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Old 21st December 2012, 04:51 PM   #11
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Can you connect the audio cable without the ground connect on one end of the cable?
This would eliminate your ground loop and the cable will still be shielded as it will be still connected to ground on one end.

Connecting the two chassis together won't change things as the loop is formed from the two chassis and the signal cable and they are connected just the same through the power plug.

If this ground pin you talk about is built in to the amplifier for this purpose then use it, as what it is doing is just separating the audio ground from the earth ground and the power supply is still connected to the earth ground.

I learned this the hard way on my Mackie 32-8 mixer as there are two ground systems in it.
One that goes to the earth ground for the chassis's and a separate one for the audio signals.

Ground loops are a PIA and just moving around cables and re-situating the power source cables can solve it too.

I had this same issue a while back and redid every thing and now I have no noise with the same machines, same cables and same computers and sound cards.

Make sure that all of the power for the devices are coming from the same source and daisy chained off of the same source and not from a different plug even though it is the same circuit.
The reason is that if the other plug has a longer run of cable it will cause a mismatch of resistance on the line compared to the other units and cause an imbalance and therefore induce a small amount of AC on the ground side.

After I had done this I did not have to lift any of the ground's on any of my equipment and it is now absolutely dead quiet even with every gain and slider turned to its maximum settings (including the amps) and no noise what so ever.
No mods and all I did was re situate the wiring.
I lucked out as sometimes it is not this easy.

Good Luck !!!

jer

P.S. My imbalance was so bad that just connecting the two chassis together with a wire resulted in some sparking as I tried to connect a wire from the mixer to the computer and it does not do this now even with a rather long run of audio cable!
FWIW

Last edited by geraldfryjr; 21st December 2012 at 05:02 PM.
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Old 21st December 2012, 07:33 PM   #12
Kei is offline Kei  Wales
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I'd already tried linking the two chassis together to no avail and they have always run from the same extension chord. (cable lengths are pretty similar too) The ground loop is sufficiently troublesome, that using a coax digital connection to my marantz AV receiver and then using the marantz as a pre-amp doesn't stop it. (switching to optical should stop it) What is peculiar is that there is no mains hum present whilst the pc is on, just a high pitch whine and various buzzes etc in time with various on screen activity. Mains hum only occurs when the pc is switched off, even if the psu switch is off.

Not sure on lifting the signal ground as i use BNC terminated coax to link them. I could make up an adaptor that lifts it.
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Old 22nd December 2012, 08:26 AM   #13
marce is offline marce  United Kingdom
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Firstly I would draw out a schematic of what is connected and how, it will be easier to pass comment then based on such a diagram.
Then you can trace the current flow, remembering that low frequency signals, may take a different path from the high frequency signals.
What worries me more is that when you mremoved the protective earth connection made things feal furry!!! I would suspect some other problem, that connection should only carru current in a fault situation.
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Old 22nd December 2012, 07:45 PM   #14
Kei is offline Kei  Wales
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This diagram shows the current wiring process for signal cables. Power is all taken from a single wall socket via a 6 way extension. (i've tried different extensions too)
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File Type: jpg GndLoop.jpg (85.2 KB, 93 views)
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Old 22nd December 2012, 07:49 PM   #15
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How cheaply can you find a mains isolation transformer over there? Probably the easiest and safest solution. Use it to power the PC.
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Old 22nd December 2012, 07:51 PM   #16
Kei is offline Kei  Wales
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Irony has it, i believe i have a mains isolation transformer somewhere in storage. Would that be a better solution than faffing with the signal cables?
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Old 22nd December 2012, 08:54 PM   #17
Kei is offline Kei  Wales
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I've just tried disconnecting the signal ground and the noise got slightly worse. Also, just to cover all bases, I have also tried enabling/disabling the various options in the bios for noise reduction. (spread spectrum and disabling cool 'n' quiet etc)

Not sure what the VA rating is for the isolation transformer that i have. The pc runs off a 650W psu, what size transformer would i need
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Old 23rd December 2012, 05:52 PM   #18
Kei is offline Kei  Wales
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From what i can find out, the mains isolation transformer that i have (somewhere) is 500VA.

The small octal socket transformer that i have spare is a Gardners AF matching transformer MU7523. (Pri. 75/500Ω Sec. 500Ω) I've attached an image of it.
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File Type: jpg GardnersTrans.jpg (200.4 KB, 82 views)

Last edited by Kei; 23rd December 2012 at 06:05 PM.
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Old 26th December 2012, 12:03 AM   #19
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Depending on what's actually in your PC, that isolation xmfr may still do fine - one likes to stay a factor of 2 away from the power rating. With a fancy graphics card or two, a fast-clocking quadcore and a big monitor, it might break into sweat under heavy load. Do give it a shot when you come across it.

I'm not really sure what the old AF matching transformer would be good for. If I had to guess, I'd say it may have been used to provide line and headphone outputs on a communications receiver way back when.
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Old 26th December 2012, 12:24 PM   #20
Kei is offline Kei  Wales
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Pc spec is as follows:

AMD phenom 2 955 running at 3.7ghz
Gigabyte 790fx ud5 board
6 hard disk drives
1 ssd
1 bluray writer
1 Radeon HD 6970
Xonar essence st deluxe
A few usb peripherals
1 Dell 24" LCD and an older neovo 19" LCD.
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