Audio Systems: Headphones/Ear Piece and Sources, radio design labs, motorola ta200

Question
I hope you can help...   I am trying to wire up a system on my motorbike.. i would like to listen to 2 systems at the same time..    1) Sat Nav (or MP3 player) 2) Radio (Bike Intercom - Motorola TA200 Radio)   I can set the levels on each device to hear the volume I want - i.e. Turn the Radio up to the level I want and the MP3 player up so that i can hear traffic but hear the radio when my wife speaks onto the radio.....    I have tried using a system where I place a 10k ohm 0.25watt resistor in-line with each uniton the signal line and join the common grounds together... e.g.  each source wire has a resistor in line... BUT the signal strength to the earpiece becomes VERY weak...   I did think about just joining the signal wires and the ground wires... but.... I keep reading that this might damage the output amplifiers in each case...   What is the best thing to to?   I am trying to avoid spending '000s $$$ on system that prioritise these different inputs etc... e.g. Autocom Starcom systems...   Many thanks,   Tony.

Answer
That's the bad thing. Unless you or someone you know can build such a device from scratch you'll have to resort to $$$.

You have to have a summing circuit that can take two sources and combine to one. Some books by Forrest Mims will provide well enough information to come up with a design.

I do know of a company that specializes in professional audio devices such as what you're after - Radio Design Labs (RDL)

http://www.rdlnet.com/product.php?page=87