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Beigel Sound Lab/Mu-FX Tru-Tron 3X Review

Mike Beigel updates his classic Mu-Tron III envelope filter.

Stompbox history is populated by crackpots and geniuses. But few pedal builders are as fit for inclusion in the latter category as Mike Beigel. In the early ’70s Beigel co-founded Musitronics, which built some of the most coveted and colorful effects ever conceived: the Mu-Tron Bi-Phase, the Mu-Tron Flanger and Phasor, the Vol-Wah, and the Mu-Tron III.

When Mu-Tron bit the dust (after acquisition by ARP in 1979) Beigel took his curiosity and engineering chops elsewhere—helping develop, among other things, miniscule radio-frequency ID chips to track animals. But like many instrument developers, Beigel found it hard to stay away from the biz. He marks his return with the Mu-FX Tru-Tron 3X, an enhanced version of the Mu-Tron III envelope filter.

World Beyond Wah
Even some seasoned players struggle to define the function of an envelope filter (also known as an auto-wah). Basically, it uses a voltage controlled frequency peak filter to emphasize a selectable frequency range. Meanwhile, an envelope follower triggers the effect in response to your playing dynamics.

Most players know the sound when they hear it—think Jerry Garcia’s bubbling mid-’70s leads, Bootsy Collins’ vocalic bass bombs, and Stevie Wonder’s funky clavinet on “Higher Ground.” All were created with an original Musitronics Mu-Tron III.

The Tru-Tron can transform a simple quarter- or eighth-note melody into something much more colorful.

The optical filter control circuitry and control set of the Mu-Tron III is at the foundation of the Tru-Tron 3X. And though the Tru-Tron eschews the elegant and colorful look of the original in favor of a more contemporary, industrial appearance, the controls (if not the layout) will be familiar to anyone who’s used an original.

There are two mini-toggles for envelope drive and filter range. The envelope drive switch selects between rising (updrive) and dipping (down drive) voltage controlled frequency peak frequency sweep. (These simulate heel-to-toe and toe-to-heel wah action, respectively, only without the rocker pedal.) The filter range control shifts the filter drive range to high-frequency or low-frequency input. The mu control (labeled gain on the original) sets the gain level of the signal to the envelope follower and filter, determining the effect’s dynamic response. The peak knob the filter Q or emphasis of the frequency sweep, while the mode knob selects between low-, high-, and band-pass filters, plus a useful new mix mode that blends dry and filtered signal.

The most significant new features are the preamp knob and switch. The pre setting effectively splits the drive function, creating separate amplitudes for filter audio input and envelope sweep, expanding the palette of available sounds.

Vowels Spoken Vociferously
Like any envelope filter, the Tru-Tron relies on picking dynamics and input gain to determine how intensely the filter performs, so it can take a little practice to adjust the settings to suit your touch. (Soft pickers might benefit from higher mu settings, while more dynamic pickers and low output pickups might benefit from a lower one.) But once you’ve established an appropriate gain level, it’s easy to hear what makes this effect intoxicating for so many players.


Authentic Mu-Tron III tones. Independent filter gain for overdriven sounds. Excellent build quality.

Miss that cool blue and silver enclosure! Not super-intuitive.


Ease of Use:




Beigel Sound Lab/Mu-FX Tru-Tron 3X

Because the effect can completely alter the character of a single note, it’s well suited to concise, percussive, and melodic phrases. (Fast flurries tend to get messy, though lower peak settings and adjustments to an internal trim pot for response help make the pedal better suited to fast picking.) The Tru-Tron can transform a simple quarter- or eighth-note melody into something much more colorful. The pedal’s Garcia-inspired setting is great for exploring this capability. It uses the low-pass filter, which lends a deep, vocal quality to each note.

Changing the filter type has a profound effect on tone and dynamics. High-pass imparts an almost electric sitar-like flavor, with a slow-blooming and fast-decaying high-midrange tone. The band-pass filter tends to remove low end, producing rich but compact vowel-like tones ideal for doubling a bass line without muddying a mix. The mix setting, new on the Tru-Tron, blends in dry signal for a subtler effect. Like all the Tru-Tron’s controls, peak is sensitive and wide-ranging. High values work well for percussive, mid-tempo leads that leave room for the filter open and close between notes. Low to medium peak settings give notes more articulation.

Preamp is one of the most significant and unique Tru-Tron features. When switched on, it drives the filter independently from the envelope, enabling muscular, overdriven tones that work with the envelope more seamlessly and harmoniously than a fuzz, distortion, or overdrive would. This opens up a whole world of heavy soloing textures. These can be dynamic alternatives to phasing or flanging, and more unique-sounding than a wah sweep working a distorted tone.

The Verdict
The Tru-Tron 3X captures the deep, rich, vocal, expressiveness of the original Mu-Tron III (no mean feat) while adding enhancements that expand its potential. It’s a very powerful stompbox—and that’s before you factor in how well it works with bass and keys. It’s not for everyone, and some players enticed by the concept may lack patience for the experimentation the pedal demands. But for those willing to tinker, the Tru-Tron 3X can provide unexpected musical potential that extends far beyond the obvious Garcia and Bootsy-styled applications.

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