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What Is An Vari-Mu (Tube) Compressor? How Does It Work? And What Should You Use It On?

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COMPRESSION DEFINITION || OPTICAL || FET || VARI-MU (Tube) || VCA || DIGITAL || DIODE-BRIDGE || MASTER BUS COMPRESSION


While compression is fundamentally the process of balancing the loudest and softest parts of an audio signal, different types of compressors operate in different ways. The majority of the plugin emulations present in today's DAWs are based on the five main types of audio compressor circuitry. The Vari-Mu compressor is one of the less well-known varieties. Vari-Mu compressors adjust the dynamics of an incoming audio signal by using a tube or valve as its principal gain reduction element. In this Vari-Mu compressor article, we will delve deep into the inner workings and applications of Vari-Mu compressors utilizing sound examples created with some of the most popular Vari-Mu compressor plugins in the market.

What Is An Vari-Mu (Tube) Compressor? (TL;DR)

A variable mu compressor is a compressor that uses a valve or tube for gain reduction. The tube circuit alters the bias from moment to moment in proportion to the incoming signal, which results in variable gain attenuation.

While it isn't uncommon to find tubes in the makeup gain section, Vari-Mu compressors specifically use tubes as their gain reduction element.

‘Mu' (which is originally a Greek word) is an engineering term for gain. Hence, Vari-Mu essentially means variable gain. Vari-mu compressors use a Variable-Mu Tube, which allows the compressor to have a wide range of gain reduction.

The gain reduction is controlled by the input signal level and the threshold setting on the compressor. When the input signal exceeds the threshold, the compressor reduces the gain of the signal to prevent it from becoming too loud.

The Origins Of Vari-Mu Compressors

The Vari-Mu design dates back to the origin of audio compressors itself. Tube compressors began being used as a way of controlling the peak audio level that went into AM radio transmitters. 

Since initial AM radio transmissions sounded quite bad when they were distorted, an extremely fast compressor with a high ratio was needed. Vari Mu compressors were a perfect fit at the time as they catered to the specifications of the remote cut-off valve, which was prevalent in the 1940s.

A lot of Vari-Mu (Tube) compressors are still used in radio production today to provide the distinctive sound people were used to.

The BA6A, in the image above, used the 6386 tubes in a Vari-Mu design which was first made in 1951 as a way to improve the quality and clarity of AM radio transmissions.

AM, or Amplitude Modulation, was the dominant form of radio transmission at the time, but it suffered from a number of issues, such as distortion, noise, and interference.

The Vari-Mu compressor was designed to address these issues by using a tube-based circuit to control the volume of the audio signal being transmitted. The gain of the circuit could be adjusted in real time based on the volume of the audio signal. This helped to reduce the amount of distortion and noise in the transmitted signal.

The Vari-Mu Circuitry

The circuit diagram of a vari-Mu compressor typically includes the following components:

  • Input transformer. This component is responsible for impedance matching between the input signal and the rest of the circuit. It also provides galvanic isolation between the input and the rest of the circuit, which helps to reduce the chances of ground loops and other types of interference.
  • Tube amplifier stage. This stage typically consists of one or more vacuum tubes, which amplify the input signal. The tube amplifier stage is responsible for providing the necessary gain to drive the compressor.
  • Vari-Mu control circuit. This circuit is responsible for controlling the gain reduction of the compressor. It typically consists of a voltage-controlled resistor or a variable transformer, which is used to adjust the gain of the compressor in response to the input signal level.

In essence, the control voltage adjusts the grid voltage of the variable-mu tube based on the level of the input signal, which determines the gain-reducing action of the compressor. The control voltage can be expressed as:

Control voltage = K * (Input signal level – Threshold level)

Where K is a constant, that determines the slope of the control voltage curve, and the threshold level is the level at which the compressor starts to reduce the gain.

