There's a number of stereo mods floating around for the Atari 8-bit computers. Most involve a custom PCB that lets you add another POKEY to the system. It's an increasingly popular modification, and virtually all software made after the early 1990s (demos, newer games, etc.) supports it. However, there doesn't seem to be much consensus on how to hook it up. Most installation instructions involve drilling out more holes in the back of the machine for RCA audio jacks, but this post outlines a better (or at least simpler) way - adding more pins to the existing A/V connector.
Simply desolder the existing DIN-5 jack and put in a DIN-8:
(pictures coming soon)
The extra pins on the connector can be clipped before they short to the PCB, and they can then be wired straight to the output of the stereo board. This is the proposed new pinout:
Old A/V cables should still work (in mono of course). New cables should be easy to make. You don't have to hook pin 8 to +5V, but I figured it might be useful for certain projects.
Unfortunately, this causes a problem. The power jack is a DIN-7 and thus it's easy to accidentally plug the power supply into the A/V jack, which would be Bad. The solution: change the power connector (and possibly get a better power supply while you're at it). Here's the proposed new pinout:
This uses a DIN-5c connector, which is kind of difficult to find until you realize that it's just DIN-6 without the center pin. So if you can't find a PCB-mount DIN-5c jack or a DIN-5c plug, just get the DIN-6 version and pull the center pin out. No changes to the board are necessary; the new connector will drop right in.
This new connector standard is also great for modding RF-only systems like the Atari 400, 5200, and 2600. It minimizes the number of holes you need to drill, and it allows for stereo support on the 2600.