Piezoelectric Drum Pad Amplifier

Charge amplifier (?) configuration for the DTXPLORER drum module. The input is a piezoelectric drum pad sensor, which oscillates and then decays when hit. This circuit represents almost all of the analog signal conditioning that goes on before the signal is digitized and sent to the CPU to be parsed. Everything to the right of the middle diode is pretty clear-cut: it's a LPF then a buffer. But on the left hand side, there's so much I don't understand. Why the three diodes in series? Is it to add a positive offset to the output voltage? Why the two diodes across the opamp inputs? Why the series resistor at the first opamp output? N.B. the resistor value on the right was lowered from 390k to 3.9k so the exponential decay on the output is visible. With the original resistor value the time constant is too large to be viewed at this simulation speed. Original schematic: https://imgur.com/jUAykUx

published 6 years ago