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Old 30th November 2010, 08:53 PM   #1
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Default Another easy to answer question that is above me. XLR vs Ground loop

Hi,

This is a simple problem but driving me nuts trying to fix.

I have a ground loop,

I have a PC directly connected to a Rotel RB-993 power amp for surround channels and to an RC-995 pre and RB-991 power amp for the front two.
An Avondale Arcam Alpha 5+ is also connected to the RC-995.

If I disconnect either the PC or the CDP the hum goes.

All the earths are connected, all one circuit, all low resistance (I'm a sparky, do not know electronics, but can fix a dedicated spur well. )

I currently use a ground loop isolator between the PC and RC-995, but it really messes up the quality (but fixes the loop).

I want to fix my ground loop, but do not like the isolator.

I just read that using an XLR can fix the issue!
I didn't know this, and not really sure how it would.

I currently have XLR between my pre and power, but could fit modules onto the CDP quite easily.

This leads to the questions.

1: Do XLR fix a ground loop issue?
2: Are there different qualities of XLR and can they affect sound quality?
3: Something like this work?
Vision Techconnect V2 XLR Male Module
or
VISION TECHCONNECT MODULE FEMALE XLR - TC-XLRF

4: Any suggestions or ideas?

I should add that my PC does leak to ground, it is a PC, they all do.
My CDP also leaks to ground, no friking idea why, but cannot afford to get it fixed, it is a small trickle like the PC, but disconencted the case charges up (that may fix the loop, but there are problems eg electrocuted cats. )
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Old 30th November 2010, 09:45 PM   #2
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It's not the XLR connector that fixes the ground loop, but that XLR connectors are often used in balanced interconnect circuits. For this to work you need a balanced output stage on the PC and a balanced input stage on the RC-995.

Before going that route, try a heavy wire from the PC case to the pre-amp case.
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Old 30th November 2010, 11:26 PM   #3
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Quote:
Originally Posted by inteificio View Post
If I disconnect either the PC or the CDP the hum goes.
...

I should add that my PC does leak to ground, it is a PC, they all do.
My CDP also leaks to ground, no friking idea why, but cannot afford to get it fixed, it is a small trickle like the PC, but disconencted the case charges up
Most things with switched mode power supplies will float up to mains voltage above true earth. The current drive is very low so it's not really dangerous.

If you cut the earth to the CDP, and have it earthed only by means of the screens of the co-axial audio cable, that might fix things.
Ideally, an audio system should be tied to true earth at only a single point.

You can buy good quality audio coupling transformers that will eliminate any ground loops, but the ones that sound good (or rather, don't have any sound) are quite costly.
Probably cheaper would be to get a 1:1 50/60Hz mains isolating transformer for the CDP alone. It's likely to have to be only about 30VA.
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Old 30th November 2010, 11:36 PM   #4
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Couldn't you just lift the ground on the PC audio cable? Seems that would be an easier fix than a transformer. Might cause a small DC offset on the hot pins but your (pre)amp probably has capacitive coupling on the line inputs anyway.
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Old 30th November 2010, 11:48 PM   #5
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Quote:
Originally Posted by inteificio View Post
1: Do XLR fix a ground loop issue?
The connectors aren't something magic. What fixes the ground loop (i.e. the hum on the output) is that the ground connection is no longer part of the signal path. The loop still remains though, via pin1 of the XLRs.

Quote:
2: Are there different qualities of XLR and can they affect sound quality?
Not so you (or I) are likely to notice. Many, many years ago I got some cheapo plastic XLRs from Maplin, they worked just as well as metal cased ones. Just the locking mechanism was weak.

Quote:
3: Something like this work?
The connectors themselves will do nothing in the absence of including a balanced input in your preamp. Does it have one already? If not, you'll need to add one - either an active (opamp) or passive (via transformer). Balanced outputs are not mandatory though.
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Old 1st December 2010, 07:32 AM   #6
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Do those modules not have the balance circuitry? If they are just connectors that is a little useless. Are their balanced modules that have the full circuitry?
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Old 1st December 2010, 08:35 AM   #7
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I haven't examined them very clearly, but it would be very unusual in my experience for balanced-unbalanced converters to be included in the connectors themselves. That's because they'd either be transformers (which tend to be bulky) or opamps (which tend to be the equipment designer's perogative). I'm not aware of balanced modules which have the full circuitry.
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Old 1st December 2010, 09:12 AM   #8
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Seemed too easy =-(

Oh well, guess this plan was a fail.
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Old 2nd December 2010, 05:58 AM   #9
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Quote:
Originally Posted by abraxalito View Post
The connectors aren't something magic. What fixes the ground loop (i.e. the hum on the output) is that the ground connection is no longer part of the signal path. The loop still remains though, via pin1 of the XLRs.
yes, but who says you have to use pin 1 in an analogue XLR cable, for AES it would be an issue, but for line level analogue you can either not use a shield at all provided your gear has good CMRR, or you can use the connector shield for the cable shield rather than pin 1, so the only connection should be between chassis grounds (directly anyway) rather than signal or power supply ground

now of course that doesnt solve our friends problem
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Old 8th December 2010, 02:49 AM   #10
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Quote:
Originally Posted by qusp View Post
yes, but who says you have to use pin 1 in an analogue XLR cable, for AES it would be an issue, but for line level analogue you can either not use a shield at all provided your gear has good CMRR, or you can use the connector shield for the cable shield rather than pin 1, so the only connection should be between chassis grounds (directly anyway) rather than signal or power supply ground

now of course that doesnt solve our friends problem
Firstly, true balanced connections dont cure ground loops but they sure simplify the grounding scheme by making it possible to get rid of the ground on each interconect. Connect the shield only on one end to pin 1. Pin 1 is almost always tied to the chasis in the device, which is usually tied to both power and audio ground.

Last edited by cbdb; 8th December 2010 at 02:55 AM.
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