For people interested in this, there are several open source projects which turn Raspberry Pis into a MIDI synthesizers:

MiniDexed: 8 DX7 Tone Generators

- https://github.com/probonopd/MiniDexed

- https://www.youtube.com/watch?v=Z3t94ceMHJo

MT32-Pi: Roland MT-32 emulator

- https://github.com/dwhinham/mt32-pi

- https://www.youtube.com/watch?v=vaSD_wbzJRw

Optionally, buy a PCM5102/PCM5122 or ES9018K2M I2S DAC to improve the output sound quality since the DAC built into the Raspberry Pi is not that great:

https://raspberrypi.stackexchange.com/questions/76188/how-to...

PCM5102 (~$7): https://www.aliexpress.com/wholesale?SearchText=PCM5102+DAC

ES9018K2M (~$12): https://www.aliexpress.com/wholesale?SearchText=es9018k2m+i2...