Optical Compressor – What It Is and How to Use It

The optical compressor, also known as an opto compressor, is one of my favorite types of audio compressors because of its ease of use, transparency, and the results I get from it. Let’s define what is an optical compressor and how it works, what instruments to use it on, and HOW to use an optical compressor in your mix.

The Optical Compressor

First, let’s define what an optical compressor even is (the hint is in the name).

What is an Optical Compressor

An optical compressor uses light to compress a signal, hence the name.

The greater the input level, or rather the lower you turn the compression threshold setting, the more intense the light becomes. The greater the intensity of the light, the greater the compression which is applied to the audio.

In other words, the compression ratio is dependent entirely on the signal being fed to it.

In that same vein, the beauty of an optical compressor lies both in its simplicity as well as the transparency of the results.

In terms of transparency, there’s no transistors as part of the process like in the case of a Vari-Mu compressor and the tube’s involved yields arguably a cleaner sound than a FET compressor.

This helps the optical compressor to achieve a transparent compression without coloring the audio as much as the optical compressor’s peers.

That’s not to take anything away from those other compressors, but when you don’t want the audio being altered in any way outside of the peak reduction with little to no artifacts, optical compression is a great choice.

Optical Compressor Settings

It’s also a great choice when you don’t need a lot of control over the compression.

I alluded to the simplicity of an optical compressor a moment ago, as well. A lot of that comes from the relative lack of audio compressor settings which you’ll find as compared to most other compressors.

This is especially true with the CLA-2A from Waves, my go-to optical compressor.

Here are the optical compressor settings you’ll find on it:

Peak Reduction

The “Peak Reduction” dial is essentially the threshold for the optical compressor. The lower you turn this counterclockwise, the more signal which gets compressed.

While the ratio, attack, release, and knee settings are all absent, all of the things which these parameters would normally control are instead dynamically shaped by the Peak Reduction dial.

Output Gain

The “gain” knob is universal and self-explanatory, but this controls the makeup gain.

Use this to make up for what you take out for gain staging purposes. It’s also important to match the input level because this ensures that when you split test with the optical compressor on and off you’re only hearing the effects of the compression to appraise the effects.

Hi Freq

The “Hi Freq” dial allows you to focus the compression more on the high frequencies if you so choose. If you want the higher end to be more compressed/attenuated than the mids and lower frequencies, turn this to the left.

If you want to compress the audio evenly and without bias, turn it to the right.

This setting is mostly specific to the CLA-2A but it’s useful in warming up your audio or providing a little taming to harsh frequencies.

I talked about this as part of my recent tutorial on hi-hat compression as I’ll oftentimes set this at 50 or farther left when the high end needs a touch of smoothing.

When I need more than that I’ll reach for a multiband compressor, but it’s great for subtle fixes and imparting warmth to tracks which need it.

Analog

Another setting which is specific to the 2A, “Analog” simulates the analog noise you get with the hardware based units it’s emulating.

This manifests as some low level white noise, so I typically leave this off.

What to Use Optical Compressors On

Optical compression works well on anything you don’t need a ton of compression on, meaning anything without a lot of dynamic range. It’s great when you just need to smooth out some small peaks and I typically don’t aim for more than 1-3dB of gain reduction when I’m using an opto style compressor.

Still, a few instrument types I’ll frequently reach for it on include:

Vocals

I find optical compressors work best as a part of my vocal chain, specifically as the second compressor in serial compression. I let a FET style 1176 compressor take the lead with an 8:1 ratio swallowing up the entire practical signal as the threshold to reduce the dynamic range to something more manageable.

After that I’ll send in the 2A to rein in the remaining peaks a bit more.

Check out my complete vocal compression guide for more information on how to compress your vocals just right.

While were at it, just grab my free compression cheat sheet for in depth guides on compressing every single instrument in your mix.

Cymbals

I alluded to this earlier, but I love the 2A in particular for smoothing out instruments which can be a bit too bright or tinny like cymbals. Just aim for a couple dB in gain reduction and adjust the “Hi Freq” knob to taste.

Guitars

Opto compressors sound nice on guitars, both acoustic and electric, particularly when we’re talking about a strummed part which doesn’t have a lot of dynamic range. Like with cymbals, if your guitar tone is a bit too bright or harsh which can be the case with amp modelers a lot of times, the 2A is great.

Buses

I’ll typically reach for a VCA style compressor when I want a little glue on a bus, but an optical compressor can be used in place of it. Just be sure to keep the frequency favoring flat/uniform.

Best Optical Compressor

It’s no surprise that my vote for the best optical compressor is the CLA-2A. Modeled after the LA-2A I referenced earlier, it’s simple to use and sounds fantastic, particularly on all of the instrument types I mentioned a moment ago.

If you’re looking for a free optical compressor, check out the Variety of Sound ThrillseekerLA. The setup is a bit more complicated as there’s more features than you’ll find on the 2A, but it’s worth checking out for a free option.