Here's a tutorial for stereo modding your NES.
Ben already made an excellent guide for isolating the 2 audio channels, so refer to it before reading this similar guide.
First, find the 3 resistors in a row just below the expansion port. Their values should be 20K, 20K, and 12K, respectively. Don't pay any attention to the first one (it just leads to the expansion port), but we are interested in the second and third resistors. Desolder and pull up the left lead of the 20K resistor (the second one), and remove the 12K resistor (but keep it!). The left leads of both resistors were connected to the 2 audio channels, which you just isolated.
The audio channels sound very distinct when they are not mixed. I suggest mixing them with 2 1K potentiometers (or whatever value you have should work). After doing that, connect the wiper of one of the pots to the left lead of the 20K resistor that you pulled up. Connect the wiper of the other pot to the 12K resistor you removed (and hopefully still have). Now, one of the audio channels is reconnected to the onboard amplifier; you are done with that one. It is now time to amplify the other audio channel!
The best way to do this is just to replicate how the NES amplifies a mono audio signal. Kevin Horton made an excellent schematic showing how this works. Here is a simpler schematic that I made for the amp we will make.
You can salvage all of these parts from another (broken) NES. The 39µh inductor is the little green thing that looks like a resistor. This circuit uses a 74HCU04 hex inverter, which is not hard to find (although it is not available at Radioshack). Anyway, connect the right lead of the 12K resistor to "Audio In" and you're done.
The final step is to connect your audio outputs to your screen/rca jacks/audio amp/whatever you want to connect it to. Just connect to the 2 spots marked "Audio Out" in the schematic and the diagram below, and don't forget your common ground.
<-- pinout for the solder points on the NES
If this doesn't work, I don't know what to tell you. It worked for me on my first try. While you have your NES on, you can adjust your potentiometers for optimum audio balance.
Credit for this mod goes to Ben Heckendorn for his guide, Kevin Horton for his schematic, and Vomitsaw for his useful thread. I apologize for the weird-looking schematic; I am still learning to use Eagle, so I just made it in Illustrator.
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