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 behavior 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.
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 about VCA and variable-mu compressors?
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.
In variable-mu compressors, tubes perform gain reduction. But 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.
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.
