Mu-Tron III

The Mu-Tron III is an envelope filter pedal released in 1972 by Musitronics Corp. of Rosemont, New Jersey, designed by Mike Beigel¹. Instead of a foot rocker like a wah pedal, it reads the envelope, or "volume picture," of every note you play and uses it to sweep a filter, producing what the original ad called "an easily-controlled, articulated, vowel-like sound" that moves as fast or slow as you play². It is the pedal that defined the envelope filter category, and its circuit, a bipolar-powered state-variable filter swept by photo-couplers, is still being reissued and cloned today.

For a player, the appeal is touch response. Pick harder and the filter opens further; play softly and it barely moves. The effect is often described as an automatic wah, and the Mu-Tron III is the circuit most later envelope filters, from Electro-Harmonix's Q-Tron to Behringer's B-TRON III, trace their lineage to³⁴⁵.

Where the Mu-Tron III came from

Mike Beigel designed the Mu-Tron III, and the pedal's core idea came from synthesizer practice: its circuit line is derived from the Guild synthesizer's "timbre generator" and one of its four voltage-controlled filters⁹. The pedal was at first called the Auto-Wah, and early units were sold at E.U. Wurlitzer in Boston, where they sold well enough that more were made⁹. As Beigel put it, "We used a dual supply instead of a single supply so the effect could have a wide dynamic range"¹⁰.

The company's run included a change of ownership: late-'70s versions of the Mu-Tron III, made when Musitronics was owned by ARP, have a built-in power supply¹⁰. For many years, the performance of any given Mu-Tron III was based on the electrical performance of its photo-couplers, which did not all behave the same, so individual units could differ in feel¹¹. The design's influence continued long after, through later pedals built on the same circuit.

Jerry Garcia is listed by Musitronics among the line's past endorsers or users¹², which is part of why the pedal stays tied to the Grateful Dead sound in players' minds.

How it works: an envelope-controlled filter with photo-couplers

The Mu-Tron III is an envelope-controlled filter, a type often called an autowah¹³⁵³. The signal path has two jobs. First, an envelope follower tracks the volume shape of your playing. Second, that envelope voltage drives a filter whose cutoff frequency moves up and down with it. The result is a wah-like sweep that follows your dynamics instead of your foot.

What makes the Mu-Tron III's version of this distinctive is how the envelope drives the filter. The original units used photo-couplers, components that combine a light source and a light-sensitive resistor in one package, sometimes called optocouplers, photocouplers or vactrols¹¹. The vintage pedals used the Hamamatsu 0805a photo-coupler¹⁴. The envelope voltage drives the light, the light changes the resistance, and that resistance sets the filter's frequency. Musitronics notes that for many years the performance of any given Mu-Tron III was based on the electrical performance of its photo-couplers, which did not all behave the same¹¹. Two original pedals could sound noticeably different, and the company's modern Opto-Mod Red module deliberately models "an older Mu-Tron III which has drifted out of factory calibration"¹⁵.

The filter itself is a state-variable type, a synthesizer topology that provides lowpass, bandpass and highpass outputs from one core circuit, which is exactly what the pedal's MODE switch selects between¹⁶.

The controls: filter modes, gain and the range switch

The original Mu-Tron III keeps its control set small. The MODE switch selects the type of filter: lowpass, bandpass or highpass². The GAIN control sets how hard your signal drives the envelope and the filter, which is the main way you tune the pedal's sensitivity to your instrument and touch¹⁷. The RANGE switch emphasizes high or low frequencies, and the DRIVE switch shifts the filter up or down for two totally different effects¹⁸.

How each mode behaves, as Musitronics describes it on its modern versions of the circuit¹⁷¹⁹:

The modern Mu-Tron III+ adds internal DIP switches: two tailor the high and low frequency ranges, three control how the unit follows the dynamics of the instrument, and one lets the low-battery LED double as a sweep limit indicator¹⁸. It also carries a true, passive bypass switch for routing the audio around the effect¹⁸.