vari-mu compressor circuit diagram
  • Output transformer. This component is responsible for impedance matching between the output of the compressor and the load (such as a recording console or an amplifier). It also provides galvanic isolation between the output and the load, which helps to reduce the chances of ground loops and other types of interference.
  • Power supply. This component provides the necessary voltage and current to power the vacuum tubes and other components of the compressor. It typically includes a transformer, rectifiers, and filter capacitors.
  • Output amplifier stage. This stage is responsible for providing the necessary gain to drive the load (such as a recording console or an amplifier). It typically consists of one or more vacuum tubes, which amplify the output of the compressor.
  • Sidechain. The Vari-Mu compressor uses a side chain circuit to control the gain of the audio signal being processed. The side chain circuit consists of a variable resistor, a fixed resistor, and a detector circuit.
  • The Variable Resistor, also known as the threshold control, determines the level at which the compressor will begin to reduce the gain of the audio signal. When the audio signal exceeds this threshold, the variable resistor allows a small amount of the audio signal to pass through to the detector circuit.
  • The fixed resistor, also known as the ratio control, determines the amount of gain reduction that will be applied to the audio signal. When the audio signal passes through the variable resistor and reaches the fixed resistor, the fixed resistor reduces the gain of the audio signal by a fixed amount.
  • The detector circuit, also known as the gain reduction control, determines how much the gain of the audio signal will be reduced based on the input from the variable resistor and the fixed resistor. The detector circuit compares the input from the variable resistor and the fixed resistor to the audio signal, and adjusts the gain of the audio signal accordingly.

The side chain circuit can be represented by the following equation:

Gain Reduction = (Threshold – Audio Signal) / Ratio

Where Threshold is the level at which the compressor will begin to reduce the gain of the audio signal, Audio Signal is the input audio signal, and Ratio is the amount of gain reduction that will be applied to the audio signal.

  • Remote Cutoff Style Gain Reduction. The remote cutoff style gain reduction curve in a Vari mu compressor is unique and different from other compressors in that it utilizes a remote cutoff system to control the gain reduction.

This style of gain reduction uses a remote control circuit to adjust the amount of reduction applied to the signal, rather than using a fixed threshold or ratio. Here, the compressor's threshold and attack settings are not directly adjusted by the user, but rather by an external signal or control voltage.

The remote cutoff style gain reduction is achieved through the use of a variable-mu tube, which allows for precise control over the amount of compression applied to the signal. When the input level exceeds a certain threshold, the Variable-Mu Tube begins to compress the signal, reducing its level.

The amount of compression applied is determined by the setting of the remote control circuit, which adjusts the bias of the tube to achieve the desired level of reduction.

This style of gain reduction is highly flexible and allows for precise control over the amount of compression applied to the signal. It is often used in high-end audio equipment, as it provides a smooth, transparent sound and allows for a wide range of dynamic control.

How Does A Vari-Mu (Tube) Compressor Sound?

The Vari-Mu compressor sounds warm, smooth, and natural because it uses a vacuum tube circuit to provide gain reduction. Since it has a slow attack time, it takes longer for the compressor to start reducing the gain, giving the signal a more gradual and transparent sound.

The way the soft knee is designed also results in a more subtle, gradual compression, as well as a more organic release.

Due to these features, the Vari-Mu compressor is also less prone to create artifacts or pump-and-breathe effects like a diode bridge when cranked or add punchy character-filled distortion like a FET.

Why Does The Vari-Mu Compressor Sound Different From Other Compressors?

While adjusting the knee is not an available function on all compressor types, it certainly has a huge impact on the Vari-Mu compressor’s sonic characteristics. 

While we could’ve included the VCA and Opto compressors as well, we decided to compare the popular Manley Vari-Mu compressor to the Universal Audio 1176 FET compressor, as their knees are noticeably different. As we compare these two legendary compressors, we’ll also see what parameters contribute to the uniqueness of the sound. 

We’ve attached sound samples for both the Vari-Mu and FET compression done on a Group Buss. We’ve applied Vari-Mu and FET compression on all individual tracks of the group, as it's easier to notice the effect of the knee on the sound.

Sound Examples

Dry Group Buss:
Vari-Mu:
FET:

Nature of Gain Reduction:

The soft knee on a Vari mu compressor means that the compressor begins to apply gain reduction gradually as the input signal approaches the threshold level. This creates a more natural and subtle compression effect, rather than a hard knee compressor which applies gain reduction abruptly once the threshold is reached.

A FET compressor, on the other hand, typically has a hard knee. This means that it applies gain reduction sharply once the threshold is reached, resulting in a more noticeable and punchy compression character. This can result in a more aggressive sound, as the compressor is clamping down hard on the signal.

Threshold Ratio:

The threshold ratio also plays a role in the difference between these two compressors. The Manley Vari mu compressor has a variable threshold ratio, meaning the amount of gain reduction applied can be adjusted to suit the specific needs of the signal. This allows for more fine-tuning and control over the compression.

