The Warm Audio Mutation Phasor II is an electro-optical phase-shifting pedal with a feedback circuit, released on 28 June 2023 alongside the ODD Box overdrive12. It is Warm Audio's accurate recreation of the 1970s Mu-Tron Phasor II, a discontinued analog phaser that now sells for over $500 on the used market31. Warm Audio describes it as one of the most desirable, 100% analog phaser circuits, made famous in the experimental era of the 1970s, and says it lets you add depth, wobble, swirl or "instant funk" to guitar, keys, bass or anything you can dream of1.
The pedal is housed in an ultra-tough, vintage-vibe steel case, and it is built with premium components including classic RC4558P op-amps, vintage-style diodes, carbon resistors and film caps14. It carries an MSRP of $1491.
A 6-stage optical phaser with feedback
The original Mu-Tron Phasor II circuit uses multiple op-amp phasing stages driven by an LFO, with LEDs and light-dependent resistors doing the actual phase shifting, plus a voltage inverter power board5. Warm Audio describes the Mutation Phasor II as an electro-optical phase-shifting pedal with a feedback circuit, and Premier Guitar's reviewer characterized it as a 6-stage voice that "excels in these lazy cycles" at slow sweep settings6. The pedal's frequency response runs from 20Hz to 20kHz, and its phasing rate spans .1Hz to 18Hz3.
Warm Audio says the feedback circuit can "send the phase shifts into overdrive by dialing up the feedback, emphasizing the 'wooshes' of the pedal"1. MusicRadar's Rob Laing noted that the Feedback control "ups the intensity of the sweeps that is especially beneficial at slower rates"7.
The controls: Speed, Depth and Feedback
The pedal keeps the same three continuously variable controls as the original Mu-Tron Phasor II, which Musitronics offered as Rate, Depth and Feedback8. Warm Audio's product page describes each knob's job:
| Control | What it does |
|---|---|
| Speed (Rate) | Sets the timing of the modulation speed, so you can sync the sweep with the tune1 |
| Depth | Fine-tunes how deep the modulation goes1 |
| Feedback | Dials up the feedback, emphasizing the "wooshes" of the pedal1 |
Pedal of the Day found the simple layout quick to work with: "With just the three knobs, you can dial in an amazing sound in seconds"9.
Power: 9V, 150mA
The Mutation Phasor II runs on a 9-volt battery or an external 9V regulated DC adapter with a center-negative plug, with a current draw of 150mA3. The gearnews reviewer listed the battery power option among the pedal's pluses2.
The original Mu-Tron Phasor II was AC powered with no battery option; Musitronics billed this as "quiet, reliable operation without the annoyance of frequent battery changes"8. The Warm Audio version moves to 9V DC, which the gearnews reviewer found easier to incorporate into modern setups, and it comes with a warranty2.
Other published specifications include a frequency response of 20Hz to 20kHz, a phasing rate of .1Hz to 18Hz, and maximum and minimum feedback figures of 2V RMS (5.6V peak-to-peak) and 4V RMS (11.2V peak-to-peak) respectively3.
How fast it sweeps, and how it works on bass
The Speed control ranges from a gentle one tenth of a sweep per second at the slowest to 18 sweeps per second at the fastest310. That slow end suits long, lazy phase cycles, while the top end reaches fast, energetic swirl.
On bass, the gearnews reviewer reported that "Trying the Phasor II on bass guitar yields a similar result; it's a texture that adds movement and interest to your existing" tone rather than replacing it2. Delicious Audio likewise notes it works well with all sorts of instruments, including guitar, keys, bass, or any instrument with a line out3.
The Mutation Phasor II and the Mu-Tron Phasor II
The pedal is a recreation of the Mu-Tron Phasor II, the second-generation model of the Mu-tron Phasor, which Musitronics says the original professionals used with great success all over the world8. Players of the original pedal include Billy Corgan, Tom Bukovac and Robin Trower, who used it to create innovative music that broke the mold, according to Premier Guitar's coverage of the launch4.
The practical differences for a player today:
- The original is discontinued and sells for over $500 used; the Warm Audio version is $14913.
- The original is AC powered; the recreation runs on a 9V battery or center-negative adapter38.
- The recreation comes with a warranty and is easier to fit into modern setups2.