Power: ±9V from two batteries

The original Mu-Tron III runs on ±9 volts, supplied by 2 NEDA 1604 batteries, or by the Musitronics Model PS-1 Battery Eliminator, which made batteries unnecessary²²⁰. The bipolar supply is part of the synthesizer heritage of the circuit and is one of the things that makes cloning the pedal more involved than a typical 9V fuzz build¹⁶.

The pedal needs a minimum input signal of 1.5 mV from a 600 ohm source for the full Mu-Tron effect, and its maximum undistorted output is 3.6 volts RMS². Its input impedance varies from 1 megohm at the lowest GAIN setting to 3.3K ohms at maximum gain, so the GAIN control also changes how the pedal loads whatever is in front of it².

Versions from Musitronics to ARP

The Mu-Tron III's production history splits into eras:

• Original run: the Mu-Tron III was made in the US, and late-'70s versions, produced when Musitronics was owned by ARP, have a built-in power supply¹⁰.
• Modern re-issue: the Beigel Sound Lab Tru-Tron 3X is described by its maker as the authorized modern all-analog re-issue and refinement of the classic Mu-Tron III envelope filter, with higher Q than the original Mu-III when needed²¹.

Alongside these, Musitronics' current lineup carries several pedals described as reincarnations of the classic Mu-Tron III envelope filter: the Mu-Tron III Vintage Silver, built with the same circuit in a modern enclosure and made in the US²²; the Mu-FX Micro-Tron III²²¹⁷¹⁹; the Micro-Tron IV, based on the Micro-Tron III¹⁹; and the Mini Mu, which the maker says is true to the original Mu-Tron III gain circuit and opto-coupler design²²¹⁷¹⁹. The Vintage Silver's bypass is selectable between "Bleed" and "Active" on an internal control, with the bleed mode letting the pedal act as a clean boost when bypassed²².

The descendants: Q-Tron, Mini Q-Tron and Tru-Tron 3X

The Mu-Tron III's design carried forward into later pedals from Mike Beigel. The Beigel Sound Lab Tru-Tron 3X is described by its maker as the authorized modern all-analog re-issue and refinement of the classic Mu-Tron III envelope filter²¹. It offers higher Q than the original Mu-III when needed, giving a player a stronger, more resonant filter sweep than the 1970s unit could produce²¹²³.

The Mu-Tron III's reach extends beyond its own brand. Behringer's B-TRON III has been described as a not-so-subtle $69 replica of the Mu-Tron III envelope filter⁵²⁴, and HeadRush's Auto Wah modeling stompbox is stated by its maker to be based on a Mu-Tron III envelope filter²⁴.

Building a Mu-Tron III clone

The Mu-Tron III is a well-documented DIY project. General Guitar Gadgets' Neutron is a kit based on the Musitronics Mu-Tron III, which the maker calls "one of the most famous and best sounding Envelope Filters ever made"²⁵. Builders who have completed the project report that it is a great pedal, with lots of cool sounds, including phaser-like sounds on the down sweep controlled by picking velocity, though one noted it is not easy to wire²⁶.

A clone builder should expect two things that differ from a typical pedal build: the bipolar ±9V supply, and the photo-couplers. The original Hamamatsu 0805a photo-couplers are no longer common parts, and Musitronics notes that the character of any Mu-Tron III depends heavily on the electrical behavior of the photo-couplers used¹⁴¹¹. That is also why original units vary so much from pedal to pedal, and why the company now offers Opto-Mod modules, including one that models a drifted, out-of-calibration vintage unit¹⁴¹⁵.

Pedals on this page

Mu-Tron III

Mu-Tron · Filter

Beigel Sound Lab Tru-Tron 3X

Beigel Sound Lab · Filter

$379.73 list price

Mu-Tron III Vintage Silver

Mu-Tron · Filter

$279 list price

Mu-FX Micro-Tron III

Mu-FX · Filter

Musitronics Mu-Tron Mini Mu

Musitronics Mu-Tron · Filter

Musitronics Mu-Tron Micro-Tron IV

Musitronics Mu-Tron · Filter

Behringer B-Tron III Envelope Filter

Behringer · Filter

$69 list price

Mu-Tron III Envelope Filter

Mu-Tron · Filter

Musitronics Mu-Tron III

Musitronics Mu-Tron · Filter

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