On the other hand, the FET Universal Audio 1176 compressor has fixed threshold ratios, with options of 4:1, 8:1, 12:1, and 20:1. These ratios determine how much gain reduction is applied for a given amount of signal above the threshold.

For example, a 4:1 ratio means that for every 4 dB of signal above the threshold, 1 dB of gain reduction is applied.

Tube vs Transistor Design:

The Manley Vari-Mu compressor uses a tube-based design, while the 1176 compressor uses a transistor-based design. The choice of design can affect the sound of the compressor in a few different ways. Tube-based designs tend to produce a warmer, more musical sound, with a slower attack and release time.

Transistor-based designs, on the other hand, tend to have a faster attack and release time, which can be useful for achieving a tighter, more controlled sound. They may also produce a slightly harsher or more aggressive sound, depending on the specific design.

The ‘‘Soft-Knee’’ Feature On Vari-Mu Compressors:

The Manley Vari-Mu compressor has a feature called “soft knee,” which refers to the way the compressor responds to signals that are close to, but not quite exceeding, the threshold. 

With a soft knee, the compressor starts to apply gain reduction gradually as the signal approaches the threshold, rather than suddenly applying it once the threshold is crossed. This can result in a smoother, more natural-sounding compression. 

The FET 1176 compressor, on the other hand, does not have a soft knee feature as it isn't designed for this trajectory of compression.

Knee Comparision Diagrams Of The Manley Vari-Mu And The UA 1176:

As you can see in the comparison diagrams below, the varying threshold and ratio settings affect the shape of the knee drastically. Comparing the Soft knee of the Vari-Mu with the hard knee of the FET compressor at similar settings is the best way to understand their impact on your audio signal.

In the Vari-Mu’s ‘Compressor Mode’ diagram below, we have the input and output set to achieve unity. We can see that lowering the threshold makes the knee roundish and, in fact, so smooth that the compression curve is almost unnoticeable. 

If you notice the gain reduction on the Y-axis, you can notice that the characteristic soft knee of the Vari-Mu is maintained till the gain reduction hits 9 or 10dB. The 1.2:1 to 3:1 to 9:1 line says it all. It shows how the knee behaves at different ratios, slowly clamping down as we hit higher ratios.

vari-mu compressor graph

In the diagram below, where the Vari-Mu is set to the ‘Limit Mode’, the input and output are set to hit 12dB. Reducing the threshold makes the knee extremely soft. It starts by behaving like a compressor, till it enters into the realm of limiting.

The 3:1 and 9:1 reference lines and the 1:1 and 12:1  reference lines best describe the Vari-Mu’s behavior. 

When we look at the FET compressor diagram, we can see how much it differs from the Vari-Mu's soft knee compression. While there is a considerable difference between 4:1 and 20:1 on a FET, you can see that even 4:1 is quite aggressive. If you actually go by the looks of it, the FET’s 4:1 compression ratio gives a curve similar to the Vari-Mu’s 8:1. 

But even if we go visually, we can see that the inherent trajectory of gain reduction is so different that it wouldn't be fair to compare them. 

Scenarios Where Vari-Mu (Tube) Compression Is Useful

Since they are known for their smooth, natural-sounding compression, and their ability to add warmth and character to a signal, they are used on vocals, bass, and instruments where transparent musical compression is needed. They are also used in mastering, for final touch-ups, and gluing mixes without introducing undesirable artifacts.

The Vari-Mu design allows for the compressor to respond to the dynamics of the drums in a more organic and musical way, which can help to enhance the swing, feel, and groove of the drum performance.

They are also used heavily on mix busses and master busses, as they have an innate ability to iron out the dynamic levels of various instruments. This helps bring the group of tracks together, making them sound cohesive, and devoid of  anamolies.

Bass:

Vari-Mu is useful on Bass to add body, add distortion, saturation, bite, and warmth to a bass signal. However, it can remove punchiness in some instances. Tube compression can additionally be used on bass to slightly enhance its stereo presence.

We went through a live example on a bass track so you're able to hear the difference.

We cranked up the input intentionally to drive the tube emulation plugin harder. We drove the input till 6 but dialed down the attenuation to -7. 

The Royal Compressor plugin that we’ve used here doesn't use tubes in the make-up gain section but instead uses tubes in the gain reduction section called the ‘Output Attenuator’.  So we made sure that the tubes were fully engaged in order to hear the Vari-Mu emulation at its best.

