Mu-Tron III

The Mu-Tron III is an envelope-controlled filter pedal that Musitronics Corp. first released in 1972¹. It reads the envelope, or "volume picture," of every note you play and translates it automatically into a filter sweep, producing what Mu-Tron describes as "an easily-controlled, articulated, vowel-like sound, as fast or slow as you play"². In plain terms: play harder and the filter opens further; pick faster and the sweep moves faster. It is one of the earliest filter pedals of its kind, and it works on guitar, bass, keys and more.

The pedal is an analog design built around photo-couplers (also called optocouplers or vactrols), which dynamically control a series of frequency filters based on the level of the input signal³. That opto-based circuit is a big part of why the pedal sounds the way it does, and why later copies have chased it for decades.

What it does: an envelope filter with vowel-like sweeps

An envelope filter, sometimes called an auto wah, is not the same as a wah pedal you rock with your foot. A wah sweeps on a fixed curve; the Mu-Tron III sweeps in response to your playing dynamics. Its patent description puts it precisely: "The filter peak response frequency is thus varied in accordance with the varying envelope of the sound generating signal"⁷.

Guitar World's Nick Wells described the range of textures it can produce:

Aion FX notes that the vocal-like effect was quickly embraced by musicians and became a characteristic effect of entire genres, particularly funk³. Notable users include Bootsy Collins on bass, Jerry Garcia on guitar, and Stevie Wonder³. Premier Guitar reports that it became one of Jerry Garcia's signature sounds⁹.

The controls

The Mu-Tron III has six controls: three knobs and three switches¹⁰. The controls and what they do:

A few points worth knowing as a player. The Low-High switch does not create distortion; it just focuses the envelope detector on bass or treble tones⁷. The drive switch's up and down settings behave very differently: one builder on the Guitar FX Layouts blog reported that "with the down sweep I can get some cool phaser like sounds controlled with picking velocity"⁷. Your guitar's volume knob is also part of the instrument here, since the pedal is reading your signal level the whole time⁷.

Power: ±9V from two batteries

The original Mu-Tron III runs on ±9 volts from 2 NEDA 1604 batteries, or from the Musitronics Model PS-1 Battery Eliminator². Guitar World describes the early versions as running on either two 9-volt batteries or an 18-volt power supply⁸. Designer Mike Beigel explained the reasoning:

History and versions

The Mu-Tron III came out in 1972 from Musitronics Corp.¹. Premier Guitar's builder profile tells the origin story: the circuit was derived from a Guild synthesizer, using the side of the instrument Beigel called the "timbre generator" and one of its four voltage-controlled filters⁹. The pedal was at first called the Auto-Wah⁹. The first units were sold at E.U. Wurlitzer in Boston; as Beigel put it, "They sold well, so we made more"⁹. A patent for the Mu-tron III was filed in 1975⁹.

The timeline of the original and its descendants:

• The Mu-tron III was originally called the Auto-Wah⁹
• Its circuit was derived from the Guild synthesizer's "timbre generator" and one of its four voltage-controlled filters⁹
• Early units were sold at E.U. Wurlitzer in Boston, and they sold well, so more were made⁹
• Late-'70s versions, from when Musitronics was owned by Arp, have a built-in power supply⁸
• For many years, the performance of any given Mu-Tron III was based on the electrical performance of the photo-couplers, which did not all behave the same¹⁴⁶

The original pedal went through revisions: Mu-Tron's own documentation for the Mu-Tron III+ lists two versions of the manual, one for serial numbers up to and including 2791 and another for serial number 2792 and above¹³. The original Mode selection was a rotary switch with three positions⁷.

Later products carry the design forward. The Mu-Tron Phasor III is described as a compact, modernized and affordable version of the classic Mu-Tron Phasor II¹⁵. The Mu-Tron III Vintage Silver offers Low pass, Band pass and High pass modes⁴. The Mu-FX Micro-Tron III has its own owner's manual, covering its operation¹²¹⁶.

One quirk of the originals: for many years the performance of any given Mu-Tron III depended on the electrical behavior of its photo-couplers, which did not all behave the same, so two vintage units could respond differently⁶. The vintage units used a Hamamatsu 0805a photo-coupler⁶.

The Behringer B-Tron III

The Behringer B-Tron III is Behringer's envelope filter pedal, and the press has covered it in connection with the Mu-Tron III¹⁷¹⁸¹⁹. Worth noting for comparison: the character of the original came from its photo-couplers, the vintage Hamamatsu 0805a part found in the vintage Mu-Tron III pedals, and for many years the performance of any given unit was based on the electrical performance of those photo-couplers, which did not all behave the same⁶.

What reviewers and builders said

Guitar World's assessment of the original's standing is blunt:

DIY builders have their own take. The Guitar FX Layouts blog, which published a modified layout for the circuit, collected builder reports ranging from "this thing is amazing! There are so many tonal variations!" to complaints that the build is "not easy to wire but it's very fun to play with"⁷. One builder found the LP mode amazing but the BP and HP modes warbly and distorted in the high end, while another heard no discernible difference between the three modes⁷. Build notes from the same page: the 15uF cap should be non-polarized, and the TL072 can be substituted with most dual opamps but not the single NE5534⁷.

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