the electro-harmonix q-tron is an envelope controlled multimode filter, the modern day version of the original 70s 'doctor q' pedal.

it offers two knobs (gain, peak) and five switches (envelope follower range, filtermode, filtersweep direction, boost, bypass). four filtermodes are available (highpass, bandpass, lowpass, mix of bandpass and dry signal). 'peak' controls the resonance of the filter, 'envelope follower range' selects the intensity of the envelope tracking the input signal, creating sharp or smooth responses.

obviously this unit is good for autowah-effects. the q-tron offers few controls only, but this also makes it really easy to use - this pedal was designed to actually play a guitar through it, so the input signal has the biggest influence on how the unit reacts. the q-tron works with all kinds of signals, but of course dynamic signals like percussion have the greatest effect. the filter sounds smooth, not too aggressive. full self-oscillation is not possible.