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?
I’d love to build this in as tiny an enclosure as possible — could this design run off of a pair of 3V CR2032 button cells? …what about running off of a SINGLE button cell? Would there be enough headroom? Any ideas about battery life in a design like this?
Thanks!
It will definitely run on a pair of CR2032 cells, and maybe on a single one, but headroom would suffer.
You could put it in a 1590A and include a power jack to run off an external power supply.
-Jack
What is the best way to use your circuit to blend both a Shadow and Fishman piezo pickup on my upright? I want to be able to control the volume of each piezo output. I have tried the above circuit and I good results on an Acoustic Image Clarus I but not on a Polytone Mini Brute III. Thanks.
I’m building a EUB, but its new to me, and i’m looking at putting a Piezo pickup on it. But I don’t know jack S~it about this side of things, Need Help. What do I need and how do I do it.
Thanks UprightCowboy
It is best to follow a piezo pickup with a buffer that conditions the signal to better interface with the following circuits while preserving the full tonal range.
-Jack
Would adding the buffer help tame some of the harshness and feedback problems from the piezo? I’ve added a Radio Shack piezo to my Cigar Box Guitar build, but the treble harshness and feedback are big problems.
I don’t think that it is going to help with feedback, but it should extend bass response, which will help balance the treble.
-Jack
Hi Jack, I recently breadboarded up this one and am experimenting with SWTC and am very pleased with the sound I’m getting with a 25mm piezo disc placed just behind the bridge inline with 5th and 6th strings. When recorded into my firewire (old motu box) DAW I,m getting very musical sounding recordings with a dash of room reverb applied. when I record simultaneous tracks with a RODE mic-very nice indeed.I notice though when I use the SWTC follower a lot of gain is lost-what would be a good recovery stage. Tried a LPB1 too much gain and will try a Tillman soon. your thoughts appreciated. Thanks for the great resource
Hey! Nice blog! I’ll dare to ask something absolutely out of place here…
I once tried to run a piezo element using a 555 timer at ultrasonic frequencies, and between the 555 output and the piezo, placed a 0.1 uF cap just because I’m used to placing caps between elements… It turns out the 555 got damaged. The same happened to a microcontroller I used for running the piezo.
The question is: Could the piezo-cap combination have worked as some kind of oscillator in such configuration so it may have produced a voltage high enough to damage the IC’s?
I’m sorry to ask something so out of place but no one seems to be able to answer such a question.
Thanks!!!
The output of a piezo pickup is usually flat in the audio region, but above the audio range there may be a very strong peak response. It is very possible that you were causing oscillation due to the peaked response.
My bbq grill has a piezo igniter that generates a nice big spark when you press the button on the front panel, and the spark has enough energy to ignite the gas coming from the burner. Also, there are cigarette lighters that have piezo igniters. Obviously, large voltage spikes are possible from piezo materials.
-Jack