iMac 21.5 sound problems

[djbattaglia]

ok so here is my problem, when i hook up my logitec 5.1 sound system to the imac the sond is perfect, when i hook up my beats by dre headphones its perfect, but when i connect my iMac to my Yamaha Receiver with my polk audio speakers and sub with a Y-RCA cord, i get a mad HUMM and BUZZ sound out of the speakers, WTF... i have tried 5 different cords including a single rca cord that would be used as a aux portable cord, i even plugged my imac into different outlets throughout my basement and i also did the same thing with my receiver, so was wondering if someone can fix this problem for me PLEASE its bothering the hell out of me....