While going for a faster attack would’ve been ideal on the bass, we went for Model ‘C’ which has the slowest attack of the three EMI RS 124 hardware models chosen by the designers for emulation.  

Going for Model C gave us a sizeable gain reduction between -7 and -8dB. While the dry sample does sound smooth, it doesnt highlight nuances int he changing of the notes. 

With this sound example being made on Impact Soundworks Fretless Bass, capturing the slight pitch variations while sliding between fretless notes is the USP of the fretless sound. To enhance this, we went for a slow ‘held’ release at 5. The Red dots in the ‘Recovery’ section hold the release times for longer.

Going for 100 percent saturation didn't make too much of a difference. There will be some cases when the saturation doesn't do much. But overall, there was a considerable amount of stereo width added, especially in the lower mids and lower frequencies.

tube compression bass settings

If you listen to the dry sample, you’ll notice how the playing doesn't sound as nuanced as the wet sample. While the Vari-Mu is adding body, it also brings more attention to the note changes in the way that it colours the signal.

Bass DRY:

Bass WET:

Drums:

Vari-Mu (or Tube) compression is useful to help add thickness, punch, and bite to drums, without introducing artifacts or creating a harsh feeling of compression. It tightly glues everything together, while adding a certain punch and warmth to the sound.

Below is a live example of tube compression being used on drums, with sound examples showing how it alters the sonic character of a recording.

tube compression drum settings

With the drums, we went for the fastest possible settings. Setting the Attack to the fastest Model A, we made sure the snare was captured precisely as Vari-Mu compressors tend to generally miss these details.

We set the release to the black coloured ‘1’ in the Recovery section which is faster than the red one. With every snare and kick hit, the VU meter was spiking back and forth between -3dB and 0.

We drove the input to around 4.7 to maintain the same level as the dry signal. We went quite extreme with the output attenuator to get more ‘tube’ colour. This made the bottom snare have more body and warmth than before. It also made the kick more defined than the dry sample, where it sounds a bit loose.

But as we’ve mentioned throughout this article, the soft-knee compression on the Vari-Mu can sometimes be a huge disadvantage.

In this case, having used the fastest available attack and release settings, we were still not able to capture the cymbals – the VU meter just wouldn't go up when the hi-hats played.  Only after crossing 6 on the input dial did the compressor begin to react to the hi-hats.

(but an input of 6 was loud and was unusable for the example below).

When listening back to the audio example, you can hear that the Vari-Mu compressor does a great job at capturing and colouring both the kick and the snare, as well as adding bite to the hats and just slightly compressing the toms.

Drums DRY:

Drums WET:

Mix Buss:

Tube compression on the master bus/mix bus is perfect for making a piece sound cohesive and applying a warm, smooth gluing effect to your mix without destroying dynamics.

We've provided a live example of Tube compression used on a mix bus below, for you to get a feel of how it affects the sound.

tube compression mix bus

We used the same bass and drums on the mix buss, and made 3 different sound examples in order to explain the example in more detail. All the tracks in the first sound example are dry. You can hear the drums without their colour and the bass without the body. 

In the second sample, we left the individual tracks dry and added a Vari-Mu compressor on the mix buss alone. This is the standard way of using a bus compressor.

We went extremely mild with the compressor, going for a gain reduction of -0.5dB to -2dB. We dialed in an input of 1.5 and drove the tubes to an attenuation of -2.9, using the slowest Model C attack and dialing the release to the red-dotted 3, with autogain switched off, and a saturation level of 60 percent. 

You can hear how the drums and bass get glued together to form a cohesive mix. While the third sound sample is bolder, this one achieves the most amount of cohesiveness without going overboard. Notice how it also doesn't add too much colour and sounds ‘transparent’.

In the third sample, we turned on the Vari-Mu on the individual bass and drum track as well as the mix buss. Using the same individual settings as before, we added the mix bus compressor to the group. You can hear how the bass drives the track and the snare attack is too noticeable.

Vari-Mu compressors don't function at their best when used on all tracks. While an FET would sound great, with added grit and character, the Vari-Mu doesn't handle it well. If you listen closely, you can hear how the bass pops in the second measure of bar 2.