- Both share the same three controls: Rate, Depth and Feedback8.
MusicRadar's reviewer Alex Lynham saw the pedal's size and the existence of the Mu-Tron Phasor III as drawbacks, while listing great classic phaser sounds, vintage looks and an affordable price as reasons to buy11. On That Pedal Show's "Five Fantastic Phasers" episode, the pedal appeared in the pedals-and-stuff list, with an MXR and Warm Audio comparison at the 21:10 mark12.
What reviewers said
The gearnews review rated the Mutation Phasor II 4.0 out of 5.0, praising its build quality, affordability, low noise and clean sound, and the battery power option2. Pedal of the Day called it "a reimagining of a vintage classic" and an accurate recreation of a 1970s analog phaser circuit9. Guitar World's Paul Riario listed the price as $149 / £15513.
The reviewer rated the Mutation Phasor II 4.0 out of 5.0, citing its build quality, its affordable price, and its low noise, clean sound as what it did well2.
For players comparing phasers, the Mu-Tron Phasor II page covers the original circuit, and the phaser pedals type page covers how phasers work and what else is available. Warm Audio's earlier Jet Phaser is another of the company's 1970s phaser recreations.
Videos
Warm Audio
Warm Audio
TheSuperFunAwesomeHappyTimePedalShow
Questions
Is the Mutation Phasor II true bypass or buffered?
It uses buffered bypass, not true bypass. That means a buffer stage stays in the signal path when the pedal is off, which keeps your tone from losing high end over long cable runs, though some players prefer true bypass switching. Warm Audio built it this way as part of the faithful recreation of the original circuit.
Does the Mutation Phasor II take a battery?
Yes. It runs on a 9-volt battery or an external 9V regulated DC adapter with a center-negative plug. That is a change from the original Mu-Tron Phasor II, which was AC powered with no battery option. Reviewers liked having the battery choice on the modern version.
Does the Mutation Phasor II work on bass?
Yes. Warm Audio says it adds depth, wobble, swirl or instant funk to guitar, keys, bass or any instrument with a line out. The gearnews reviewer tried it on bass guitar and found it adds movement and interest to your existing tone rather than replacing it.
How fast can the Mutation Phasor II sweep?
The Speed (Rate) control sweeps from about one tenth of a sweep per second at the slowest setting up to 18 sweeps per second at the fastest. That slow end is well suited to lazy, ambient cycling, while the top end gets into fast, energetic swirl territory.
What is the difference between the Mutation Phasor II and the Mu-Tron Phasor II?
The Mutation Phasor II is Warm Audio's modern recreation of the vintage Mu-Tron Phasor II, which is discontinued and now sells for over $500 used. The Warm Audio version is affordable at $149, runs on a 9V battery or adapter instead of AC power, and comes with a warranty, while keeping the same three controls.
Sources (13)
- Warm Audio Mutation Phasor II Electro-Optical Phaser Pedal With Feedback, Warm Audio
- REVIEW: WARM Audio Mutation Phasor II, gearnews.com, 2023-06-28
- Warm Audio Mutation Phasor II, Delicious Audio, 2026-08-04
- Warm Audio Unveils ODD Box V1 & Mutation Phasor II, Premier Guitar, 2023-06-28
- Guitar FX Layouts: Mutron Phasor II, Guitar FX Layouts (tagboardeffects.blogspot.com), 2014-06-25
- Warm Audio Mutation Phasor II Review, Premier Guitar, 2023-09-09
- Warm Audio's new ODD Box and Mutation Phasor II pedals look familar, MusicRadar, 2023-06-28
- Mu-Tron Phasor II, Musitronics Mu-Tron (mu-tron.com)
- Warm Audio Mutation Phasor II Electro-Optical Phase Shifter, Pedal of the Day, 2024-06-16
- Warm Audio Releases the ODD Box V1 and Mutation Phasor II, GuitarPlayer, 2023-06-28
- Warm Audio Mutation Phasor ll review, MusicRadar, 2023-09-08
- Five Fantastic Phasers Plus Phaser Tips & Tricks, That Pedal Show, 2026-05-15
- Warm Audio ODD Box V1 and Mutation Phasor II review, Guitar World, 2023-09-14
