The reason is likely with the way your power cord plug connection is made/broken to your electrical outlet-and has nothing to do with your unit. It seems your plug's ground may not be making a solid connection or your receptacle is faulty. I would recommend checking and cleaning each with particular attention to the ground connection. By any chance had you disconnected or floated the ground? You may be able to test this idea by simply using a $10 or $15 polarity/ground tester to start, at your receptacle.If things are ok try using any grounded extension cord and retest the receptacle on the extension cord. If all is ok you plug into the extension , listen a while and power down and wait 5 minutes. Capacitors, after having been warmed and played, start to discharge immediately upon turn off of incoming current with their energy dissipating mostly in the 30 seconds after turn off. I'm assuming your amp is connected to speakers of course. If there is no pop you may have confirmed the problem is related to the plug/receptacle interface. If so I would upgrade your power plugs; usually an easy worthwhile improvement for any power cord. If not lets discuss more. Good luck. Pete