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This page describes a method by which you can modify Nintendo's 8-bit consoles, the NES, Famicom, or any compatible system, to produce stereo sound.

• How do I do that?

Well, in a nutshell, you do it by opening up your Nintendo system and tapping some wires from the CPU.

• Is that hard to do?

It depends on your expertise with electronics. If you know how to wrap wires, use a screwdriver, and identify basic electronic components, then you can probably do this with ease.
However, I will make it clear right now that any damage that you cause to your NES, yourself, your brothers or sisters, your house, your city block, or your country as a result of inexperience or ineptitude on your part shall be your problem and yours alone. Having said that, I don't see how that could really happen. But…

• So, before I do this, I want to know how it sounds. Does it work as you say?

Okay, well, I should say that as the NES is a rather old system, it was not intended to be in stereo. Thus, NES games do not make use of stereo effects or anything like that. Here's basically how you can get stereo sound:
The NES has five sound channels: two pulse wave, one triangle wave, one white noise, and one sample channel. These sounds are generated directly from within the NES' CPU, a modified 6502 processor known as the 2A03. From two pins on the 2A03 come these sound channels, the two pulse channels on one, and the rest on the other. They then get mixed together and go to the audio jack and RF modulator of the NES. Simply by taking the two channels before they get mixed into one, you can get stereo sound. Hooked up to an amplifier, and listened to on headphones, the effect really does sound pretty neat (although this is highly subjective). This can also be a way to give your NES' audio output a MEGA BASS BOOST!! All is explained below.

Open up your NES system using a Phillips head screwdriver (I'm using an oldstyle NES system, but they should all be the same; this hack worked with my Famicom clone also.) If you have an NES top loader or AV Famicom, you will need to use a 4.5 mm gamebit screwdrive to open the console. Remove all the metal shielding from the outside and underside of the circuit board. You don't really need all that shielding anyway. Pull the board out from the plastic case and turn it over to reveal all the chips 'n stuff. Locate the CPU chip. It is one of the largest chips on the board and should have “U6 CPU” written above it. Now, the first two pins on the lower left side of the 2A03 (if you imagine the CPU as being an x-y plane with the writing on the chip being right-side-up) are the two audio channels. These pins are the ones that are connected to resistors R3 and R4. Connect two small shielded wires to each of these pins, making sure that they don't touch each other or anything else on the board. What I did was wrap the bare end of the wire around the pins of the CPU, then solder them down. Do not tape down your wires unless you want them to come loose and wreak havoc inside your system.

The leftmost pin from which you tapped the audio carries a (weak) audio signal from the NES' audio channels 0 and 1. These are both from the pulse wave oscillators, and are most often used as the lead instruments in NES music. I'd generally make that the left channel. The pin to the right of the leftmost pin carries the audio signal from channels 2, 3, and 4. These are from the triangle wave oscillator, and the noise channel. These are almost always used as an accompanying melody and for percussion. Any digitized samples will also come out of this pin. I have that mapped to the right channel. Ground can be tapped from a multitude of points on the motherboard, including the outer edges of the motherboard.

After this, you can do pretty much whatever you want to the audio. I'll tell you what I did. First, I connected an electrolytic capacitor, rated at 1 uF (1 microfarad) rated for 50V, to each of the two new sound lines in order to protect the NES hardware from power surges or shorts. The positive lead of the capacitor goes to the audio pins, and the negative lead goes to the audio out. I highly recommend that you do this too! Next, I bought some gold-plated RCA jacks so I could connect proper RCA cables to the outside of the NES. I drilled two small holes into the back of my NES unit just to the right of the big RF module. Through that I connected the jacks to the outside of the unit, and then the wires to the jacks. The ground lugs of both RCA jacks are wired to ground on the motherboard. Don't forget this step, as you will get a loud buzz mixed in with your audio. That sounded good, but I still wanted to mix the audio so that it wasn't so separated between the two speakers. I bought two 47K ohm micropotentiometers for audio mixing. I tapped regular mono audio from the inductor labeled “FC1” on the NES circuit board, as it was the final component before the audio went into the RF modulator. This component is also present on the top loader, AV Famicom, and some revisions of the original Famicom. I used this mono signal as the line with which to mix each of the stereo channels, because not only did it contain the sound information of the opposite channel for which to mix, but also it contained some sound information for the corresponding channel; this boosts the volume of the (relatively low) stereo channels. Because they are potentiometers, you can now adjust them to provide the degree of mixing that you desire. Take a look at the wiring diagram that I included. It's not a real symbolic diagram, but just an illustration.

Here's an even better trick. You don't want to mix the channels all the way, obviously, because you just did a bunch of work to get them separated. Instead, slowly lower the resistance of each of the pots while you play a game with suitably high bass (I would recommend Solstice or Silius ). As you lower the resistance, you should notice that at a point, the bass tones of the mono channel get mixed in before the rest of the tones. With a little bit of tweaking, you can get much-amplified bass sounds from your modified NES. When I play Solstice on my modded NES for instance, I can make the walls of my room vibrate. Ain't that great?

From: http://www.zyx.com/chrisc/nesstereo.html

FIXED NEW LINK: http://www.disgruntleddesigner.com/chrisc/nesstereo.html