Although I am an electrician, I am after some advise.
I have put my sat nav into my car (mio c710 portable) and given it a power supply via cig outlet. I have it running through my autocom headsets (3.5mm stereo plug input).
When I run the sat nav or mp3 programs on the internal battery, the sound is perfect, but if I plug in the power, I get a high pitched whine.
This whine is constant and does not vary with engine speed.
Could this be a suppression problem or some sort of earth loop cross over?

The whine is almost certainly originating from a switching PSU within the PPC adapter.

The interference could either be put onto the 12volt line and is then picked up my the autocom, or it could be superimposed on the headphone output of the PPC. Does the whine dissapear when you unplug the connection between the PPC and the autocom?

Make sure that the grounds for the PPC adapter (i.e. ciggy lighter) and the autocom are in close proximity with a good return to the battery.

Whit the engine off did disturb disappear? If not then the problem could be on the adapter.
....and did you try it in another car?

Just checked again and it does it with the engine off, so not an engine suppressor problem.
I have altered the backlight settings on the mio and the whine gets quieter as it is dimmed.
It only happens when the power lead is connected to the mio.
If I just use the autocom without the power lead, then no whine.
Could I put a radio suppressor in the autocom circuit?



[Edited on 14/9/2006 by nitram38]

After searching 10 pages of google, I found a forum with users talking about the same problem.
Their solution was an isolation lead from autocom, that goes between the gps output and the autocom input.
I ordered one today, £24 (ouch) so hopefully it will sort it.

Sounds like an earth loop problem then, I suspect the heaphone output on the MIO and the input on the autocom both have signal grounds that are not galvanicly separated from the power ground. This means power currents can circulate through the signal cable ground. Surprised the autocom doesn't have an isolated intput to be honest.

£24 notes is a bit on the wallet shrinking side, though it's nice to be able to have something that plugs straight in and works.

If you ever need another, the £8.99 job from Maplin http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=33172&doy=14m9 should be fine, though you'll need to put the correct connectors on it.

I did see the maplin option but it looked too untidy . The autocom is smaller and is ready with 3.5mm jack plugs.