Grounding Issues - Speakers Humm

[JPNorth]

[FONT=Arial]I have a denon pma-700ae amp.[/FONT]
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[FONT=Arial]During a recent renovation I tried to get a bit clever and installed an RCA in wall face plate (to plug my ipod into) as the amp is hidden away in one room and the speakers are in the ceiling. I have soldered cat 5 cabling to the back of the sockets and rca plugs on the other end, as the cabling is in the wall the total run for each side is approximately 10m. When I turn on the amp the speakers now hum( only when cable from face plate is plugged in), so I realise that there is a grounding problem somewhere. Looking on the forum already I tried a couple of things:[/FONT]
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• [FONT=Arial]Attach ground wire to hifi and place other end into mains earth: that made the hum half as loud.[/FONT]
• [FONT=Arial]Attach ground wire to hifi and unplug 1 rca from the hifi, place other end of ground wire to centre pin of rca plug: this eliminated the hum totally. [/FONT]

[FONT=Arial] [/FONT]
[FONT=Arial]I am relatively new to this game and am not wanting to spend stacks of cash on a solution, if anyone has any ideas they would be much appreciated as it is really annoying me now. I realise the easiest way would be to eliminate the extra cabling to place the face plate in wall and simply plug the ipod directly to the hifi (this works without any hum), however that would defeat the purpose of what I am trying to do. In case my ramblings are unlcear, I have attached a doodle of the setup. [/FONT]

[bsuttie]

One solution may well be a small ground loop isolator which will break the ground connection altogether.

However this may not help if you have used unscreened CAT5 cable which is designed for balanced computer networking. Particularly if the cable runs close to any mains power which will be induced onto it.

You should really have used screened audio cable rather than CAT5 .

regards

Brian

[JPNorth]

I am away to purchase one now. will let you know if this solves the problem.

As a newbie to all this I am confused about the cable type I should have chosen, I was of the belief that more and more people are using CAT5E UTP to make custom cables, could anybody advise something more suitable as I don't make the same mistake again.

Many thanks,

JP

[Mark.Yudkin]

If you want to use CAT5e, you should use S/FTP, F/FTP oder SF/FTP (see ISO/IEC-11801), not UTP.

UTP per se is rather uncommon over here (CH). It's more popular in US, although given its inherent problems that remains rather surprising.

[bsuttie]

Originally Posted by JPNorth

I am away to purchase one now. will let you know if this solves the problem.

As a newbie to all this I am confused about the cable type I should have chosen, I was of the belief that more and more people are using CAT5E UTP to make custom cables, could anybody advise something more suitable as I don't make the same mistake again.

Many thanks,

JP

You can use CAT5 cable for carrying A/V signals, but you really need baluns at each end to convert the signal into a balanced one, i.e. in the same way as a computer network signal is balanced. This then means that any interference is induced equally onto both wires of a twisted pair which then gets cancelled out.

The transformer I pointed you to is effectively the same as a balun, and with a portable piece of kit like an ipod which has no ground reference will make the signal path balanced. The other thing to mention is that you have to use the right wires within the CAT5.

There's no point in using blue and green for the left channel for example - you have to use say blue and blue/white as they are twisted together.

A screened cable will be better than UTP for rejecting any interference as long as the screen is connected to ground (earth).

regards

Brian