VCA, Opto, Vari-Mu, FET compressors… When to use which ?


As you probably already know, I’m a big fan of analog compressors. I owned several of them in the last few years and I keep rotating and accumulating them in my rack. My objective was to have at the very least one of each kind into my rack, an objective I have accomplished only relatively recently.

Being a fanatic of compressors, I was amazed to hear some people ask questions like “Should I buy a LA-2A or a 1176 ?”. I mean, these are completely different machines. What are you trying to achieve in terms of texture exactly ?

The truth is that each type of compression will have a distinctive sound and one of the secret of achieve textures, is by having the right combination of compressors doing the right things. You cannot interchangeably switch a LA-2A with a 1176, they have little in common.

The more I search on the net, the more I realize that people do not master the differences between each compressor and don’t seem to know when to use which and most importantly, WHY?

I had this guy who bought one of my 1176 recently. This guy is a freelancer and does a lot of speech recording. He is obsessed with clean and pure sound. Yet, he wanted absolutely to have an LA-2A for some reason. Well, it’s not because one piece of gear is popular that it suits your needs. LA-2A has a very nice distortion content, which makes a vocal cut through mixes, but that effect wouldn’t really appropriate for speech recording.

Back to the subject, there are 4 big families of compressors, and I would like to review each of them with you. I added a picture of the compressor I’m using for each type.  Here they are:



VCA stands for “Voltage Controlled Amplifier” and its compression behaviour is based on PEAK, with fast attack and release.  We will start with that one as it is arguably the most used one. Most of the compression plugins are based on its principle.

This family of compressor tend to react almost too quickly, in my opinion, as they are very sensitive to micro-dynamics and transients. VCA tend to be very efficient for some applications and completely inappropriate for others. Their response curve is generally linear (hard knee), but some design integrated the soft knee in order to adapt them to mix bus compression purposes.

When to use it: You should use it when you have transients that are some order of magnitude out of the dynamic range where they should be. For example, a very percussive recording will benefit greatly from having a VCA controlling the peaks as it will do it efficiently and transparently.

When not to use it: You should not use a VCA when you try to adjust the average volume of a song. This thing has no macro-dynamic effect whatsoever. It’s good for instantaneous drastics changes and peaks, and that’s it. It doesn’t really smooth out stuff neither as would do a varimu or an opto.

The advantages of using VCAs: It can take care of intense transient with transparency. It can give a sense of punch and aggressiveness.

The limitations: It will always sound “thin”. It’s rather hard to warm up a signal with a VCA. It feels sterile. It’s also harder to make it feel smooth as cake. To do so, longer release time are required, but by doing so feels like covering the speaker with a sheet.

Exemple of compressors using this design: SSL, Neve and API mix bus compressors, Focusrite RED, DBX 160, Alan Smart C1.



I’ve recently read a funny quote in book about mastering. The worst thing is that It was written by someone knowledgeable… It was basically saying something like “Since there is nothing faster than speed of light, the opto compressor acts very rapidly”. This makes me laugh, as the opto is about as slow and smooth as a compressor can possibly get.

Opto uses photocells as a detector and a light bulb to determine the gain reduction. As the signal passes through the light bulb, it will make the light bulb glow more or less depending on the intensity of the signal. Since the intensity of the light is function of the temperature of the filament, the light intensity will vary as a smoother version of the signal. In other words, if the detector in the VCA design sees the exact signal, the opto one will see an averaged over time version of it.This makes the opto compression much less sensitive to transients, peaks and sudden spikes. For this reason, much higher ratios can be used.

In the digital world, the opto effect can be simulated using an “RMS” based compressor. As opposed to peak compressors, the detector will calculate the “average” (or the area under the curve) over a certain amount of time and will base its decisions on it.

