What Is A FET Compressor? And How To Use It To Better Mixes

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While compression is simply the process of balancing the loudest and softest sections of an audio signal, different types of compressors work in various ways. The bulk of plugin emulations in today’s DAWs are based on one of the five main types of audio compressor circuitry. One of the most used ones is the FET compressor. FET compressors use an FET to control the level of the audio signal being processed. We will go deep into the inner workings and applications of FET compressors in this FET compressor article, using sound examples made using some of the most popular FET compressor plugins on the market.

What Is An FET Compressor? (TL;DR)

Basically, as the voltage increases, the FET becomes more conductive, allowing more current to flow through it and increasing the gain of the signal. Conversely, as the voltage decreases, the FET becomes less conductive, reducing the gain of the signal.

The FET helps to control the amount of gain applied to the signal, allowing the compressor to reduce the dynamic range of the audio signal and create a more consistent and controlled sound.

How Does The FET Work?

There are four primary components that make up an FET compressor. They are:

1. Source
2. Drain
3. Gate
4. Body

Source:

The source is the input terminal of the FET compressor, and it is connected to the audio signal that is being processed. The source is responsible for providing the input signal to the FET, which is then used to control the flow of current through the device.

The source, which is the terminal through which current flows into the transistor; is typically connected to a voltage source, such as a battery or power supply, and is used to provide the current needed to drive the transistor.

Drain:

The drain is the output terminal of the FET compressor, and it is connected to the output of the device. The drain receives the processed audio signal from the FET and sends it on to the next stage in the audio chain.

It is typically connected to a load, such as a speaker or an amplifier, and is used to receive the current from the transistor.

Gate:

The gate is the control terminal of the FET compressor, and it is responsible for controlling the flow of current through the device. The gate is connected to a control voltage, which is used to adjust the resistance of the FET and, in turn, the level of gain reduction applied to the audio signal.

It is responsible for the flow of current between the source and the drain. When a voltage is applied to the gate, it creates an electric field that modulates the resistance of the transistor, allowing the flow of current to be controlled.

Body:

The body of the transistor is the part of the device that surrounds the source and drain. It is the substrate upon which the source, drain, and gate are built. It is typically made of silicon or another semiconductor material and serves as a conductor of current between the source and drain.

The body is also connected to a voltage source, which is used to control the resistance of the FET and the amount of gain reduction applied to the audio signal.

How Do The Components Aid In FET Compression?

It all starts with the audio signal being applied to the FET’s source terminal. This signal is then transferred via the FET’s body and into the drain terminal before being sent to the next step in the audio chain. The flow of current through the FET, on the other hand, is not constant; it is controlled by the voltage applied to the gate terminal.

When the voltage connected to the gate terminal is low, the FET’s resistance is high, limiting the passage of current through the device. As a result of the FET effectively working as a voltage divider, the audio signal level is reduced.

The resistance of the FET lowers as the voltage supplied to the gate terminal increases and the flow of current through the device increases. As a result of the FET enabling more of the audio signal to flow through, the audio signal level rises.

In reaction to the level of the audio signal, the FET compressor continually adjusts the voltage delivered to the gate terminal. When the audio signal exceeds a specific threshold level, the voltage connected to the gate terminal increases, lowering the FET’s resistance and allowing more of the audio signal to flow through.

When the audio signal goes below the threshold level, the voltage provided to the gate terminal is reduced, increasing the FET’s resistance and lowering the audio signal level.

What Goes On Inside A FET Compressor? How Does The FET Circuitry Work?

Since the 1176 FET design is so well known and well documented, we’ll use its FET circuit design as a framework to elaborate the inner workings of a FET compressor.

Power Supply: The power flows in from the AC on the wall frame into a large power transformer. It’s then directed from a power switch to the main board. Acting as a main power rail, a Zener diode(breakdown diode) on the board, maintains the voltage at 30Volts.

There is a smaller auxiliary Zener at -10DC, which is used as a reference for the Gain Reduction part. There are also some caps which act as a noise filter for the power supply and as a power reservoir.

Input Transformer: The audio flows from through the Input attenuator, onto the balanced input transformer. You can change the volume at the input attenuator before it goes to the transformer. From here, the transformer steps down the signal before it reaches the FET to gain control.

Class ‘A’ Signal Preamp: The Class ‘A’ signal preamp provides 26dB of gain and allows the transistors to display their non-linear qualities, which is crucial to the colour added during compression.

Output Pot: The signal is now fed to a passive attenuator, which is the Output pot. From here it’s fed to a signal line amp, which is a discrete Class ‘A’ stage amp.

Sidechain: Called the ‘Gain Reduction Control Amp’ on the board, the side chain section is where the FET’s control voltage is generated and controlled. The Sidechain also hosts the Ratio, Attack and Release controls.

As seen in the block diagram below, the signal flowing from the signal preamp to the output pot, has an auxiliary route that the side chain taps into.

Feedback Style Compression: 

While it may sound confusing and counter-intuitive, the Sidechain receives the signal after the FET and feeds this signal back to the FET. So, in essence, the side chain does two things at the same time.

