I am planning on putting some piezo saddles in my fretless bass. All blending should be done onboard, meaning a regular mono output. Obviously this will require an active buffer for the piezos but I'd like to keep the magnetic pickups fully passive if possible. What kind of buffer should I get (or build) to match the piezo impedance to the magnetics in order to allow for passive (master) blending? The whole circuit should be as simple as possible. I don't need automatic stereo switching, EQ or anything like that. The power source should be 9V DC.