When to use it : Opto will do a wonderful job at taking care of macro-dynamics. Basically, it can even out the average levels of a song. For example, if a song is very quiet at the beginning but quite loud at the end, a VCA would do absolutely nothing during the first section and then smash everything during the loudest part, where an opto would work a bit all the time and even out the song levels without even be noticable. It also can be used when you want to tighten up a bit the mix without killing the transient and leach the life out of it.

When not to use it: When you have intense peaks and spikes, it will simply not be able to handle them. It will let them pass for once, but it will also make the opto pump in an obvious way. Also, bass heavy program will make the compressor pump as well. JLM Mac Opto comp I use has a high pass side chain filter for this purpose.

The advantages of using Optos: Very transparent. Tightens up a mix without getting noticed. Doesn’t flat out the transient.

The limitations: Pumping is really the big issue in presence of low end content, so make sure you have a high pass filter in sidechain when you use it on a mix.

Exemple of compressors using this design: LA-3A, JLM Mac Opto Comp, LA-2A, TubeTech CL1B.

Variable Mu (Tube Compressor)


Although it’s the earliest compressor design you can find, the Variable Mu design is still very popular for high end audio application. Manley’s Variable Mu has been used on countless platinum records and is here to stay. Very few compressors have become an industry standard for mastering as did the Manley.

(Just to make things clear, an opto compressor with a tube stage at the end IS NOT a Variable Mu. In the variable mu design, the compression is actually achieve using the tube itself. )

Variable Mu compressors produces incredibly smooth compression. It’s transfer curve is far from being linear. The actual ratio increases with gain reduction. That means that louder a transient is, the harder it is going to be compressed.

Another characteristic of this type of compression is the time constants. It simply doesn’t respond as fast and impulsively as a VCA or FET. The tube compressor takes its time and never over-react. It has this ability to glue a mix together like no other type of compressor because of that.

When to use it: When a mix has reach its dynamic coherency, just pass it through a tube compressor. It will tighten up and smooth it up. The whole mix will start to blend properly until it become homogeneous. It can be use to make things softer and smoother. For example, a guitar that has thin and aggressive sound can be smoothed and warmed up with a variable mu.

When no to use it: To solve dynamic issues or to get punch. That’s simply not the compressor for that. The time constants are too slow to make it agressive or punchy. This type compressor has no aggressiveness whatsoever in its compression behavior. To be aggressive is just no part of its character. It’s also too slow to handle intense dynamic problems.

The advantages of using Vari-mu: they have a very warm, rich sound. It has a sound that simply cannot be achieved using plugins. It brings depth, texture and definition… well probably the sound you’re looking if you like it smooth.

The limitations: Operation without a sidechain filter can be troublesome as it will kill the bass. This compressor cannot do punchy.

Exemple of compressors using this design: Fairchild 670, Altec 436C, Manley Variable Mu. I personally use the Manley and the Altec (Edit: I recently acquired a HCL Varis, which is an incredible unit!).


2015-05-30 09.34.13

The last, but definitely not the least, the FET!

I personally love the sound of FETs. To me, 1176 is clearly one of the best sounding compressor in the history. Not surprised to find a bunch of them in every studio. Very few compressors can be placed on every tracks like this one.

So, if you are looking for punch, that’s the compressor. What Opto and variable mu simply can’t achieve, this one does. The slowest attack time available on the FET is usually faster than the fastest attack time on a variable mu! Yet, it’s far from being as transparent as a VCA. It definitely has more character. Usually, when you use a FET, you want to hear it working, because it’s a sound we all like. (Think of a typical rock snare).

(BTW, I personally bought DIY kits of 1176 clone. I went a bit crazy at the time and I bought 10 of them! Now I have 4 of them in my rack and the remaining ones are available for sale at discounted rate if you’re interested:!/1176-clone-Matched-pairs-available/p/48424840/category=12631169)

When to use it: For that punch, a 1176 can be used on drums, vocals, bass, and everything else that needs bite and punch. It’s known for its “Snap” on drums. The distortion on this compressor is really rich and warm. I personally have not tried any other FET models than the 1176, but I know Slate Technology also produced a version with a side chain filter (what a great idea!).

