What Is A Variable-Mu (Tube) Compressor & How Does It Work?

The variable-mu (tube) compressor or “delta-mu 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 variable-mu compressors work; have a look at some examples of variable-mu compressors, and consider their strengths, weaknesses and typical applications.

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A Primer On Compression

In advance of the main discussion on variable-mu/tube compressors, I figure we should go through a more general explanation of compression.

Click here to skip ahead to the section What Is A Variable-Mu (Tube) Compressor?

Dynamic range compression (often simply referred to as “compression” in the audio world) is the process of decreasing the dynamic range of an audio signal. The dynamic range is defined as the difference in amplitude between the highest and lowest amplitude points.

Since the noise floor is often 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 order to attenuate the “loudest parts” of a signal, there are two questions that must be answered:

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

These questions are effectively answered via the threshold and ratio parameters of a compressor, 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 variable-mu/tube compressors, the gain reduction circuit is centred around a remote cut-off vacuum tube.

With that primer, let's get into variable-mu/tube compressors and how they act to compress the dynamic range of audio signals!

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What Is A Variable-Mu (Tube) Compressor?

Before we begin, I should mention that the term “variable-mu” has actually been trademarked by Manley as the title of the brand's Variable Mu Compressor. That being said, the compressor type is still often referred to as “variable-mu,” though perhaps we should be paying Manley royalties every time we use the term.

Other names for variable-mu compression include:

• Delta-mu compression, which is my favourite.
• Vari-mu, which is just shorthand.
• Variable-gain compression, which is perhaps confusingly similar to variable-gain amplification or voltage-controlled amplification (VCA).
• Tube compression, though any compressor with tubes in its design could be called a “tube compressor”.

Synonyms aside, let's get into this article.

A variable-mu or tube compressor, as the name would suggest, is a compressor that utilizes a vacuum tube as the central element of its gain reduction control circuit.

Note that many other compressor types utilize tubes in their design, especially as amplifiers in the makeup gain stage. The difference in a variable-mu compressor is that the tube is actually used specifically as the gain reduction component. Of course, variable-mu or delta-ma compressors can also have tubes elsewhere in their designs to offer additional gain.

The term delta-mu contains two Greek letters. Delta, in mathematics, generally refers to the change of a changeable quantity. Mu, in electronics, often refers to gain. So then, putting these two terms together yields the phrase “changing gain” or “variable gain”. For the remainder of this article, I'll refer to “variable-mu compression”.

This is fitting since the tube will apply variable gain to the audio signal depending largely on the strength of that signal. As the input signal amplitude increases, so too do the amount/ratio of the gain reduction. This makes these compressors program-dependent, which we'll get to momentarily.

Before doing so, let's quickly go over how tubes work in general.

What is a vacuum tube? A vacuum tube (or simply a “tube) is an electrical device that controls current flow in a sealed vacuum between electrodes with a voltage applied. The basic triode tube can control the rate by which current flows with an additional electrode called a “grid”.

The basic tube amplifier (which, in the case of variable-mu compressors, acts as an attenuator rather than an amplifier) is known as a triode. Triode tubes have 3 terminals: the anode, cathode and grid. These terminals, along with the heater, are illustrated in the following image:

As a voltage is applied across the tube, the anode and cathode become charged, and current begins flowing between the two terminals within the vacuum.

The control grid acts as a sort of mesh between the cathode and anode. Its holes allow electrons to pass through it. By adjusting the current applied to the grid, we control the number of electrons flowing from the cathode to the anode and modulate the current across the vacuum tube.

In simple terms, by altering the signal at the grid (input) of the tube, we can alter the signal between the anode and cathode (output) of the tube. If the base signal between the anode and cathode is stronger than the “input” signal at the grid, the tube can effectively amplify the “input” signal.

So how can tubes be used for compression? Let's begin by discussing what a variable-mu compressor does, followed by how it does it.

First off, variable-mu compressors utilize a particular class of tubes called remote cut-off tubes. These compressors exploit the natural property of such tubes that allows the gain of the to be widely varied. As more negative voltage is applied to the tube (via a control signal), the amount by which the plate current falls gets less and less, which is the basis of tube compression.

The voltage/amplitude of the variable-mu compressor input signal has an effect on the bias voltage of the remote cut-off tube. As the input signal level increases, so too does the ratio of gain reduction.

In this way, we can say that tube 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 ratio control.

With variable-mu compression, we have the situation where the input signal is sent to the grid of the tube. As the input signal (AC) increases, so too is the negative bias (DC) sent to the grid. This reduces the current at the tube’s grid, thereby attenuating/compressing the signal at the output.

To understand why this happens, we must understand the feedback compressor design topology. Let's begin with this simple signal flow diagram:

So the audio signal flows to the tube (grid), and the tube applies some amount of gain to the signal. The tube output is then fed back (this is a feedback compressor diagram) to its input.

Before the tube output 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 tube's grid.

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

As the negative bias voltage at the grid increases (made more negative), the current at the grid decreases. In this case, the tube is biased “colder”.

Put differently, the “sidechain signal” of a variable-mu compressor continuously adjusts the bias of the tube to alter its gain appropriately.

