Hooking up computer to home theatre problem pls help

Old 12-30-02, 04:49 PM
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Hooking up computer to home theatre problem pls help

this is going to be long winded , pls bear with me.

i've hooked up my hercules game theater sound card up to my home theater setup , the problem i'm getting is a loud hum from my speakers in the h/t.

i've tried plugging the computer into the same curcuit as the h/t but the hum is still present , made sure the patch cable is nowhere near any power source.

the odd thing is that when i turn off the computer the hum is still heard in the speakers , but when i unplug the computer and the monitor from the wall the hum goes down to almost nothing.

i tried plugging the monitor and computer into another curcuit together still the hum.

any solutions that i can try , i've asked different people and one suggests a ground loop isolator , others say better quality cables to the h/t and each other say the others solution won't fix it so i'm really stumped. after reading up on hums on the web it seems to be a ground loop isolator that will fix it up but i'm really stumped now.

thanks gord
Old 01-06-03, 10:09 AM
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I would try turning off just the computer or monitor it is most likely just one of them. The monitor will sometimes put a hum in speakers and the computer fan can also do the same thing. once you find out let me know I have some tricks that I know but need more idea on the layout.
Old 01-06-03, 01:15 PM
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You have an audio ground loop. The audio grounds are not the same on both systems, or the electrical ground is longer where it's plugged in on one system than it is on the other.

You can try lifting the ground on one of the two systems (even when plugged in to an extension cord), but that will probably be unsafe.

Audio (and video) wants to have all of its associated components grounded at one and only one point -- just like the electrical system in your house. That is NOT to say that you have to run separate ground wires to the nearest water pipe!

Things to try:
-- Unplug the cables and re-connect them one at a time until the hum goes away. When you find the culprit ...
-- Lifting the ground of the offending cable will stop the hum, but it will also stop the signal. That's because you're running and "unbalanced" signal, which means that the ground is also carrying signal. Bad for audio.
-- Get rid of any cheepo cables that you purchased from Radio Shack and replace them with two-conductor braided, foil-wrapped shielded cables.
-- Make sure your computer's circuit breaker is on the same leg of the electrical input panel as your sound system. You'll need to ask the experts in the Electrical forum how to do this.
-- An isolating transformer will correct the problem. They are not expensive, but they require common-mode rejection (2-wire-plus-ground) wiring instead of the usual RCA stereo wires that are common with stereo wiring.

I run my computer's sound card ouptut 50 or so feet into the basement surround system with Cat-5 telephone cable (twisted pair). When I first connected it, it hummed like a two dollar ...er ... Lady of the Evening. The culprit was a parallel run for about five feet next to some electrical wiring that was inducting voltage into the audio signal. You never know ...

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