A compressor is one of the most useful tools an engineer can have on hand during a session. They have been the glue that holds recordings together since their creation and engineers still heavily rely on them for vocals, voiceover, guitars, bass, drums, the mix bus and more.

When young engineers get their start, they are usually aware of compressors, but don’t know how to properly use them or know of the wide variety of functions they offer while tracking or mixing. It is very easy to overuse a compressor and crush all the dynamics of whatever source you are running through them, but when used in the right way, they expand the depth and sound of the overall recording or mix.

What is an audio compressor?

The basic definition of a compressor is “an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds thus reducing or compressing an audio signal’s dynamic range.” That is the definition that most of us learned from our mentors or in audio school and is completely accurate in the basic function of a compressor. It tames the louder sounds to avoid intense transients from vocals, drums, etc, and also brings up the softer sections of the source which ultimately makes a more even, less dynamic recording.

I think that definition is very vague on the variety of functions a compressor has to offer. A compressor does much more than tame dynamics and, when used properly, can greatly expand the overall frequency range, add warmth or glow to the source and improve the harmonic content even if no compression is actually happening.

Every style of compressor has a “sound,” and some engineers prefer the way the audio is affected by just running through a compressor without any actual compression happening. Since they are loaded with transformers, resistors, wiring and capacitors that add color, engineers can use them as a subtle EQ. One of the top mixing engineers on the planet, Michael Brauer, uses multiple compressors in a send and return set-up to fill in different frequency ranges of a vocal rather than trying to use an EQ. Since each compressor has their strong points, he uses one compressor for the air of the vocal, one for the mid range, one for low end or body, one for aggression and so on.

Audio compressor types

In this blog, I’m going to go over the basic functions of the three main types of compressors; FET, optical and variable-mu. In addition, I'll offer up some tips/tricks on how to get the most out of them and what compressor to use for certain sources. One thing to remember when working in the audio world is that there are no set rules on how to use any certain piece of gear. There is no golden standard telling us that "this is the compressor you always use for this source." Every project is different and should have a unique approach so everything doesn’t end up sounding the exact same. Every engineer has a different style and opinion in the way they work, but there are some basic operations that can help you easily dial in a sound that will sound good on almost any style of recording.

Compressors work as an insert on an audio channel, but can also be used in parallel, which I will explain more in depth in a later blog. In a DAW, you place the plug-in directly to the channel through the insert path. This will turn the dry audio into a processed signal and the compressor will always be engaged. On a large format console, you will set up the compressor on the insert path through the insert send and insert return points on the patch bay, which you can then engage or disengage using the “insert” button on the audio channel. If your console doesn't have an insert point, you can route out of the mic preamp to the input of a compressor, then out of the compressor to the input of your DAW or tape machine, then the output of your DAW or tape machine will feed the line input of your console.

Both hardware and plug-in compressors have a unique sound to them. With the advancements of digital technology, companies like Universal Audio and Waves are making plug-in models of the industry's top compressors that can make you forget you aren't using the real hardware version. The plug-ins aren't made to replace the hardware versions, just add more variety and flexibility to your workflow if the budget isn't available to have racks of outboard gear. I recommend trying a variety of hardware and plug-in options to find the right choices for your engineering or production style.

What is a fet compressor?

A FET compressor offers the quickest transient response and is solid state by design. These are the go-to compressor when taming the dynamics for aggressive sources such as drums, heavy electric guitars, percussion and aggressive vocals. FET compressors are commonly the choice for parallel processing because of the aggression and vibe they add. They work by passing audio through a “field effect transistor" for their gain control (hence the abbreviation FET). Some modern compressors have more advanced controls, but let's go over the basics which are Input, Output, Ratio, Attack and Release.

The Input determines how much signal is being fed into the compressor from the preamp, the more you drive the input of the compressor, the closer you get to crossing the threshold and engaging the compression. These compressors tend to add a lot of “color” to the sound you run through them, which is usually on the more aggressive or edgy side of the sound spectrum. When driving the input of one of these units, you can saturate the sound source in a pleasing way.

The Output control is used to fine tune how much level you are sending to your DAW or tape machine after the input is set. If you were to drive the input and hit the compressor hard, you would use the output to back down the level so you're not clipping the DAW converters or input to the tape machine. If you are gently hitting the compressor, but aren't quite getting enough level to the converter or tape machine, you can use the output for make-up gain.

The Attack control is used to determine how quickly the compression will engage when the signal passes the threshold. For more transparent compression, you will want to use a slow attack, which will allow the softer signals to pass through without engaging the compressor, but catch the loud transients. With a slower attack time, you won’t hear the “pumping” effect that often occurs when too much compression is added, which is impossible to remove if it is printed into your DAW or tape machine.

The Release control determines how long the compression will stay engaged after crossing the threshold. A fast release will result in the most transparent compression, it will reduce the transient of the incoming source, then quickly go back to zero without affecting the softer portions of the signal. A slow release time will ensure that the compression stays engaged through the transient all the way until signal stops coming into the compressor.

If you want something to stay at a consistent level the entire time, a slower release time might be the way to go, although it will drastically change the dynamics of the player. The “bread and butter” setting is the most common use on a FET compressor, which is the slowest attack time and the fastest release possible. This will ensure that dynamics are preserved in the recording, tame the loudest transients that come through and quickly go back to zero before you notice compression ever happened.

Ratio determines how many decibels will be reduced after the source crosses the threshold. For example, a ratio of 2:1 indicates that a signal exceeding the threshold by 2 dB will be attenuated down to 1 dB above the threshold, or a signal exceeding the threshold by 8 dB will be attenuated down to 4 dB above it, etc. If you are looking for very subtle compression while tracking, a lower ratio will be the way to go and give you more room if you choose to heavily compress in the mixing process.

