When using a piezo transducer on an instrument such as a guitar, or a mandolin, in order to get good tone, and output from the piezo transducer, a preamp is a good idea. This is because most piezo transducers are high impedance devices. Often times when a Piezo transducer is plugged directly into an amplifier, it usually will have low output, and sound very thin.
A piezo transducer will have an impedance of around 3 Megohms. This impedance is typically much higher than even the high impedance input of guitar amps, and mixer boards. additionally, being a high impedance transducer, the capacitance in the cable feeding the signal from the piezo transducer also greatly affects the sound in a negative way as well.
The solution to this problem is to use a preamp. Specifically one that uses a Jfet. Using a Jfet as opposed to an OpAmp, or a transistor is that the input impedance of a jfet’s theoretical input impedance is infinity. As such you can define your own input impedance of your Jfet preamp by simply selecting the value of the gate resistor.
For my Jfet preamp and piezo transducer, I chose to use a Radio Shack Piezo buzzer (Piezo Transducer 273-073) for the transducer that many others have used, and can be easily searched for. I found this link, which is an excellent explanation for using the Radio Shack buzzer as a piezo transducer. Specifically how to open up the buzzer without damaging the piezo element. However he doesn’t use a preamp for his piezo transducer.
Once you have your piezo element removed from the buzzer, I use the existing red and black wires already attached to the piezo transducer. However in order to keep noise and hum to a minimum, I twist them together as shown in the photo below:
Typically, the Jfet preamp will be installed into the instrument. So this allows me to keep the leads short and allow me to not use shielded cable. I wanted it this way because I didn’t want to introduce additional capacitance by using shielded cable.
Now as far as the Jfet preamplifier circuit, I chose to use the MPF-102 N channel Jfet. When I build my circuits, I like to use readily available components, which for me means Radio Shack parts. I have seen Jfet Preamp circuits using the J-201 Jfet, which has more gain, but I can’t obtain it locally.
I am very pleased with the results of this jfet preamplifier. It greatly improves the sound quality, as well as gives increased output of a piezo transuder.
My Jfet preamplifier circuit has a few additions than other Jfet preamplifier designs, such as a volume control, and the use ot a ¼” stereo jack for the on / off switch.
Here is my Jfet preamplifier using the MPF-102 Jfet:
The two zener diodes on the input are 5.1 volt Radio Shack part number 276-565. I also used a typical 1/4 inch stereo jack for the input, which also doubles as the on/off switch when a cable is plugged into it.
I am pleased with the results of building this piezo preamplifier circuit.
As far as an enclosure for the jfet preamplifier circuit, I just used an Altoids tin as shown in the photo below:
Here is the inside of the altoids tin. In order to keep the jfet preamplifier circuit from shorting out against the altoids tin, I cut a piece of vector board as big as would just barely fit inside of the Altoids tin. This provided insulation against the jfet preamplifier circuitry.