Using an Arduino as an HID, Evan Kale turned a “gently used” analog mixer into a computer interface.
Older audio equipment may not have the interfaces that you need to make totally electronic music, but they can be very well-built, so are perhaps worth salvaging. In the video below, Kale salvages potentiometers from an old mixer, then hooks them up to a Pro Micro. This allows the Arduino to take these 12 inputs, and output them as a USB MIDI signal.
Along the way, Kale points out a few very important hacking tricks, including that the library may have a printer ready for you to use, and that analog slider pots many times are logarithmic (or close to it) and need to be calibrated. Also, around 5:25 he introduces viewers to analog multiplexers which can give you eight analog inputs at the cost of three digital and one analog pin.
You can check out more of Kale’s Arduino-based hacks on his YouTube channel here!