ludi wrote:Heiwashin wrote:Have an isolator and a new cable on order but I'd still like to know exactly what the isolators doing.
Automotive grounds are incredibly noisy. A passive isolator typically includes a 1:1 isolation transformer for each channel, the idea being to NOT have a direct shield ground connection between the source and destination devices. It also breaks the loop if there is more than one path from the equipment back to ground (as can happen when both the power supply and the audio cable create separate paths back to the same ground point, in this case your truck's frame).
Ludi got the basics. A ground loop is caused when a signal has two different paths to ground and due to distance, poor connection, or numerous other reasons, the two paths have different voltage potentials. Then end result is that a current flows over the ground cable, this allows all sorts of noise to ride along with the good signal you want to amplify. In home audio, it generally presents as a 50/60Hz hum, though you can get other noise, up to crazy stuff like receiving AM broadcasts. A ground loop isolator removes the direct ground connections, by, as ludi noted, using a pair of isolation transformers. The result is that there is no direct electrical connection between input and output. This removes one of the two ground paths.
If you have the option to power the keyboard from batteries, try it. If the noise goes away, that strengthens the ground loop theory. It doesn't prove it though as there are other ways noise can get in the line output but not be audible in the onboard speakers. Still, trying the ground loop isolation will be the easiest and cheapest thing to try first and has a high probability of addressing the problem.
--SS