attack and release behave
Attack and release mean the same thing on every type, but they act differently. OPTO is slow by nature, so a given setting feels looser. FET is fast, so the same setting feels sharper.
When you switch types, don't keep the old numbers. Set them again by watching the GR meter and listening.
Saturation after the compressor
A compressor's main job is controlling level. If the track feels a little flat after compression, you can put saturation after the compressor to add harmonics. Put it before, and the compressor also reacts to the peaks the saturation adds, which changes how it compresses.
- TUBE adds even-order harmonics and a warm, round thickness. Try it on vocals and acoustic guitar.
- XFMR (transformer) adds weight to the low end. Try it on bass and buses.
- TAPE softens the highs and helps parts blend. Try it on buses and drums (more in Saturation and Tape Basics: How Harmonics Are Created and What Makes Tape Sound Like Tape).
- FET gives a slightly edgy push. Try it on drums and guitars.
Too much gets muddy, so stop a little short of where it sounds just right. Saturation also raises the level, so match loudness again before comparing.
Letting the source pick the type: AC-1 Adaptive Compressor
AC-1 Adaptive Compressor from AIDE AUDIO is a compressor with OPTO, FET and VCA modes. Its LEARN function picks the type and the starting values of the parameters from your material.
How to use AC-1
- Choose a SOURCE: Pick the kind of material. If you're not sure, Universal is fine.
- Press LEARN and play 8 seconds: Play your actual material until 8 seconds of sound have been measured. Silence and very quiet passages aren't measured.
- Get your settings: From the measurements, AC-1 sets the compressor type (OPTO / FET / VCA) and values such as THRESHOLD, RATIO, ATTACK, RELEASE and KNEE.
LEARN sets the values once and keeps them. It doesn't keep adjusting during playback. Treat the result as a starting point and adjust it by hand.
After LEARN, TARGET lets you set how much gain reduction you want. Drag the marker on the GR ring anywhere from 1 to 20dB, and RATIO, KNEE and THRESHOLD change to match. If you already know you want about 3dB, you don't have to turn each knob.
After the compressor comes DRIVE, with four saturation types: TUBE, XFMR, TAPE and FET. LEARN doesn't switch DRIVE on, so turn it on yourself if you want it.
AC-1 is a one-time purchase, currently at a launch price of $14.99 (regular price $29.99). One serial covers up to three computers by default. The trial gives you every feature for 7 days.