What Is VariMu Compressor: Key To Achieve Classic Sound! (2023)

What Is VariMu Compressor


Welcome to yet another informative guide on another type of compressor. In this article we are going to talk about the VariMu Compressors, also found around the web like Variable Mu compressors.

It was originally a hardware unit designed to produce a creamy, tube-based analog squeeze, variable mu compressors have been used in mixing and mastering for decades. Having a smooth and clear compressing style is perfect for gluing mixes together without adding too much saturation or altering the original signal too much, which makes it a favorite tool among mastering engineers.

What Is Variable Mu Compressor?

A tube-based (or valve-based) compressor was the only machine-modifying gain device available in the early days of audio. The level detector circuit transmitted voltage to the tube in accordance with the principle they were working on. As a result of this voltage, the tube’s gain is directly affected. In the early days of compression, Fairchild and Altec were the first devices to implement this principle.

It is today that Manley provides an update on this approach based on the state of the art. This type of electronic system uses variable voltage input in place of a transistor, in which the variable voltage input changes the bias of the tube instead of changing the gain of the transistor. Vari-mu is also a trademark of Manley.

Tubes - What Is VariMu Compressor

Manley Variable Mu Stereo Compressor Limiter

There is no doubt that the renowned Manley Vari Mu Limiter/Compressor is the choice of producers and recording engineers worldwide – and you will be a believer as soon as you hear what it can do for your sound!

Its capabilities are well known to those who have heard the Fairchild 670 Series limiters in action – yet the Variable Mu produces an even sweeter sound than the classic limiters. Mixing and mastering are optimized by its adjustable attack and release settings, but many tracking situations can be accommodated by it as well. In terms of versatility and reliability, Manley’s Variable Mu is the best-selling device in the company’s product line.

There is no clone of the Variable Mu, but the Fairchild 670 series uses a similar limiting principle, and those who know this gear know it’s very special. Due to the variable gain, “mu” means gain or amplification factor, unlike most devices, the Variable Mu provides true variable gain.

With the vacuum tube rectified side-chain control voltages, the 5670 dual triodes are continuously re-biased so the peak-reducing and compression action can happen. By doing so, the tube’s gain will be changed smoothly.

Manley Variable Mu Stereo Compressor Limiter

A soft-knee 1.5:1 ratio is used in the Variable Mu’s Compress mode, whereas a sharper knee 20:1 ratio is used in the Limit mode when limiting signals of greater than 12dB. It is interesting to note that the knee actually becomes softer as more limiting is applied. Input and output can be modulated in a creative manner by turning up the input and reducing the output while using little or no compression.

Vari-Mu is ideal for dual-mono and stereo applications. You may notice that it has ganged input controls, but it is certainly not mono-incompatible. Additionally, you can always adjust the levels of your individual sources elsewhere, since there are separate threshold and output controls.

As soon as you switch to Link mode, you will realize the advantage of the stereo input control. That is what the Variable Mu excels at limiting and compressing final mixes, 3-track mixes, or mastering mixes.

For EU
Manley Variable Mu Stereo Compressor Limiter

Characteristics Of Variable-Mu Compression

As part of the tube compressor sound, tube-based gain states introduce sometimes musical and appealing distortions. In addition, the level detector circuit changes the tube’s gain after detecting a change in its gain. Gain may be increased or decreased in a non-instantaneous manner.

There is a claim that it is more “creamy” or “organic” in its action, and it has a somewhat different action. The unique attack and release times of tube-based compressors can be attributed to this characteristic. In order for the compression effect to be as effective as possible, the rate at which the gain increases or decreases is highly influential.

Compressors respond quickly to changes in input audio signal and levels based on the attack time, while they stop compressing as soon as they fall below the threshold based on the release time. Compression for vocals and acoustic instruments can sound more natural with slower attack and release times.

In addition to their harmonic distortion and saturation, variable-mu compressors have the characteristic of tube coloration and slight harmonic distortion. This warmth and harmonic richness are imparted to the sound by vacuum tubes in the compressor’s circuitry. The tube saturation of compressed signals increases as the compression ratio increases, adding to their pleasing coloration.

