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Ground Loop Hum coming from HTPC

post #1 of 19
Thread Starter 
I have stripped down my entire entertainment center trying to find the cause of a ground loop hum in all of my speakers/ sub.

My HTPC is connected to my Receiver using an HDMI cable from the Video Card in the HTPC. The second the HDMI cable is unplugged the hum stops.

So what can I do to stop or track down what in my HTPC is causing the ground loop electrical noise sound. I also hear a cracking in the speakers when I use the scroll wheel on the mouse.

When I use an Spidf cable to the receiver from the HTPC the scroll wheel cracking stops but the hum is still there since the HDMI cable is still connected for Video.

Both the computer, receiver, tv, modem and sub are all connected to the same power strip. And the coax is unplugged from the HTPC and Modem for testing.
post #2 of 19
Try another hdmi cable?
Try unplugging power from modem and sub. Just pc, tv and receiver plugged in
Edited by ripsaw - 2/18/13 at 9:20am
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post #3 of 19
Thread Starter 
Tried 3 different HDMI cables all different brands.
post #4 of 19
Thread Starter 
Tried that I still get the ground loop hum in the speakers.
post #5 of 19
you could try taking an earth from the AV amp to the computer casing.
The AV amp / Receiver should have an earthing terminal and you could use one of the screws on the computer case that bolts the PSU to the chassis... just use a length of copper speaker cable.

EDIT: just another thought, you've got all analogue inputs (to a sound card or TV tuner) muted I take it?
Edited by pharcycle - 2/18/13 at 2:04pm
 
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post #6 of 19
Quote:
EDIT: just another thought, you've got all analogue inputs (to a sound card or TV tuner) muted I take it?

This is a good question. A digital signal should never cause a ground loop hum. What analog connections do you have in the system?
post #7 of 19
Quote:
Originally Posted by VABuckeye View Post

This is a good question. A digital signal should never cause a ground loop hum. What analog connections do you have in the system?

the speakers themselves are driven by an analogue signal though so if the amp plug doesn't have its own earth pin (quite common) and the PC does, then the difference can be coupled into the amp output i.e. the speakers... Is the PC earthed, I guess a 3-pin plug may not be standard for all countries?

In fact, with the HDMI cable unplugged, try commoning the PC and amp chassis together via the method I mentioned previously - does the hum return?

you could try stripping the hdmi cable 10mm or so and cutting the shield braid at the amplifier end - the shield along the cable should still provide immunity to outside noise as it's earthed at the PC end but should stop the ground loop with the amp if that's what it is. Only do this to a cheap 1.99 ebay cable though! You probably don't want to common the PC and AMP chassis if doing this but could be worth playing with the combinations to see if anything helps

PC power supplies are inherently noisy due to the switched mode of operation (PWM) so you could try a different one and see if that helps
 
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post #8 of 19
Thread Starter 
Alright with further testing. It is definitely a ground loop issue.

I came home and grabbed one of my hundreds of extra power cables laying around and pulled the ground pin out of it. Hooked up the sub using this power cable and the hum is gone. I then put the correct power cable back on the sub and tested the same cable on the HTPC. And again the hum was gone.

So removing the ground at either the sub or the PC removed the hum. Also from further testing I found out the sub will hum even with the PC powered off (what does this mean?).

And again I also tested running a cable between the PC and the Receiver and with no change. I will have to test the HDMI cable unplugged again shortly.

And I will test a deshielded HDMI also. I have like 10 of them from monoprice like $3 bucks each.
post #9 of 19
Quote:
Also from further testing I found out the sub will hum even with the PC powered off (what does this mean?).

The HDMI cable is not the source of the ground loop. It can't be as it is a digital signal. Ground loops are analog. You need to look at the cable connecting the sub to the amp and your other speaker connections. Are you using speaker wire for all these connections? Look for a rogue speaker wire that is getting pinched or compromised somewhere in the chain.
post #10 of 19
Thread Starter 
I have determined the cause but you are incorrect. It is a ground loop Caused by the PC and the Subwoofer. Now I need a solution. How can I stop a ground loop when using an HDMI cable to connect my PC to the Receiver? Is there a device that will kill the ground loop in the coax between the receiver and subwoofer?


PC <---hdmi---> RECEIVER <----coax---> Subwoofer

If I remove the ground at either end device the humming stops. The receiver Is not grounded it has a 2 pin power cable and does not contain a grounding post. I have removed all other possible devices out of the equation.

I also tested the previous test and yes when I disconnect the HDMI cable and connect a speaker wire between the PC and the Receiver the humming continues.
Edited by jdouglas - 2/18/13 at 4:43pm
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