Variable compressors (or vari-mu in short) are classic compressors made up of many tubes and transformers. They are often referred to as “space heaters” by producers due to the amount of heat produced by the several tubes in the unit. When it comes to sound, vari-mu compressors have a very warm and gentle sound signature.
How Does a Vari Mu Compressor Work?
Vari-mu compressors make use of tubes to control the gain reduction stage. The bias of those tubes is altered by the input voltage which results in a gentle, and rich sound signature which is comparable to optical compressors in terms of musical characteristics and sound style.
The uniqueness of vari-mu compressors comes in their soft-knee style of compression where the attack and release times are generally slower which gives a softer compression perfect for vocals and guitars. While some vari-mu compressor designs have variable attack and release times, the tube circuits within the unit help maintain a slow attack at all times.
Additionally, the tube circuits allow the vari-mu to add a lot of compressions without much gain reduction or loss in sound character. The ratio parameter is dependant on the program whereby it increases when the input voltage increases.
The 7 Best Vari-Mu Compressor VST Plugins of 2021
1. Kazrog True Iron Review
True Iron is a fabulous-sounding transformers emulation with a minimalistic interface.
As with any similar plugins, there aren’t many features we can talk about other than the sound itself. And fortunately, Kazrog’s True Iron is a beast when it comes to sound. It adds weight and even subtle brightness to the sound you could use for individual tracks or even full mixes. Further, you can crush the audio to make it sound distorted as well. Also, consider checking their other great variable mu-plugin compressor True Dynamics.
- Input Impedance
The Unity/Boost toggle controls whether you want to listen to the effect without any volume change or whether you want the Strenght knob to increase the volume as well. While Kazrog recommends the latter, the first can be helpful when you want to listen to what’s happening to your audio without the loudness changing your perception.
- Six Transformers
The plugin features six transformer emulations, including UTC 108 X, Malotki E4M – 4001B, Western Electric 111C, Haufe V178, Marinair LO1166/A, and the UTC O-12. Each has a subtly different harmonic distortion that becomes more obvious with the 2x crush.
- DNA Mode
The DNA button under the transformer selector activates a deeper frequency response emulation. It makes the sound even more authentic. However, it does use some extra CPU, so you might want to use it sparingly.
This plugin is available for Windows 7 or higher and macOS 10.9.5 or higher, both 64-bit only. It comes in VST 2/3, AU, and AAX formats.
In conclusion, we think if you are after a saturator that sounds highly close to the hardware transformers, True Iron is difficult to beat. Furthermore, the plugin is CPU efficient enough for moderately powerful computers to handle an instance in every track with ease. The simplicity and the sound quality make it an easy recommendation.
2. Pulsar Mu By Pulsar Audio Review
Pulsar Mu is based on the classic vari-mu compressor by Manley and is one of the most accurate emulations of that.
Just like the hardware, the Pulsar Mu can function in either L/R or M/S modes. The developers of this plugin say that it’s made for mixing and mastering, but from personal use, it can definitely be used for tracking as well.
In addition to all the features and controls present in the hardware unit, the plugin comes equipped with additional functions for better controllability and versatility. Firstly, it has an external sidechain input and a sidechain EQ with a low cut filter and adjustable mid bands and high shelf which allows for more precise control over the compressor’s response for a variety of ranges in the frequency spectrum. Furthermore, it has both look-ahead and look-behind features which is responsible for anticipating transients and treating them accordingly.
- External sidechain
Allows you to set an external sidechain input. This paves the way for a wide range of applications including de-essing, ducking, sidechain with EQ, or apply dynamic processing
- Mid/Side processing
Enables you to process independently sound that is panned to the middle and sounds panned to the sides of the stereo image. Moreover, it comes with a “Listen” button which allows you to listen to the Mids only or Sides for better monitoring
- Look-ahead and Look-behind features
Work by changing the timing of the gain reduction signal to follow the input. Look-ahead allows the Mu to start treating transients before they start playing which is helpful to prevent any sharp transients. On the other hand, look-behind delays the gain reduction which leaves fast transients unprocessed. This allows you to apply various effects to the transients like clipping after the Mu is done
- Flexible sidechain equalizer
Allows you to adjust the response of the compressor in accordance with the incoming signal
- CPU-friendly plugin since it is optimized by the developers to make the most efficient use of any CPU it’s used in
Pulsar Mu works on Windows 7 or higher for PC and OS X 10.7 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports 32 and 64-bit systems for PC, but 64-bit systems only for Mac.
The Pulsar Mu by Pulsar Audio is considered by many engineers and producers as one of the best variable compressors and is an excellent emulation of the classic Manley Mu compressor. Its functionality makes it perfect for a wide range of applications from vocals to drums.
