[strogg]

FOR CLIFFS VERSION, LOOK AT POST #4

Ok... i'm going to say this now: i have NO idea where to post this. it has to do with, well, all sorts of stuff. this is the closest thing i can think of.

anyway, i have a 1212m on my computer, which is incidentally hooked up to a comcast box (for aux input so i can watch tv), 2 monitors, a grounded headphone amp, an ungrounded subwoofer, and a rotel rb-1072 speaker amplifier. i was listening to music through the speakers while i was making the wiring behind my computer all neat, then i start hearing this awful buzzing sound. it basically sounds like a HUMONGOUS ground loop problem, basically this annoying buzzing sound, to the point where my speakers are not listenable.

but here's the thing: the subwoofer makes no noise, and nor does my headphone amp. and i know it's not the rotel amp, because plugging my ipod into the amp was just fine and dandy, until i used the computer power source as primary power, then it became unlistenable again. but for some reason, if i use the usb port to power the battery on the ipod (instead of bypassing the battery), it's just fine.

turning off the computer but leaving it plugged in reduced the noise significantly, but it was still there. i took out the analog part of the sound card and disconnected it from the digital part. the noise disappeared. but when any part of the grounded part of the card (eg the bracket) touches computer ground, the buzzing comes back.

now here's the interesting part. realizing that the negative on the rotel is isolated, i decided to take my TRS wire and wire up T as signal and R as negative, while S (earth ground) was isolated, basically taking advantage of balanced outputs. the noise became a LOT louder when i did that, even with just one channel plugged in.

so... what's wrong? is my 1212m becoming toast? but why do my other devices work? anyone know how i can fix this? if anyone can even try to point me in the right direction, i'd be happy.

thanks for the help

 

[FasterThanEver]

I think you are right to think that you have a ground loop.

It doesn't work to think of a ground loop as caused by one component. It is the combination of components connected electrically that produces a ground loop.

The computer and at least some other device have a difference in ground level. One solution is to insert an isolation transformer at some point.

You mentioned that you have a Comcast box connected to the computer. Cable TV connections are frequent contributors to ground loop problems. Try unplugging

In general, try unplugging single components to see what removal eliminates the hum. Then you will have identified one end of the ground loop. (Note that sometimes, a single component may be in the middle of an chain of components. It can take several trials to identify the end of a ground loop.)

Different power circuits in your home may have different ground potentials. Even different outputs on the same circuit can have a difference in ground potential. Plugging most of all of your equipment into the same power outlet is always good practice to reduce the chance of a ground loop.

Bill

 

[strogg]

ok, after some careful testing, i came to the conclusion that it's the computer. mainly because unplugged everything from the wall and computer, then plugged in the amp and computer together, then straight to the wall. buzzing sound was immediately heard when i hit the power button for the computer.

so i guess i'm definitely on the right forums. what can i do to reduce this noise? and why is it the ONLY thing affected is the speaker amp, while the headphone amp and subwoofer aren't affected?

 

[strogg]

ok, now it's come to this as a summary

the audio coming out of my speakers were scrumptious, as was the audio out of my headphones, but something new happened: i hear a ground loop noise from my speakers. I unplugged everything except the computer and speaker amp, and ground loop noise is still there. i then connect speaker amp to ipod. no noise. i then connect headphone amp to computer. no noise.

now here's the kicker: i then plug the computer to BOTH speaker amp and headphone via Y cable (which i've always used). there is noise though both (speakers a lot louder than headphones). i turned off the speaker amp. there is same amount of noise through headphones. i UNPLUGGED speaker amp. no more noise.

so bottom line is that i think there could be a power supply issue on either the computer or speaker amp. and of course, the speaker amp is not earth grounded while the computer is. so... any ideas? and no, ground loop isolator for the audio did absolutely nothing.