What Is A VCA Compressor & How Does It Work?

The VCA (voltage-controlled amplifier) compressor is one of the most common types of hardware compressor (or plugin emulation) on the market and should be understood on our way to audio mastery.

In this article, we'll learn about how VCA compressors work; have a look at some examples of VCA compressors, and consider their strengths, weaknesses and typical applications.

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A Primer On Compression

Before we get to the specifics of VCA compressors, let's quickly define what compression is.

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Compression (and more specifically, dynamic range compression) is the process of reducing the dynamic range of an audio signal. That is, reducing the difference in amplitude between the highest and lowest points of the signal.

Compressors achieve this by attenuating only the loudest parts of the signal.

To attenuate the “loudest parts” begs two questions to be answered:

• What constituted the loudest parts?
• By how much should the loudest parts be attenuated?

A compressor answers these questions with the threshold and ratio parameters, respectively.

What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor kicks in (with its given attack time). As the input drops back down below the threshold, the compressor disengages (according to its release time).

What is the ratio of a compressor? The compressor ratio compares the number of decibels the input signal is above the threshold to the number of decibels the output signal is above the threshold. In other words, it is the relative amount of attenuation the compressor will apply to the signal.

Other compressor parameters worth mentioning are the following (I've added links to in-depth articles on each parameter):

• Attack Time: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release Time: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the input.
• Makeup Gain: the gain applied to the signal after the compression takes place (typically used to bring the peaks of the compressed signal up to the same level as the peaks pre-compression).

All compressors work with a gain reduction circuit that effectively compresses the audio signal in response to a control signal. This control signal (also referred to as the sidechain) is derived from the input audio signal (common) or via an external audio signal (less common). It is manipulated via the aforementioned compressor parameters.

So every compressor will have two critical signal paths:

• The audio signal path, which sends program audio through the gain reduction circuit.
• The control signal (sidechain) path that reads manipulates the sidechain signal (input or external) and controls the gain reduction circuit.

In the case of VCA compressors, the gain reduction circuit is centred around a voltage-controlled amplifier.

With that primer, let's get into VCA compressors and how they act to compress the dynamic range of audio signals!

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What Is A VCA Compressor?

A VCA compressor, as the name would suggest, is a compressor that utilizes a VCA (voltage-controlled amplifier) within its gain reduction circuit.

The first question that comes to mind when learning about VCA compressors is something along the lines of “if a compressor needs to attenuate signal level in order reduce dynamic range, then why is a voltage-controlled amplifier used?”

Well, as we'll discuss in this section, in addition to amplification, a VCA can also act to attenuate a signal.

In order to fully understand VCA compressors, let's define VCAs in more detail.

What is a voltage-controlled amplifier? A VCA (voltage-controlled amplifier) is n amplifier that alters its gain (applied to an input/carrier signal) in proportion to a control voltage (CV) or modulator signal, hence the name. VCAs can be used to amplify, attenuate or otherwise control the amplitude of the input/carrier signal.

So a VCA will be involved with three signals. They are:

• Carrier: this is the input AC signal (audio signal) which is generally bipolar (having negative and positive amplitudes). It is the signal that becomes altered by the VCA.
• Modulator: this is the control voltage/input which is, in VCA compressors, is unipolar (having only positive amplitude). This is the signal that effectively alters the audio signal/carrier.
• Output: the instantaneous product of the carrier and modulator so long as the modulator is positive.

The word “amplifier” in the term voltage-controlled amplifier is misleading. The majority of VCA circuits do not exceed a gain ratio of 1, meaning that VCAs actually attenuate rather than amplify the signal.

So a VCA works like this: the stronger the control voltage/modulator (the highest the amplitude), the more carrier signal is passed through to the output. This means a few things:

• At some control voltage/modulator level, the full carrier signal level is passed through to the output.
• When the control voltage/modulator level is zero or negative, no carrier signal is passed through to the output.
• At all other points between zero and “full”, the modulator will have the effect of attenuating the carrier between the input and output of the VCA.

This can be visualized in the following illustration of a basic VCA with a bipolar carrier and bipolar modulator:

In terms of compressor lingo, the carrier of the VCA is considered the audio or program signal and the modulator of the VCA is considered the sidechain. It's the sidechain/modulator that triggers any and all compression, and it is the audio/program/carrier that is affected by such compression.

Here's a simple signal flow chart to express compressor sidechain:

When it comes to VCA compressors, the audio input of the compressor feeds both the carrier and modulator.

More precisely, the input signal is split through an integrated circuit (IC chip) into an audio path and a detector path. The audio path is what will ultimately get compressed and passed through to the output, and the detector path controls the compression effect.

