Get to Know the Gear: Compressors

Effectively applying compression can be confusing when you’re starting out your mixing journey. There are different types, sonic characteristics and the natural tendency to over compress. We’ll discuss compressors’ signature sounds, most common application and compression series to help mitigate the over-achiever.

A modern track isn’t stamped complete without a healthy amount of compression. Whether that compression is used as an obvious effect for the soundscape or as a transparent dynamic glue. We’ll be diving into the basics, different types of compressors, applications and effective use of serial compression.

Here are the 4 types of analog compressors:

  • FET (Field Effect Transistor) Compressor
  • VCA (Voltage Controlled Amplifier) Compressor
  • Tube (Vari-Mu) Compressor
  • Optical Compressor

The Basics

Compression is used to control dynamic range, reducing hard peaks and boosting softer sound intensities. The model of the gain reduction circuit is what gives each compressor their signature sound, imparting unique colors with slight saturation. Faster compression, slower attack, more glue or better transient response etc. will create a different reaction with each compressor type. Some compressors are great for single instruments, giving detailed control. Others are great to use on buses (Bus compressors).

The three main controls you need to fundamentally understand are the Ratio, Attack and Release. We hear about these parameters all the time and how they are set depends greatly on the transparency you are going for and the source type. Here are some guidelines for each parameter that will switch your compression transparent to audible.

High Transient audio source type (Drums, Active Bass lines, Vocals): Ratio. A ratio of 4:1 is common, depending on the compressor you can go to a ratio 6:1 and remain transparent. Attack. The attack will depend greatly on the type of compressor you are using so these numbers are definitely guidelines. 5ms-15ms on a transient source will generally keep a transparent sound. Release. 30-50ms or below will start to yield some sustain noise thus, staying above 50ms will maintain transparency.

Sustained audio source type (808, Synth, Buses): Ratio. Cut the ratio in half, 3:1 is where you’ll start to hear the compressions on sustained sources. Attack. In the range of 25-50ms you’ll start to hear the attack. Release. Between 100-300ms is the range for audible release time.

When the goal is transparency, only one of these parameters should be at or near the threshold of audible, ie; to enhance those lows, release fast and lower that ratio and attack time.

FET (Field Effect Transistor) Compressor

  • Good for a variety of instruments ie; Vocals, Guitar, Bass, Drums (especially Vocals)
  • Good for parallel compression (mixing a heavily compressed signal with a dry signal)

FET compressors are unique, applying a field-effective transistor to the circuit to perform gain reductions. This adds an exciting and aggressive color to the signal that is similar to tube compressors but has a faster-acting transient response (faster attack time, faster release). The slowest attack time on an FET Compressor will still act faster than its analog counterparts, the VCA, Tube and Optical compressors. The addition of harmonic distortion and non-linear compression, differentiates it from digital counterparts.

Arguably, the most famous FET compressor is the UREI 1176 and its stereo version, the 1178. Software emulations have capitalized on the desirable effects, expanded past hardware limitations, and eliminated unwanted effects. Softube’s FET Compressor and Arturia’s Comp FET 76 are perfect examples of software emulations matching and expanding all the desirable effects of the FET Compressors.

Softube’s FET Compressor adds an internal ‘Parallel inject’ knob which allows parallel compression right from the plug-in. In the ‘Detector’ section, are the ‘External Sidechain’ for sidechain input sensitivity, the ‘Filters’ which are wide spread and versatile (can be used for general clarity or frequency targeting) and the “Look Ahead’ feature. The ‘Look Ahead’ feature allows the plugin to add up to 1 millisecond in your DAW to ensure no peak is missed. This feature also makes it viable as a master limiter, but check those True Peak levels.

Arturis’s Comp FET-76 has the classic look with double arrows that open up a new page with all the features you need for extra control. Arturia also features a sidechain and adds an ‘Input Detection Mode’ and ‘Input EQ’ to control and scope the input signal. The ‘Time Warp’ is quite cool, moving the effect forward acting as a look ahead function and moving it back will provide a looser compression of transients.

VCA (Voltage Controlled Amplifier) Compressors

  • Good for Drums and Master Busses.

VCA Compressors may not be versatile in the instruments but they sure do glue your mix together with a crisp sheen! With snappy transient response, they have great control of intense peaks and versatility in their parameters, but if you’re looking for a smooth ride, then an optical or tube compressor would be what you’re looking for. Hitting too hard with these compressors can quickly lead to distortion so application must be done carefully (PRO TIP: Series Compression!)