The side chain monitors what the FET does. Based upon what the FET just did, it simultaneously uses that information as feedback to tell the FET what to do next. So the side chain is sending commands and reacting to the FET at the same time.

This principle is extremely crucial to the sound of the compressor and compressors that use them are known as ‘Feedback Style Compressors’.

If you’re interested, you can learn to make a DIY 1176 FET compressor here.

Voltage Divider: A voltage divider in simple terms divides and reduces the voltage in specific desired fractions of the total voltage applied between its two resistors. The FET in our compressor is one part of a voltage divider and basically replaces R2.

If you’re interested, you can find more information on Voltage Dividers here.

Gain Control FET: Here we have a three-pin configuration with Gate, Source, and Drain. The drain-to-source resistance is generally quite high and sometimes there is no gain reduction if R2 is infinite. But when you apply DC voltage to the Gate, you can reduce the resistance at R2, which will in turn lower the voltage at R1. So basically instead of having a volume knob, we have a DC voltage control. To understand the signal flow better, let us understand where the DC comes from.

Sidechain: This DC is fed from the Sidechain. The Sidechain interferes with the signal after the preamp section. The sidechain establishes the ratio and sets the attack and release while converting it to DC. It now sends it back to the FET as a control voltage. The incoming voltage sends signals to the FET, allowing it to set the desired Gain Reduction.

Ratio: The ratio control is one of the most interesting parts of the FET design and understanding its functioning is crucial to understanding how its characteristic sound is generated. Basically, the FET doesn’t receive information in a logarithmic way. However, we as humans perceive loudness in a logarithmic way. For instance, doubling the voltage doesn’t sound twice as loud to us. We just notice the decibel increase of 6dB.

For example, our input audio and threshold are coming into the FET at 2Volts in the intro of a song. When the verse kicks in at 4Volts, the compressor begins to compress and we perceive a 6dB increase.

Now when we have an energetic chorus coming in at 8Volts, we hear a 6dB increase again. While it’s only a 6dB boost for us, the voltage has gone high above the threshold for the FET. If you continue this process, the difference keeps increasing, until you reach 20dB. At this time, the voltage is 100 times the initial voltage.

The FET doesn’t see it as a 20dB increase. it’s seeing it as 100 times the DC voltage compared to the initial level.

So the Ratio in the FET is input dependent. So you can’t specify a single ratio with the ratio switches. This alone is quite instrumental in shaping the sound of the FET compression.

Primarily, the Ratio is controlling the AC threshold, or the level of the signal entering the Sidechain and the DC threshold, where the rectifier diodes begin to transfer DC to the transistors.

How Does A FET Compressor Sound?

How Does Non-Linear Distortion In A FET Compressor Make It Sound unique?

The non-linear harmonic distortion of FET compressors is what gives them their unique sound. FETs (field-effect transistors) are known for their ability to create distortion when driven hard, and this is what gives FET compressors their characteristic sound.

In a FET compressor, the nonlinear currents topology refers to the way in which the FET transistor is used to control the signal. When the signal is below the threshold, the FET acts as a linear amplifier, allowing the signal to pass through without any distortion.

However, when the signal exceeds the threshold, the FET becomes saturated and begins to distort the signal. This distortion is what gives FET compressors their unique sound.

The nonlinear current topology in a FET compressor is achieved by using a voltage divider circuit to control the gate voltage of the FET. This allows the FET to be driven hard, creating the desired distortion.

The amount of distortion can be controlled by adjusting the ratio settings on the compressor. Though the Threshold setting is generally fixed on a FET compressor, it is dependent on the changes in the Ratio.

Scenarios Where FET Compression Is Useful

Since FET compressors like the UA 1176 have been used for decades on some of the best records, there are a set of standard settings which are used as a starting point by engineers and producers.

We have listed the frequently used settings on the 1176 compressor along with its industry-standard names or terms. These settings can be used on any 1176-style compressor hardware or emulation plugins.

Quick Tips To Get The Best Settings On Your FET Compressor

We’ve demonstrated the difference between 4:1 and ALL in the drum track below. Notice how the kick and snare are more pronounced in the ALL mode. Notice how the kick comes ahead in the mix. This is the same way vocals would react within a dense mix.

So you can use this trick to highlight any element that needs more attention.

4:1 Ratio

ALL Buttons In

Adding Attitude With Distortion:

• This trick comes from the era when overdrive was recognized as a desirable sound, instead of an anomaly. It basically follows the philosophy of ‘More Is More!’
• Set the Release and Attack to their fastest values of 7. (Remember that the 1176 knobs are based on speed and work in reverse. So if you’re turning the knob to 7, you’re getting the fastest attack and fastest release. If you’re setting it to 1, you’re getting the slowest attack and slowest release.

• Setting it to 7 makes the attack and release times so fast that the small fluctuations that occur, sound like distortion.
• Add the ALL buttons mode for a more pronounced compression distortion.
• Use this on your bass, or vocal chorus when the vocalist is stretching to hit a high note. The distortion colours the voice in a way that adds attitude and character to the sound.

