Originally Posted by BEhrler
All of the above sounds like great advice. Where do I start and where do I get GLI cables. I have no clue.
Start by figuring out what you need. How many devices do you need to isolate? iPod, cell, radar detector, GPS, Sat receiver, FRS/GMRS, CB? Then determine which are stereo sources and which are mono.
If you are good with a soldering iron, you can roll your own pretty cheap, if not, you will need to buy them at a car stereo shop, Radio Shack, or some other supplier (I roll my own...). The store bought isolators will likely have the wrong connectors for your devices, so you will need the appropriate adapters too. Then you have to find a place to hide them all.
In my opinion it is not worth the money to buy high quality "audiophile" isolators. The signal to noise ratio on your bike (70MPH wind blowing past your helmet), and the quality of your helmet speakers means you won't hear the difference.
The device from Radio Shack, part 270-054, works fine, but has the wrong connectors for most devices (it has RCA type instead of the typical 3.5mm plugs). It's not to hard to change them if you can handle a soldering iron. If not, you will need adapters to get the right connections, but these will increase the size of the package you need to hide somewhere. The 270-054 is a stereo isolator, so it can handle one stereo device like a sat receiver, or two mono devices like GPS and radar detector.
There are companies on the web that sell isolators with 3.5MM plugs, google a bit and you will find them.
Try to keep the isolators away from other electrical cables on the bike, since they can actually pick up noise from them. A transformer is an inductive device. It is essentially two coils separated by a ferric core. The audio from your device is fed to one coil which creates an electro-magnetic field that is inductively coupled to the other coil, which is then connected to the amplifier. This is how it isolates the ground, there is no direct connection between the two coils. But, placed too close to a source of strong electrical noise, like say a spark plug cable, some of that spark energy can be induced into the coils as well.