Drum Master Schematic
Below is a portion of the analog schematic of Drum Master rev 2. (Click on it to view the entire schematic at full size in a new tab). It may look a bit confusing, but it is actually quite simple; below we discuss each section in more depth.
For the discussion, open the link to the schematic PDF in another window, so you can compare back and forth.
Along the left side of the schematic you can see the 12 analog circuits with peak detection and amplification. The ICs along the left side (U2 - U7) are the op amps (an amplifier chip); each chip has 4 op amp circuits. Two op amp circuits are required for each analog channel; the first is used (in conjunction with a diode and a capacitor) as a peak detection circuit, while the second is used (in conjunction with a potentiometer) to amplify the output and provide a low impedance source for later components.
The MCP46x1 chips (U10 - U15) are digital potentiometers. These let us calibrate the output so that different pads can all provide the same output for strikes of the same velocity. Since they are digital, we can easily adjust the values in software without needing to open the case and adjust trim pots with a screw driver.
At the right side, starting at the top, we show the power supply. This is a simple affair with a poly fuse, a MOSFET to protect against reverse polarity (plugging in the power backwards), and a couple of linear regulators to provide 5v and 3.3v.
Next down we have the display. This is a 20x4 character HD44780 display, which you can buy on eBay for about $10.
Next down is the connection for the rotary encoder + pushbutton, which (in conjunction with the display) provides the user interface for the drum set.
Next down is the Teensy 3.1 with audio board. Labels at the end of green wires are connectors to the audio board (which may also be used for other things, e.g. the SPI pins). Labels connected directly to the IC are not used on the audio board.
Finally, we have the SPI flash chip, which is used to hold the digital audio samples.