Mu-Tron Phasor II

The Mu-Tron Phasor II is a phaser pedal released by Musitronics in 1977, a second-generation model built from the technology the company developed for the Mu-Tron Bi-Phase¹². It is an LDR-based optical phaser: a low-frequency oscillator drives LEDs whose light falls on light-dependent resistors, sweeping notches through the signal to create the effect known as phasing²³. Six phase-shift stages provide a maximum of 1080 degrees of phase-shift, and the pedal can be switched between 4 and 6 stage operation¹⁴.

The vintage Phasor II is discontinued and now sells for over $500 on the used market⁵. Musitronics today sells a reissue, listed at $199.00 as of October 2026, and Warm Audio sells a recreation, the Warm Audio Mutation Phasor II⁴⁶.

What the Phasor II does

Musitronics positioned the Phasor II as a phase-shifting device that adapted the advanced electronic technology developed for the Bi-Phase into a single-pedal format¹. It carries the feedback circuitry first introduced on the Bi-Phase, which feeds part of the effected signal back into the effect for a stronger, more pronounced sweep¹. The company says the circuitry has extremely low noise and signal-handling capability that keeps distortion out of the signal, with unity gain and less than 1db of insertion loss¹.

Aion FX's technical write-up traces the circuit's lineage further back: the designers drew inspiration from the Uni-Vibe and replicated its LDR and lamp setup, but with six phase stages instead of four, op-amps instead of transistors, an LED instead of an incandescent lamp, and a redesigned LFO². The 6-stage mode is what Musitronics calls the creamy and classic Mu-Tron sound⁴.

The signal path is straightforward: one 1/4" input jack and one 1/4" output jack, with an input impedance of 390K ohms unbalanced and an output impedance of 600 ohms unbalanced¹⁴. Signal-to-noise ratio is better than 86 db below a 2V input, and the phasing rate sweeps from 1Hz to 18Hz¹.

The controls

The Phasor II gives the player three continuously variable controls plus a stage switch¹⁴:

This control set is a step up from the original Mu-Tron Phasor, which used a Depth switch with L, M and H positions rather than a continuously variable knob⁷. The Bi-Phase's feedback and depth controls work the same way, which reflects how much circuitry the two pedals share⁸.

Power and connections: 115V AC, no battery

Musitronics advertised AC power as a feature, describing the Phasor II as "AC powered for quiet, reliable operation without the annoyance of frequent battery changes"¹. That sets the vintage unit apart from most pedals of its era and from the current reissue, which runs on 9V DC in a rugged Mu-Tron enclosure⁴.

The housing is made of heavy gauge steel and thick, rugged epoxy-finished aluminum, weighing 3½ pounds and measuring 8¾ inches long by 3⅛ inches high¹. The pedal handles 4V RMS (11.2V peak-to-peak) at minimum feedback and 2V RMS (5.6V peak-to-peak) at maximum feedback¹.

The Phasor II and the Bi-Phase

Aion FX sums up the relationship plainly: the Phasor II is essentially one-half of a Bi-Phase, with a nearly identical LFO and phase-shift topology, but without the second phaser². The Bi-Phase runs two phasers, A and B, with a selector that feeds Phasor B from input A, from Phasor A's output, or from its own input B, and a sweep switch that puts Phasor A under pedal control or LFO control⁸. The Phasor II offers none of that routing, just one phaser with its stage switch.

Guitar.com's reviewer notes that Mu-Tron currently produces the Phasor III, which combines the Phasor II and the Bi-Phase in one unit⁹. Musitronics describes the Phasor III as a compact, modernized and affordable version of the classic Phasor II, adding square and sine wave operation, priced at $279.00 as of October 2026¹⁰.

Designers, builders and the Phasor II today

The Phasor II came from Mike Beigel and his team at Musitronics. Premier Guitar's reviewer, writing about the Warm Audio recreation, credits Beigel's team directly¹¹¹:

¹¹

The DIY community has kept the circuit alive. IvIark's Guitar FX Layouts project documents the phasing circuit with its LFO-driven LEDs and LDRs, and notes that ticking, a common artifact in LDR phasers, can be reduced by proper lead dress, shielded wire for input and output, cables crossing at 90 degrees, short input and output runs, and an enclosure larger than a 1590BB³. He also suggests that with two of these one could build a bi-phase³.

For players who want the design new, there are two routes: the Musitronics reissue at $199.00, and the Warm Audio Mutation Phasor II, a recreation whose knobs and control color fields are identical to the original's⁶. Guitar.com's Andy Grant rated Warm Audio's version 8 out of 10 and called its price "half the cost of a Mu-Tron III and barely a quarter of what an original" costs⁹¹.

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