Musitronics Mu-Tron Micro-Tron IV

The Musitronics Mu-Tron Micro-Tron IV is an envelope filter pedal: a filter whose frequency peak moves in response to how hard you play, the effect often called an autowah. Musitronics calls it "a compact, convenient and affordable version of the classic Mu-Tron III™ envelope filter," and says it "provides all the classic Jerry tone and Bootsy funk, in a pedalboard-friendly size and powered by industry-standard 9V DC"¹.

The pedal is a compact, convenient and affordable version of the classic Mu-Tron III envelope filter, as its maker describes it¹. Like the Micro-Tron III before it in the line, it packs the envelope filter effect into a small enclosure, measuring 4.75" x 2.5" x 2.25" and weighing 1 lb, with a single ON/OFF footswitch that engages or disengages the effect¹.

An envelope filter with LP, BP and HP modes

An envelope filter listens to the level of your playing and uses it to sweep a resonant filter up and down, so a hard-plucked note opens the filter and a quiet one lets it close. The Mu-Tron III circuit that the Micro-Tron IV builds on is the classic example of the type, sometimes called an autowah⁴.

The MODE switch selects one of three filter types, and each shapes the sound differently¹:

LP is the most common choice for the familiar funk quack, since it keeps the low end while sweeping the top. BP narrows the effect to a vocal-like band around the moving peak, and HP thins the signal out by cutting lows as the filter moves¹.

The controls

The control set is small and shared with the rest of the Micro-Tron line¹:

The RANGE switch is where the pedal adapts to different instruments and playing positions. LO sets the filter sweep to a low range for most rhythm guitar, bass and lower fret uses; HI sets it to a high range for accenting string harmonics or for up-the-neck and solo work¹. GAIN works with this: more gain makes the filter react to a lighter touch, less gain demands a harder attack before the sweep opens up.

Power and connections

The Micro-Tron IV runs on 9V DC with a 52 mA draw¹, which means it works with the standard center-negative power supplies most pedalboards already use, unlike the original Mu-Tron III, which was a larger AC-powered unit. Alongside the 1/4" instrument input and output jacks, the pedal carries a 1/8" CV input jack and a 1/8" CV output jack¹. These control-voltage jacks let the filter's sweep be driven from outside the pedal and sent to other CV-equipped gear, an option the original design did not have⁵.

A Mu-Tron III variant with Opto-Mod parts

The Micro-Tron IV is the newest reincarnation of the classic Mu-Tron III envelope filter, based on the award winning Micro-Tron III¹. Aion FX describes it as "an updated version of the Mu-tron III, with modernized circuitry, CV input and output, and swappable optical modules for different reaction times and filter characteristics"⁵.

The original Mu-Tron III circuit used photo-couplers, the light-dependent components also known as optocouplers or vactrols, to control the filter sweep⁶. Those original Musitronics parts are no longer made, so for the Micro-Tron IV the company developed its own replacements, which it calls MU Opto-Mod technology³.

The Opto-Mod parts are also sold separately as the Mu-Tron Opto-Mod Modules for Micro-Tron IV, in Gold, Red and Blue versions. Because they are swappable, a player can change the overall ADSR (attack, decay, sustain, release) behavior and filter character of the pedal without tools or soldering, tailoring the reaction time and filter characteristics to their playing⁶.

Where it came from

The Micro-Tron IV comes from Musitronics, the maker behind the Mu-Tron III envelope filter it is based on²⁷⁸. The pedal itself is a small unit, at 4.75" x 2.5" x 2.25" and 1 lb, built around one control for the player: an ON/OFF footswitch that engages or disengages the envelope filter effect¹. It carries a 1 Year Limited Warranty from the maker¹.

A Vintage Silver finish version, the Micro-Tron IV Vintage Silver, shares the same circuit, controls and specifications².

What reviewers and players say

Published reviews of the closely related Micro-Tron III give a sense of how the family sounds in use. Pedal of the Day found that "it sounds great not only on guitar, but on bass, keys, horns, harmonica and more as well"⁹, which matches the maker's own description of the line's range⁹¹⁰. Musitronics itself frames the pedal's sound by reference to its best-known users, promising "all the classic Jerry tone and Bootsy funk"¹.

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