Originally Posted by jpeaslee
Did you ever figure this out?
Looks like he has no progress! Allow me to give you a few hints. First off, the circuit as shown will NOT work. The input impedance is WAY too low even for magnetic pups. (and gain way too high) Note that bass requires an input impedance of about 10 megohms to get the low notes! A guitar doesn't have as great a problem because it doesn't go as low.
I've built PASSIVE piezo/magnetic guitars and they get by. They are JUST on the verge of not working with normal amp input impedance. But this is impossible on a bass.
The PROPER way to do this is to use a Fishman device called a "power chip". It's actually a smallish circuit board with a volume control (for the piezo) that allows you to have both piezo and magnetic pickups and even select them. I have one in one of my guitars. They have figured it ALL out for you! In fact if you plug in a stereo cable it has stereo output. With a mono cable it mixes them! Too cool.
Downside is NOT cheap! ($80 US) plus NOT shielded. Hence if putting in a wooden guitar you have to build a copper shield around it. In a shielded bass cavity, it will be fine, though. I highly recommend it for what you are doing!
But if you REALLY want cheap, you can mod the circuit you show. You don't seem to know too much so I'd advise you to get help on this. Basically what you have to do is change the power supply from one battery to TWO 9 volt batteries so you can remove the phantom ground going into the + input. You use THAT input for your Piezo. (10 megs to ground). You then cut the gain back on the other channel (input R to - about 50-100k)
and use that for the magnetic input. You could experiment, but note that designing preamps like this is tricky and you have to watch out for oscillations etc. You may be in over your head. I'd recommend the Power Chip. It's one battery and all worked out for you.