Moreover, variable-mu compressors tend to have a dynamic compression ratio, which varies with the input signal level and dynamics. The dynamic nature of these compressors makes them useful for processing sources like vocals, drums, and bass guitars when a fixed compression ratio is not appropriate.

Vari Mu Manley Inside Setup

‘‘Soft-Knee” On Vari-Mu Compressors

Variable-mu compressors are characterized by their soft knee, which means that as the input level approaches the threshold, the compression ratio gradually increases. The variable-mu compressor is well suited to achieve compression with subtle and transparent compression due to this characteristic. They also provide smooth and natural compression because of their relatively slow attacks and releases.

In other words, with a decrease in the audio signal and level, the compressor starts reducing the compression amount, but with an increase in signal level, it gradually increases. A soft-knee compressor refers to its response curve having a smooth shape.

There is a gentle slope to the curve, allowing for a smoother transition between the uncompressed and compressed signal levels. Compared to “hard-knee” compressors, which have a sharp transition between compressed and uncompressed signals, this model has a softer transition.

When compressing musical material, this feature is especially useful since it allows for more naturally and musically compressed dynamic ranges. A vari-mu compressor is commonly found with this characteristic, which is known for its warm and musical sound. Vari-mu audio compressors also are favored by recording studios because of their soft knees, which make them ideal for processing vocals and other acoustic instruments.

what is knee in compression

Scenarios Where Vari-Mu Is Useful

Compression using variable-MU is a common technique used in mastering, as discussed earlier. It is possible to create a cohesive feel with variable-mu compression by bringing dynamic elements together and adding a sense of cohesiveness to the track. Vari-mu units’ program-dependent nature, which we have already discussed, plays a significant role in this.

In addition to gluing capabilities, modern vari-mu compressors are also excellent at handling sub-mixes. It is particularly beneficial to use a vari-mu on drum buses, as it allows separate drum tracks to be unified and makes the kit sound like one instrument.

While these instruments and signals can benefit the most from such compressors, that doesn’t mean that you cannot add it to everything you wish if you are after some creaminess for the signal. You just need to properly set the compression, get a little bit of gain reduction, and adjust the attack and release to your liking and you are good to go.

This is the big benefit of tube compressors or vari mu compressors because they are versatile and can be used on many different positions in your mix or master. Now that we looked into the basics and characteristics of variable mu compressors, let’s continue with some plugins you can get.

Vari Mu Compressor Plugin Emulations

1. Softube Summit Audio TLA-100A

Softube Summit TLA-100A Compressor plug-in lets you recreate the legendary Summit Audio TLA-100A sound on your computer. For over three decades, Summit Audio TLA-100A compressors/limiters have been a staple of studios and live rigs.

The TLA-100A was the first set-and-forget compressor and one of the most user-friendly compressors on the market. With the combination of solid state and tube technology, the compression produced is smooth sounding and warm in appearance.

A modern section of this plug-in includes features such as parallel compression, saturation, and a low cut in addition to the original TLA-100A sound. This studio standard can now be found on your computer with the Softube Summit TLA-100A Compressor Plugin

There are many reasons why the TLA is so popular, including its superb leveling, smooth operation, and ease of use – all of which can be found in the software version, as well. The ‘set and forget’ slogan by Summit Audio isn’t just marketing hype: you simply set the peak reduction knob to the desired compression amount and it practically does the rest.

Since the TLA is a valve-leveling amplifier, it has a soft knee response, which gradually increases compression as the signal increases. It is extremely smooth because of the valve process.

Softube Summit Audio TLA-100A

2. Native Instruments Vari Comp

Based on an analog device renowned for its excellent sound, the Vari Comp provides a wide range of sound options. Because of the device’s analog nature, overdriving the effect in order to produce tube distortion is possible by increasing the input gain while lowering the output gain. This plugin was modeled in collab with Softube.

This unit can be used for mono as well as stereo tracks, with an automatic joint stereo setup (the same compression amount is applied to the sound samples both left and right channels). Furthermore, it offers easy parallel compression with the Dry mix knob.

Vari Comp functions as a versatile compressor that can be applied to instrument groups or individual tracks as indicated by its name. As an analog emulation, Vari Comp limits and compresses sounds to produce a clear and warm ‘analog’ sound.

