I have a kit-built KPA500 that has the same issue. It's well grounded and all the bolts are tight. The humm is barely noticeable when off but definitely there. 73 Stan AE7UT -- View this message in context: http://elecraft.365791.n2.nabble.com/KPA500-Hum-when-off-tp7575069p7575086.html Sent from the Elecraft mailing list archive at Nabble.com.