I use the Electric Avenue, Amplirider ( http://www.electric-avenues.com/amplirider.html ) in many installations. I've installed dozens of them for myself and friends. This device is a simple headphone amplifier designed for motorcycle use. This unit has three inputs and a headphone output with a remote mounted volume control. At $65 it is an absolute winner and Gary, the Electric Ave owner is a joy to deal with.
But motorcycles, especially heavily "electroniced" new bike have lots of electrical noise whenever you try to add amplifiers and such. For years I've been using simple DC line voltage filters with 92% success. This is usually good enough because with wind noise you can't hear the 8% noise.
But Gary at EA recommended I try a 418-EC1TA11N Cincon DC/DC Converter available through Mouser Electronics ( http://www.mouser.com/ProductDetail/...2fm03LRw%3d%3d )
I'm hear to tell you that this little device is simply amazing. On my K1200R, which has more electronics than the Space Shuttle, it removed 99.3% of all the alternator and ignition noise.
The beauty of it is that it is about the size of a thumb nail and about 3/16" thick. You need to solder four wires to the device and then just wire it in series with the 12V pos and neg. going to the Amplirider.......Done.
Having my GPS, MP3/Phone and Radar Detector all going into a set of ear buds is a wonderful thing. Listen to music, hear GPS commands, hear the phone ring, get messages from the phone and be alerted to speed traps is just great! And all for well under $100!