Nice. Fun fact: I authored this as part of my PhD.
jamesstaub It is essentially a bank of 80 magnetic coil pickups, so I'd like to design a circuit to digitally control the attenuation of each pickup individually.
What organ model is your generator from? It should be either 76 or 91 tonewheels as far as I know.
Unfortunately, the multiplexer is not going to work nicely with the analog outputs (it is designed to work with the inputs). Basically, you'd be driving each output high only for a fraction of the sampling period, so this would be useful for little more than driving LEDs or other application where a low sampling rate, low power, and a lot of high frequency noise is ok. For a bit more detail on what the multiplexer can do on the outputs, see here.
On the Hammond, the tonewheels are mixed together on the busbars passively via the resistive wires used within the manuals (which are in the order of a few tens of ohms, a different value per each contact), and then the drawbars attenuate the output of each busbar using a stepped transformer before going into a matching transformer whose output is plugged into the preamp. In my experiments, I tried to get a voltage output from each of the busbars (that is, after several tonewheels had been summed together), but I had no luck, as the voltage was very small. My understanding is that the signal is high-current but low-voltage from the generator through the wires to the contacts, busbars and drawbars, and the matching transformers after the drawbars and before the preamp is what increases its voltage to a level where it can actually be useful.
So, if my understanding is correct, unless you place one matching transformer on each tonewheel, you won't be able to mix them together with anything that is not some variable resistors with very small values (< 100 ohm, similar to those of the resistive wires).
