I don't think I'd hear anything with computer off since I can hear the noise associated with what is on the screen. Move things around on the desktop and I get noise. Leave things alone and I can hear things like one second pulses of noise as various routines kick in, especially Task Manager. Plus that will of course turn off the sound card which is USB powered.
But all is good. I dug out an old Denon AVR-2500 (which has direct input to the power amps) and hooked it up. Being a Class II device, no ground loop, no noise - could not hear anything even with ear pressed against speakers. Sadly, my rebuilt vintage amp has been relegated to driving the surround channels - they are further away, and without tweeters aimed directly at my ears, I don't hear the noise.
But all is good. I dug out an old Denon AVR-2500 (which has direct input to the power amps) and hooked it up. Being a Class II device, no ground loop, no noise - could not hear anything even with ear pressed against speakers. Sadly, my rebuilt vintage amp has been relegated to driving the surround channels - they are further away, and without tweeters aimed directly at my ears, I don't hear the noise.
OK, maybe that will be a task for some day in the future. Thanks for your help.
I could hear letters being typed in linux terminal 
I am glad you have sorted it out. I believe ground loops are the very reason for all the "fine tuned" computer setups where people try to shut down all processes, screen, everything and claim to hear a difference. Such changes are futile because they do not fix the very cause - the ground loop in their setup.
I am glad you have sorted it out. I believe ground loops are the very reason for all the "fine tuned" computer setups where people try to shut down all processes, screen, everything and claim to hear a difference. Such changes are futile because they do not fix the very cause - the ground loop in their setup.
I could hear letters being typed in linux terminal
I am glad you have sorted it out. I believe ground loops are the very reason for all the "fine tuned" computer setups where people try to shut down all processes, screen, everything and claim to hear a difference. Such changes are futile because they do not fix the very cause - the ground loop in their setup.
I've tested dozens of computers on my professionnal sound cards and they were all as silent.
My last post implied perfection once I went to a Class II amplifier. That is not completely true. Apparently my sound card mutes until something comes along for it to output, so when I did my initial experiments with a Class II amp the sound card was muted. If I start playing music, then pause, I can still hear some computer noise, although at a much lower level than with the Class I amplifier. It is only noticeable if I have no music playing and am messing around with computer stuff.
Not one to leave things alone, today I cobbled up a little interface board to power the sound card from a separate (floating) power supply. That seems to have largely eliminated the noise. With ear pressed against the tweeter while moving windows around the desktop I can maybe hear a little something, but certainly inaudible from my listening chair and probably comparable or less than the amplifier hum/noise. So maybe I'll build up something a little less kludgy.
Not one to leave things alone, today I cobbled up a little interface board to power the sound card from a separate (floating) power supply. That seems to have largely eliminated the noise. With ear pressed against the tweeter while moving windows around the desktop I can maybe hear a little something, but certainly inaudible from my listening chair and probably comparable or less than the amplifier hum/noise. So maybe I'll build up something a little less kludgy.
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