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4 Types of Audio Compressors You Need to Know About

November 5, 2020
by
Charles Hoffman
Learn about VCA compressors, optical compressors, FET compressors, and tube compressors. Hear the difference with audio examples.

A compressor is a device that reduces the dynamic range of an audio signal, making the difference between the loud and quiet parts of the signal less significant. This concept is simple enough, but the compression applied by one compressor isn’t necessarily going to produce the same results as compression applied by another compressor.

Coming from the digital audio world, this might seem a bit odd. Why would one compressor sound different than another? Aren’t they all just doing the same thing?

Well, hardware compressors are broken down into different topologies (types) that make use of unique internal designs. Based on the topology that a compressor belongs to, you can make a few assumptions about the character it will impart on the signal you compress.

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For example, one type of compressor may provide “clear” and “transparent” results, while another may apply compression in a way that leads to a “thick” and “creamy” processed signal.

Plugin companies often create hardware compressor plugin emulations that provide you with the sound of famous hardware compressors within your DAW. Even if you don’t own a hardware compressor, you may be in possession of one or more hardware compressor plugin emulations.

Becoming familiar with the sound of compression, typical of different compressor topologies, will allow you to make more well-informed decisions when choosing between compressors when mixing and mastering your music.

The four types of compressors you should become familiar with include VCA compressors, optical compressors, FET compressors, and tube compressors. In Nick Messitte’s article called “4 Types of Analog Compression—and Why They Matter in a Digital World,” he dives into the design elements unique to these different types of compressors.

Messitte’s article is somewhat technical, so I’ve summarized and simplified the key takeaways for you. I want you to be able to decide when it makes sense to reach for a compressor from one topology versus another. Additionally, we’ll take a look at a few popular hardware compressors and the companies emulating them the best.

1. VCA Compressors

VCA compressors, or voltage controlled amplifier compressors, use a control signal to dictate whether or not gain reduction is applied to an audio signal. These are some of the most common types of compressors and often include all the controls you’re used to seeing on plugin compressors, like attack, release, threshold, ratio, and sometimes a knee.

An image of Solid State Logic's G Comp 500 Series Stereo Bus VCA Compressor.
Figure 1: Solid State Logic's G Comp 500 Series Stereo Bus VCA Compressor.

Generally, VCA compressors provide a very precise level of control, and a sound that’s predictable and clean. In addition to being a popular choice for vocals, VCA compressors are often used for mastering purposes because of the transparency they tend to provide.

The API 2500+ Stereo Bus Compressor is a popular VCA compressor that includes a unique “THRUST” feature that places a filter in front of its RMS detector. This filter imbues each octave with equal energy, instead of half the energy as the next lowest one. The result is a “punchy” sound that lacks the undesirable “pumping” effects that other compressors often provide.

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Another unique feature you’ll find on the API 2500+ is the ability to toggle between two types of compression modes, which include Old and New. When toggled into Old mode, the API 2500+ uses a feedback type of compression, in which the detector is placed after the VCA. With New mode engaged, the compressor uses a feed-forward type of compression, placing the detector before the VCA, resulting in a faster response to transient material.

Arguably one of the most popular VCA compressors used to make mixes sound “snappy,” “punchy,” and “huge” is the Solid State Logic (SSL) G Comp. This compressor is designed after the legendary center section module from SSL’s G-Series analog console of the 1980s. You’ve undoubtedly heard the original unit used on the stereo bus of countless hits.

2. Optical Compressors

Optical compressors, or opto compressors, rely on a light-dependent resistor and a light source to dictate the way in which compression is applied. An optical compressor’s input signal illuminates a light source—the more powerful the input signal, the brighter the light will shine. As the light source illuminates, the resistor causes the compressor to apply compression, and when the light source dims, the amount of compression applied is reduced.

The thing to keep in mind about optical compressors is that the materials used in their construction greatly affect how they behave. One type of light source may illuminate faster than an other, while something as simple as the material the resistor is made of can alter the way compression is applied as well.

As Messitte states, “…the harder you hit an optical compressor, the quicker its initial release time can be—but the slope back to a normal, uncompressed sound will not fall in a linear fashion. It will “curve.” So if the circuit gives you 10 dB of gain-reduction, the first five decibels might release much more quickly than the following five.”

The result of this is that optical compressors tend to sound quite musical and “smooth.” To control sharp transients, an optical compressor is likely not the first choice for most audio engineers. Although, optical compressors tend to compliment ballads and folk vocals quite well.

