I couldn't find a commercial solution, so I came up with this little passive audio mixer that takes 1/8" headphone outputs from up to four devices and safely mixes the signals into one line that can feed a car stereo Aux input, powered speaker or any other amplifier that can take 1/8" stereo output.
No batteries are required, it's sonically transparent and most audio gadgets have their own volume control so the mixer can be done very simply and cheaply.
This little mixer also works great for connecting multiple computers to one set of amplified speakers and has many other possible uses.
Note: Soldering is required. If you don't know how, please search for Instructables on how to solder, as that's beyond the scope of this project.
Step 1: Materials
Qty 1 --- Altoids Tin. See Note 1.
Qty 5 --- 1/8" (3.5mm) Stereo input Jacks, Radio Shack part 274-249 or equiv.
Qty 8 --- 1k ohm Resistors, Radio Shack part 271-004 (5-pack) or equiv. see Note 2.
1 foot --- 22-30 gauge solid hookup wire, stripped bare
Note 1: A normal Altoids Tin can take 4 or more inputs, this example assumes 4 inputs. The Altoids Gum tins can fit 3 inputs comfortably. Adjust the number of jacks according to:
# of Jacks = 1+ # of inputs
and number of resistors by:
# of Resistors = 2 * # of inputs
Note 2: All resistors (2 per input channel) should have the same value, which can be anything between 1k and 10k ohms. Higher values result in a higher volume drop. Also, the resistor power rating can be 1/8 or higher. This circuit runs no power through it, so there's no need for bigger than 1/8 w, but use whatever you have handy. Bigger power rating = bigger size.