What Is A FET Compressor & How Does It Work?

The FET (field-effect transistor) 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 FET compressors work; have a look at some examples of FET compressors, and consider their strengths, weaknesses and typical applications.

A Primer On Compression

To start this article off right, we should briefly discuss compression in a more general sense.

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Dynamic range compression (which is the full technical term for what is more generally referred to simply as “compression”) is the process of decreasing the overall dynamic range of an audio signal. In terms of audio signals, dynamic range refers to the difference in amplitude between the highest and lowest points of the signal.

In the majority of cases, it's the noise floor that is the lowest point of a signal. Compressors will work by attenuating only the loudest parts of the signal (rather than bringing the quiet parts up, which is technically upward compression).

In determining how a compressor should attenuate the “loudest parts” of a signal, there are two basic questions that must be answered:

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

The threshold and ratio parameters of a compressor answer each of these questions, 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 passes through the gain reduction circuit and gets compressed.
• The control signal (sidechain) path that reads manipulates the sidechain signal (input or external) and controls the gain reduction circuit.

In the case of FET compressors, the gain reduction circuit is centred around a field-effect transistor.

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

What Is A FET Compressor?

A FET compressor, as the name would suggest, is a compressor that utilizes a FET (field-effect transistor) at the core of its gain reduction circuit.

What is a field-effect transistor? A FET (field-effect transistor) is a semiconductor device that uses an electric field to control the current flow between its source and drain terminals. Applying a voltage to the third terminal (gate) alters the conductivity between the source and drain, thereby altering the current flow/signal.

As mentioned, a field-effect transistor has 3 terminals: the source (S), drain (D) and gate (G). These terminals are illustrated in the following image:

To reiterate, FETs control the flow of current between the source and drain (which we can call the output) by altering the conductivity between the source and source with the application of a voltage to the gate (which we can call the input).

More specifically, by altering the bias voltage applied across the gate and source (V_GS) input, we can alter the voltage across the source and drain (V_DS) at the output.

FETs are often operated within the constant-current portion of their output characteristic.

They can also be used as switches with “on” and “off” outputs. The point at which the transistor reaches “off position” is called “pinch-off,” where the resistance between the source and gate is very high.

However, with FET compressors (and many other audio applications), the FET will operate in its linear region just before pinch-off, where V_DS is relatively small. Within this linear region, the FET will work as a voltage-variable resistor.

In this region, the drain-to-source resistance R_DS can be controlled by varying the bias voltage V_GS. By varying the voltage at the input of the FET, we can control the resistance between the source and drain and, ultimately, the voltage at the “output”.

The gain reduction circuit of a FET compressor can be simplified (albeit oversimplified) as a voltage divider with a variable resistor (the FET itself):

Where we have the following general equation:

V_{\text{out}} = V_{\text{in}} \cdot \frac{R_2}{R_1 + R_2}

• V_out: Audio Out
• V_in: Audio In
• R₁: resistance of the fixed resistor (audio circuit before the FET)
• R₂: variable resistance of the FET

By breaking it down like that, we can see that as the resistance of the FET increases, more signal transfer occurs. Therefore, as the resistance of the FET decreases (as the input signal increases), there will be more attenuation/compression of the output audio signal.

Note that the output signal must be very small to work within the “linear region”. FET compressors will often have to reduce the signal level before the compression and provide a gain stage after the compression. This is often achieved via transformers that will add some amount of distortion to the signal.

Okay, so how does a FET compressor work?

Well, the input signal doesn't only drive the input of the FET, but it also has an effect on the bias voltage of the FET's grid. In other words, the input signal is responsible makes up the program/audio signal and the sidechain signal. So far, so good.

Within the linear range, the gate's bias voltage (and V_GS) becomes more negative, and the resistance between the drain and source (R_DS) increases.

As the program signal experiences more resistance, it will be effectively attenuated/compressed.

Since the voltage/amplitude of the FET input controls the FET output, it will have an effect on the bias voltage of the FET. This effectively means that as the input signal level increases, so too does the amount gain reduction.

In this way, we can say that FET compressors are program-dependent: the amount of compression applied is dependent on the signal level of the program/input signal. In fact, they're so program-dependent that most don't even have a threshold control, as the threshold will be largely determined by the linear range of the FET itself.

To understand why this happens, let's consider the feedback compressor design topology of a FET compressor. Let's begin with this simple signal flow diagram:

So the audio signal flows to the FET, and the voltage-variable resistor/FET produces an output signal. This output signal is then fed back (this is a feedback compressor diagram) to the FET input.

However, before the feedback signal reaches back to the input, it passes through a peak detector. The peak detector is essentially a series connection of a diode and a capacitor that outputs a DC voltage equal to the peak value of the applied AC signal. In this case, the DC voltage is a negative bias voltage for the FET's gate.

Therefore, as the input signal level increases, so too does the negative bias voltage.

Put differently, the “sidechain signal” of a FET compressor continuously adjusts the FET's bias and varies the FET's resistance to alter its attenuation appropriately.

