Select portion of Drum Master Schematic
Below is a selected portion of the schematic. This comprises the filter and amplification circuit for four of the 32 analog channels.
The sensors are connected via short jumper cables from the RJ45 breakout headers to P3. J1 is used to short out unused sensors in case of operation in a high electrical noise environment (probably not needed for most people). The schottky diodes D5 - D9 are used to rectify the AC signal from the piezo (see the design page for some piezo theory and tests I performed), and charge capacitors C1 - C4. The voltage on these capacitors are amplified by the op amp U2, with a gain determined by the ratio between RV1 - RV4 and RR4. If you use the values shown here (200k variable resistor / 20k resitor), this provides up to 10x gain on the input signal.
The resistor array RR2 is used to drain any accumulated negative voltage, which would reduce the sensitivity of the circuit over time. Resistor array RR6 is used to drain the capacitors quickly and drop the signal to zero within about 10ms of the initial strike, to avoid double triggers.
Note that to keep the board design as simple and small as possible I used bussed resistor arrays wherever possible.
Entire Drum Master Schematic
Below is the entire Drum Master schematic. You can see the above filter / amplification circuit repeated eight times, as well as the multiplexers which combine the channels into one for sensing on the Arduino.
Towards the top right of the schematic you can see the trigger circuit - this uses an op amp to compare the voltage between the incoming signal and the voltage drop across a schottky diode (about 0.3 volts). If the signal is higher than this, the trigger gives a logic high; otherwise, a logic low. This trigger is connected to digital inputs on the Arduino to quickly tell if there is a signal to read, as reaing a digital input is much faster than reading an analog input, since the analog input has to be run through an ADC to find the actual analog value.
Other places of interest include the digital inputs and associated pull up resistor (ground the line at the pad to send a digital signal), at the bottom left; also note the RJ45 connectors and breakout headers, along the right side.