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Manley Labs Nu Mu

Stereo Valve Hybrid Compressor-Limiter By Bob Thomas
Published February 2021

Manley Labs Nu Mu

We check out a modern take on Manley’s classic valve compressor.

Developed to deliver transparent, fast and punchy audio without losing the characteristic sound qualities of their much‑admired Variable Mu stereo compressor, Manley’s Nu Mu retains the transformer‑balanced inputs and valve‑based gain controls of its ancestor. It employs discrete, solid‑state electronics, powered by the proprietary high‑voltage, low‑noise, switch‑mode power supply that Manley have used in most of their recent products. This allows the Nu Mu to achieve its design objectives for a lot less money than the Variable Mu.

In With The Nu

With the two channels’ controls laid out in mirror‑image fashion, on black backgrounds set into the blue‑anodised 2U fascia, the Nu Mu’s look is pure Manley. However, a central, blue‑lit, analogue, twin VU meter with opposing needles lends a practical, and strikingly different, dimension to the company’s signature aesthetic.

A blue LED inside the front‑panel power switch glows faintly when mains power is present and brightens up when the Nu Mu is switched on, but powering up the Nu Mu is an exercise in delayed gratification. It takes approximately 30 seconds to power up and during this time the meter’s backlight pulses to indicate that the audio outputs are muted. Once the power‑on procedure has been completed it switches to a steady glow, but being a valve device, the Nu Mu can take another 10‑15 minutes to fully warm up; you can tell when that’s been achieved, as the gain‑reduction meters will settle precisely on their calibrated 0dBVU positions.

Controls common to both channels are a three‑position (‑3/0/+3 dB) input‑level toggle switch (corresponding to maximum levels of, +25, +21 and +18 dBu), three non‑detented potentiometers covering Output, Threshold and Attack, and a five‑position rotary switch that controls the Recovery (release) time. I’d personally have preferred detented or switched pots, for easier channel matching and recall, but it’s not a deal‑breaker. Each channel also carries three push switches that illuminate to indicate a change in function or functional status. In the left‑hand stack you’ll find buttons for Compress/Limit, In/Bypass and Link on/off, which switches the Nu Mu between stereo and dual‑mono operation. On the right the buttons cover HIP on/off (more about that particular function later), Output/Gain Reduction meter display select and High‑Pass Side‑Chain in/out. At the rear lie the XLR connectors for the unit’s balanced audio I/O and TRS jacks for each channel’s side‑chain insert loop.

The input stages feature the same combination of Manley ‘Iron’ line transformer inputs and T‑Bar valve‑based gain control cells as the latest version of the older Variable Mu, which the cleaner‑sounding Nu Mu is intended to complement rather than replace. The T‑Bar valve circuit is based around a closely matched pair of 6BA6 single pentode valves, each wired in triode mode, which, in effect, gives us a ‘twin triode’ with a performance that is virtually identical to that of the 6386 twin triode used in the original version.

The ‘old‑school’ transformers and T‑Bar tubes of the Nu Mu’s twin signal paths are followed by the solid‑state circuitry of feedback‑type compressor side‑chains (with 100Hz high‑pass filters) and impedance‑balanced output stages. These solid‑state side‑chains endow the Nu Mu with a minimum attack time of 13ms that is approximately twice as fast as that of the Variable Mu, and give its recovery time range a similar performance delta. While that’s obviously nowhere near the microsecond response of a fast FET compressor, it does make the Nu Mu much more versatile than its predecessor. Compression ratios in the Nu Mu range from 1.5:1 to 12:1 in Limit mode and 1.2:1 to 3:1 in Compress mode.

The switchable HIP function is billed by Manley as allowing compression to happen at lower levels, whilst preserving the original dynamics in the louder sections and compressing the signal overall. The result is described as an increase in the apparent level of quieter sections, giving one‑button access to an effect similar to that of parallel compression.

Both channels feature a side‑chain insert point on the rear panel.Both channels feature a side‑chain insert point on the rear panel.

On Test

With no input meters, switched input gains that set operating levels (there’s nothing to stop you using these more creatively), and only Threshold knobs to control the amount of compression or limiting (and thereby the compression ratio), operating the Nu Mu is a compressor that repays the time you invest in listening to it and understanding the relationship between the controls.

It has a precise, transparent and detailed character...

Having grown familiar with the relationship between the various controls, though, I found that balancing the incoming signal level and Threshold setting with the timing set by the Attack and Recovery controls was pleasingly intuitive, and that I could easily shape a source’s transient response to create the character that I wanted to hear. Since the Nu Mu can handle up to +25dBu at its input and deliver up to +27dBu at its output, there was, in my case, no real risk of exceeding its limits in normal operation, but that’s not to say you can’t find colour here. In fact, adding more colour, warmth and depth is simply a matter of increasing the level of the signal going into the Nu Mu until you achieve the effect you’re looking for, whilst using the Output control to avoid overdriving the next link in the signal chain.

The HIP facility turned out to be effective and easy to use: it was able to deliver results that both felt and sounded quite natural. In fact, the only essential effort required from me was to lower the threshold so that compression was happening more or less constantly, apart from on the quietest parts of the track. Since HIP doesn’t over‑compress the signal, Manley describe it as a ‘Safety’ or ‘Easy’ mode, and I found it useful as a quick way to check the effect of compression on a track or bus without having to get my settings precisely correct.

Fanfare For The Modern Manley?

The Nu Mu is intended to be cleaner‑sounding than the company’s Variable Mu, and it is definitely a worthy addition to Manley’s product line. It has a precise, transparent and detailed character, courtesy of its 112dB dynamic range and 20Hz‑50kHz frequency response. Yet, the transformer‑balanced inputs, valve gain controls and solid‑state side‑chains combine to give its compression and limiting a speed, punch and appealing character that, for me, make the Nu Mu well‑suited to tracking, submixing and mixing duties across most instruments, vocal styles and musical genres (with the possible exception of those working with classical music), including electronic ones. To my ears, though, the Nu Mu is at its very best when operating in the sub‑4dB gain‑reduction range, in which it excels at controlling the level of a source or mix, enabling you both to add colour to it and to sculpt the character of its transients. Thus, in addition to tracking and mixing, I could easily envisage having a Nu Mu sitting permanently across my main mix bus, both to add a touch of colour and to gently massage any wayward peaks. Having said that, the Nu Mu is no sledgehammer, and it’s unlikely to be the only compressor that you’ll want to deploy on a typical session.

Although a Manley Nu Mu isn’t quite as ‘reassuringly expensive’ as the Variable Mu it isn’t exactly ‘cheap’, but the level of performance, the sonic character of its compression, and the high build quality we’ve come to expect of Manley more than justify its price tag. Should a valve compressor‑limiter lie within both your future and your budgetary constraints, then I’d highly recommend spending some time getting to know the Nu Mu.


  • Tight, fast and punchy compression for a valve compressor.
  • Precise, transparent and detailed sonic character.
  • Warmth and grit available, when desired.


  • Detented pots would have been nice.


A combination of transformer‑balanced inputs and valve‑based gain cells that brings vacuum tube compression up to date — and without losing that technology’s essential characteristics.


£2635.00 including VAT.

Studiocare +44 (0)151 707 4545


Manley Laboratories +1 909 627 4256.