A couple of people have asked me how I bypass the Twin Famicom audio circuit and properly mix expansion audio into the NES RGB sound circuitry.
This isn't a tutorial, but a very crude picture coupled with a brief explanation that should thoroughly explain the concept and process.
Simply tap into pins 1 and 2 of the 2A03 and terminate to the A and B inputs of the NESRGB board.
Cartridge expansion audio is also cake: Tap into pin 46 (expansion audio out) of the cartridge connector and terminate it to the NESRGB (depending on PCB revision) with a 100K resistor.
This takes care of everything, except for the FDS RP2C33 audio channel. You
can not wire it directly into pin 46 as you did with the cartridge audio.
FDS audio needs to be disconnected from pin 46 while in cartridge mode.
The connection from pin 54 of the 2C33 (FDS audio out) to pin 46 of the cartridge connector needs to be active
only when the Twin Famicom is in FDS mode.
I mapped out the existing cartridge/disk system switch, and wired the audio-out from the 2C33 to the switch, and from the switch to pin 46. When you slide the switch to FDS mode, the connection will be live, when you flip the switch to cartridge mode, the connection is disconnected, thereby not interfering with cartridge expansion audio.
EDIT: I entirely missed this on the picture diagram. In order to keep the 2A03 audio-channels from premixing and outputting on pin 46 (expansion audio out), either entirely remove or lift a leg on the resistor cited in the picture above. Normally the audio from the 2A03 is pre-amped and fed into pin 45 and mixed with expansion audio, outputting on pin 46. The NESRGB will be handling all of the sound mixing, it's important to do this.
