"pops through speakers"

when I turn my pre-amp on or off there are static pops in the speakers, any idea why?

amp = Denon POA 1500
Pre -Amp Denon PRA 1500
Normal. The preamp puts out a glitch when you turn it off, and the power amp amplifies the glitch. Solution is to turn off the power amp before the preamp. And turn the power amp on after the preamp.