Mu-Tron Phasor

The Mu-Tron Phasor is a phaser pedal made by Musitronics, the company whose stable also produced the Mu-Tron III envelope filter, the Bi-Phase and the Mu-Tron Flanger¹. It was in production from 1974 to 1976, and Musitronics' own material from the era says it was "used with great success by professionals all over the world"²³. It is a large, heavy, AC-powered floor unit rather than a compact modern pedal, and it was discontinued in favor of the more popular Phasor II⁴⁵.

Phasing is the effect the Phasor produces. In Musitronics' description, the circuit "periodically varies the length of delay of a time-delayed signal and re-combines it with the original signal," creating notches that move through the frequency spectrum¹. The company's original ad copy put the sound in plainer terms:

What the Mu-Tron Phasor does

The Phasor splits the guitar signal, delays one copy by a varying amount, and recombines the two. As the delay length changes, the combined signal develops cancellations at frequencies that sweep up and down, which the ear hears as movement. Musitronics described this as the effect known as "phasing"¹.

The original Phasor used CA3080 operational transconductance amplifiers (OTAs) for its phase stages, which Aion FX notes were relatively new technology at the time². When the Phasor was discontinued, it was replaced by the Phasor II, an LDR-based design originally released in 1977².

Pedal of the Day, writing about the unit in 2016, summed up its physical character: "This pedal is huge, but with a small control section"⁵.

Rate, depth and the footswitch

The Phasor's control panel is small for the size of the box. Two controls shape the effect, plus a switch to turn the unit on and off⁵.

The rate range is worth comparing with what came after. The Phasor sweeps from 0.16Hz to 12Hz¹; the Phasor II covers 1Hz to 18Hz³; and the Bi-Phase's sweep generators run from .1Hz to 18Hz⁶. The original Phasor therefore reaches slower at the bottom of its range than the Phasor II, but not as fast at the top.

Power and connections

The Phasor runs on mains power, not a battery or a modern DC adapter: its AC power requirement is 115V AC, 60Hz, 5 watts¹. That alone marks it as a product of its era, and it is a practical consideration for anyone using one today, since 115V 60Hz mains is the North American standard.

Connections are unbalanced at both ends, with a 390k ohm input impedance and a 600 ohm output impedance¹. The unit handles signals up to 1V RMS, 2.8V peak-to-peak¹.

Musitronics and the later Mu-Tron phasers

Musitronics was best known for the Mu-Tron III envelope filter, but that, as Guitar Player's Dave Hunter wrote, "wasn't the only classic to come out of the Musitronics stable," which also included the Bi-Phase and the Mu-Tron Flanger⁹. Premier Guitar's reviewer credited designer Mike Beigel and his team with products that offered "unique, deeper, and different sounds and functionality"¹⁰.

The Phasor line continued after the original was discontinued:

• Mu-Tron Phasor II (1977): a six-stage phaser with Rate, Depth and Feedback controls, made possible by adapting the electronic technology developed for the Bi-Phase². Aion FX describes it as "essentially one-half of a Bi-Phase, with a nearly identical LFO and phase-shift topology"². Its six phase-shift stages provide a maximum of 1080 degrees of phase-shift³.
• Musitronics Mu-Tron Phasor II (current reissue): a 9V DC version in a rugged Mu-Tron enclosure, made in the USA, with externally selectable 4 or 6 stage phasing and a modular optical design⁷.
• Mu-Tron Phasor III (current): a compact, modernized version of the Phasor II, made in the USA, running on 9V DC and drawing 115 mA, with square and sine wave operation and a Stage switch for 4 or 6 stages⁸¹¹. MusicRadar's Alex Lynham noted it "has the same circuitry as one of the two stages of the Bi Phase, as well as a control voltage (CV) in and out"¹².
• Mu-FX Phasor 2X: an enhanced reissue of the Phasor II with the same analog electro-optical phase-shift circuitry, and dual independent rate, depth and feedback controls for separate 4- and 6-stage settings¹³¹⁴.
• Warm Audio Mutation Phasor II (2023): Warm Audio's recreation of the Phasor II, released in June 2023¹⁵.

MusicRadar reported in 2023 that "the Phasor III iteration is currently being produced under the Mu-Tron banner, along with other classics from the brand"¹⁶.

What reviewers said

Published reviewers have been positive about the Phasor line, with size and complexity as the recurring criticisms¹⁷¹²⁵.

Pedal of the Day, reviewing the original Phasor in 2016, called it "an incredible effects pedal, even if you're not a huge phaser fan," judging it worth both the cash and the pedalboard real estate despite its size⁵. Of the modern Phasor III, the same site wrote that Mu-Tron was "not cutting any corners to ensure it sounds right on par with the vintage units…if not better"¹⁸.

MusicRadar's Alex Lynham, rounding up phasers in 2021, credited the Phasor III with "unique vintage phaser sounds" while listing complexity as its downside¹². Reviewing Warm Audio's Mutation Phasor II, the same writer listed size as a drawback, noting it matches the footprint of the original and that Mu-Tron currently makes the Phasor III¹⁷.

The DIY world has also weighed in. On the Guitar FX Layouts blog, a builder of a Phasor II clone reported getting it "fired up and sounding amazing"¹⁹, and another builder, TW, called it "quite possibly the best pedal I've built thus far out of about 30"¹⁹.

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