3. Audified U73b Compressor V2 Review
This plugin is a precise and authentic emulation of the German compressor/limiter unit, the U73b.
The U37b was traditionally used for mastering between the years 1960 and 1980. It is a vari-mu compressor and a limiter with adjustable release times and an all-tube circuitry. The plugin version of the U37b is almost exactly the same design-wise with the exception of the additional input and output gain controls to regulate the compression levels.
In addition to that, the plugin has a VU meter and selectable side-chaining. Overall, the U73b is amazing for a wide range of applications including mixing of drums, vocals, guitars, bass, as well as mastering.
- The plugin U37b has external sidechaining to expand its functionality
- Auto output parameter in the plugin functions like an AGC which automatically sets output gain level
- The plugin is built with modern IDEs using more effective coding, resulting in a very robust software
- A dialog to control the overall “color” of a track
- High quality oversampling
- Highly efficient GUI with retina support
The U37b is compatible with Windows 7 or higher for PC and OS X 10.9 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports both 32 and 64-bit systems.
Overall, the Audified U37b Compressor V2 is another vari-mu compressor that offers all the basic controls that you need to achieve a sound signature similar to that of older vari-mu compressor hardware units with additional controls for versatility and quality.
4. Kush Audio AR-1 Review
This plugin seeks to throw back to the roots of American rock and roll with its precise emulation.
Kush Audio AR-1 is a model of the Lisson Grove AR-1, a hardware model of the 50s classic Altec 436. The Altec unit was known for providing the sound for The Beatles and early Pink Floyd. Kush’s digital iteration offers all of the traits the AR-1 is known for but with digital convenience.
The AR-1 features a Link switch to link or unlink the Input and Output knobs, a threshold knob, and a mix knob, none of which were available in the original hardware. Furthermore, you can now see the exact values of each parameter thanks to the labels. And finally, to counter the pumping sound caused by the low frequencies, it also features a high-pass filter at 60 Hz with a 6 dB/oct slope.
- Tone or “O/P Load”
The Tone knob is inspired by the O/P Load knob found on the Lisson Grove AR-1 hardware. The original Altec compressor used a 200 Ohms output impedance, resulting in a darker tone, and the Lisson Grove AR-1 improved on it by making the output impedance variable from 200 to 600 Ohms. However, taking it further still, Kush Audio has added a high and a low boost to make the brighter sound even more apparent.
You can have both internal and external side-chain input in this plugin. Using the external side-chain, you can have another input like the bass-drum controlling the compression amount on your bass.
As with any Kush Audio plugin, the AR-1 also features an oversampling toggle. Interestingly, this toggle is session-wide, which means if you disable oversampling in one instance of AR-1, it will do so in every instance of the plugin in your DAW session. It works well for manually activating the feature before rendering.
This plugin is available for Windows 8 or higher and macOS 10.13 or higher, both 64-bit only. It comes in VST 2, AU, and AAX formats.
The AR-1 feels like a true extension of the original hardware. We particularly like the Tone control and the Release #1-4 options for emulating the hardware’s fixed release times. The plugin works well for leveling your track without the effect being too noticeable, thanks to the fixed 2:1 ratio. Similarly, the tone control combined with internal saturation can make the sound distinctively brighter and popping.
5. Royal Compressor Review
Emulating a classic compressor that goes by the same name, the Royal Compressor plugin provides you with all the tools you need to achieve that vintage, warm, analog sound of a vari-mu compressor.
The original Royal Compressor was an all-time classic and was used by several popular artists including The Beatles. Its popularity was due to its harmonically rich sound that consisted of a pleasant saturation alongside sensitive compression.
- Simplistic design
Inspired by vintage hardware with a few knobs which aided users by having a larger impact leading to faster analog results
- 3 circuit types
You can choose from using the A/B/C knob with each circuit offering a different value for the fixed attack
- Automatic Gain Control (AGC)
Helps maintain output level at a set amount despite any changes that might take place
- Valve saturation
You can add at an adjustable level to add a more pleasing tone to your sonics
- Smart bypass feature
Makes sure that there are no harmful clicks or sudden loud noises when adjusting parameters
- Low on CPU thanks to its modern simulation technology
The Royal Compressor is compatible with Windows 8 or higher for PC and OS X 10.10 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports both 32 and 64-bit systems for PC and 64-bit systems only for Mac.
In conclusion, the Royal Compressor plugin is among the best emulations of older, vintage hardware with additional features to make it compatible with the digital market. It has a very vintage sound that a lot of users may find pleasing and is normally lesser in other vari-mu compressor plugins.
6. IK Multimedia Dyna-Mu Review
Available in the T-Racks5 bundle, the Dyna-Mu by IK Multimedia is an emulation of a classic American compressor known for its warm sound and crystal clear colored tone.
