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Old 26th January 2007, 10:35 PM   #1
mikee55 is offline mikee55  United Kingdom
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Exclamation UK How can I take away safty Earth and still keep it?

Hi all,

I'm running a Car amplifier and my Linkwitz Transform circuit off of an old "AT" PC powersupply. I have an issue with hum. I can eliminate it only by dis-connecting the mains Earth on the AT supply. Simple enough, but I have a Rugrat and 2 young children and safty is Paramount. Can I use this...

Cheers Mike
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Old 27th January 2007, 05:46 PM   #2
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Not a good idea, frankly.

You're getting hum because somewhere in your audio system there are two (or more!) links between the audio system 0v and Earth (mains ground). If you can find one or all of them, pick (n-1) and cut them.

The bond between earth and audio 0v is recommended for safety in mains-powered systems. It should only happen at one point in the entire system, or earth loops and hum results; I like the bond at the source or at the preamp.

Note that this is entirely separate from earthing the cases. If you use metal/conductive cases on mains-powered equipment, then it must be bonde to earth. The signal 0v within does not, though, for the reasons given above. Hope this helps!
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Old 27th January 2007, 08:12 PM   #3
Nordic is offline Nordic  South Africa
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Would it be a problem to cut the ground on the preamp side of things (as thats what I have open) ... I'm haveing a little problem with hum useing a new pre(buffer), both amp and preamp connected to mains earth via disconnect network... both have their circuits connected to the networks... wich I think is what you are describeing... No hum with headphones..
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Old 27th January 2007, 08:24 PM   #4
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No matter what - the metal chassis of the PSU MUST be connected to mains earth directly.

Any fault in the switching PSU could make this live otherwise.


Cliff

up the road in Wimborne.
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Old 27th January 2007, 09:03 PM   #5
Nordic is offline Nordic  South Africa
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Lol, thats why I don't use metal chasis... I do like my transformer chassis grounded though...
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Old 27th January 2007, 10:19 PM   #6
mikee55 is offline mikee55  United Kingdom
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Default Hummmmmm

Hello Cliff, you in Wimborne? I'm in Corfe Mullen, nice to know locals are here too.

I did try to remove the 0v but then I lost the signal and got a buzz.
I must mention that I'm using my PC as a music source. I designed the system with the help of richie000boy,( I don't like to ask too much of him-he's been very kind in helping and I don't like to take the P*!S). It was designed using this PC which sits in our kitchen, so I've left it here for now, we listen to music and watch TV with it. It is designed for my lounge, and richie000boy said it should be ok running off the DVD player in my main system, but whilst I'm still tweaking it, its residing in the kitchen and is attached to the PC. I guess I should have mentioned this as it must have an Earth loop via the PC's PSU. oops!sorry!

But I still need to deal with this earth, before something goes wrong and the kids touch it.

Cheers Mike
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Old 27th January 2007, 10:30 PM   #7
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A ground loop isolator for the preamp line may work. You should be able to find one at any car audio shop.

Some amps don't work properly with the shield floating so you may need to connect the amp side of the RCA shield to the amp's chassis/power ground (when using the GLI).
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Old 28th January 2007, 10:57 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
Some amps don't work properly with the shield floating so you may need to connect the amp side of the RCA shield to the amp's chassis/power ground.........
I don't think this is correct.
What is GLI?

I would prefer to see the smps chassis directly earthed.

Your chassis is also directly earthed. Keep it this way.

If the smps is directly earthed does that mean the audio ground must also be directly earthed? i.e. is the smps chassis connected to the 0v reference going to the power amp Central Audio Ground?

Is there a way to earth the smps chassis but float the 0v reference coming from it?

If the above looks to be the problem then Cordell's solution may be the answer (I have not tried it yet).
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Old 28th January 2007, 11:40 AM   #9
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Quote:
What is GLI?
GLI = Ground Loop Isolator

It is simply a small 1:1 isolating transformer.

Click the image to open in full size.
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Old 28th January 2007, 12:28 PM   #10
Nordic is offline Nordic  South Africa
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If I had to guess a transformer in a box...
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