i have a Tech 21 Power Engine in the shop. It was reported to be buzzing really bad. It is apower amp intended to amplify one of those pedal board thingy’s which the player goes directly to the house sound. It has only a couple small knobs on the back. It's basically a powered speaker. I verified that with nothing plugged in it did hum when the levl control was turned up. I tried it with a guitar and it sounded really bad. I ordered some caps, but didn’t use them. I did some research, and others had the sme issuie. But Tech 21 explained that the input went thru an isolation transformer. They said it will be noisy, unless you have an active low impedance source driving the input. So I took. Phone plug and shorted out the wires. When I plugged that in, the amp was pretty much silent. I then took a cord and connected the output of an ipad to the Power Engine. It also did not buzz or hum. So I contacted the customer and told him there isn’t anything wrong with his amp. I will do the two demos for him and try to get him to understand.