Beigel Sound Lab Tru-Tron 3X

The Beigel Sound Lab Tru-Tron 3X is an all-analog envelope controlled filter pedal, the authorized modern re-issue and refinement of the classic Mu-Tron III envelope filter¹. It is available in the USA, direct only from Mu-FX, for $379.73¹. An envelope filter listens to how hard you play and sweeps a filter in response, so the Tru-Tron 3X turns picking dynamics into the vowel-like, squelchy filter motion heard on classic funk and rock records.

The pedal keeps the optical filter control circuitry and control set of the original Mu-Tron III at its foundation², and its maker describes it as "the modern all-analog refinement of the classic Mu-Tron III™ envelope controlled filter"³. It is made in the USA to the ISO 9001 standard³.

An all-analog filter built on the Mu-Tron III circuit

The Tru-Tron 3X is an all-analog pedal, an authorized re-issue and refinement of the Mu-Tron III rather than a digital emulation¹. The original Mu-Tron III circuit was derived from the Guild synthesizer's "timbre generator" and one of its four voltage-controlled filters, the configuration Mike Beigel carried into the pedal⁴. Of the filter configurations tried during its development, Beigel has said the electro-optical approach "just sounded best"⁴, and the optical filter control circuitry and control set of the Mu-Tron III sit at the foundation of the Tru-Tron 3X².

The core of the design is electro-optical. Beigel tried many filter configurations before settling on the opto approach, saying "we'd tried out all kinds of filter configurations, but the electro-optical thing just sounded best"⁴. Guitar World's history of the pedal notes that using opto-isolators to control the filter was central to the design⁵. In practice, the filter's peak response frequency follows the envelope of the input signal, so the harder you pick, the more the filter sweeps⁶.

The Tru-Tron 3X refines that circuit rather than replacing it. Premier Guitar's review puts it plainly: "The optical filter control circuitry and control set of the Mu-Tron III is at the foundation of the Tru-Tron 3X"².

The controls and indicator lights

The control set follows the original Mu-Tron III, with a few additions. The maker's product page describes each control³:

The MU and PRE controls give independent control of envelope drive and audio gain, a change from the original, where one drive control did both jobs¹. The mode knob also adds a "MIX" position on the filter mode switch, alongside low-pass, high-pass and band-pass¹². The Peak control offers a larger maximum output peak than the original for more control and tonal variation⁷; the maker says the Tru-Tron 3X peak is much stronger, from about 9 to 11 on the dial³.

The pedal carries indicator lights for envelope saturation and audio distortion, which show when the input is overdriving the envelope follower or the audio path, a practical aid when setting the MU and PRE controls¹. A power indicator light shows green for bypass and blue for effect¹.

Premier Guitar's reviewer described what the three filter modes sound like: the low-pass filter "lends a deep, vocal quality to each note," the high-pass "imparts an almost electric sitar-like flavor," and band-pass "produces rich but compact vowel-like tones"².

From the Mu-Tron III to the Tru-Tron 3X

The Tru-Tron 3X is a variant of the Mu-Tron III envelope filter from Musitronics, the classic effect used by numerous musicians⁷. Mu-FX introduced the Tru-Tron 3X as the first of its upgraded line and is now shipping it¹. The optical filter control circuitry and control set of the original Musitronics Mu-Tron III remain at the foundation of the new pedal, which was designed to capture the character of the original while adding enhancements that expand its potential². ===

Premier Guitar's 2021 review of the pedal, built around comparisons with an original Musitronics Mu-Tron III, was positive about how the reissue holds up²:

The same reviewer found it works well with bass and keys, not only guitar². For players weighing it against other filter options, the filter pedals category covers the wider field, and the Mu-Tron III page covers the original circuit in more depth.

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