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.
What FET do you recomend for Q1?
The FET is not critical. Use a 2N5457, MPF102 or other n-fet transistors that you might have.
Just built this one into a little box wired as a plug-in-power unit using an MPF102. It really worked wonders with my piezo equipped Breedlove C250. There is more presence and a more refined bass response. Not so boomy as without the buffer. Thanks for posting this little circuit!
can i use a 2n7000 FET GORDON
You could but that is a mosfet, and you would be better off with a jfet like a 2N5457, MPF102, 2N3819, or J201.
-Jack
I just picked up a Mosfet booster board from you store. I intend to make this to buffer a piezo bridge pickup from an upright bass. I have a few questions about this application:
I intend on blending this with a fretboard piezo (I am going to build a simple jfet buffer for that as it is only for fingerboard clicks) so I do need to contol the output volume / level so can I:
1) Use both the gain pot and the output volume control or should I use just the output volume control or use the gain trim pot but route the output of the buffer to a blend pot?
2) I would like to put a phase invert switch in the signal I assume I would do this before the inputs into the board?