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The Universal Audio Teletronix LA-2A is a wildly famous electro-optical compressor/limiter with tube circuitry that applies gain reduction with zero increase in harmonic distortion. When you take a look at an LA-2A, you’ll notice that the primary controls consist of just a Gain knob, Peak Reduction knob, and Compress(3:1 ratio)/Limit(100:1 ratio) switch. There are no attack, release, or threshold controls.

An image of Universal Audio's Teletronix LA-2A Electro-Optical Compressor.
Figure 2: Universal Audio's Teletronix LA-2A Electro-Optical Compressor.

The way in which the LA-2A applies compression is highly program-dependent. When strong signals are run through the compressor, you can expect long release times, and when weak signals are run through the compressor, you can expect short release times. The uncalculated nature of the LA-2A is exactly what makes it desirable.

If you don’t want to spend $4,000+ on an LA-2A, Warm Audio and Klark Teknik have each designed their own compressor based on this iconic unit, at a fraction of the cost. For an affordable hands-on experience, these emulations are worth considering.

Another popular optical compressor is found within the Avalon VT-737sp Tube Channel Strip. This compressor is frequently used to compress pop, hip-hop, and R&B vocals. It’s been used by Jay-Z, Dr. Dre, Babyface, Beyonce, Eric Clapton, and even The Rolling Stones.

The VT-737sp tends to be described as “glossy,” and since this is the most-used vocal compressor in my mixing arsenal, I can confirm that. When I pair my Shure SM7B with this channel strip, there’s very little processing I need to apply in-the-box to get that polished “radio-quality” sound.

3. FET Compressors

FET compressors, or field effect transistor compressors, make use of transistor circuits. They respond quickly to the signal you run through them, and they’re also quite punchy, colorful, and bright.

There’s some debate online about the distinction between VCA compressors and FET compressors, according to Messitte. However, he states that “In a VCA compressor, the transistor is housed within an integrated circuit (an IC) which responds to the voltage of your incoming signal. The FET, however, works with the electrical field as a whole, and gain changes are the result of electrical charges in addition to voltage.”

In practice, FET compressors are capable of extremely fast attack times, but they tend to introduce some color and distortion to the signal you apply them to—especially when driven. For mastering purposes, this isn’t typically something you’re looking for, but FET compressors tend to suit a wide variety of aggressive rock and rap vocals.

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Perhaps the most popular FET compressor of all time is the 1176 Limiting Amplifier, which was originally introduced to the market in the late 1960s. This compressor has lightning-fast attack and release times, and it delivers a bright, present, and energetic sound.

One of the iconic features of the 1176 is that when you push in all four ratio buttons in at once, the results are aggressive and musical. Putting the 1176 into this state is commonly referred to as “all button mode” or “British mode.”

Over the years, there have been over 13 revisions and variations of the 1176—each revision is unique in its own way. If you’d like to learn more about the differences, this Universal Audio article provides a comprehensive overview of each revision. Universal Audio’s 1176LN reissue is what you’re most likely to find in stores now, but you can hunt down previous revisions of the 1176 by searching websites like Vintage King.

4. Tube Compressors

Finally, there are tube compressors. These types of compressors, unsurprisingly, rely on tubes to tame dynamics. Some compressors make use of tubes to color the signal running through them, but tube compressors specifically make use of tubes during the gain reduction process. Tube compressors also tend to provide a slow response to transients, relative to other types of compressors.

The Fairchild 670 (stereo) and Fairchild 660 (mono) are two of the most sought after compressors in the world, making use of 20 tubes and costing upwards of $50,000! They deliver tremendous silky warmth, and will find themselves right at home on vocals, guitars, drums, or your mix bus.

Manley Lab’s flagship Variable Mu Stereo Compressor Limiter is a go-to mix bus compressor for many engineers. It delivers exceptional transparency, smooth gain reduction, and warm harmonic content free of noticeable compression artifacts.

Choosing Your First Hardware Compressor

If you’re looking for an incredibly versatile hardware compressor, I personally recommend Empirical Lab’s EL8 Distressor. It can deliver the speed of a VCA, the personality of an FET, the warmth of an optical, or the coloration of a tube compressor—depending on the settings you use. A jack of all trades, the Distressor is the first hardware compressor that many audio engineers incorporate into their studio.

The EL8 Distressor is a digitally-controlled compressor that makes use of 100% analog circuitry, which provides a substantial warm vintage flavor. When you engage the compressor’s Dist 2 mode, you apply tube-like 2nd harmonic distortion to your signal, while in Dist 3 mode, 3rd harmonic distortion is applied—similar to tape distortion.

You can also stereo link two Distressors together to process stereo signals. Since forming this link simply involves plugging in a few cables, you can start with one Distressor, and then integrate an additional Distressor later on as budget permits.

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