VCA is an acronym for ‘voltage-controlled amplifier’, which is the component at the heart of the circuitry in this type of compressor. A VCA compressor reacts to peaks that are above the user-set threshold. VCA compressors are known for having fast response and are therefore a good choice on peaky, rhythmic or transient-heavy material.

Many VCA compressor designs include precise control of a wide range of compression parameters including threshold, ratio, attack and release time, makeup gain and sometimes knee. This abundance of control makes VCA compressors versatile jack-of-all-trades dynamics processors. Depending on how you set them, VCAs can be transparent or not to the original tone & harmonic characteristics.

Among the most renowned VCA compressors are the API 2500, which can be used as a buss compressor or on individual sources, the buss compressors built into SSL consoles, and the dbx 160. The latter has been a long time go-to compressor, especially for drums, adding its own unique snappy character to the drum transients.

Good for:

• Compressing drums, percussion and other transient-heavy sources
• Adding punch
• Smoothing out peaks in a transparent way—for example, restricting the dynamic range of a vocal or instrument performance without squashing transients
• Buss compression—both master and subgroup
• Virtually any compression task, because they're so versatile

Not as good for:

• Adding warmth or colour

Like the VCA, FET compressors are solid state but use a particular kind of component called a "field-effect transistor," which was designed to emulate the behaviour of tube circuitry. FET compressors offer even faster reaction times than VCAs. Many FET compressors have no threshold control. The amount of compression applied is governed by the combination of the amplitude of the input signal, and the setting of the input level control. The louder the input, the more signal gets compressed.

A FET compressor is not what you'd choose if you want transparent gain control. It imparts a distinctive sonic fingerprint on the source material. Probably the most famous FET compressor is the Urei 1176, which is heard on thousands of classic albums and offers an aggressive, fast compression that's great on vocals, drums, guitars and more.

The CLA-76 is a plugin emulation that not only mirrors the control set of an 1176 but also offers you the option to switch between models of two different iterations of the well-known compressor.

Good for:

• Adding an aggressive sound and excitement to vocals, drums, bass, guitars and more—from a little extra vibe to super pumpy
• Limiting, due to fast attack time
• Parallel processing; their ability to create extreme compression is useful when you're blending it in with the uncompressed sound

Not as good for:
• Transparent compression

Compressors typically split your input signal into two parts: One is sent through a detection circuit, which determines how the compressor will act, and the other is the audio that's operated upon by the compressor and sent to the output. In an optical compressor, the detection circuit is unique; the audio signal is turned into light, which triggers an electro-optical sensor that governs the amount of gain reduction. The response of this setup is smooth and transparent. Unlike other compressor types, hardware optical compressors have fixed ratios, typically 3:1.

Perhaps the most famous optical compressor of all time is the Teletronix LA-2A. Technically referred to as a "leveling amplifier" (hence the "LA" in the name) it combined both electro-optical circuitry and a tube amplifier for a smooth and pleasing compression that was particularly useful on vocal tracks, but also great on other sources. The LA-2A hardware unit is ubiquitous in commercial studios and has been heard on countless recordings. It is available in faithful plugin form as the CLA-2A.

The LA-3A model offers similar functionality but without the tube circuitry, giving it a cleaner sound. This one is also available as a plugin: the CLA-3A.

Good for:

• Transparent compression
• Adding warmth and sheen to vocals, guitars, basses and other instruments (especially true with the CLA-2A)
• Improving the vibe of vocal tracks

Not as good for:

• Controlling hard transients
• Aggressive-sounding compression

A tube compressor produces smooth compression with warm and pleasant colouration. The circuitry achieves its attenuation through re-biasing of the tubes. These units are not super-fast acting, so they aren’t as good as FET or VCA units for transient control. However, they add warmth and depth to just about anything; you can get nice sounding aggressive compression if you push the input and threshold controls to extremes.

The most legendary tube compressor was the Fairchild 670 Tube Limiter (which also came in a mono version, the Fairchild 660). Original Fairchilds are incredibly rare and really expensive. Fortunately, there are excellent tube emulations including the PuigChild 660 and 670 plugins, plugins, which provide realistic sounding versions of the original Fairchild units.
On both the original and emulations, the attack and release controls are linked together under the Time Constant parameter, and you can choose between six different pre-set attack/release time combinations.

