Musitronics Mu-Tron Phasor II

The Musitronics Mu-Tron Phasor II is a phaser pedal, originally released in 1977 and now reissued by Musitronics Mu-Tron as what the maker calls "the newest and purest reincarnation of the classic Mu-Tron Phasor II™ phase pedal"¹². It is an optical design: multiple op-amp phasing stages driven by an LFO, using LEDs and light-dependent resistors to sweep the notches, with a voltage inverter power board supplying a consistent negative voltage³².

The headline feature of the reissue is the ability to externally select between 4 and 6 stages of optical phasing. Six phase-shift stages give a maximum of 1080 degrees of phase-shift, which the maker describes as "a deep, satisfying phasing effect"⁴. In the maker's words, 6-stage phasing "allows for the creamy and classic Mu-Tron sound," while 4-stage phasing "works well with saturated signal"¹.

A 4- or 6-stage optical phaser

The Phasor II's phase-shifting is done optically. The LFO drives LEDs paired with light-dependent resistors (LDRs), and the changing resistance in each stage sweeps the filter notches that create the phasing sound. A voltage inverter board provides a consistent negative voltage for the circuit's transistors³. Aion FX, which has studied the circuit closely, describes the Phasor II as "essentially one-half of a Bi-Phase, with a nearly identical LFO and phase-shift topology"².

The stage count matters to the character. Four stages give a shallower, more subtle sweep that the maker recommends for saturated signals, such as a distorted or fuzzed guitar, where a deeper sweep can turn muddy. Six stages double the depth of the notch pattern, and it is this mode the maker ties to the classic Mu-Tron phaser sound¹⁴.

The controls

The reissue carries four controls, three of them continuously variable on the original design¹⁴.

Feedback is the control that sharpens the resonant peaks of the notches. The original Phasor II's exclusive feedback circuitry was one of the advances it carried over from the Bi-Phase, and the maker's vintage page notes that the three continuously variable controls for Rate, Depth and Feedback were a selling point of the 1970s design⁴.

From the original Phasor II to the reissue

The Phasor II was the second-generation model of the Mu-Tron Phasor. The Phasor I preceded it, in production from 1974 to 1976, and used CA3080 operational transconductance amplifiers, which were relatively new technology at the time². The Phasor II followed in 1977 as an LDR-based phaser, adapting technology from the Bi-Phase, adding six phase-shift stages and the feedback circuitry²⁴.

Musitronics was a pioneering 1970s effects builder, known for the Mu-Tron Bi-Phase, the Mu-Tron Flanger and Phasor, the Vol-Wah, and the Mu-Tron III envelope follower⁶⁷. GuitarPlayer's history of the era's builders noted that "Some models, like the Mu-Tron III envelope follower and Phasor II, worked and sounded great"⁸. The company ceased trading after its acquisition by ARP in 1979⁶, and the vintage Phasor II is now discontinued, selling for over $500 on the used market⁹.

The current Musitronics Mu-Tron company reissued the Phasor II with a modular optical design the maker says allows for consistency across units, true bypass, and a 9V DC supply in a rugged enclosure, made in the USA at $199.00⁵. The related Mu-Tron Phasor III is described by the maker as "a compact, modernized and affordable version of the classic Mu-Tron Phasor II" that re-creates the creamy phase of the Phasor II while adding a Shape control that selects square or sine LFO waveforms¹⁰¹¹; the Phasor III Vintage Silver lists at $279.00¹².

The Warm Audio alternative

The Warm Audio Mutation Phasor II is a recreation of the Musitronics Mu-Tron Phasor II¹³. Warm Audio builds it with classic RC4558P op-amps, vintage-style diodes, carbon resistors and film caps, and it offers a Feedback parameter for the wet phase-shifting signal along with a battery power option¹⁴¹⁵¹⁶.

MusicRadar's reviewer Alex Lynham found the Warm Audio "close to unbeatable" on its own merits and on price, though beaten by the Mu-Tron Phasor III in practicality and expansion terms¹⁷. On That Pedal Show, the Mutation Phasor II appeared in a five-phasers comparison, with an MXR and Warm Audio comparison segment at 21:10¹⁸.

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