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Old 5th April 2012, 08:59 AM   #1
UV101 is offline UV101  England
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Default TomTom Iphone Ground loop

Hi,

I'm using the tomtom iphone kit in my car. The problem I have is that when using the charging socket and line out, there is a ground loop that results in alternator noise through the speakers. I believe this is as probelm with the dock as the same problem does not exist if you power the iphone direct and use ther headphones out.

I've previously resolve the problem using 1:1 transformers in a std off the shelf ground loop isolator.

What I'd like to do is understand what is actually causing the problem and see if the tomtom cradle can be fixed. Is it just a question of bonding the gnd's in the tomtom or is it a bit more complex??

Just to add a bit more, is it possible that the problem is being caused by the cigar ligher adapter? Its a std USB mini-B type so I guess its dropping 12v to 5v in the adapter.

I can just fit another loop isolator but I'd rather try and understand the problem. Thanks, Ian
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Old 5th April 2012, 11:13 AM   #2
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The noise is being caused by a very slight difference in the ground potential between the various units. You either have to have precisely the same ground potential or you have to break the DC ground connection (through the isolators).
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Old 5th April 2012, 12:01 PM   #3
UV101 is offline UV101  England
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OK so if I was to link the gnd's in the cigar lighter and the head unit directly do you think this will resolve the issue? I guess its possible that the gnds are separate in the cradle.
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When Bitstream came out, I thought, “my God, what are we going to do...?” Ken Ishiwata http://www.hifisounds.co.uk
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Old 6th April 2012, 12:38 AM   #4
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It's difficult to get the same ground if the components don't have precisely the same ground point. The head unit is likely grounded by it's casing to the chassis of the vehicle. The MP3 player of the navigation likely have different ground points. The grounds are all connected but slight differences in ground potential can make a big difference. Even if they all have the same ground point, it's possible that they will produce noise. Amplifiers have specially designed signal grounds that prevent noise problems but event those will give you noise if you connect two different sources to the amp.
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