The compression ratio is affected by a unique automatic ratio and knee adjustment in limit mode, which alters the compression ratio from 4:1 to 20:1 automatically as a function of the input signal.

Native Instruments Vari Comp

3. Waves PuigChild 670

Its pristine compression makes PuigChild a valuable bus compressor. PuigChild maintains its open sound even with fairly heavy compression, unlike most modern compressors which enclose sound with more aggressive compression methods. There are very few original Fairchild units left in the world. It would be nearly impossible to find one.

There is not much to notice visually about the Puigchild, and its layout is a faithful recreation of the original, with the exception of a switch that controls the amount of mains hum that is emitted. This can be completely bypassed. The master outputs became highly distorted when this feature was enabled on several tracks in the project.

Hum was one of the details we liked, but you can turn it off if you did not want it to play. It differs from some modern replacements in that it doesn’t have a dedicated ratio setting. Additionally, there is a dial that switches between six attacks and releases contours and a time constant control.

As well as linked stereo, dual stereo, and lateral vertical operation (the left and right channels process mono respectively), the 670 is capable of controlling multiple channels at the same time.

Waves PuigChild 670

4. Pulsar Audio Mu

With its unprecedented transparency, even when the gain reduction is pushed to the limit, the Mu is a perfect match for the hardware that inspired it. Its ability to contribute cohesion – the elusive “glue” – to your busses, mixes, and masters is unmatched.

The emulated tube signal path gives this plug-in a touch of analog warmth and allows it to be used as a leveling, limiting, or just warming effect. The plug-in is armed with Pulsar’s Topology Preservation Technology. Compared with its predecessors, Mu includes look-ahead limiting, a flexible sidechain EQ, mid-side processing, a sidechain external output, and modern metering.

There is more to the Pulsar Mu than simply imitating vintage hardware – that is just the beginning. Moreover, the compressor has a full-featured sidechain EQ section that allows you to have precise control over how it responds to frequencies specified by you. With the use of a modern metering section, you will be able to adjust the compression settings in real-time, representing both the waveform and the gain reduction over time.

The emulation performed by Pulsar is outstanding. Despite all the bells and whistles, the workflow enhancement is not to be underestimated. The sound quality is excellent. Can it be compared to the real thing in terms of sound? A person with a good ear and an understanding of how to work in this situation would be hard-pressed to refuse.

Pulsar Audio Mu

5. Klanghelm MJUC

There are basic controls and one observing element in the MJUC plugin. In addition to the compression knob on the left, the makeup knob on the right controls the gain of the makeup. An attack and release timing switch with four positions, as well as the rate and saturation, are also included.

There is a meter that displays additional information and provides a suitable image of an old-school compressor. There may not be any unique features in MJUC, but it does a pretty good job with the basic tasks despite being a simple-interfaced plugin.

Depending on your objectives, Klanghelm MJUC can produce smooth leveling effects or heavy pumping effects. Using natural compression sounds along with more straightforward sounding technologies and harmonics, we are able to achieve a sound that has great depth and energy. Our usual use of it is on the bass guitar and vocals, as it tames the peaks, resulting in an even and pleasing volume level.

We believe that MJUC is another excellent plugin from Klanghelm at an incredibly low price, which makes it a strong competitor to the best high-end compressor plugins.

Klanghelm MJUC


The versatility of vari mu compressors has led to their growing popularity among audio engineers and producers as a result of their ability to produce a warm, natural sound for recordings. Providing a subtle compression that enhances the tonal balance of the source material, these compressors incorporate a unique circuit design that utilizes vacuum tubes.

It is important to consider vari mu compressors when building a mastering studio from scratch. If you are at the point of purchasing a versatile compressor, vari mu compressors are ones that you should examine. The sound quality and versatility of these compressors make them a worthwhile investment for any serious audio professional, despite their higher price tag.

If you are having any questions about the topic we covered here, please let us know in the comments and we will be happy to elaborate and explain for all.



Nikoloski is the founder and main content writer and editor of Mixing Tips. With his experience in audio engineering, mixing, and mastering for over 15 years, will provide hands-on experience and expertise in all the matters covered on this website.

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