JFETs have a very high input impedance and are less fragile and noisy than MOSFET, so that's what we'll use. MOSFETs have an extremely high input impedance, but tend to be fragile (static electricity can destroy them) and also can be noisy in audio applications. Bipolar transistors don't have a high enough input impedance. There are three types of transistors to consider, bipolar (BJT), MOSFET, and JFET. It also provides more current to drive signals on the cable to the amp, which can also improve the highs.Īmplifiers use transistors (or tubes) to amplify or buffer a signal. The role of the piezo contact mic preamp isn't really to amplify the signal (the output voltage of the piezo is plenty high), rather the goal is to provide a buffer with a very high input impedance of 10M ohms or so, which is more than 10 times or so greater than the input impedance of a typical guitar amp, so that more of the bass gets through. In order to reduce the cutoff frequency so that more bass tones get through, we need to increase the input impedance that the piezo is driving. For typical values of Cpiezo and Ramp, the cutoff frequency is well into the audible playing range of a guitar. Together, Cpiezo and Ramp form a high-pass filter that cuts out low frequencies. The input of an amplifier acts like a resistor to ground (Ramp), known as its input impedance.
PIEZO PREAMPS SERIES
Because of the series capacitance (Cpiezo), virtually no DC current flows through the piezo. This voltage may be high, but the current is very small.
To understand how it works, we'll first take a look at the problem we're trying to solve, and then build up a solution in steps.Ī piezo element operates like a voltage source in series with a capacitor-when the piezo crystals vibrate, they produce a voltage.
The schematic above is the complete design of the preamp circuit.
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