In 2011, we set out to ‘hack’ MIDI guitar pickup systems and enable access to the audio outputs of the individual strings of a guitar. These MIDI guitar pickup systems generally sense string activity with piezo transducers, which can be used as accurate microphones if amplified correctly.
A: Resistors R1 and R2 divide the voltage in half. This sets the operating point of the JFET (C). This allows the maximum voltage swing before clipping can occur. These resistors also set the input impedance to a high value required (approx. 1.1 Megohm)
B: Resistor R3 and Capacitor C1 form a low pass filter at 194kHz. To limit the chance of picking up radio interference whilse just above the ‘studio standard’ of 192kHz.
C: This is the JFET. It carries out the impedance conversion duties.
D: Resistor R4 limits the amount of current that is drawn from the battery.
E: Capacitor C2 blocks the battery supply voltage from passing into the output.
F: Resistor Rz is a pseudo-balancing resistor. Its function is to present a resistance to ground that matches the output impedance of the main circuit. This allows better interfacing with balanced equipment that may follow.