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Topinio
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PC 5.1 set ground loop/short: repairable?

Sun Oct 29, 2017 5:02 pm

I have a set of Logitech Z506's, which I bought in April to replace a 2011 purchase of the same thing, and both have always crackled a bit on plugging one of the 3.5 mm mini phone cables into the soundcards of my PCs. I thought nothing much of this.

Then today, plugging in a mains powered USB hard drive into a front-of-case USB port caused a PC crash and there were some sparks between the port and the USB A cable as it went in. Neither the Z506's nor the drive have a real earth pin on their plugs. Both were in the same 6-way PDU, along with the PC and monitors.

This was reproducable, then was not once I unplugged the speakers from the PC.

Questions I have, any answers gratefully received:

  1. Is this a ground loop or something else?
  2. Is it fixable, and if so how?
  3. Should I RMA the speakers?
  4. Is there an alternative set of PC 5.1's that I can buy for under £100 which will not have this problem?
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Re: PC 5.1 set ground loop/short: repairable?

Sun Oct 29, 2017 8:26 pm

Crackling when plugging powered speakers in to the soundcard is not unusual. You really should turn the speakers off (or at least turn the volume down) when plugging/unplugging them anyway, to prevent potential damage to the speaker drivers/cones. Even in the absence of other issues, the sudden charging of the DC blocking capacitors on the audio output when the connection is made can cause extreme excursions of the subwoofer driver; slamming it into its mechanical limits isn't good for it.

It can't be a ground loop if neither the speakers nor external HDD have a 3-wire plug, Since they're all plugged into the same power strip, that should take care of any ground loop issues even if they did have 3-wire plugs. Regardless, sparking in a situation like this is definitely abnormal.

There are multiple potential issues here. Yes, it sounds like there may be a defect in the power adapter for the speakers (leakage current), which is causing some abnormal voltage to be present on the audio cable. But even if this is the case, your PC's chassis ground should be draining that away, eliminating the possibility of sparking. Audio and USB connectors are all low voltage and low current; sparking is an indication of a potentially serious problem.

I suspect you may have a defective 3rd (ground) wire on your electrical circuit. This is a potential safety issue, and needs to be checked out. Know any electricians?
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 2:08 am

just brew it! wrote:
I suspect you may have a defective 3rd (ground) wire on your electrical circuit. This is a potential safety issue, and needs to be checked out. Know any electricians?

Oh dear. Thanks.
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 4:10 am

You could put a decoupler to place in between the computer and the speaker to isolate them electrically.

Try googling for a car audio decoupler, it should be fairly cheap.

This is of course not fixing the root cause but that might be tricky with computers...
 
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 11:51 am

I would vote for a defective power supply in either the hard drive or the speakers.
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 6:19 pm

ludi wrote:
I would vote for a defective power supply in either the hard drive or the speakers.

Shouldn't the chassis of the PC be at ground potential (via the protective ground) though? That's why I said I thought there could be multiple issues here.
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 8:17 pm

Topinio wrote:
just brew it! wrote:
I suspect you may have a defective 3rd (ground) wire on your electrical circuit. This is a potential safety issue, and needs to be checked out. Know any electricians?

Oh dear. Thanks.


Don't panic here or waste money. The ground wire system is there to protect you in the extreme case where something has gone very wrong and provides a path from the metal of the power supply chassis to ground. So, if you had an short from the hot line to the case it would go to ground through the wire not your body. In normal operation there ground wire is not a factor.

You can easily check this yourself with a cheap simple tester. No need to call in professional help unless this tester reveals a problem.
https://www.amazon.co.uk/13-Amp-Plug-So ... B004BLPIBK

And all of the above is likely not related to your problem as you noted neither of the suspect devices has a ground connector.

Is the spark you see a one-time quick event or continuous? If one time and causes a reboot, that seems more like a static discharge. The USB hard drive is isolated via its power supply transformer. So, effectively it is floating ground relative to the computer and gets brought to the same ground potential when you plug in the cable. It could be possible those two grounds are far enough apart that you see a small spark when you initially connect them, but no real current should be flowing.

Just as an aside, I assume you are in the UK based on your currency comment. I was over there this year an noticed several outlets that had switches for the outlet right there on the outlet face. Is that a fairly common arrangement?
 
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Re: PC 5.1 set ground loop/short: repairable?

Mon Oct 30, 2017 9:20 pm

Well, he did say "sparks" (plural), so I was assuming it was more than a one-time static discharge. If something is putting enough voltage on the chassis or cable to cause multiple sparks when plugging the cable in, something is amiss.
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Re: PC 5.1 set ground loop/short: repairable?

Tue Oct 31, 2017 2:07 am

just brew it! wrote:
Well, he did say "sparks" (plural), so I was assuming it was more than a one-time static discharge. If something is putting enough voltage on the chassis or cable to cause multiple sparks when plugging the cable in, something is amiss.

