Slightly OT: stereo and mono audio splitting

[Castellen]

Hi Ali,
It really depends on what audio level your external RF modulator requires and it's input impedance.  Do you have the specs on it?

The Amiga's audio output at the RCA connectors is approx 1V RMS.  You can make yourself a passive combiner (a simple resistor network) but the combiner's output will be down by 6dB, i.e. about 0.5V RMS.
If that's what you need then fine.  If you need less, you can design the combiner network to easily give you the required attenuation.

If you need a higher signal (greater than 0.5V RMS) then you need an active combiner, which you can easily make with an op-amp.  It can run from the +12/-12V available at the RGB or serial port, so you can generate up to 8V RMS of audio.

The A1200 already contains an internal audio combiner for output to the video section.  You could make it available externally by linking it to the audio out pin on the serial port (pin 11 from memory).  That relies on your modulator handling quite a low level signal and it needs to have a high input impedance (>10k ohm) for that to work.

If you get stuck, give me the modulator's audio input specs and I'll scribble you up a quick schematic and do the value calculations.

[Castellen]

Hi again, is the A3000 still going OK?

Since you're looking at using other audio sources, it's probably cheapest and easiest just to buy yourself a simple audio mixer.  That'll do everything you want, all input levels are easily catered for and stereo to mono is no problem.
That also takes care of your connectors, a box to house it in, a power supply to drive it.

Dick Smith used to do something ideal for about $30, but they no longer list it.  Keep an eye out on TradeMe as there's bound to be something turn up on there sooner or later.
There's always Ebay if you like giving money to dodgy weirdos who take pleasure in ripping you off.