The Mu-tron III was "the world's first envelope-controlled filter", first made in 1972 by Musitronics. The unit soon became an essential effect for many funk musicians. It was taken in production again, in a modified version, in 2014.
The Mu-Tron III was based on a Guild prototypes called the Timbre Generator. Musitronics' founder, Mike Beigel, said he chose the envelope-controlled filter over other synthesizer elements, such as ring modulation, because it sounded more musical; it was a more general effect that would lend itself to a variety of applications, and it was easy to use.
The Mu-tron III became an instant success and was used by a variety of musicians for a variety of instruments, especially guitar, Clavinet, and bass. Perhaps the best-known use of it is by Stevie Wonder, who used it on his Clavinet for the song "Higher Ground".
In early 1995 Beigel lent his expertise to Electro-Harmonix, creating an update of his original design, the Electro-Harmonix Q-Tron, and he also designed a Bi-Filter for E-H. Three other pedals, the Mini Q-Tron, Micro Q-Tron and Q-Tron+, are available from EHX as well, who now also offer the Bi-Filter, a modern version of Beigel Sound Lab's Envelope Controlled Filter, made in 1979. Michael Dregni, in Vintage Guitar, noted that none of the "clones, copycats, and other attempts to bring it back...sounded quite like the real deal". But in 2014, a renewed version of the Mu-Tron III, now called the Tru-Tron 3X, was made by Beigel's new company, Mu-FX.
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