I figure a defective switching supply could be building a charge imbalance on its own floating ground, or even leaking mains voltage through, but not enough to trip anything. That, in turn, might get pulled back into the other device when the USB connection is made. While the chassis ground should protect the user from shock or catastrophic damage, it's often not as "clean" as one might hope -- especially if OP's UK residence is old enough to be using a ring circuit.
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Re: PC 5.1 set ground loop/short: repairable?

Tue Oct 31, 2017 9:28 am

ludi wrote:
just brew it! wrote:
Well, he did say "sparks" (plural), so I was assuming it was more than a one-time static discharge. If something is putting enough voltage on the chassis or cable to cause multiple sparks when plugging the cable in, something is amiss.

I figure a defective switching supply could be building a charge imbalance on its own floating ground, or even leaking mains voltage through, but not enough to trip anything. That, in turn, might get pulled back into the other device when the USB connection is made. While the chassis ground should protect the user from shock or catastrophic damage, it's often not as "clean" as one might hope -- especially if OP's UK residence is old enough to be using a ring circuit.


This. My first suspect would be the power supply on the external hard drive. You can find all sorts of online reviews of USB chargers that let mains voltages through.

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Re: PC 5.1 set ground loop/short: repairable?

Thu Nov 02, 2017 5:00 pm

videobits wrote:
Don't panic here or waste money. The ground wire system is there to protect you in the extreme case where something has gone very wrong and provides a path from the metal of the power supply chassis to ground. So, if you had an short from the hot line to the case it would go to ground through the wire not your body. In normal operation there ground wire is not a factor.

You can easily check this yourself with a cheap simple tester. No need to call in professional help unless this tester reveals a problem.
https://www.amazon.co.uk/13-Amp-Plug-So ... B004BLPIBK

And all of the above is likely not related to your problem as you noted neither of the suspect devices has a ground connector.

Is the spark you see a one-time quick event or continuous? If one time and causes a reboot, that seems more like a static discharge. The USB hard drive is isolated via its power supply transformer. So, effectively it is floating ground relative to the computer and gets brought to the same ground potential when you plug in the cable. It could be possible those two grounds are far enough apart that you see a small spark when you initially connect them, but no real current should be flowing.

Just as an aside, I assume you are in the UK based on your currency comment. I was over there this year an noticed several outlets that had switches for the outlet right there on the outlet face. Is that a fairly common arrangement?

Thanks, this is really helpful. Thanks to everyone else too!

I still suspect the amp is at issue, maybe leaking through the 3 mini phone cables. Any idea how to test this anyone?

The spark was continuous, came as I touched the metal of the USB A end of the cable from the hard drive to the metal of the USB A socket on the front top of the PC, and went as I moved it away; rinse, repeat. Definitely not static. Ni reboot, just locked up with black screens.

Yes, I'm in the UK, and yes those switches on sockets are nearly everywhere https://en.wikipedia.org/wiki/AC_power_ ... et-outlets
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Re: PC 5.1 set ground loop/short: repairable?

Thu Nov 02, 2017 5:02 pm

SecretSquirrel wrote:
This. My first suspect would be the power supply on the external hard drive. You can find all sorts of online reviews of USB chargers that let mains voltages through.

Interesting. Any way I can test this?

The drive is a relatively new Seagate Expansion 4 TB unit.
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Re: PC 5.1 set ground loop/short: repairable?

Thu Nov 02, 2017 5:20 pm

Well, you could try touching the mini-phone cables from the speakers and the drive's USB connector housing to a grounded object (e.g. metal water faucet), to see if you can draw a spark. That would let you know if there's an appreciable leakage issue.

Wear rubber-soled shoes and keep one hand behind your back (standard safety procedure for handling high voltage), just to be safe. You don't want any chance of the current passing through your body.
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Re: PC 5.1 set ground loop/short: repairable?

Thu Nov 02, 2017 7:05 pm

just brew it! wrote:
Well, you could try touching the mini-phone cables from the speakers and the drive's USB connector housing to a grounded object (e.g. metal water faucet), to see if you can draw a spark. That would let you know if there's an appreciable leakage issue.

Wear rubber-soled shoes and keep one hand behind your back (standard safety procedure for handling high voltage), just to be safe. You don't want any chance of the current passing through your body.

Might give that a go in the morning then, ta.
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Re: PC 5.1 set ground loop/short: repairable?

Wed Nov 08, 2017 1:47 pm

Um, are you still alive?
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Re: PC 5.1 set ground loop/short: repairable?

Wed Nov 08, 2017 2:01 pm

morphine wrote:
Um, are you still alive?

Forum reports that he was last online at noon today. Unless he touched the wrong wire and got his consciousness transferred into Skynet, and is browsing from there.
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Re: PC 5.1 set ground loop/short: repairable?

Wed Nov 08, 2017 2:07 pm

morphine wrote:
Um, are you still alive?

:D Yes. And thanks!

I actually started thinking that it must be the sub/amp unit at fault, because this doesn't happen when it's disconnected from everything (mains, soundcard, satellites), and that the safest thing to do might be to get it checked out before anything else. Probably going to retrieve the box (for easier transport) from the loft tomorrow and take it in on Friday.
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