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:
 * 5) *On a NES, the points marked AA or BB on the schematic are not only equivalent, but actually the same.
 * 6) *On a Famicom, AA produces cleaner sound, but doesn't include expansion audio, in which case BB has to be used.