Electro-Harmonix Q-Tron Plus

The Electro-Harmonix Q-Tron Plus is an analog envelope filter pedal: a filter that sweeps automatically in response to how hard you play, rather than to a foot pedal like a wah. Electro-Harmonix describes it as the same design as the original EHX Q-Tron, with two additions that set it apart: a dedicated effects loop and an Attack Response switch¹. It was released in 2008 and carries a list price of $192.30¹.

An envelope follower reads the level of your playing and uses it to drive the filter's sweep, so picking harder opens the filter further and playing softly closes it down. That makes it a dynamic effect: your touch, not a knob turn, shapes the sound in real time.

The controls: Mode, Range, Response, Boost, Peak and Gain

The Q-Tron Plus front panel gives you controls over the filter's behavior: the Gain control, which in Boost mode functions as both a volume control and the filter sensitivity control; the Boost switch, which passes the input signal through the filter at its original level in Normal mode and increases the signal gain to the filter in Boost mode; the Peak control, which determines the resonance peak or Q of the filter; the Mode switch, which selects what frequency range the filter will pass; and the Response switch, which changes the sweep response between two optimized settings. A Bypass switch toggles between effect mode and bypass¹.

The Mode switch is the biggest tonal decision. LP (low pass) lets lows through and sweeps the cutoff upward, BP (band pass) isolates a moving band of frequencies for the classic quack, and HP (high pass) thins the sound as the filter opens. Mix blends the filtered and dry signals¹.

The Peak control is worth attention from players new to envelope filters: turning it up sharpens the resonance at the cutoff point, which makes the sweep more vocal and pronounced, while lower settings give a gentler, more subtle filter movement¹.

The Gain control's dual role in Boost mode matters in practice. Because it sets both the level hitting the filter and the filter's sensitivity, a single knob adjustment changes how strongly your playing drives the sweep as well as how loud the effect is¹.

What the Response switch does

The Response switch selects between two sweep responses. RockBoard's PedalPedia entry describes the two modes as "a slow, smooth, vowel-like attack" and "the fast, snaked response of the original" Q-Tron². In FAST the filter reacts quickly to your attack, in SLOW it opens and closes more gradually, giving a rounder, more vocal character.

The effects loop

The Q-Tron Plus's defining feature over the original Q-Tron is its effects loop, which lets you place an additional effect, or a chain of effects, between the pedal's preamp and its filter section without changing the envelope drive¹. In practical terms, whatever you insert in the loop colors the sound feeding the filter, but the envelope follower still reads your original playing dynamics, so the sweep stays consistent.

The loop uses two jacks:

• FX Loop Send jack: musical instrument signal output to an external effect¹
• FX Loop Return jack: from an external musical effect's output back into the Q-Tron Plus filter process¹

The rest of the pedal's connections are an input jack for your instrument and an effects out jack to your amplifier¹.

How it differs from the Q-Tron

Electro-Harmonix positions the Q-Tron Plus as the same design as the original EHX Q-Tron, with the effects loop and the Attack Response switch as the additions¹. If you want to insert drives or other effects ahead of the filter without disturbing the envelope, the Plus is the one that does it. Players who just want the basic envelope filter in a smaller or cheaper package also have the Micro Q-Tron and Nano Q-Tron in the EHX range.

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