Electro-Harmonix has been a modern leader in the classic envelope filter effect ever since the day the creator of the Mu-Tron and the creator of the Bassballs teamed up to produce the Q-Tron, now itself a classic, must-have effect pedal.

Known for their vowel-like liquid quality, envelope filters are devices that sweep the peak frequency of a filter by analyzing the amplitude of the notes the musician plays. The loudness and softness of the musician's playing alone controls the filter. It is an effect that is effortless for just about every guitarist.