Manley Vari Mu Compressor


Manley Vari Mu

The Manley Vari Mu stereo compressor is one of those instantly recognisable pieces of pro audio equipment. It made it in at number 8 in Attack Magazine’s list of the 20 Best Compressors Of All Time, and has been a long time favourite of mastering, mix and recording engineers all over the world since its introduction over 20 years ago. It is no surprise that it has been Manley’s best-selling product.

Under the hood the Manley Vari Mu is elegantly put together. The thick PCB construction sits in a strong extruded rack frame with removable perforated top and bottom panels that allow easy access to both sides of the PCB and plenty of ventilation. Just as well… the Vari Mu runs pretty damn warm! Circuit-wise, the Vari Mu uses the same compression technique as the renowned Fairchild 670. “Mu” being the scientific term for the gain factor of a vacuum tube, a variable mu compressor works by constantly re-biasing a valve to create the required gain reduction. While it has the standard threshold, attack and release controllers, you won’t find a familiar ratio knob. Instead there is a two pole switch that will flick the ratio between ‘compress’ – a gentle 1.5:1 ratio to ‘limit’ – a higher 4:1 ratio that increases towards 20:1 as you start hitting -12dB of gain reduction.

Manley Vari Mu Guts

So how does it sound? I love it. It is the perfect complement to faster and grabber SSL or VCA style compression. Compared to the Smart C1, I find I tweak the input and output volumes a lot more than most of the other controls on the Vari Mu. Doing it this way opens up a lot of sonic variation with changes in harmonic content as you hit the valves harder, and changes in compression knee and ratio as you increase the level of gain reduction. The Vari Mu can remain quite clean, or be the ultimate mojo piece with attributes very akin to some of my favourite old Motown records. Try it on everything you can, from single tracks to overheads and stereo busses. Ride that input and output level, and toggle that limit/compress switch!

Vari Mu Meter

Modding it for more!

Like most equipment in long-term production, the Manley Vari Mu has had numerous updates, revisions and alterations. Changing from Sowter to Manley’s own transformers, Replacing the hard to find 6386 valves with 5670 tubes, then the subsequent T-Bar modification. My Vari Mu is date stamped with a month in the year 2000. As such, it is equipped with 5670 valves and incandescent festoon bulbs for the meters. As they all do eventually, both festoon bulbs blew. I contacted Paul in the spare parts and special orders department at Manley and he organised two LED meter modification boards for me for the lowly sum of $25. If you had the time and inclination you could make your own if you wanted to save a penny or two, but it almost isn’t worth the effort. The modification itself took about 10 minutes. It is simply a case of removing the festoon holders by snipping and isolating the wires, then soldering the new LED boards to the 5670 valve filament traces. There is a 5751 valve directly behind each VU meter. The result is classy – while the backlight isn’t as warm in colour as the old festoon bulbs, the meters are clear and easily read… and most importantly, won’t blow for a very long time!




  • Vari Mu side
  • Manley Vari Mu Guts
  • VU Meter OLD and NEW
  • Vu LED Mod - Left Meter
  • Vu LED Mod - Right Meter
  • Vari Mu Meter
  • Manley Vari Mu

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