Musitronics Mu-Tron III

The Musitronics Mu-Tron III is an envelope filter pedal, the box that reads how hard you pick and sweeps a filter accordingly, the effect often called an autowah. Released in 1972 by Musitronics Corp. of Rosemont, New Jersey, it was the first commercial envelope filter pedal¹². Instead of rocking a footswitch like a wah, the Mu-Tron III opens and closes its filter from the volume envelope of your own signal: pick hard and the filter sweeps up, let the note decay and it settles back down.

The vocal-like effect was quickly embraced by musicians and became a characteristic sound of entire genres, particularly funk². Notable users include Bootsy Collins on bass, Jerry Garcia on guitar, and Stevie Wonder². Musitronics even hired guitarist Larry Coryell to record a Mu-Tron III demo album, released in 1975³.

The first commercial envelope filter

Mike Beigel and Aaron Newman founded Musitronics in 1972, and the Mu-Tron III came out that same year¹. Aion FX describes it as "the very first commercial envelope filter pedal, also called an autowah"². The circuit used optocouplers to dynamically control a series of frequency filters based on the level of the input signal², and its patent describes a voltage-controlled filter whose peak response frequency is varied in accordance with the envelope of the input signal².

The circuit uses optocouplers (also called photocouplers or vactrols) to dynamically control a series of frequency filters based on the level of the input signal². In the words of the patent description, "the filter peak response frequency is thus varied in accordance with the varying envelope of the sound generating signal"⁶. That is the whole trick: the pedal is a voltage-controlled filter whose cutoff tracks your playing dynamics, so no footwork is needed.

The vocal-like effect was quickly embraced by musicians and soon became a characteristic effect of entire genres, particularly funk². Despite the name, versions "I" and "II" never existed: the Roman numeral III was Beigel's favorite number, adopted from the start⁷. The original unit's PEAK control had its maximum at about 8 on the dial⁸.

The controls

The Mu-Tron III has six controls, three knobs and three switches, which allow you to tweak the filter in a number of ways⁷.

The mode selection was originally a rotary switch with three positions⁶. The Drive switch chooses between up and down sweep¹¹; in the down position, builders on the Guitar FX Layouts blog found they could get "some cool phaser like sounds controlled with picking velocity"⁶.

What reviewers said

Guitar World's history of the pedal found that "the phenomenal sound of this pedal makes it very easy to love. Every one of its settings leads to one of those classic filter tones reminiscent of P-Funk tracks"⁷. A separate Guitar World review noted that "the wah sounds go well beyond those of a typical wah pedal, producing a wide variety of textures that include duck-like quacks, vowel-like vocal tones, piercing synth squeals and bowel-shaking bow wows"¹². The same article reports that the pedal works fantastically well with any other pedal, including an octaver⁷.

DIY builders on the Guitar FX Layouts blog, working from a verified layout of the circuit, were enthusiastic but pointed out quirks. One described it as "a Mt. Everest build" that is "amazing" with "so many tonal variations"⁶. Another warned that the circuit is picky about its LDRs (light-dependent resistors), working well with GL5539 parts but sounding bad with different ones, and one builder found the low pass mode sounded amazing while band pass and high pass came out warbly and distorted in the high end⁶.

Later versions and reissues

The original pedal is long out of production, but the circuit has been revived several times⁵⁷²:

• Mu-Tron III+ by HAZ Laboratories, a reissue of the Envelope Controlled Filter with a depth control described as "a sensitivity control for optimum operation from any musical source," plus an inverted snap the maker calls perfect for funky bass¹³
• Mu-FX version (2014), a modified version released by Beigel's then-new company Mu-FX⁷
• Mu-Tron III Vintage Silver, the current reincarnation from Mu-Tron, built with the same circuit as the 1972 original and aimed at guitar, bass, clavinet and synths¹¹¹
• Mu-FX Micro-Tron III, a pedalboard-friendly reincarnation that keeps the original's Mode, Peak and Gain knobs and Range and Drive switches, and adds an OUTPUT level control⁹¹⁴¹⁰

Pedal of the Day reviewed the Micro-Tron III as "an innovative take on a classic, and one that we've all been wanting to see reissued"¹⁰. The maker says it provides "all the classic Jerry tone and Bootsy funk, in a pedalboard-friendly size," and that it sounds amazing on guitar, bass, keys, horns, harmonica and more⁹.

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