Here's is a step-by-step guide to bypassing the internal electronics in your Yamaha CP-70 piano, allowing it to be a passive instrument like an electric guitar.
Benefits of the bypass:
1. Sonic improvement. The stock preamp, EQ and FX are tone killers
2. Negates need for the rare and expensive 2-prong XLR power cord
3. It's an easy and non-destructive mod. Only very basic tinkering (and some soldering) are required
1. First, locate the piano's pickups. To do so you have to remove both the top covers so all the piano internals are visible. The pickups are little shiny metal capsules about 1/2"W x 1/4"D. Each string has its own and they are about arms-length from the keyboard, toward the back of the piano. You'll notice that all the pickups are strung together with one wire that runs off to the left-hand side of the piano and into the circuit boards. This wire is usually white, and it carries the piano's pure audio signal. You may have to undo some twist-ties to free it up. That is the wire you want to interrupt, and ultimately solder to the positive terminal of a 1/4" phone jack. (If you're not sure which terminal is positive, just look closely at the jack and you'll see that one of the terminals is connected to the ring of the jack and the other is connected to the extruded tip—this tip is the positive terminal.)
2. Put the new ¼” phone jack in place of one of the original ones, that way you don't have to modify the chassis of the piano. You can save the original jack if you like, but use a new metal jack for this mod rather than the original plastic one. Take the wire from the pickups that you traced earlier and cut it such that it runs directly to where your new jack will be, allowing some slack for safety. Strip it, and solder it to the positive terminal. (clip it or bend on and test it first) You may also decide to solder both ends of the wire you just cut to the jack. That way the signal still runs to the circuit board and you can still use the internal electronics if you like.
3. Now you'll need to ground the other terminal of the new ¼” jack to the chassis of the piano with a piece of hookup wire. You could also steal a ground from one of the original outputs, but I seem to remember that it was easier to just make a fresh one.
4. And that's it! You're done! Your piano should sound much bigger!
Mono. These pianos are mono instruments, the stereo output is created artificially with the tremolo effect. This can be done much more effectively with a high-quality outboard FX unit.
Will an external power supply be needed to power the pickups?
No, you won't need a power-supply at all. Electronically the piano operates like an electric guitar so it's totally passive and ready to be plugged into an amplifier or PA via a direct box.