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Manley Variable Mu® Stereo Limiter/Compressor
Reviewing: Manley Variable Mu® Stereo Compressor/Limiter
by By Bobby Owsinski

A classic gets hot-rodded

When does a unit become a "classic"? That’s easy -- when it becomes so widely used that it’s considered standard studio fare. One piece of gear that definitely fits this description (certainly in mastering facilities) is the Manley Laboratories Stereo Variable-Mu limiter/compressor. To illustrate this point, while doing research for my latest book, The Mastering Engineer’s Handbook, I asked most of the major mastering facilities exactly what equipment they were using. As you’d expect, the answers were all over the board in terms of gear preferences, but one piece consistently showed up on virtually all their lists: the Manley Variable Mu. In fact, the Variable Mu (or "Vari-Mu," as it has fondly become known) was one of the units that most mastering engineers singled out as indispensable to their work. That certainly spells standard by any definition! And now, more than ever, I’ve been seeing the Vari-Mu show up not only in the racks of major studios, but in independent engineer’s racks as well.

So why is the Vari-Mu everywhere? From my personal experience, I can truly say that it’s because it makes a mix sound better. The Vari-Mu adds a certain "glue" to a mix when used as a bus compressor that’s difficult to get with all but a few other boxes.

The Basics

The Manley Variable Mu is an all-tube compressor/limiter featuring real transformer balanced input and outputs and three dual triodes in a fully symmetrical all-tube circuit. This is the basic circuit configuration made famous by the hard to find and outrageously expensive Fairchild 670. Although originally designed around a 6386 variable mu tube, Manley switched some years ago to the more easily obtainable 5670. A 7044 or 5687 is used in the output section, which features a higher output current and better consistency than the original 12BH7 that was used.

The unit is built like a tank, yet with a craftsmanship and precision way beyond the norm -- which is the level of quality we’ve come to expect from Manley. The meters, attenuators, and input and output transformers were all custom designed, with the all-important mu-metal encased transformers wound in-house at the Manley Lab’s magnetics department (a very impressive part of the facility). And, indeed, the specs are equally impressive, with the input of the Vari-Mu capable of up to +36 dB (that’s 52 volts RMS) with only 1% distortion. Output is capable of +30 dB, which, in this world of transformerless output stages, is an impressive spec that harkens back to pre-IC days.

Though brief, the manual is one of the best in the industry, covering everything from installation to example settings and operational tips, to service adjustments, and even including a section about correct interfacing and cable wiring.

Simple Features

One thing about the Vari-Mu is that it’s not over the top feature-wise. In fact, the unit has exactly what’s needed to get the job done -- nothing more and nothing less. Essentially, there are two channels that are mostly independent except for one important exception -- they share a common input gain control (more on this later). Each channel has a threshold control, output gain, attack and recovery (release), and a switch that selects between compression at about a 1.5:1 ratio and limiting at about 4:1. The ratios are deceiving in that they actually increase as the amount of compression/limiting increases, with the ratio climbing up to 20:1 at 12 dB of limiting, hence the name "variable-mu" or variable gain.

A nice thing about the Attack and Recovery controls is that they have "slow" and "fast" panel designations that make them much easier to quickly set than those on other units that only have timing numbers. The envelope section provides quite a lot of control with the Attack settings available from "fast" at 25 ms, to "medium" at 50 ms, and the "slow" position at 70 ms. The Recovery control switches between five selections, starting with "very slow," with a time base of eight seconds, to "very fast," with a recovery time of 2 ms. There’s also a hard-wire bypass and two large precision meters that indicate gain reduction. A link switch connects the left- and right-channel control circuits for stereo operation, although the individual controls aren’t over-ridden, so you still need to set up both channels to similar parameters.

There are several hot-rod options available for the Variable Mu from the factory: the mastering version features precision 1% metal film resistors, Greyhill rotary switches with gold contacts, and stepped threshold attenuators and output controls. The surround version enables three units to be linked together via a common detector (see the upcoming review in Surround Professional on this version).

Another version contains a true M/S feature that allows for Mid/Side recording, playback, and processing. And, lastly, the M/S or Vertical/Lateral Mod allows for Mid/Side recording, playback, and processing, which can be used to give a wider sound by making the unit only compress the in-phase information, leaving the out-of-phase info unscathed. Conversely, as in the Fairchild 670, one can achieve a more mono sound (helpful in disc cutting to minimize groove liftout) by setting the Vari-Mu to compress more out-of-phase info.

In Use

Over a period of months, I used the Vari-Mu on numerous projects. On bass, the tubes of the unit helped round out the sound, while controlling the peaks and evening out the quieter notes. On vocals, the tubes enhanced the sonics in a way that only tubes can do, adding both clarity and warmth. As stated before, the left and right channels share a common input gain control, which means that you have to adjust the source feed into the channel and work the threshold control a bit more than usual when in dual-mono mode, but this isn’t much of a problem after you get the hang of it.

But it's across the mix bus where the unit shines. At 4 to 6 dB of limiting, you don’t even know that the unit is there (except for its effect on the dynamics); it’s that transparent. Kick it up to something obscene, like 15-20 dB, and it squeezes the track like a silk glove, getting the effect without any nasty artifacts. There's something to this unit that just glues everything together and makes the track sound, for want of another term, "better." No wonder it's been the secret weapon of mastering engineers around the world for so long.

At $4,000 retail, the Manley Labs Vari-Mu isn’t what you’d consider inexpensive. But there’s a reason why everyone uses one and it’s because it has that "sound." It’s not a smash-you-in-the-face type of sound, it’s more gentle and subtle, yet equally as effective. You can’t really compare it to anything else because it is indeed unique, a quality that’s getting more and more difficult to find in this digital age. Once you try one, you’ll be hooked forever.

MANUFACTURER: Manley Laboratories, Inc.

SUMMARY: A stereo vacuum tube compressor/limiter with a classic variable-mu design.

STRENGTHS: Excellent design and workmanship. Highest quality sonics. The ultimate "glue" for a mix.

WEAKNESSES: Ganged input control makes it slightly difficult to use in dual-mono mode.

PRICE: $4,000


The following review is reprinted with permission from the July 1997 issue of Pro Audio Review magazine. Pro Audio Review can be found on the web at http://www.proaudioreview.com.
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