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Old 27th December 2012, 05:19 PM   #21
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From your description is sounds as if your PC is not grounded.
*edit* or the monitor.

Most PC PSU and many SMPS comes with two caps that goes from phase and 0 down to protective ground. While this shorts out some RF noise (when grounded) it also effectively makes the protective ground float 50% in between phase and 0 when not. Not good when different equipment have to interconnect.

I took safety ground and connected it to the nearest radiator (water) as i did not have a protective ground supplied in this room. You could get a lovely spark if you touched the radiator but didn't make a good enough connection.

I have also had lovely sparks form when connecting for example a VGA monitor to a PC.

Isolating the PC with an ungrounded transformer is not a good solution for this as it will be free to float at high potential, not fun when you happen to connect something at the wrong potential. Especially as you, the user will be at ground potential.
Ground it properly and it might be good but you should still be able to solve your problem with other means.

Proper investigation into what generates the voltage potential and which path the current takes is important here.
Redirecting the current through a big wire somewhere else could be anything from a good idea to a bad one depending on what is happening.

We can safely say that you don't want the current to flow through signal cables so you have to find a way to prevent that.

Last edited by David_Web; 27th December 2012 at 05:28 PM.
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Old 28th December 2012, 11:35 AM   #22
Kei is offline Kei  Wales
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Even with the pc signal cables unplugged, the pc chassis still has continuity with the luxman amp chassis, which suggests that it does have a connection with earth. I have noticed that the resistances between the various points i've tested with my meter are different. The computer power supply is a corsair TX650 V2 if that helps at all and the case is a coolermaster CM690II, which is painted (or coated) in a non conductive finish. There is an intermittent grounding issue somewhere in the front panel connector as i occasionally get a spontaneous repeating restart when the usb plug of a device touches the port. (this is quite rare though, hence it being an intermittent issue)

I've had my pc connected up to my other hi-fi downstairs which used to use a LEAK 2200 amp and the ground loop noise was even worse. (intolerably bad) Using a passive balancing box and a quad 240 power amp stopped the noise. I've drawn up some improved diagrams of how everything is connected. I've also included a part of the amp schematic showing the mains in section.
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File Type: jpg MainsConnections.jpg (118.0 KB, 69 views)
File Type: jpg Signal-Connections.jpg (183.4 KB, 65 views)
File Type: jpg L215_RectSchema.jpg (90.4 KB, 50 views)
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Old 28th December 2012, 12:10 PM   #23
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Do you have a grounded outlet?
PC should not carry any potential if you do.
Is there any potential between PC an radiator for example?
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Old 28th December 2012, 12:22 PM   #24
Kei is offline Kei  Wales
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There is -13.4mV between my pc chassis and the nearest radiator. Resistance of 30ohms. There is 11mV between the pc chassis and the luxman chassis. There is 14mV from the pc chassis to the earth pin in the extension and 2mV between the luxman and the same earth pin.

As far as i know, the only devices that are not earthed are the cd/dvd/bluray players, the PVR and the marantz AV reciever. (all 2 pin mains connections) Everything else has an earthing pin that is connected.
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Old 1st January 2013, 09:16 PM   #25
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Do the ungrounded devices have a polarized plug on them?
Sometimes if the polarity of the power signal is wrong on a device it can introduce hum as well.

jer
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Old 1st January 2013, 09:21 PM   #26
Kei is offline Kei  Wales
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No they aren't polarized.
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Old 2nd January 2013, 07:51 PM   #27
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Ground loops are tricky, you have to figure out where a current draw from one device across a common impedance causes a voltage drop across the signal path of the other device. Then break that current path by redirecting it though a lower impedance path, by adding one, reinforce the offending path or increase the other one like putting a resistor across signal ground lead. Which may or may not be a good idea depending on what is happening.
Also taking care of which order devices are pulling current so that the amplifier won't cause the preamp to pick it up and with all that gain cause positive feedback. Unlikely but possible.
I guess this is one of the times one wish that consumer electronics was using balanced interfaces. Or was that all the time...
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Old 9th January 2013, 09:06 PM   #28
Kei is offline Kei  Wales
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Found some monacor ground loop isolation transformers reduced and decided that they were at a sufficiently good price that i'd give them a shot. Bought 2 and fitted one between the xonar and the luxman, the other between the marantz and the luxman. All noise is gone completely, I now have absolute silence at maximum volume. (noise became audible above about 1/3rd)

Can't hear effects on the sound so far. Will keep listening and see how it goes.
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Old 9th January 2013, 09:16 PM   #29
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Sweet!!!!

Glad to hear that you have solved it!!

jer
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