This modern-classic compressor inherits the same characteristics of gluing a mix together at the highest quality as in the hardware unit. It fits perfectly everywhere in both the mix and the mastering chains. In addition to that, it can be used to add a distinct color to your mix through its added saturation when pushed to the limits.
The Dyna-Mu by IK Multimedia features a very simple interface with all the basic controls that can process the signal in both L/R and M/S modes. These controls include input, threshold, attack, release, and output. In normal use, the Dyna Mu is a gentle compressor with a ratio of 1:5:1. However, it can be pushed to have a medium compression with a ratio up to 4:1 by using the HARD switch.
The HARD setting is perfect for individual tracks, whereas the normal softer compression is best for mixes. Additionally, there is a GANG switch that can be used to control both the rows together. Then you have the LINK switch which controls the sidechain detection mode for the two channels and you can set it to either be linked or independent. Finally, there is the SC (sidechain) switch, and engaging this will insert a high pass filter to reduce the processor’s sensitivity to signals below 100 Hz, which makes it perfect when using it on bass or kicks.
- This plugin is a perfect emulation of the all-tube circuit present in vintage hardware
- Can operate in both L/R or M/S modes
- Comes as either an individual processor or part of the processing chain by T-Racks
- Latency compensation support
- High quality oversampling
Dyna-Mu is compatible with Windows 7 or higher for PC and OS X 10.9 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports 64-bit systems only.
In conclusion, Dyna-Mu works magic on both individual tracks and full mixes. Its medium compression allows you to use it on drums to make it hit harder and its usual gentle compression is perfect for treating your vocals to have a warm, vintage tone.
7. Manley Variable Mu Limiter Compressor Review
The hardware version of the Manley Variable Mu compressor is considered by many to have the best implementation of vari-mu compression in a unit.
This plugin is developed by Universal Audio (UA) in collaboration with developers at Manley to come up with a highly accurate emulation of the classic compressor. The interface of the plugin is almost a carbon copy of that of the hardware and comes in 2 sections, one for the left channel and the other for the right. The main controls include; Recovery speed knob which consists of 5-time settings between slow and fast which are 8s, 4s, 0.6s, 0.4s, 0.2s. Then, you see the output level knob to be used to manually compensate for any gain loss that might occur due to compression. Next, you have the Dual Input knob which controls the input gain for both the left and right channels. Below that, you find the knobs for threshold and attack time which can be set from 25ms to 75ms.
Apart from the control knobs found on the original hardware, the plugin version also comes with a Dry/Wet control knob which provides you with parallel compression by allowing you to set the blend level of the dry and wet signals. In addition to that, there is a switch to link both channels together and a knob to control the headroom. Furthermore, the plugin version includes an M/S mode which allows you to process the signal in Mid/Side channels as opposed to Left/Right.
- This plugin model the entire circuitry of the original hardware in great detail including the custom transformers, input gain, as well as gain reduction characteristics
- Additional features in the plugin include a Dry/Wet control knob as well as a headroom control knob as discussed
- L/S or M/S operation mode with a sidechain filter
- Control the dual input, stereo output, threshold, and attack of your track continuously
This plugin requires Windows 7 or higher for PC and OS X 10.12 or higher. It comes in VST/AU/AAX formats and supports 64-bit systems.
All in all, there’s no going wrong with this plugin. It is not only one of the best compressors in the market, but it also models an all-time classic which is the Manley vari-mu compressor unit.
The 2 Best Free Vari-Mu Compressor Plugins 2021
1. Analog Obsession Varimoon
While offering few controls, the plugin delivers massive sonic changes with a few tweaks and adjustments. Its highly modern engine is capable of adding a lovely saturation distortion on many different applications.
Although the plugin can make drastic changes to any signal, it won’t require too much processing power from your computer, which is always a valid point, especially if the plugin is on budget.
You can adjust an input knob, which will grow a pleasant saturation/distortion and character. A release knob will feature six fixed settings to indicate how the release will behave, and the release will offer two auto and four-second-based times. A DC threshold knob will let you adjust between an excellent soft compression and a hard limiting. The AC threshold will control how you want the compression to kick in. A high-pass filter, side-chain compression, and wet/dry segments are also present in the interface.
While monitoring through the detailed metering display on the plugin’s interface, you can apply VariMoon and quickly hear its smoothness. Basses can benefit particularly well from the plugin’s behavior; I could instantly listen to it becoming mellow and fatter by using the plugin on a bass track.
Analog Obsession once again offering producers and musicians high-quality free plugins. Varimoon is a brilliantly designed compressor/limiter that has intuitive and easy-to-use controls. The power of VariMoon is outstanding; it can shape and fatten any part of the mix using dynamic principles. Its modest interface is highly optimal for fast-paced producers who want an authentic compression/limiting on their tracks while saving a lot of processing power from whatever device they’re using.
