Manley Laboratories: Nu Mu Stereo Limiter Compressor

Manley makes audio gear in Chino, CA, for audio enthusiasts and audiophiles. In the same manner that drivers who love to drive (and have lots of money) drive Aston Martins, audio professionals who love audio use Manley gear. Manley graces the racks of top mastering engineers, recording engineers, and hi-fi enthusiasts around the world. For close to three decades, the Manley Variable Mu® stereo limiter compressor has helped engineers shape the tone and dynamics of the recordings we love. In the mastering studio, the Variable Mu adds the final glue and sheen to mixes, while in the mixing studio, the Variable Mu tightens up heavy guitars and drum buses and subtly yet adeptly reigns in dynamics on vocals and bass. The concept of a remote-cutoff tube compressor is decades old, with the Fairchild 670 being perhaps the most well-known and coveted example. The 670 and other tube compressors using a similar circuit typically have very soft knees and no ratio control. In the case of the Manley Nu Mu, the ratio begins around 1.2:1 and increases as more gain reduction is applied.

The Manley Variable Mu, arguably the most prolific mastering and mixing compressor used in professional studios, comes with a price tag that somewhat limits its accessibility to many producers and studios. EveAnna Manley [Tape Op #101] tasked her company with designing the Nu Mu, a modernized and more affordable incarnation of the Variable Mu, which takes advantage of newer manufacturing techniques, less labor-intensive assembly (while still being hand-assembled in Manley's Chino factory), and the integration of a custom-designed, audiophile, internal, switching power supply, which is also deployed in the newer Manley CORE, ELOP+, and FORCE rackmount units. Furthermore, the sidechain control circuit, tube gain stage, and final transformer-coupled output stage have been redesigned using high-voltage discrete amplifiers and transformerless FET output circuits. All of these changes save manufacturing costs, which result in a retail price about 35% lower than the old-school Variable Mu. The input stage retains the Manley Iron transformer and special TBAR mod circuit from the original Variable Mu (with two 6BA6 pentode-wired-as-triode vacuum tubes per channel). Great care was taken by Manley to achieve the audiophile standards and beloved personality of the original Variable Mu. The (extremely generous) headroom and maximum output level of the Nu Mu are spec'd as a few dB lower than the Variable Mu, which, in practice, may be offset by the way the Nu Mu's transformerless outputs interface with modern DAWs and other electronically balanced inputs. In will reach +27 dBu, which is about 6 dB above what an Avid HD I/O interface can handle. Noise figures remain the same for the Nu Mu as the Variable Mu.

The Nu Mu looks bold and modern, with comfortably bright, blue LED pushbutton switches and a unique, round, stereo, analog VU meter that shows either gain reduction or output level. Each channel boasts large, finely adjustable knobs for threshold, attack, recovery, and output, as well as an input level selector toggle switch with three settings. Buttons facilitate stereo-linked or dual-mono operation, a 100 Hz sidechain filter, limit/compress modes, metering mode, and a unique HIP function (more on that later). When the unit is first powered on, the VU meters flash for about 30 seconds until the vacuum tube operating points have stabilized. The rear of the 2RU-height chassis provides XLR connectors for I/O, as well as a 1/4'' TRS jack for each channel's sidechain insert. An IEC power connector accepts worldwide voltages.

I have considerable experience with the original Variable Mu, and I looked forward to trying the Nu Mu on my masters and mixes. The three-position input selector switch seemed curious at first, but the level choices of −3, 0, and +3 dB are referenced to +18, +21, and +25 dBu, which allowed interfacing with my Pro Tools HD interface without any concerns. The Nu Mu imparts the same lower-midrange detail and push that the Variable Mu is known for, but the Nu Mu leaves the highs a bit more open, and the lowest octave remains deep and punchy. The original Variable Mu had a sweet character that pushes low-mids and slightly damps the lowest and highest octave. I found the Nu Mu more appropriate for EDM, fusion, and bright pop mixes. On vocals, guitar, and bass, the Nu Mu worked exactly the way the Variable Mu would — as much dynamic control as you could want with an open and even harmonic structure.

Attack and recovery times for the Nu Mu, like most mastering compressors, are what I would consider medium-slow to slow. Attack times range from tens of ms to more than half a second, and recovery times range from 100 ms to almost 4 seconds. When the manual states, "Use a fast recovery time for transparent mastering," the reader should take the word fast with a grain of salt — 100 ms is not typically called fast! That said, I could always dial in an effective attack and release time, and when mastering, I often used the Nu Mu for its subtle tonal coloration with almost no gain reduction showing on the meter. On a lead vocal, bass, or guitar, 5 or 6 dB of gain reduction holds the dynamics steady with almost no tonal change.

Manley has thoughtfully included a button labeled "HIP," which essentially achieves the effect of parallel compression, bringing up the level of softer passages, while not overcompressing the loud moments. I found that on instrumental and orchestral music, the loud moments could be slightly compressed while the HIP function brought out some details during softer moments — especially noticeable on pianos and acoustic guitar, where the transient detail and harmonics seemed to become slightly exaggerated giving those instruments their own space in the master. For mixing, HIP worked well to bring up softer details on picked acoustic guitars and even on uneven rap vocals.

I love the venerable Variable Mu, but for my money, the Nu Mu will probably prove more useful and flexible in my day-to-day productions, mixing, and mastering. I consider the Nu Mu very close to, but slightly less colored than the Variable Mu and a bit more colored than other tube compressors, like the Summit Audio DCL–200 or Pendulum Audio ES–8. Like the Variable Mu, Massive Passive, and other Manley gear, the Nu Mu can make a great recording into a great master.