The carrier is simply the audio input signal itself.

The detector pathway of the VCA, however, has several stages that affect the incoming audio.

In order to have a continuous output signal without any flatlines at zero amplitude (relative to the input signal), the modulator much be unipolar.

This requires a sort of RMS (root mean square) detector and rectifier that will effectively read the “average” amplitude of the input signal and output a varying DC voltage that will always be positive.

Different VCA compressors will be designed differently. However, the main idea is that the detected DC voltage will effectively lower the modulator signal level to attenuate the carrier.

So the detector path will manipulate the modulator signal to achieve the typical compressor parameters. These parameters include the following (I've provided links to in-depth articles on each control):

• Threshold: the amplitude limit that dictates when the compressor will engage and disengage.
• Ratio: the ratio of input signal amplitude above the set threshold to the output signal amplitude above the threshold.
• Attack: the amount of time it takes for a compressor to engage/react once the input signal amplitude surpasses the threshold.
• Release: the amount of time it takes for the compressor to disengage (to stop attenuating the signal) once the input signal drops below the threshold.
• Knee: the transition point around the threshold of the compressor where the output becomes attenuated versus the input.

Note that the detected RMS signal will not lower the modulator signal/control voltage if the input signal is below the set threshold.

Of course, the number of available parameters and the amount of control of each parameter are always dependent on the particular VCA compressor.

This compressor type can be designed into a small form factor due to the fact that the IC chips used in the design can be rather tiny. These chips, along with many other components, can also be pretty cost-effective, making some VCA compressors relatively affordable.

VCA compressors are among the most commonly used compressors not only for their popularity on the market but also because they excel on everything from vocals and percussion to master bus tracks. With plenty of controls, VCA compressors offer superb versatility and functionality.

As the control voltage changes, there is a chance that it may leak into the audio path, which would lead to artifacts and distortion. The result of such distortion is dynamically changing. This is unlikely to cause any significant colouration, but in particularly bad cases, the unwanted artifacts can become overly noticeable.

It's also worth noting that while VCA compressors are certainly capable of high levels of gain reduction, pushing them too hard will certainly cause distortion. It's up to you to decide if this distortion is pleasing or not.

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Characteristics Of VCA Compressors

In this section, we'll consider a few of the typical characteristics of VCA compressors:

• Very low distortion
• Flexible threshold, ratio, attack, release and (sometimes) knee parameter controls
• Capable of very fast attack and release times
• Transparent sound
• Capable of very high levels of gain reduction
• Typically less expensive than delta-mu/tube and optical compressors
• May lack high-end clarity

Before moving on to the VCA compressor examples, I'd like to point you toward my video on the seven different compressor circuit types. You can check it out below:

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VCA Compressor Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 5 different VCA compressors to help solidify our understanding of this type of compression.

In this section, we’ll discuss:

• 500 Series VCA compressor: Shadow Hills Dual Vandergraph
• 19″ rack unit VCA compressor: Solid State Logic XLogic G Series Compressor
• VCA compressor effect pedal: Retrograde FX Underthruster
• VCA compressor Eurorack module: ST Modular Stereo Comp
• VCA compressor plugin: Arturia Comp VCA-65

Other notable VCA compressors include:

• SSL G-Compressor (Quad-Compressor)
• dbx 160A
• Focusrite Redd3
• API 2500
• WesAudio Dione
• Vertigo VSC3
• Shadow Hills Mastering Compressor

Shadow Hills Dual Vandergraph

The Shadow Hills Dual Vandergraph is a 2-channel (stereo) class-A VCA compressor in the 500 Series format.

Whether you choose to use the Dual Vandergraph on a single channel or two, the signal path/paths will be affected by the same superb compressor circuits.

This VCA comp offers 4 different ratios selectable via a 5-position lever switch (off, 2:1, 2.5:1, 4:1 and 8:1).  Each of the four ratio presets engages its own predetermined attack and release times as well. These time constants are as follows:

• Ratio: 2:1
  • Attack time: 30 ms
  • Release time: 100 ms
• Ratio: 2.5:1
  • Attack time: 30 ms
  • Release time: 500 ms
• Ratio: 4:1
  • Attack time: 10 ms
  • Release time: 500 ms
• Ratio: 8:1
  • Attack time: 0.5 ms
  • Release time: 250 ms

The “Compression” knob control (scaled from 1 to 24) controls the threshold of the compressor, and the other knob, labelled Output Level, controls the makeup gain of the compressor.

