Warm Audio Mutation Phasor II: A fantastic vintage effect for synths and studios

Warm Audio has released a brand new emulation of the legendary Mu-Tron Phasor II, favourite of Billy Corgan, Tom Bukovac and more. Has the ‘Warm Formula’ scored again?

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Warm Audio Mutation Phasor II

Warm Audio Mutation Phasor II. Image: Warm Audio

Review Overview

Our rating


Our verdict

Highly flexible and editable
Elegant, studio-quality sound
High headroom – suitable for a range of signals

Takes up a fair amount of space on the pedalboard

Warm Audio is on a mission: to repackage legendary sounds at an affordable price point. The Texan company swears by the ‘Warm Formula’, keeping price points as low as possible while using quality components; such is the case with Warm Audio’s new electro-optical phaser pedal, the Mutation Phasor II.

‘Did I miss the Mutation Phasor I?’ We hear you ask. No, you didn’t: there is no Mutation Phasor I. The pedal seeks to accurately recreate the now-legendary Musitronics Mu-Tron Phasor II from 1976, a swirling and lush sounding pedal that has furnished the setups of many a musical icon.

From its orange, red and purple colour scheme and the font emblazoned across the shelf of the chassis, the Mutation Phasor II is loud and proud of its heritage. Similarly, Warm Audio’s ODD ‘Over Driver Disorder’– released at the same time – is just as transparent about being inspired by the Fulltone OCD.

The Mu-Tron II first appeared in the mid-1970s to succeed the mark one Mu-Tron, presenting three controls for depth, rate and feedback on top of the original’s two, which were for depth and rate. Just like the Mutation Phasor II is today, it was originally intended for players on a budget. It has stood the test of time to become synonymous with those stridently experimental guitar sounds of the 1970s. Original Mu-Trons on the second-hand market now carry a price tag to match that prestige.

“Mu-tron Phasor II is a new, advanced phase-shifting device designed to give the musician true state-of-the-art performance in a rugged, compact package at a very affordable price,” reads the original advert for the pedal.

“Six phase-shift stages provide a maximum of 1080 degrees of phase-shift for a deep, satisfying phasing effect […]. The new circuitry has extremely low noise and unequalled signal-handling capability for freedom from distortion. This makes Mu-tron Phasor II ideal for use after a pre-amplifier or with high-output electric keyboards, and also for critical use with electric guitars or other instruments in the studio or on stage.”

Warm Audio Mutation Phasor II
Warm Audio Mutation Phasor II. Image: Warm Audio

It’s in this realm of keyboard instruments and synthesizers that we’re keen to put the Mutation Phasor II through its paces.

First up: a 1979 Rhodes Mk 1. Happily, we can report, an authentic-sounding 70s vibe is instantly achieved upon plugging in the pedal. Warm and watery on one end of the spectrum and more aggressive in the midrange at the other (thanks to the Mutation Phasor II’s RC4558P op-amps), the MPII responds superbly across the electric piano’s considerable range, from deep bass notes to chiming high tones. Steadily increasing the depth to widen the frequency range of the phaser elegantly deepens the low end and stretches the highs, offering anything from subtle movement to undulating modulation. This can create evolving, phrase-spanning sweeps or it can threaten ring-mod-style disruption to your signal– all the while still feeling very much controlled.

This feeling of control partly stems from the way the Mutation Phasor II presents a clean and classy, three-dimensional sonic character that you might sooner associate with a 19-inch piece of studio hardware than with a guitar pedal. Perhaps this is because other famous phaser pedals, take the MXR Phase 90, for example, tend to deliver palpably more grit– you might even say more of a ‘lo-fi’ character– as they move through their phase stages.

Warm Audio Mutation Phasor II
Warm Audio Mutation Phasor II front. Image: Warm Audio

There is absolutely room on the pedalboard for both schools, of course – especially when considering the relatively compact footprint of the Mutation Phasor II. Although chunky, the pedal is still significantly smaller than the aluminium casing of the Mu-Tron Phasor II, which would undoubtedly need to travel separately to a main pedalboard if you included it in your rig.

We can happily report that the Mutation Phasor II’s headroom is just as high as the original’s advertisement promises. But it also responds nicely to the variable input levels from a mixer’s aux send, with the feedback knob primed to add more character.

Dialling up the feedback adds electric feedback to the wet signal, emphasising the filter-sweeping character of the phase shift and adding to the vocal formant-like resonance and intensity of the sound.

At extreme settings, the MPII doesn’t advance into particularly gnarly territory but offers a little bite to go with its bark. We test this with a bright and fast synth arpeggio from the Melbourne Instruments 12-voice Nina, to which it responds excellently.

Phaser effects often reach incredibly fast rates of modulation, but at the other end of the spectrum, they can leave us frustrated by not being able to go slow enough. Another hazard is the way many pedals react to those fastest modulation rates. In honesty, this can rarely sound particularly musical, since it often comes at the expense of accurate harmonic tracking unless there’s enough flexibility in the tone control to make adjustments.

Thankfully, not only does the MPII’s rate range from a dizzying 18 sweeps-per-second all the way down to a truly languid 10 seconds-per-sweep, its accompanying pair of sound-sculpting controls offer just enough control to keep things sounding psychedelic. It’s workable and typically highly musical right across the board, particularly with low-depth, high-feedback settings. Swirling and smooth on one end, shimmering and warbling on the other, and fantastic-sounding at either.

Suffice to say that when it comes to the Mutation Phasor II, it’s very much a case of ‘mission accomplished’ for the Warm formula.

Key features

  • Swirling, characterful recreation of the Mu-Tron Phasor II
  • Flexible controls for depth, rate and feedback
  • Rate range: 0.1 Hz to 18 Hz
  • Buffered bypass
  • Battery or 9v operation
  • Dimensions: 17 cm x 9 cm
  • Contact: Warm Audio

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