A lesser known (but highly esteemed) tube compressor is the Abbey Road RS124, considered the “secret weapon of Abbey Road engineers.” The smooth-and-silky sounding unit was most famously used to compress Paul McCartney’s bass parts on all Beatles’ albums, and many more records made at Abbey Road in the 1960s.

The plugin is equipped with a number of additional controls. These include a Sidechain High-Pass filter, a Mix knob for creating parallel compression, a Monitor section that includes Stereo, Dual Mono and Mid-Side adjustments, HF Roll-Off, and “Super Fuse mode” that lets you dial in highly aggressive compression.

The plugin offers two different flavours of the RS124: One is called Studio, which models a unit used mostly in Abbey Road’s studio control rooms. The other, Cutting, has a slower attack and release and a slightly different sound due to an alternate tube configuration.

Good for:

• Adding warmth, colour and fatness to any source
• Parallel compression
• Adding glue on the mix bus
• Mastering limiting

Not as good for:

• Precise attack and release control (on Fairchild emulations specifically)
• Controlling hard transients

While analogue-modelled compressors are very popular, there are plenty of compressor plugins on the market that aren't designed to simulate the sound and behaviour of specific vintage units and have capabilities that take advantage of the precision and versatility of digital technology.

For example, the Waves eMo D5 Dynamics plugin uses the Waves' Parallel Detection technology, which is only achievable on a digital plugin, to provide incredible precision, plus other one-stop-shop dynamics controls. The H-Comp plugin offers the ability to dial in different types of analogue-compression characteristics in ways that wouldn't have been possible in a hardware unit.

The Waves Renaissance Compressor is another example of an original design digital compressor. It offers the versatility of both “Warm” and “Smooth” character types, which engage low-frequency harmonic warmth, or bypass it for transparent compression more true to the signal’s original tone. Also built in are “Electro” and “Opto” behaviours, which respectively engage more quick or slower release times.

Good for:

• Its versatility
• Extra features not possible on analogue units
• Comprehensive and precise control

Not as good for:

• Emulating specific hardware compressors

Multiband compressors, which allow you to focus on several user-definable frequency bands, are used most commonly on the master buss and in mastering situations but can also be quite helpful in a mixing context. They're a bit more complicated to use because you have to set crossover points to define the frequency zones and configure the compression parameters for each one independently. They can be beneficial for many types of tasks that single-band compressors wouldn't be appropriate for.

By giving you the ability to target specific frequency areas for compression, multiband compressors can not only control dynamics but can be used for frequency manipulation as well.

Waves offers three different multiband plugins: the six-band C6, four-band C4 and the Linear Phase Multiband Compressor.

Good for:

• Targeting specific frequency bands for compression
• Mastering
• Smoothing out sources—such as vocals—that change in timbre when they get louder or softer during a mix
• Reducing problem frequency areas in a source

Not as good for:

• Simple operation

Another type of compressor that exists only in digital form is a low-level compressor (a.k.a. "upward compression"). Like a standard downward compressor, it reduces dynamic range but does so by bringing up soft sounds rather than lowering loud ones. If, for example, you have a vocal track where the singer gets too quiet on some words, you could use the low-level compressor to bring those words up automatically.

You can also use it to accentuate parts of a signal that were recorded lower. So, for example, if you have a room or overhead drum mic, you could accentuate the room sound with a low-level compressor.

Waves offers a few different compressors with low-level capabilities: MaxxVolume and the MV2. These models both include not only low-level compressors but downward compressors as well, allowing you to squash a source from above and below if you want to. MaxxVolume also includes a noise gate and a leveller.

Achieving good results on an upward compressor typically requires that you use subtle settings. Heavy settings will not usually yield pleasant sounds. Typically, you'd use a low-level compressor on individual sources, although they're sometimes used on the master buss to thicken the sound a little. For example, the "modern" limiter mode in the Abbey Road TG Mastering Console features an original-design VCA-based compressor that incorporates some low-level compression elements, perfect for a modern mastering sound.

Good for:

• Bringing up low-level aspects of a track such as room sound
• Reducing dynamic range by raising quiet sections

Not as good for:

• Adding warmth
• Controlling transients
• Peak reduction
• Parallel compression