The Variable Mu Limiter Compressor from Manley Laboratories has been an industry standard for decades. ("Variable Mu" is even a registered trademark of Manley.) Inspired by the famous tube limiters from Fairchild and Gates, the Variable Mu employs a "remote cut-off" compression scheme. In simplistic terms, the input tube is constantly re-biased by the tube-rectified side-chain control voltage. As the bias changes, so does the gain control, resulting in a smooth compression profile, with a sound that is augmented by tube and transformer harmonics. The unit imparts a thick, warm, almost syrupy sound to anything it processes. The best description I've ever heard comes from engineer Sean McDonald; in his opinion, the Variable Mu "makes everything better, just like a big hug from mom." 

Released in 1994, the original Variable Mu used a single GE 6386 input tube per channel, the same as in Fairchild and Gates models. When audio-grade 6386 tubes became harder to source, Manley implemented a new design using a 5670 tube. For light compression, say 6 dB or less, the 5670 mimicked the 6386 convincingly. In fact, most people were completely satisfied with the 5670-based units, and to this day, the 5670 remains the input tube in all standard Variable Mu hardware units. 

But some owners (including this writer) missed the saturation characteristics offered by the original tubes. Fortunately, Paul Fargo, the senior repair technician at Manley, came up with an alternate design. Dubbed the "T-Bar mod," the Fargo scheme replaces each dual-section 5670 with a pair of single-section 12BA6 tubes wired as a triode, which better matches the character of the 6386. Better yet, high-quality 12BA6 tubes are relatively affordable and widely available. 

Manley is known for maintaining an active dialog with its users. When recording engineers asked for features, Manley responded by offering various modifications and upgrades, including a high-pass filter for the side-chain (to reduce pumping induced by low-frequency energy), mid-side processing, and surround linking. Originally, you had to choose between the HPF or M-S mods; there was not enough room on the front to do both. A workaround has since been made available, but it involves moving the power switch to the rear of the unit. The Variable Mu plug-in for UAD-2 and Apollo is based on the aforementioned "T- Bar Mod," and it includes many of the custom upgrades while adding a mix control, which allows users to blend the unprocessed sound with the compressed audio. 

After testing the plug-in against my hardware Variable Mu, I believe this title ranks as Universal Audio's finest effort to date. Instantiating the plug-in immediately imparts the warm, rich syrup that has made this unit so popular. In terms of the compression, I tried setting the knobs on my hardware to match the plug-in's, but 15-year-old pots don't want to play nice with new computer versions. Blind testing is even harder because the test material requires different stages of D/A and A/D conversion, depending on which product you're hearing — the analog hardware or the digital plug-in. You end up comparing converters more than the products. Ultimately, I turned knobs without looking and was able to get the compression response to be comparable. 

So what else is good about the plug-in? Obviously, the number of Variable Mu channels on a project is now limited to UAD-2 processing power. (E.g., one stereo instance uses 8% of a UAD-2 QUAD at 96 kHz.) Want a mono version on the bass, an M- S limiting version on the drum room mics, and a stereo high- passed version on the master fader? Do it. (Try that in the real world, bank account!) Also, having the mix feature is outstanding. A real T-Bar Variable Mu can impart too much color for some sources — not a problem for the plug-in. Load a stereo version, set the threshold to max, and adjust the mix percentage to taste. Moreover, unless you have the mastering version with detented controls, recalls on the hardware Variable Mu can be tricky. Digital versions recall perfectly. Plus, tubes never burn out in a plug-in, nor do meter bulbs. (I'm starting to get angry.) 

The plug-in also permits more experimentation. Although I rarely use the limit mode on my hardware, having multiple digital instances allowed me to investigate where time and budget might have demanded a quicker solution. For example, the input control is before the tubes and after the input transformer. Increasing the input value hits the input tube harder. Recall from my topology lecture at the beginning of the review that the input tube also varies the compression. It should become clear that the threshold control is not the only way to coax performance out of this unit. While I tend to keep my I/O constant on my hardware, I realize now that I've been accessing only a small portion of the device's personality. 

So what are the limitations of the Variable Mu plug-in? Well, I have a real Variable Mu. I spent a lot on it. I've also spent a lot on tubes, upgrades, and repairs. Now, anyone with a UAD-2 system and $299 can get a Variable Mu. That makes me cranky. But I can't sell my hardware unit due to the way it pairs with other outboard units. For mastering, the plug-in cannot replace my hardware, but we'll see how long that lasts. Tracking people will note that it is often difficult to record through a plug-in. (I hear Apollo owners snickering — they don't have that problem.) And while I'm not 100% convinced this plug-in is exactly like my rack unit, it is darn close. I mean darn-close. 

People who may not have the budget for a real Manley Variable Mu can now purchase a terrific emulation of this unique compressor for a fraction of the original's $5000 price. I applaud all of the individuals who labored to bring the Variable Mu plug-in to market. But don't expect me to be happy about it. Now, get off my lawn. 

Tape Op is a bi-monthly magazine devoted to the art of record making.

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