Electro-Harmonix Q-Tron PlusElectro Harmonix Q Tron Plus XO Envelope Follower Pedal Same design as the original Q Tron but with extremely useful Effects Loop and an Attack Response Switch. The Effects Loop gives the musician the ability to place an additional effect, or chain of effects between the Q Tron Plus's preamp and filter section without changing the envelope drive. This is particularly important with most distortion devices, as most all do not have much of an amplitude