A while ago, I modified both of my electric guitars with piezo disc microphones to get a mix of electric and acoustic sounds. The first circuit I made had no controls, other than a mix knob for the rest of the guitar. It ended up being way to loud and harsh compared to the normal pickups, and recently it broke, so I decided to make a better one.

I used a 2n7000 for the mosfet since the level was too far below the cutoff of the jfets I had, but the circuit would probably work with minimal changes for any fet. The gate resistor should probably be somewhere above 1M, otherwize it will sound harh. The fet and gate resistor are soldered as close as possible on the piezo disc to minimize noise and effects of capacitance, especially from the long lead wires normally installed on these discs.

On a separate perforated prototyping board, I soldered a volume and tone control. I used 3x3 pin trimmers instead of full potentiometers, since I just use these to balance the tone with my guitar's pickups, then use the guitar's original tone and volume controls for both. The trimmer to 9v is 5k, but make sure not to turn it all the way to 0 ohms, or it will burn out the fet. You could add a small resistor in series if you want to adjust it a lot. The 10u is a multi layer ceramic capacitor, but could probably be an electrolytic to save parts. The tone trimer is 200 ohms, and forms a low pass from about 1.7khz and up. If you ground the other side, it would form a high pass at about 80hz, if you don't want as much low string noise. The only value that really matters a lot is the first trimmer, since it sets the gain, so experiment before soldering to match the volume and find a good tone.

In my guitar, I glued the piezo to the inner wall of the electronics cavity and installed a new full size potentiometer as a mix knob. All the way left is electric, right is piezo acoustic. The pot I chose also includes a switch that closes when turned right, which I connected to the battery. This means that when the mix is all the way set to the original pickups, the battery won't be drained. Looking at the potentiometer from the top with the pins facing me, I soldered the guitar's pickup switch output to the left terminal, piezo preamp to the right, and mixed output to the middle. This mixed output then goes to the guitar's volume and tone like normal. Make sure to connect the circuit's ground to either the back of a potentiometer or the output jack's ground!