Here's a simple signal flow chart to express the compressor sidechain. Note that, with FET compressors, the peak detector would be the level detection circuit, and the DC bias of the gate would be the control signal:

If the bias exceeds the natural threshold of the FET, the circuit will cause some amount of gain reduction.

By applying a small DC offset to the control voltage of a FET, we can achieve different gain reduction characteristics (input/output curves), making the compressor more or less sensitive to input signal variation at the gate.

The attack and release controls of a FET compressor are made possible by altering the performance of the peak detector.

Another note on the nature of FETs and feedback circuits is that they won't be able to provide a true ratio control. They will, instead, have a ratio of just over 1:1 just above the threshold, and then it will approach ∞:1 well above the threshold. Ratio controls, then, essentially adjust the above-threshold level before the inevitable ∞:1 ratio.

These parameters, in a bit more detail, are as follows (I've provided links to in-depth articles on each control):

• 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.

A FET acts similarly to a triode tube, making FET compressors somewhat similar to variable-mu/tube compressors in the way that they offer compression. Of course, there are plenty of differences in character and speed (a FET gain reduction circuit reacts much faster than a tube). While tubes are rather non-linear in their compression curve, FETs act very linearly when properly set up.

FET compressors will certainly colour the sound due to their inherent distortion characteristics. They’re cherished for their aggressive sound and are popular choices for drums, guitars, vocals and other sources that need a bit of edge to pop out in a mix.

Characteristics Of FET Compressors

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

• Very fast attack and release times
• Non-linear compression that adds character via harmonic distortion
• Additional saturation due to the inclusion of transformers
• Requires low-level input signals
• Requires more output gain which often raises the noise floor

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

FET Compressor Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 4 different FET compressors to help solidify our understanding of this type of compression.

In this section, we’ll discuss:

• 500 Series FET compressor: BAE 500C
• 19″ rack unit FET compressor: Universal Audio 1176LN
• FET compressor effect pedal: Origin Effects Cali76 Compact Deluxe
• FET compressor plugin: Arturia Comp FET-76

Other notable FET compressors include:

• Chandler Limited Germanium Compressor
• Daking FET III

BAE 500C

The BAE 500C is a 500 Series single-channel FET compressor inspired by the many superb FET compressors from the 1960s and 1970s.

This well-laid-out FET compression is pretty straightforward. It features a FET gain reduction circuit with 3x 2520-style opamps and a transformer-coupled output. Let's run through the controls.

First, we have continuously variable input and output gain controls along with continuous-variable attack and release time parameters. The ratio of the 500C can be set to 2:1, 4:1, 8:1, 12:1, 20:1 and the revered “All Buttons Mode” (ABI) that offers a new sound (the ratio ends up being somewhere between 12:1 and 20:1).

In addition to these controls, there is also a switchable high-pass filter for the side chain signal with a 6 dB/octave slope at 125 Hz. Alternatively, the unit can be bypassed fully, or only the gain reduction circuit can be disabled (allowing the input/output gain controls to remain functional).

Universal Audio 1176LN

The Universal Audio 1176LN is a recreation of perhaps the most legendary FET compression of all time: the Universal Audio 1176 Limiting Amplifier.

This rack-mount FET compressor has a FET gain reduction circuit with ultra-fast attack time (20 µs to 800 µs) and a class-A line level output amplifier.

The release time is adjustable between 50 ms and 1.1 s. The threshold of the compressor is program-dependent.

As for the ratio, it can be set to 4:1, 8:1, 12:1 or 20:1 via the front panel buttons. Alternatively, all four front panel buttons can be engaged for extreme limiting where distortion increases and a plateaued slope and a lag time are introduced in response to transients.

Gain reduction or output level can be monitors via the VU meter, and the unit offers up to 50 dB of gain.

Origin Effects Cali76

The Origin Effects Cali76 Compact Deluxe is another compressor based on the legendary Universal Audio 1176. This one takes the form factor of a stompbox.

This studio-grade FET compressor pedal is designed with high-current, low-noise, discrete Class-A circuitry.

The compressor control panel features 6 knobs and offers a ton of flexibility. The controls are:

• Dry: adjusts the amount of direct signal in the output and, therefore, the amount of parallel compression.
• Out: adjusts the amount of makeup gain applied to the signal post-compression.
• In: adjusts the gain of the input preamplifier and, therefore, the level of the signal being sent into the compressor.
• Ratio: adjusts the compression ratio.
• Attack: adjusts the attack of the compressor (the time it takes for the compressor to kick in once the threshold is passed).
• Release: adjusts the release of the compressor (the time it takes for the compressor to disengage once the input signal drops back below the threshold).

The pedal also features the classic 3-colour jewel lamp as the gain reduction meter.

Arturia Comp FET-76

The Arturia Comp FET-76 is yet another FET compressor based on the legendary 1176 (did I mention how important the original was yet?). This compressor plugin offers the same sound and functionality as the original 1176, only in a digital plugin format.

In addition to the input, output, attack, release, ratio and meter controls we've discussed above, the Arturia Comp FET-76 offers a dedicated mix control and an advanced sidechain control to “modernize” the functionality of the plugin.

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.

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)