Hi, I got Synthcart for the Atari and today attempted to do the stereo audio out mod.
Well, guess what, a PAL Atari has a different TIA ("Stella") chip. The 2 channels of audio come out of pin 13, mixed together. Pin 12 is called PAL-S.
On an NTSC Atari pins 12 and 13 carry a channel of audio each. (And so it can be modified with 2 outputs)
Was pretty bummed out, thought I'd save you the frustration of finding out the hard way like I did.
Now you know!
Oh by the way, the Atari & Synthcart sound pretty incredible on big speakers.
(http://img169.imageshack.us/img169/2109/tiapalqi6.png)
Here's the pinout :-)