This is variation of the Minidexed 8xDX7 (Yamaha-DX7-Synth) by Probonopd and Circle which runs the RPi as a if it is an MCU. (Circle is a C++ bare metal programming environment for the Raspberry Pi - i.e. it is not a RTOS.)
Two case variations were used. The first used the the snap-close 3d-case made available at the Github Minidexed site, and the second was based on a Minidexed case by RomixNL on Thingiverse. The STL files for the modified RomixNL case are in the STL folder here. The second rotary encoder present in the the second case will be used as a volume control and mute (all notes off) control.
In both cases two changes were made: (1) This uses the i2s + i2c Waveshare WM8960 DAC as discussed here and here, and sounds great. (2) A DIN 6N137 Midi-In was also added. A list of the hardware and build intructions are presented in detail on the Minidexed Github.
The Minidexed can be used as a USB Midi host i.e. it can play via a Midi USB keyboard plugged into the Minidexed Raspberry Pi USB ports, but needs something such as a ATMega32U4 (Arduino Pro Micro) or RPi Pico, to act as a a USB Midi device i.e. appear to a PC as a USB Midi device.
It should be noted that the 3v3 regulator on the Pi3B(+)/4B can easily supply the 1602LCD current (about 80mA) - refer to this discussion where a value of 800mA maximum for the 3v3 supply is suggested.
MT32-Pi with a Raspberry Pi 3A+ using a Waveshare WM8960 DAC hat and an SH1106 128x64 OLED display and a 6N137 MidiIn
The MT32-Pi Roland-MT32, similarly runs on Circle, and is also a baremetal MIDI synthesizer (for a Raspberry Pi 3 or above), based on Munt and FluidSynth, but the interface needs further development. It is currently only constructed on breadboard and used a Raspberry Pi 3A+, and the same WM8960 DAC but used an SH1106 OLED 128x64 display. The mt32-pi.cfg used is included here.
Note the warning when using i2c Backpacks with 5v LCD displays and 3v3 MCU's or Raspberry Pi's