The SSL G Series ,and its software emulations from Waves, has made its name synonymous with VCA Compressor, many engineers referring to the “glue” of the mix as the “SSL sound”. Giving tight low-end, clear mid range and a full and punchy glue, this title is well deserved, even without any gain reduction! It’s a must on your master bus.

The API 2500, and its amazing software emulation from Waves (are we surprised), is up there in popularity as well. With more controls, the API 2500 is not as user friendly as the SSL G but it adds a ‘Thrust’ control that allows for a high pass filter to act as a detection input to keep low frequencies from hitting the compressor too hard. Waves API 2500 gives such solid punch, it’s a must for your drum bus.

Tube (Vari-Mu) Compressors

  • Good for a variety of instruments ie; Vocals, Guitars, Bass, and Drums
  • Good for transparent compression

Tube Compressors or Variable-Mu (Vari-Mu), are adequately named as the amount of compression applied is variant, depending on the loudness of the input signal. Adding exceptional warmth, depth and color, their unique saturated sonic characteristics make them a great addition to any instrument and a well loved tool of mastering engineers. The harmonic distortion and noticeable tone comes from the tubes re-biasing. In other words, the harder you hit the compression, the more your input signal and gain reduction interact with each other, creating a warm saturation.

The Fairchild 670 is among the most well known Tube compressors, being used by the Beatles and no a well established sound in the ultra-compressed, clear and warm soundscape of modern music. The Fairchild 670 from Universal Audio is a beautiful software emulation of the original hardware but unfortunately is only available to UAD audio interface owners. However, Waves very own Puigchild is up there with its own solid emulation and widely available.

The Manley Vari-Mu popped up later in the 90’s and gives mastering busses that cohesive, clear and precise stereo imaging. It’s a wonderful addition to your soundscape and the Pulsar-Mu plugin is a great emulation of it.

Optical (Opto) Compressors

  • Good for Vocals, Guitar and Bass
  • Good in Combination with FET Compressors
  • Good for Softer genres and the Acoustic sound

Optical Compressors give a smooth, transparent, clear compression at a high ratio with a slow-acting attack on intense transients. Opto compressors are unique in that they apply compression when audio hits light-sensitive resistors, converting audio into light. Fundamentally, Opto’s utilize the increase in audio amplitude to increase light emission, which causes the optical cell to attenuate amplitude peaks. This even tone makes them great in combination with FET compressors and VCA compressors to level out some of the aggression of these compressors.

Teletronix LA2A from Universal Audio is the iconic tube and optical compressor. The precise gain reduction and next to zero harmonic distortion has secured its spot as industry standard for transparent compression.

Softube’s Summit Audio TLA-100A is a go to leveling-amplifier for vocals and acoustic instruments. With a “set and forget,” organized design and workflow, the TLA-100A, by far, is one of the most user friendly Opto compressors out there. With a built in parallel compression effect, high pass filter and saturation knob, the analog feel of the hardware is maintained while increasing the engineers control of parameters.

Serial Compression

Using serial compression, multiple compressors in a row, is great for two reasons: mitigating over-compression and utilizing the most desirable attributes of each compressor. Most handy for precision mixing and mastering, serial compression creates transparency as well as clarity. Splitting the workload between two plug-ins creates more opportunities for control of the overall dynamics and quality of the compression. Having one compressor performing 4dB of gain reduction will be noticeable in comparison to two different compressors performing 2dB of gain reduction.

The functional purpose of each compressor needs to be taken into account when setting up. If your goal is a transparent, clean mix, starting with an FET or VCA compressor set relatively fast at a high ratio to catch those peaks is preferable. Followed by a slower Opto or Vari-Mu compressor with a lower ratio will give that consistent dynamic with a beautiful tone. A good example for this kind of dynamic and tonal compression would be on a pop vocal.

iZotope’s Neutron has a serial compression preset that can help with understanding these concepts.

In conclusion, “less is more” in terms of compression on all accounts. Avoid overworking your individual compressors and serial compression! Compression can either color your track beautifully or crush its essence. Serial compression should be used sparingly with purpose.

Natalie Gould is an independent songwriter, producer and mixing engineer based in Los Angeles, California. Frequenting writing rooms across genres, she pulls from personal experience and lessons from mentors to deliver the full package and quality in her work.

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