NES:"Stereo" Audio Mod

While the console as designed offers mono sound, it is possible to bypass the mixer and get true 2-channel sound (although not properly called, spatially related stereo) out of it.

This is possible because of an implementation detail in the console's sound synthetizer: the two square wave oscillators are tied to one output (AUX 0, called AUX A on Famicom), while all other three digital channels generate a separate signal (AUX 1/B).

Limitations

• As deducible from the above schematic, the plain mod described hereafter will not adequately include sound originating from the microphone or the NES expansion port, and none at all from a FC game card. This is a point in favor of the mixback type mod, however...
• If the mixing resistors are not removed (which is a requirement for mixback mods, as well to continue using the regular mono output), channel separation will be reduced and further noise may be fed back in.
  • It is of course possible to replace the resistive mixer with an active one, as done by certain third party kits (currently not documented in this article).

Plain mod

1. The CPU sound output is completely disconnected from the mixing/buffering components. (Optional step: see above.)
2. An inline capacitor (1 microfarad, negative towards console) is added to each channel, for protection and minimal low-pass filtering (8 kΩ at 20 Hz).
3. The desired output connector is added.

Mixback mod

1. The CPU sound outputs are tapped, without disconnecting them from the PCB.
2. Filter capacitors and output jack(s) are added, as above.
3. Variable resistor(s), separate or dual and preferably logarithmic, are also connected at one extreme to the outputs.
4. The existing mono signal is brought to the center tap of the potentiometers or trimmers:
   • On a NES, the points marked AA or BB on the schematic are not only equivalent, but actually the same.
   • On a Famicom, AA produces cleaner sound, but doesn't include expansion audio, in which case BB has to be used.