Kenneth Marut designed and developed the KELPIE, a portable monphonic and polyphonic digital synthesizer.
This incredible little synth is a great way for beginners to get started in the world of synthesizers. It only requires a power supply and a MIDI connection that can plug right into MIDI device such as a keyboard. It also include a 1/4″ mono output and 1/8″ stereo output. The workmanship and attention to detail on this project is impressive.
Kenneth rolled his own Teensy 3.2 and Audio Shield to create the KELPIE and used Teensy Audio Library. It includes USB input so that you can update the code.
More details on the project can be found on this project page.
This project is open source with hardware files and code available on GitHub.