2. MJUC by Klanghelm (FREE & Paid) Review
This product by the German developers, Klanghelm is a tribute to simplistic designs and vari-mu compressors.
This plugin offers you a choice between 3 models that have different sound characteristics, color, controls, and GUI:
- MkI – a gentle, ‘early’ design. This model has the most straightforward interface that consists of a ratio mode and 6 timing constant settings. The timing here is comparable to the Fairchild 670 with fast attack and release for the first option, slow attack and long release for the fourth option, and fast attack with a program-dependent release for the sixth option.
- MkII – this is slightly inspired by the 1960s US in its design. The MkII has a more conventional layout of controls including Attack (0.8ms – 35ms), Recovery (20ms – 3.6s), and Ratio (2:1, 4:1, 8:1, Limit), as well as Density. MkII has cleaner compression through the way it processes the signal for the vari-mu compressor by adding a second stage of variable mu compressor and an IStage that engages an interstage transformer which separates gain make-up from gain reduction.
- MkIII – this model keeps the Density and the IStage parameters from the MkII and offers you a choice between two compression ratios i.e High and Low. In addition to that, it has a Punch setting that manipulates the attack and release. Moreover, there is a separate section with Dry/Wet controls, a sidechain filter, and 2 tone-shaping controls i.e. Drive and Timbre.
- Automatic gain compensation (AGC) feature to maintain a constant output level
- 2x oversampling provides you with high-quality output
- Consists of 3 modes that offer different sound characteristics, tone, and user interface
- Resizable interface for better compatibility with different display sizes and DAWs
The MJUC is compatible with Windows 7 or higher for PC and OS X 10.9 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports 64-bit systems only.
The MJUC is known to be one of the best vari compressors and one of the best compressors in general due to its detailed design and the 3 modes that it offers. Moreover, it has a very high quality when it comes to compression. Finally, it comes in both free and paid versions which is its biggest selling point.
Arturia Comp TUBE-STA Review
This impressive plugin by Arturia is a modern reimagination of the vacuum-tube powered Gates STA-Level compressor which was considered a secret tool for vocals and bass in the 50s.
Gates STA-Level compressor was a giant when it came to valve-driven compressors. The attack and release times were non-linear and rather quirky which made the compression gentle on vocals and instruments. Its amazing controls as well as its harmonically rich sound made it an instant favorite for radios and recording studios. On the other hand, the STA-Level was a signal processor that was well-known for its addition of slight harmonic saturation to the sound which resulted in a warm and sonically pleasing character.
The plugin version manages to replicate all the features that were present in the Gates STA-Level compressor with the addition of a few more features for better efficiency and controllability. These included:
- Input-Output link
The original hardware did not feature a “threshold” parameter which is a vital control in compressors. The plugin version is the same, but the developers added a “Link” button that linked both the input and output gain knobs in order to keep the volume levelled even after boosting the input signal which maintains a steady compression.
- New York
Older compressor units often featured a compression trick known as “New York” which splits the signal path into two and heavily compress one of them and then mix them together. The developers at Arturia implemented this technique also and added a “mix” knob which makes things a lot easier.
- Advanced sidechain controls
The plugin by Arturia features a sidechain advanced panel that consists of advanced sidechain controls as well as an SSL-style EQ which were not present back in the 50s.
- Highly accurate modelling of the tube electronic path present in the original hardware. This was done using Arturia’s state-of-the-art TAE® analog modelling
- 4 added features that were not present in the original hardware: 1. External sidechain, 2. Five different detection modes including M/S processing, 3. Advanced sidechain EQ, 4. Sidechained signal can be directly monitored
- Resizable window to fit multiple displays and DAWs
Arturia Comp TUBE-STA is compatible with Windows 7 or higher for PC and OS X 10.10 or higher for Mac. It comes in VST/VST3/AU/AAX formats and supports 64-bit systems only.
Overall, the Comp TUBE-STA by Arturia is an amazing vari-mu compressor that models one of the most desirable and popular compressors and does that immaculately. Its additional features transform the hardware giant into a giant in the digital world when it comes to vari-mu compressors.
To sum this up, most vari-mu compressor plugins tend to generate a similar-sounding, warm, analog sound due to their emulations of classic hardware units like the U37b, Manley Mu, or the Gates® STA-Level. They differ in their controllability, additional features, as well as price. This article covers a range of plugins both free and paid that are considered to be the best vari-mu compressors in the market and the choice is then yours.
K. M. Joshi is a multi-award-winning composer and sound designer, specializing in film, game, and TV audio. He enjoys making cinematic music, rock, blues, and electronica.