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

This all happens very quickly, though the tube will act relatively slowly (compared to solid-state devices), which gives variable-mu compressors their “slow sound”. Any increase or decrease of the gain is not instantaneous.

This gives tube-based compressors their unique attack and release time properties. The rate at which the gain increases or decreases has a major influence on the overall quality of the compression effect. More on variable-mu compressor characteristics later.

Note that the peak detector may very well have additional control parameters to affect the attack and release parameters and that the threshold can be altered by controlling the bias voltage via a potentiometer between the peak detector and the grid.

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

• Threshold: the amplitude limit that dictates when the compressor will engage and disengage.
• 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.

The nature of vacuum tubes produces a sort of non-linear saturation/compression with a soft knee that extends indefinitely (within reason) past the threshold. This input/output graph is visually represented in the following image:

The result is a smooth compression above the tube's threshold that offers sonic character in the form of harmonic saturation. Variable-mu compressors sound great as bus compressors and mastering tools and are perhaps less effective on tracks with fast-acting transients.

In practice, tube/variable-mu compressors are set up like push-pull amplifiers with two identical tube compression circuits that each act upon the same input signal only in opposite polarity from one another.

The peak detector/sidechain will have a differential amplifier to effectively sum the differences of the two tube outputs and feed the same gain reduction bias voltage back to the grid of each tube compressor circuit.

The output will also have a differential amplifier to effectively sum together the signal (opposite polarity) and cancel out any noise, including the low-frequency bias variation (same polarity). This allows for a much cleaner output.

Note that push-pull setups like this are designed with the same concept as the balanced audio lines of microphones.

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Characteristics Of Variable-Mu (Tube) Compressors

In this section, we’ll consider a few of the typical characteristics of variable-mu compressors:

• Soft knee
• Relatively slow attack and release times
• Slight harmonic distortion/saturation
• Add tube colouration (see point above)
• Program-dependent ratio

Before moving on to the variable-mu 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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Variable-Mu Compressor Examples

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

In this section, we’ll discuss:

• 500 Series variable mu compressor: IGS Audio Tubecore 500
• 19″ rack unit variable mu compressor: Manley Stereo Variable Mu Compressor
• Variable-mu compressor effect pedal: Lightning Boy Audio Opti-Mu Prime
• Variable-mu compressor plugin: United Plugins Royal Compressor

Other noteworthy variable-mu/tube compressors include:

• Manley Vari-Mu (mono)
• Handcrafted Labs Varis
• Fairchild 670 Stereo
• Fairchild 660 Mono
• Retro Instruments 176

IGS Audio Tubecore 500

The IGS Audio Tubecore 500 is a tube/variable-mu mono compressor in the 500 Series format.

The Tubecore 500 achieves its compression using the remote cut-off of tube biasing. The JJ tubes off a warm colour and the versatile compression settings make this 500 Series comp a superb choice on individual track and mastering busses alike. Two audio transformers (Sowter and Carnhill) have been chosen to improve the compressor's sonic performance further.

This compressor has all the typical variable-mu controls, including attack and release times, threshold, input gain and output/makeup gain. Note that there is no ratio control.

Two Tubecore 500s can be linked together to produce a stereo pair if need be.

Manley Stereo Variable Mu Compressor

The Manley Stereo Variable Mu Compressor is the trademarked “Variable Mu” compressor. This tube limiter/compressor is a renowned choice when it comes to gluing a mix together. It's no wonder it's Manley's best-selling product.

The Manley Stereo Variable Mu Compressor works using the “remote cut-off” or re-biasing of a unique 5670 dual-triode vacuum tube to achieve compression.

Additional 5751 or 12AX7 (x2) tubes are used for amplification. New models use 12BH7 (x2) output tubes, while older models use 7044 or 5687 (x2) tubes.

Each channel of the Stereo Variable Mu Compressor can be set independently of the other or, alternatively, the two can be stereo linked via a toggle switch. The Dual Input control adjusts the input level of both channels.

Each channel can be set to limit (sharp-knee with a 4:1 ratio that changes to 20:1 when limiting over 12 dB) or compress (soft-knee with a 1.5:1 ratio) the signal.

Each channel also has its own attack and release time parameters along with threshold and output gain controls.

Lightning Boy Audio Opti-Mu Prime

The Lightning Boy Audio Opti-Mu Prime is a variable-mu compressor pedal designed for musical instruments like electric guitar and bass.

Most stompboxes stay away from designs involving [relatively fragile] vacuum tubes. The Opti-Mu Prime goes against this caution and utilizes two 12AU7 tubes to produce a sort of variable-mu/optical compression hybrid. The first tube drives an optical compressor element, which in turn controls the gain of the second tube.

This pedal offers options for a soft or hard knee along with continuous control over the threshold (compression) of the circuit and the output volume of the pedal.

United Plugins Royal Compressor

The United Plugins Royal Compressor is a vari-mu compressor plugin designed to emulate variable-mu hardware compressors.

This relatively straight-forward compressor sounds great and has simple controls.

There are three models accessible via the A/B/C Model knob. Each has its own circuit emulation with a different value of the fixed attack.

The input level is adjustable as is the output attenuation. This plugin offers autogain to maintain signal level post-compression. Recovery (release time) is adjustable and the plugin offers a saturation knob to control the amount of soft clipping in the output signal.

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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)