ground loop in AV amp to Int. amp AV bypass

tlee1

I'm using Yamaha 767 and front channel preout to AV bypass of Onix Int. amp.  When I play AV system, both Onix and Yamaha turn on and there are a very little high frequency sound.  It seems to be ground loop, and after searching from some previous post, I've tried all the 4 methods:

1. remove all (digital and analog) inputs, and even remove the HDMI to TV, only connected the speakers cable, preout RCA to int amp and sub-woofer cable to sub-woofer.
2. connected the case of the AV amp and Int amp
3. connected the case of AV amp to the "earth" of the power bar
4. remove the "earth" of the PC of the int amp

But the high frequency sound from the front speaker still exist and even no improvement, any CHing can help?

QPRanger

Do you have a cable box?  Or did you try to unplug the TV signal cable?

tlee1

I've even tried to unplug the HDMI out to tv, that means no connection between av amp and any tv signal cable.