i have a persistent and annoying hum emanating from my alpha 9 power amp ,through the speakers. the volume of the hum stays the same regardless of volume control position. the mains block is plugged into a double socket . in the other side of the socket ,a small lamp is plugged in. when i either switch this lamp on or off at the socket ,the hum disappears, only to return about 1 and half to 2 minutes later. can anyone explain why this should happen and if it can shed any light on to the source of the hum?