A piezo pickup as used on guitar can be thought of as a voltage source in series with a small value capacitor, as shown here. The capacitance of a typical piezo pickup is in the range of 500pF to 800pF.
Let’s add the input impedance (Zin) of the circuit to which the piezo is connected. It becomes apparent that the capacitance of the pickup into the following circuit will form a high pass filter. If the Zin impedance is too low, then the bottom end of the signal range will be lost. Example: with a 1M impedance and a 500pF pickup, the corner frequency is about 318Hz… the bass frequencies below that will be cut and the sound will be thin and weak.
It will take an 8M input impedance or greater to pass the full range of a guitar. Very few pedals have this kind of input and they will all suffer from low frequency attenuation.
The AMZ Mosfet Booster has a 10M input impedance and will interface nicely with a piezo pickup. It will have a full rich sound with no bass loss.
Another advantage is that the Mosfet Boost has variable gain and since a piezo can have a large output signal, the gain can be adjusted to eliminate distortion that would be present in boosters that always run in full gain mode.
Alternately, if you just want to buffer the piezo signal to interface with other pedals, the jfet buffer design that was shown previously can be modified slightly for the task. Here I have eliminated R1 and changed the values of R2 and R3 to 22M. The circuit can still be built on the stripboard layout I have posted and will be a good companion to any piezo device.
You can also build this circuit with my Multi-Purpose transistor pcb. Complete instructions are included with purchase.