When not to use it: Unless you have a side chain filter, I wouldn’t recommend to use it on a mix with the compressor on, unless it’s the bass or the kick you want to give the punch to. I personally use it in my mastering chain, but with the compressor turned off. That’s actually an old trick, I’m not the only one to use it on a mix bus only for it’s color. The output transformer sounds very warm, so although it’s not compressing, there is a great benefit of having it in the chain as a line amplifier. It brings the “vivid” effect so hard to achieve in digital, even using the best saturation plugins available. The 1176 just does it more colorful than reality, it’s that intense.

The advantages of using a FET: They rock… The punch is really hardcore. They have a very warm, rich sound. It has a sound that simply cannot be achieved using plugins. It brings depth, texture and definition. Also, best of all, high quality clone of the beast can be found at reasonable price. I personally use the hairball. While they are tough and sensitive to calibrate properly, once done well, they will deliver the sound.

The limitations: Most of them don’t have a sidechain filter, so it doesn’t seem suitable for mix bus compression.

Exemple of compressors using this design:  1176 and all its clones!


I hope this helped you have a better idea of what the different type of circuitry can bring to your mixes. Indeed, the question isn’t which one is best; each of these design have their advantages, strength and application, but really when to use which. I hope I have done my job well, if not, let me know if you have any questions.

Now let’s finish on a short home made mantra :)

Stop putting VCAs on everything simply because that’s the default compressor design that comes with every DAW.


31 thoughts on “VCA, Opto, Vari-Mu, FET compressors… When to use which ?”

  1. Hi Chris

    Great ! Could you give us more combinations of compressors ? especially for mastering. VCA + Opto etc.




  2. This is great! I’ve seen a lot of articles about how to use compression on a track or mix, but very few that talk about the real differences between the types of compressors. Thanks for the article.

  3. Hi,
    And thanks for this article. I read a lot about compressor in the last couple of months, but your explanation kind of opened my eyes.
    Since I’m fairly new in mixing (and everything that comes with it) I am grateful for everything and everyone, who helps me trying to find my way in this audio-jungle.
    Thumbs up :-)

  4. Wow. Most concise and helpful article I’ve found on compression. Thanks!

    When purchasing compressors, do the different types naturally lend themselves to different instruments? Or should I think about musical style and then choose what to buy? Or, lastly, does it benefit to have the same type of compressor on every channel to create a more “cohesive” sound?

    1. Hello Elijah,

      It does not benefit to have the same type of compressor on every instruments, actually, it is quite the opposite.

      Each compressor you have on your mix or master needs to fill a very specific purpose. You need a range of different compressor because they are different tools doing different things.

      Sometimes you need a combination of them on the same channel. For example, a vocal might need a deesser to control the sibilances in the sss, a VCA to control the fast peaks, a RMS to make the whole thing more firm, and finally a limiter on the top of all that to make it compete in terms of loudness against the other instrument in a mix.

  5. Thanks for the fast response! What types would you recommend for electric guitar, bass and drums, respectively? The music is rock but very dynamic in terms of volume. I like to retain as much of the music’s dynamics as possible but I am told time and time again that “compression is the sound of rock”.

    Cheers and thanks again.

  6. Hi,
    I´ve been reading through your article and I think there is some misunderstanding when you talk about the VCA compressors.
    The VCA is placed in the gain reduction part of a compressor. The detection circuit is the one that makes all the processing to attack the VCA (a control voltage in this case). Therefore, VCA compressors can be slow and can also compress RMS (not only PEAK as you stated).
    It all depends on what is placed in the detection circuit. If you place a RMS converter to DC in it then you can have a RMS VCA compressor.

