FET vs opto compressor: choosing the right response
Use fast FET compression to catch peaks and optical compression to level phrases. Compare FET, opto, VCA and tube compressors with a practical vocal workflow.
The useful FET vs opto compressor distinction is response: a fast FET design can catch short peaks, while an optical design often smooths longer phrases with a release that depends on the preceding signal. Choose according to the movement you need, then level-match the result. Neither family is automatically better for vocals or drums. Vitric's Caliper Compressor puts FET, Opto, Tube and Clean responses in one plugin, so you can make this comparison on the same track without changing plugins.
What are the types of compressors?
Compressors are usually grouped by the part that turns the level down, the gain element. Four analog families cover most hardware and the plugins modeled on it: FET, a field-effect transistor, fast enough for peaks (the 1176); opto, a light and a photocell, smooth and program-dependent (the LA-2A); VCA, a voltage-controlled amplifier, fast and common on the mix bus (the SSL bus compressor); and variable-mu, where a vacuum tube in the audio path is re-biased to lower its gain, so compression deepens gradually as the level rises (the Fairchild 670). Not every tube compressor is variable-mu. Digital compressors calculate the gain change in software; some model one of these families, others aim for clean, precise control.
FET, opto, VCA and tube at a glance
| Type | Useful starting job | Listen for |
|---|---|---|
| FET | Vocal peaks, drums, parallel excitement | Whether the attack catches too much of the initial hit |
| Opto | Vocal phrases, bass sustain | How the release recovers after a loud or long note |
| VCA | Bus control and deliberate envelope shaping | How attack and release work with the groove |
| Variable-mu | Progressive bus leveling | The way compression deepens as level rises |
| Clean digital | Precise control with little added coloration | Pumping, transient loss and the chosen detector behavior |
These are starting points, not specifications shared by every model. Feedforward or feedback detection, the knee, stereo linking and the control signal can matter as much as the gain element.
What do FET compressors do well?
FET means field-effect transistor, a solid-state component used as the gain-control element in classic designs such as the 1176. The fast response is useful when a vocal or drum has brief peaks that jump ahead of the rest of the performance. A slower attack can retain more of the hit; the fastest setting is not a default recommendation.
Loop a phrase with both strong and soft syllables. Lower the threshold or increase input until the strongest peaks trigger reduction. Adjust attack while listening to the beginning of each word. Set release so the compressor recovers between phrases without making breaths surge. Compare with bypass at a similar output level.
Why choose an opto compressor for vocals?
Optical compressors use a light source and light-sensitive element to control gain. In the classic LA-2A-style response, recovery depends on the recent level and duration of the signal. That makes opto compression useful for evening out a sustained vocal or bass part rather than catching every tiny transient.
Listen through a whole verse and chorus, not just one loud note. A setting that sounds smooth in the verse may hold the chorus down for too long. If a single consonant escapes, edit that peak or add a gentle peak-catching stage instead of forcing the opto to do every job.
What does a VCA compressor do?
VCA stands for voltage-controlled amplifier. A control signal sets its gain, allowing designs with repeatable attack, release, ratio and knee behavior. VCA compressors can be clean or colorful; the circuit around the gain element decides that. They are useful when timing the gain reduction to the groove matters more than choosing a named vintage sound.
How does vari-mu (tube) compression work?
“Mu” is the tube term for gain. In a variable-mu compressor the detector re-biases a tube in the audio path, so the tube itself turns the gain down as the input gets louder. There is no separate gain cell. Manley describes its Variable Mu this way, and names the Fairchild 670 as the classic design that uses the same technique.
Because a tube's gain changes gradually with its bias, vari-mu compression starts gently and deepens as the level rises. That suits a mix bus, a group or a master, where you want to narrow the dynamic range a little without hearing each hit being caught. The timing is slower than a FET's: Manley lists a variable attack of 25 to 70 ms, against 20 to 800 microseconds on the 1176. For a snare that needs a fast attack to tame its peak, reach for a FET instead.
One caution: a tube compressor is not necessarily variable-mu: Tube-Tech describes the CL 1B as an all-tube optical compressor. The amplifier technology and the gain-control mechanism are separate facts. Do not infer attack speed, release memory or knee from “tube” alone.
Classic examples
Three pieces of hardware gave these families their reputations. The names are useful for reading tutorials and session notes; Caliper's modes are behavior models, not copies of any of them.
- 1176: FET. Universal Audio released the 1176 Limiting Amplifier in 1967, with an attack from 20 to 800 microseconds. That fast attack is why it became the usual choice for pushing a vocal to the front of a mix and for explosive drum room sounds, including its “all buttons in” setting.
- Teletronix LA-2A: opto. Its gain reduction comes from an optical attenuator: Teletronix founder Jim Lawrence first used photocells to control audio gain in the early 1960s. It is known for smooth, musical leveling on vocals and bass.
- Fairchild 670: vari-mu. A tube limiter whose tubes do the gain reduction by being re-biased, the technique the variable-mu name describes, and the design the gradual vari-mu response is known by.
So “an 1176 on the vocal” means fast peak control, and “an LA-2A on the bass” means slow, smooth leveling. How much of the dynamic range a track keeps depends far more on the settings than on the name.
1176-style FET or LA-2A-style opto on a vocal?
- Identify the problem. Are individual peaks too sharp, or does the whole phrase change level? Fix major recording or edit problems before adding more processors.
- Try one stage. Use FET for the peaks or opto for the phrase. Keep the output close to the untreated take.
- Use two only if needed. A restrained FET into opto can divide the work: peaks first, slower leveling second. Recheck both stages because the first changes what reaches the second.
- Judge in the mix. Check consonants, breaths, sustain and the space between phrases. More gain reduction is not the goal.
A fast release can distort a low note when gain changes significantly within its waveform. For bass-heavy material, lengthen release or try a program-dependent setting. A detector high-pass can reduce how much the low end drives compression; it does not remove bass from the audio path.
Compare the behaviors in Caliper
Caliper Compressor offers FET, Opto, Tube and Clean modes. These are behavior models, not a claim to reproduce a particular hardware unit. It has no VCA-labeled mode. The shared Mix, output and sidechain utilities let you compare responses in one session.
FET provides ratio buttons including ALL; Opto uses Peak Reduction and its two-stage release. Tube has a soft knee, while Clean offers an adjustable digital response. Start with the mode-specific controls in the manual, then try Caliper in your DAW. Its 3-day trial uses the full plugin and needs no payment card.
Sources and further reading
- Universal Audio: compression circuit families.
- Tube-Tech: CL 1B optical gain control and tube amplification.
- Universal Audio: 1176LN Classic Limiting Amplifier (released in 1967; attack 20 to 800 microseconds; vocals, drum rooms, all buttons in).
- Universal Audio: Teletronix LA-2A Classic Leveling Amplifier (optical attenuator; photocells for audio gain since the early 1960s).
- Manley Variable Mu (HHB) (re-biasing a vacuum tube for compression, as in the Fairchild 670; attack 25 to 70 ms).
- Choosing saturation before or after compression.