Vintage King carries a variety of FET compressors, some of our top hardware models are the Universal Audio 1176, Empirical Labs Distressor, DBX 160, Purple Audio MC77, API 2500, Neve 33609, Spectra Sonics V610 and SSL bus compressor. For plug-in options, the UAD-2 platform offers models of the 1176LN and Rev A, SSL bus compressor, API 2500, DBX160 and Neve 33609. Waves does their own representation of the 1176 with the CLA-76 Series and offers a licensed API 2500, DBX160, and SSL bus compressor.

What is an optical compressor?

The optical compressor has a very basic design, but tends to be the smoothest and most transparent of the three models in this blog. An optical compressor typically has a fixed ratio of 3:1, which tends to be the most preferred compression ratio when tracking vocals, bass and acoustic instruments.

While tracking a vocal, an optical compressor hitting around -3dB can really help tame the dynamics of the louder parts of the performance and be subtle enough to where you won’t really notice it was compressed on the way in.

On an upright or electric bass, tapping the compressor around -1dB at the loudest notes can help smooth out the loud attack when the player digs in. It can also help sustain the tail end of the notes, especially when using flat wound strings. If there are tubes in the optical compressor, it can also help to add a little bit of depth and warmth to the low end.

When used on acoustic instruments, compressing around -2dB or -3dB can help bring out the harmonic content of the instrument and make the high and lower strings have a consistent volume. This will add more power, body and depth to the recording. It’s a great way to make fingerpicking on a guitar very present and consistent between all the strings or make a full bodied, very present, even sound with loud strumming.

When an optical compressor is loaded with tubes and transformers, they add a nice warmth and glow to the signal even if no compression is happening. They can be slammed pretty hard without destroying the dynamics of the recording. The “sweet spot” usually falls between -3dB and -5dB. Optical compressors have very simple controls, Threshold and Gain. Some modern designs have expanded features, but let’s go over the basics.

Threshold determines at what level the compressor will begin to engage. Unlike the FET compressor, the input to the compressor typically can’t be raised within the device, it is strictly determined on the input from the preamp. If you are driving the input of the compressor without any compression happening, you will need to bring down the output level of the preamp (unless the drive sound is what you are looking for). If you want a large amount of compression, you will set a lower threshold, if you want less compression, raise the threshold.

The Gain control is placed after the compression. It is a common mistake that novice engineers think this is an input gain to the compressor, but is actually an output gain. This is used to control the output of the compressor to your DAW or tape machine. You can also use an optical compressor as a level booster. If the source audio was tracked at a low level, you can run through an optical compressor and keep the threshold all the way up so there is no way compression will be engaged, then use the gain control to add more level to the signal.

Some of the most popular hardware optical compressors are the Universal Audio LA-2A,  Retro Instruments Sta-Level, Tube-Tech CL-1B, Acme Audio Opticom XLA-3 MKII, and the Buzz Audio SOC-20. For plug-in options, the UAD-2 platform provides models of the LA-2A and Tube-Tech CL-1B. Waves does their interpretation of the LA-2A with their CLA series.

What is a variable-Mu Compressor?

Variable-Mu compressors are tube based, “Mu” is tube speak for gain, and they work by using remote cut-off or re-biasing of tubes to achieve compression. They tend to offer the most “color” of any other style of compressor, mostly because they are designed with more tubes and warmer transformers. Most engineers believe they offer the smoothest compression available, but if you are looking for something really transparent, they might not be the best option due to all the added color. Most models have similar controls to the FET design but have slightly different functionality, Input, Output, Attack, Release, and Threshold.

Variable-Mu compressors are a great compliment to vocals, they offer a very smooth and subtle compression, as well as add a lot of depth and warmth to the signal. They are the signature sound on classic records by The Beatles or anything done at Abbey Road Studios, heavily used in the Motown recording studio and was still used on every mix at Capitol Records by senior engineer Al Schmidt before he passed. They are a powerhouse on drums and can really open up the depth of the room mics and overheads, or tighten them up by evening out the dynamics between the kick, snare and toms and warming up and smoothing out the cymbals.

The Input control works the same as it would on a FET compressor, it will accept the input from the preamp, but can then be boosted to hit the sweet spot. Driving the input can result in some very pleasant tube overdrive which can be great to add some attitude to drums, vocals, electric guitars, or electric bass.

A variable mu compressor such as a Fairchild 660 or 670 adds the addition of Threshold, which works similar to the functionality of the optical compressor. After the input is determined, you can then adjust the threshold to dial in the right amount of gain reduction. This gives a little more flexibility than the FET compressor that has a fixed threshold, allowing you more possibilities to drive the compressor really hard, and then choose the desired amount of compression based on the input gain.

On older models such as the vintage Fairchild 670, the attack and release times are bundled together through a parameter called “Time Constants.” Using the Fairchild as an example, the set time constant will have attack and release times that can’t be adjusted, but are set to compliment each other. If you are on the lower setting, you will have a quicker attack and release time, if you are using the higher value time constants, you will have a slower attack and release time. On a newer model such as the Manley Variable-Mu, you have a variable attack control similar to the 1176 which ranges from slow to fast, but have set “recovery times” which are locked in four modes from slow to fast.

I hope that clears up a little mystery on compressors and how they function, and the overview of the main types can help you find which compressor will be the best fit for your recording or production style. To keep this blog less than 30 pages, more advanced features of compression will be introduced over the next couple months such as parallel compression, mastering compressors, side chain compression, end bus compression and mix bus compression.

If you have any questions on finding the specific compressor that will be the right fit for your studio, feel free to contact one of our Audio Consultants via email or by phone at 888.653.1184