It is a mechanical noise from the transformer. The transformer hums with the speaker selector "on" or "off." I did not try it to see if there is a difference with the speakers disconnected.


Edited by Clarinet (01/26/15 06:10 PM)
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Outlaw RR2150, Definitive BP8040 speakers (2), Behringer 9 band graphic equalizer. Music sources: CDs, ripped CDs, MP3, some FLAC via desktop computer; tablets via Bluetooth receiver on Outlaw amp.