  7. Nice article !

    Anyway, if I’m not mistaken, the SSL Gcomp is a VCA comp and is used A LOT on mixbus processing. And it’s fairly slow compared to, let’s say a DBX 160.
    So I don’t think that the SSL Gcomp really react as VCA as described in your article.
    Anyway, it is known to bring that punch and squash transients also. But it doesn’t feel sterile and like a sheet in front of the speaker as you said.
    But it can sounds thinner as it tends to squash the bass too much (they introduced a sidechain in the rack version anyway).
    Kind of unique.

  8. Great article. What are your thoughts on the diode-bridge family of compressors (2254, TG1, etc.) and what space of usefulness they occupy when compared to opto/fet/vca/vari-mu?

  9. Rock, Pop Rock, Hip Hop, EDM, Techno House, all this gender requieres a VCA in the 2 buss, most used are the SSL as someone pointed above as well the API 2500,
    The article is good cause they are lot things to know about compressors, but there are lot more to said: each brand independtely if they are VCA Opto Fet, or Vari mu have different Attack and Release times/constent.
    for a two buss compressor IMO it is better to place it before you begin the mix, cause they change color and balance, unless you plane to be very subtle with your compression it is not desirable use them at the end of the mix, the choice: here are the factors: Gender of music, in modern music, all genre specified at first in my post we want to ear the Snap, Pump, breathing, a noticable effect of the compression, that´s why a VCA will be the first of the chain, there lot of modern choice today has mencioned in the article hard and soft knees, drawmer 1978 is an example of FET compressor which work very well in the two buss, Opto can be used as well but taking care of the release, an Opto compressor take same releases time in VCA and Opto is not the same result, the Opto release faster at first the he slow down at the end of the release satge, this behaviour is a little weired, Balade or a Vocal driven song benefit a lot of a Vari mu compressor, a little of my adquiered knowledge, to complete a little more this excellen article

  10. i forget if BPM is too high slowish compressors don´t be desirable, i general think before what you need, do you want Pump? do you want Snap, do you want agressiveness, is the song guitar driven or is the song vocal driven, all this factor most to be planned before to choice the right tool, the option VCA vari mu is very good for modern music, Vari mu is the best glue compressor choice, Also keep in mind they are lot of creative uncomun choice like the Beatles song She said, the attack is hyper fast and the release is very long that make a curious effect listen to the song, listen to Billie Jean by Michael Jacson the snap is heavy VCA long attack slow realease, Updown Funk is always snappy, they are also some songs with a long attack which means strong transients and snap and very short realease which means gritti very saturated and artifacts but that´s an effect and it could work in fact they are example of this.

  11. EDM you need internal sidechain cause there lot of bass content drive in the low end is not desirable, frecuency content is also an important factor when it comes to choice compressors in the 2Bus, Guitars and drums VCA and FET could work very well, vocals a FET then and Opto or Vari mu is a good choice.
    Sorry maybe i have bee too large :-)

  12. Sorry again, just repassing over the article, the Opto works very well for high guitar peaks in funky music (half muted strings picking) as well in country for chicken picking (this are fast peaks but in this styles i.e. it works pretty well

  13. Not all VCA compressors have a Peak detection circuit. The DBX160 is not a PEAK compressor , it uses an RMS detector.

    The Peak and Rms has nothing to do with the topology of the compressor (Opto, Vari-Mu, VCA, FET). The detector itself determines weather is a Peak or Rms compressor, regardless of the topology.

    1. And also you are wrong in that all VCA compressor are hard knee, again the DBX 165 is a Soft Knee compressor, it says it on the front panel. “Over-easy Compressor”.

    1. Sure! Hello Tuan.

      I am not against the concept of using a FET on the mixbus per se. However, most FET machines do not have a High pass side chain filter so you end up with having the low end pumping the whole mix. There is one that works well on the mixbus; it’s the one from Slate, the dragon I think.

      I also used to use 1176 replicates on the mixbus, but I was using them in multiband to avoid this problem.


      1. Thank you for your reply, sir. Many people like to use LA-2A on Bass without side chain filter, what do you think about this?

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