Another 5-position lever switch engages filters to act upon the sidechain. These positions of the switch are as follows:

• Off
• High-pass filter @ 90 Hz
• High-pass filter @ 150 Hz
• High-pass filter @ 250 Hz
• Band-pass filter

The amount of gain reduction is shown on the Dual Vandergraph's VU meter.

Solid State Logic XLogic G Series Compressor

The Solid State Logic XLogic G Series Compressor is the 1U rack-mounting version of SSL's amazing G Series console master bus compressor.

The VCA compressor features multi-positional attack time, release time and ratio knobs along with continuously variable threshold and makeup gain controls.

The XLogic G Series Compressor circuit sidechained can be driven either by the loudest of the two input signals or, alternatively, by an external (mono) signal applied to the key input on the rear of the unit. The external sidechain can be engaged via a button on the front of the unit.

The interesting Autofade switch and its rotary control can be engaged to produce a timed fade-up or fade-down in the output signal across a time frame between 1 and 60 seconds.

The VU meter shows the amount of gain reduction.

Retrograde FX Underthruster

The Retrograde FX Underthruster is a VCA compressor in the form of a stompbox effects unit. It's designed primarily for electric and bass guitar.

The Retrograde FX Underthruster brings studio-grade VCA compression to the pedalboard. It is super-easy to control with only 3 knobs.

The threshold can be set between -40 and +20 dB. The ratio can be set between 1:1 (no compression) and 20:1 (practically limiting). Finally, the gain knob adjusts the output level between -20 and +20 dB.

This compressor pedal has an LED meter to show the amount of gain reduction.

ST Modular Stereo Comp

The ST Modular Stereo Comp, as the name suggests, is a stereo compressor. More specifically, it's a stereo VCA compressor in the Eurorack modular synth format.

This VCA has left and right channel inputs and outputs. Each signal path is affected by the same compression circuit with continuous threshold (-40 to +20 dB), ratio (1:1 to ∞:1) and makeup gain (-20 to + 20 dB) controls.

Arturia Comp VCA-65

The Arturia Comp VCA-65 is a superb VCA compressor plugin based on the renowned dbx 165A.

Arturia’s advanced TAE modelling makes it possible to achieve the tight, punchy sound of the original 165A hardware in a software plugin. In addition to analog VCA sound, the Comp VCA-65 offers several advanced/modern features.

This plugin featured the typical threshold, ratio (compression), attack, release and output gain controls. The attack and release times can be set to “Auto mode” for the original fast attack & release parameters of the 165A.

In addition to the compressor, the Comp VCA-65 also features a peak stop limiter to prevent clipping. Parallel processing is made possible with an intuitive mix knob that continuously adjusts between 100% dry and 100% wet.

The meter can be set to meter the input, output or gain reduction.

The advanced side-chain control can be set to act upon the internal (input signal) or an external side-chain signal. The detection circuit can be set to read reverse, stereo channel or mid/side-channel information. An advanced time warp function offers lookahead control.

This powerful plugin also features a sidechain EQ section complete with a low-pass filter, high-pass filter and a variable-frequency bell/peak filter with boost/cut gain control.

The maximum amount of compression applied can be set via the compression range parameter.

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Related Questions

What are the different types of audio compressors? The term “type” can have a few meanings so let's look at a few different “types of compressors.

In terms of circuit topology, compressors will generally fall into one of the following types:

• Variable-Mu (Tube) Compressor
• FET Compressor
• Optical Compressor
• VCA Compressor
• Diode Bridge Compressor
• Pulse Width Modulation Compressor
• Digital Compressor
• Compressor Plugin

In terms of how a compressor will perform when compressing an audio signal (and the typical tasks it will be set to do), we can think of the following types of compression:

• Multiband Compression
• Peak-Metering Compressoion
• RMS-Metering Compression
• Feedback Compression
• Feedforward Compression
• Upward Compression
• Limiting Compression
• Parallel Compression
• Bus Compression

Should compression be used on every track? As a general rule, compression should be used with intent and, therefore, only be used on every track in the case that every track would require it. More often than not, there will be certain tracks in a mix that sound perfectly fine (and better) without dynamic range compression.

Once again, the typical benefits of using compression on a track include (but are not limited to) the following:

• Maintaining a more consistent level across the entirety of the audio signal/track
• Preventing overloading/clipping
• Sidechaining elements together
• Enhancing sustain
• Enhancing transients
• Adding “movement” to a signal
• Adding depth to a mix
• Uncovering nuanced information in an audio signal
• De-essing
• “Gluing” a mix together (making it more cohesive)