Now we’ll delve deeper into using the FET compressor on Piano, Guitar, and Vocals with dedicated before and after sound examples.

Piano:

We’re going to try to soften and enhance the tail in this slow piano piece and make it sound more introspective. This technique can work great for lo-fi, classical, and jazz piano settings. We’ve set the Ratio to 4:1. In the Waves CLA-76, this means that the 4:1 ratio affects all transients that pass -18dBFS.

Since we’re going for an introspective piano sound, we’ll go about softening the elements that are coming in the way. The 16th notes followed by the rests here have sharp transients that need to be smoothened.

Piano (DRY)

Piano (WET)

We’re going to set the inherent fast attack of the FET compressor to 6. Setting the release time to 7 on the dial or 50ms gives us the fastest release possible. We set the hum at 60Hz, which is modelled on the original hardware units.

As you can hear in the sound sample, the 1176’s FET compression adds subtle distortion, while letting the notes ring out longer. You can notice the analogue character by focusing on the bass notes.

The ultra-fast attack time of the FET is best suited for such cases. We’ve driven the input gain to -12dB here. You can be more modest if you feel the compression is too colourful.

Guitar:

In this guitar example, we would like to reduce some of the spiky transients and increase the sustain so that it extends till the next chord change.

With guitars, you might run into this quite often. Being a highly dynamic instrument, it’s sometimes challenging to fit the guitar in a mix in its entirety. These are the times when a FET compressor plugin becomes invaluable. 

Since the Blacky is warmer than the Bluey, we chose the Blacky here to soften the high end when the chords are strummed. We also drove the Input all the way to -3dB to feed more signal and set the ratio at 20:1 . The 20:1 ratio gives us a Threshold point of -12dBFS.

While we did start by going for an ultra-fast attack time, we quickly realised how the fast attack was washing away the lively dynamics. While it worked on the piano, the fast attack takes the life away from this guitar recording.

So we dialled the attack down to 3, where the crispness of the guitar strum is maintained but the spikes are toned down just enough to fit in with the other instruments.

Guitar (DRY)

Guitar (WET)

Since slower release times were numbing the guitar, we went for the fastest release time of 50 microseconds by dialling it up to 7. Though the difference is minuscule, we went for the 60Hz hum instead of the 50Hz, as it helped the bass move well with the chord change.

While the CLA76 did add some distortion, our aim was to add warmth to the spikes and make the guitars blend with the other instruments. This technique can be quite useful while working with funk, R&B, and reggae guitars.

Vocals:

You can set the attack to 5 and the release to 3, creating a V-shaped diagram. This is a common trick used by a lot of engineers to come out of the slow release in a gradual way. But the vocals gradually lose focus in this technique. This works great when there is another instrument also playing a counter-melody, as the primary vocals can come in and out.

Vocals (DRY)

Vocals (WET)

We didn’t go for this technique here. Our vocal phrase in 4/4 rhythm is broken into two distinct parts- a dotted quarter note followed by an eighth note + half note. So our airy vocal would go out of focus on the rhythm changes if we let go of the fast release.

We wanted our atmospheric vocals to stay afloat at all times. So we decided to keep the release time at 7. Since 1176 has an ultra-fast attack time, it doesn’t sound uncompressed, even at 1.

We also sent another copy of the vocals to hit the preamp section alone. By clicking on the ‘Comp Off” mode, we bypassed the compressor and our signal passed through the FET gain reduction element of the CLA76, giving us a characteristic FET distortion.

By blending this distorted copy along with the original compressed vocals with a volume fader ratio of 1:3, we found a good sweet spot for the vocals to lie in. Ignore the compression and try listening to the subtle distortion in the second audio sample.

Besides the vocals, we used a CLA76 Blacky on the guitar and bass tracks. We slapped an Elephant (Large Wooden Room) reverb along with a Blacky on the group bus as well. While the guitar and the bass used a Ratio of 4:1, we compressed the group bus at a Ratio of 8:1, bringing everything together.

Examples Of Popular FET Compressors:

Here’s is a short list of the top three FET Compressors, which are loved and used by sound engineers and musicians alike:

1. UAD 1176LN Rev E
2. Softube Drawmer 1973
3. Pulsar 1178

1. UAD 1176LN Rev E:

2. Softube Drawmer 1973:

Compatibility: macOS 10.13+, Windows 10+. 64-bit. VST2, VST3, AU, AAX.

Price: Drawmer 1973 Multiband FET Stereo Compressor $1699 (for hardware) Drawmer 1973 Multi-Band Compressor$99 (for emulation)

3. Pulsar 1178:

Compatibility: macOS 10.8+, Windows 7+. 64-bit. VST2, VST3, AU, AAX.

Price: $2500-5700- Urei 1178 Stereo Peak Limiter (for hardware) Pulsar Audio 1178 FET Compressor $129 (for emulation)

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