Electro-Harmonix Q-Tron

The Electro-Harmonix Q-Tron is an analog envelope filter pedal, a circuit that sweeps a filter in response to how hard you play, in the same family as an auto-wah. It first appeared in 1996 as an updated take on the Mu-Tron III, using the same opto-electronic design as the 1970s original¹². Electro-Harmonix describes it as "The phattest, funkiest envelope filter ever made!"³. It was the first all-new Electro-Harmonix product in 15 years when it arrived⁴.

The original Q-Tron was discontinued in 2017³, but the design lives on in the Q-Tron+ and Micro Q-Tron, both of which have remained in production⁵, and in the smaller Nano Q-Tron added in 2021².

An envelope filter based on the Mu-Tron III

An envelope filter, also called an auto-wah, tracks the volume envelope of your playing and moves a filter's peak frequency up and down with it: play harder and the sweep goes further. The Mu-Tron III, designed by Mike Beigel and Aaron Newman, defined the category in the 1970s⁷⁸⁴, and its patent describes a voltage-controlled filter "whose peak response frequency is varied in accordance with the envelope of the input signal." The Q-Tron carries that opto-electronic design forward in pedal form⁸.

Where a wah pedal moves the same kind of filter with your foot, the Q-Tron does it automatically from your attack, which is why the design has appealed to funk and bass players. The pedal offers selectable low pass, band pass and high pass filters, plus hi and lo frequency response modes, so you can aim the sweep at the bass, midrange or treble part of your tone³. It also carries two outputs, normal and effected⁴.

The controls

The original Q-Tron's control set follows the Mu-Tron layout. The core controls, as described across the Q-Tron line, are:

The Q-Tron+ adds two things the original lacks: an effects loop and an attack response switch, on top of the same core design¹⁰. A Sweep toggle on the Q-Tron+ lets you choose the direction of the filter sweep¹¹.

Analog Man also offers a mod for these pedals: an added MIX knob that sets the blend of dry versus effected sound, functional only in MIX mode. The mix knob is available on the classic Q-Tron or the XO version of the Q-Tron and Q-Tron+, but not on the classic Q-Tron+⁶.

Power: 24V AC adapter

The original Q-Tron is not a standard 9V pedal. It comes with a 24DC-100 power supply³, a 24-volt AC adapter, and it will not run on batteries⁴. That power requirement rules out the common 9V daisy chain and means the pedal needs its own supply.

History and versions

The Q-Tron arrived in 1996, developed by Electro-Harmonix as the successor to the Mu-Tron legacy¹. GuitarPlayer reports the pedals were developed in the mid-'90s with assistance from Musitronics co-founder and electronics engineer Mike Beigel¹³, the same designer behind the original Mu-Tron III⁷. The line then branched into several versions:

The Micro Q-Tron trims the control set to filter mode, Q and drive¹⁵ in a tough, compact die-cast chassis⁹, and lists at $112.50⁹. The Nano Q-Tron, announced in December 2021, is a shrunk and improved version of the Micro Q-Tron¹⁸, adding a volume control¹⁷, with true bypass¹⁸. Reviewers noted the Q-Tron series is known for its organic, analog sound and expressivity¹⁶.

What reviewers said

Writing about the Mu-Tron III's legacy in Guitar World, Nick Wells called the Q-Tron "a fine filter for those on a budget, and well worth considering if you can't justify the hefty price of the Mu-Tron III"¹⁹. Pedal of the Day's reviewer found the Q-Tron+ a responsive, funky envelope filter and highlighted its Peak control, which "adjusts the resonance peak (or 'Q') of the filter, creating a more dramatic effect as the control is increased"¹¹. Of the Mini Q-Tron, Pedal of the Day's Mike B wrote that it is "super responsive and funky as can be," though noted these are now discontinued and relatively inexpensive if you can find one¹⁴. On the Nano Q-Tron, Guitar World's Trevor Curwen found plenty of control over the sound¹², and gearnews' Jef found the sounds in the demo video pleasing¹⁸.

Who uses it

The Analog Man mix knob mod came about for Mark Karan, who uses the Q-Tron with RatDog⁶. In Premier Guitar's rig rundown of the Red Hot Chili Peppers, Flea's Electro-Harmonix Q-Tron is listed as used for "Sir Psycho Sexy"²⁰. The Q-Tron series is documented as effective on guitar, bass, keys, horns and more²¹, and Premier Guitar has described the Q-Tron as a legendary Electro-Harmonix envelope filter used by guitar and bass players since its inception²².

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