This is a classic example of how NOT to use tube compression in your mixes

All Tracks DRY:

Individual Bass & Drums DRY. Mix Buss WET:

All Tracks WET:

Synth+Bass:

tube compression synth settings

In this synth+bass example, we wanted to show how the vari-mu can be used to musically smooth out a transient heavy ‘hot’ track. You can notice how the intensity is maintained, but the edges are smoothed without sounding artificial. 

To achieve this, we simply went for the fastest settings. We managed to get a constant gain reduction of -6.5dB to -8.5dB with this simple trick.

Synth+Bass DRY:

Synth+Bass WET:

Popular Optical Compressors:

Here's a short list of popular Optical Compressors that have become a studio staple over the years:

  1. Manley Variable Mu Stereo Compressor Limiter
  2. Fairchild 670
  3. CHANDLER LIMITED RS124 COMPRESSOR or Royal Compressor Plugin

1. Manley Variable Mu Stereo Compressor Limiter

Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA, UA Connect compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only. 
Price: $4499 (for hardware), $149 (for emulation)

2. Fairchild 670:

Compatibility: macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU, SoundGrid.
Price: 
Fairchild-670-compressor-limiter $50000+ (for hardware) Waves PuigChild 660 & 670 Plug-in$29 (for emulation)

3. CHANDLER LIMITED RS124 COMPRESSOR or Royal Compressor Plugin

Compatibility: macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU
Price: 
CHANDLER LIMITED RS124 COMPRESSOR: $2995 (for hardware) Royal Compressor$62 (for emulation)

FAQ

What Are The Disadvantages of Vari-Mu (Tube) Compression?

The threshold control on tube compressors is typically not as precise as that of solid-state compressors, which can make it difficult to fine-tune the amount of compression. Due to the tube in the gain stage and a ‘soft-knee’ control that makes the compressor work harder, the signal gets colored with added harmonics and overtones. 

While the unique coloration is where the real mojo lies in vari-mu compression, a gain stage tube and soft knee setting may also result in an increase in noise and hiss in the audio signal, as the compressor will be working harder to process the signal.

Tube compressors can be more expensive, complex, and harder to use compared to solid-state compressors. They also generate a lot of heat and can be physically large, which can be an issue in small studios or recording spaces.

What's The Difference Between Optical and VCA Compression?

The main difference between the two types of compressors is that the gain reduction in a vari mu compressor is based on the level of the input signal, whereas the gain reduction in a VCA compressor is based on the control voltage generated by the compressor's circuit. This means that the gain reduction in a vari mu compressor is more closely tied to the characteristics of the input signal, whereas the gain reduction in a VCA compressor is more closely tied to the characteristics of the compressor's circuit.

Vari mu compressors use a variable mu tube or vacuum tube to control the gain reduction. These tubes have a variable amount of gain based on the level of the input signal, which allows the compressor to adjust the amount of gain reduction in response to the signal.

VCA compressors, on the other hand, use a voltage controlled amplifier (VCA) to control the gain reduction. A VCA is an amplifier that has a control voltage input, which allows the gain of the amplifier to be adjusted based on the level of the control voltage. In a VCA compressor, the control voltage is generated by a circuit that responds to the input signal, which allows the compressor to adjust the amount of gain reduction in response to the signal.

Vari-mu compressors tend to have a smooth and warm sound, with a natural and organic attack and release. They are often used for subtle compression on vocals and instruments, and are known for their ability to add character and warmth to the sound.

VCA compressors, on the other hand, use voltage-controlled amplifiers to control the gain reduction of the signal and tend to have a more precise and accurate sound, with a faster attack and release. 

This makes them well suited for a wide range of applications. They are often used on instruments and vocals in recording and mixing, as well as on busses and masters in mastering. 

Are Plugin Emulations of Vari-Mu (Tube) Compressors Good Enough?

Unlike Opto and FET compressor emulations, high end Vari-Mu emulations are surprisingly good.  If you are using the compressor for a subtle, transparent sound, a plugin emulation may be sufficient. However, if you are looking for a more dramatic, colored sound on individual tracks, then you should go for the hardware version. Each unit is different and will provide an unexplainable tube mojo.

That being said, the Royal Compressor that we’ve demonstrated here sounds fantastic on most things we added to. Having tried the Universal Audio Fairchild 670 plugin during vocal recordings, we were surprised how well it treated the changes in microtones/cents on the vocal vibratos. They truly made the vocals more intimate.  

However, you can’t just leave the plugin On and expect it to work its magic like how the hardware unit would. 

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