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

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.

OPTO

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!).

FET

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: http://www.quantum-music.ca/store.php#!/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!

Conclusion

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.