Question from a Recording Artist in Memphis,

What are the different types of compressors, and what do they sound like?

Simply put, compression turns down the loudest parts of a given signal, thus reducing its dynamic range. Once you’ve squashed the loud peaks, you’re able to increase the volume of the entire signal. The gain reduction properties of a compressor/limiter are expressed as a ratio of input to output. Lower ratios of dynamic control, like 3 to 1, or 4 to 1 are called ‘compression’, while ratios above 10 to 1, are ‘limiting’. There are basically four different types of compressors used in Pro Audio, each with its own sonic-signature consoles were designed around the dbx VCA, including the famous SSL G384 on-board master compressor, the quintessential smack-box for mixing.

The Vari-Mu (aka Delta-Mu):
The odd name refers to a special type of high-gain, low distortion tube used by this device. It was one of the first types of compression circuits, and the prime example of a classic Vari-Mu is the almighty Fairchild. The all-tube Fairchild 660 mono and 670 stereo units were made in the early 1960s, and are highly sought-after today for their very euphonic and high fidelity sound. The Fairchild 670 is considered the ultimate vintage comp! Manley Labs, Pendulum, and others make modern Vari-Mu devices. The sonic marquee of these is a softer knee and a fluid dynamic control. Vari-Mu compressors are widely used by recording engineers for compressing the mix-buss.

The FET:
A FET compressor is very much like an opto, but with out the opto-cell. Instead, it uses a “field effect transistor” to essentially do the same thing. Using an FET as a gain-control element has its own challenges, but some very successful FET compressors have been designed and used in audio production. How successful you ask? How about the all-time favorite Urei 1176? The most popular compressor in the history of audio is a FET type, and it’s as fast as lightning. FETs do tend to be faster than other types of compressors, making them great for all around studio use. The very musical Neve 33609 is another example of a FET circuit.

The VCA:
Around 1970 an engineer named David Blackmer invented an audio component called the voltage-controlled amplifier, or VCA. This all-in-one device revolutionized audio in many ways, not the least of which was in the field of compression. Perhaps you’ve heard of the company he started? It’s called dbx. His VCA chip is at the heart of the great dbx compressors like the 160 and the 165. Perhaps the most audible of all compressor types, they are loved for drums and percussion because if the crack, smack, and punch they give. The original SSL Quad Compressor is a VCA style compressor..

The Electro-Optical Attenuator (aka “Opto”, aka “ELOP”):
In the early sixties another type of compression was developed, using an “opto-cell”. Indeed, an opto-comp uses light energy to create gain reduction. Inside the optical cell, which is literally a little can or box, an electro-luminescent panel brightens and dims as the audio input signal goes up and down in volume along with the input signal’s voltage, causing the amplifier to change the gain. Cool, huh? The all-tube Urei/Teletronix LA-2A is the most well known opto compressor in history. The Urei LA-3A and LA-4 are also optos, which use transistors for gain make-up instead of tubes. The action of an opto is a little grabbier, and at the time they were introduced, they were considered exceptionally fast. The opto-comp is very well suited for vocals, as well as bass guitar.