It's not everyday you come across a synthesizer that sounds as good as this as looks as cool as this and is a DIY project.
Otem's Teensy Polyphonic Synth was built for teensy 3.2 and the teensy audio adapter. As Hackaday note, "The Teensy audio library supports several output devices including several built-in options and external boards like the audio adapter used here. The library does CD-quality sound, supports polyphonic playback, recording, synthesis, mixing, and more."
You can find the code and more details for Teensy Polyphonic Synth on Otem's Github here. And what continues to impress me about this project is the possibilities with Teensy. Back in mid 2016 Liam Lacey demonstrated how easy it is to build a DIY MIDI controller using this platform.
More to the point here is how good this synthesizer sounds. The way it looks is down to cool, retro-looking knobs all 3D printed.
But, Otem, the maker of the Teensy Polyphonic Synth hasn't stopped there. Here's his latest synth, Mr Touchy, also a Teensy polyphonic synthesizer. This time using Nintendo DS touch screens, Teensy 3.2, Teensy Audio Adapter and some Neopixels: