Slightly OT: Mixing audio components into FM transmitter.
 

Apologies for the slight "forum drift" - but I bet you guys know the answers.

I drive a plethora of motor vehicles - rarely my own, often rented from Hertz or any one of numerous "company" vehicles.

I carry an XM satellite radio with me, which has a built in transmitter that allows me to hear the output through any standard car radio.

I also have a stand alone transmitter, with the standard mini jack (3.5 mil?) input. I can connect that to either my pda (for satellite navigation voice commands) or to an MP3 player for my own choice of toons.

A recent experiment to combine both the pda and the mp3 player, through a simple "2 into 1" cable, worked A BIT - but the audio quality of the mp3 was seriously degraded and, after a few minutes, the mp3 would cease to operate - but would reset and start again for a short while.

I'm told that the problem may be something to do with the impedance of the two units and that the pda "pulls down" the mp3 - which I believe but find confusing if they both work through the same cheapie headset.

Any thoughts as to how to mix the two signals - pda and mp3?

I know one answer would be to put my toons on the pda, start windows media player and then start the TomTom navigator - but that's the chicken way out.

They may work through the same headset, but that is one at a time, if the impedance of each unit is the same, when connected together (in parallel) the impedance will be approximately halved, and may cause problems.

What would be needed is a small impedance matching circuit, so that all should be ok.

Maybe MAPLIN or RS would have the bits and circuit details required.

What I meant was, if the parallel circuit opens up a box of Ohms Law and screws up one device, then surely it should screw up the other device at the same time.

That's the bit that throws me.

I was about to finish with the line "Bring back valves" - but I didn't understand those either, so no difference.

There are several other potential problems. Impedance and signal mismatch can do odd things to small solid state stuff. If the sound level drops a lot, or the sound quality goes for the chop, that's a good warning sign not to do that.

If you want to mix two different signals into a third device without such problems, all you need is a small active mixer device - effectively, a tiny stereo amplifier with two inputs matched to the "driver" devices, plenty of separation, and a stereo output at the right impedance for the following device.

You can probably find something suitable in one of the gadget shops.
I built one last year for a PPRuNer whose requirements were a little more specific, but I'm not going to volunteer to make any more - the bits cost about a quid, but the box to put it in, and the cables and plugs were absurdly expensive.