MIDI synthesizer built from two YM3812s for 18-voice mono or 9-voice stereo. More info: https://tholin.dev/ym3812d/
The gerber file for the synthesizer can be found here and you should be able to use this with all the major board houses. Just make sure they correctly recognize it as a 4-layer PCB. The dimensions of the PCB are low enough to where it should be cheap despite it being 4-layers (8€ on JLCPCB).
The YM3812s and their DACs are the most annoying parts to obtain. They can be easily found on eBay and AliExpress by simply using "YM3812" and "Y3014" keywords, but the condition of the parts can vary. The AliExpress listings I bought from are here and here.
The nanoCH32V003 mini dev board you can maybe even find at your favorite maker shop, but also on AliExpress. If there is an option to buy the board plus a "WCHLinkE" you may use that (recommended), but I will also show you how to use an Arduino Uno to flash the synthesizer firmware later on (though this requires a Linux install or WSL).
All the other integrated circuits can be found at various electronics retailers:
6N139 (or 6N138): TME - Mouser - LCSC
SN74LV1T34DBVRG4: TME - Mouser - LCSC
MX25L1005 (or MX25L2006 or 25Q32 or any spiflash that supports the 25xx command set and is made by Winbond or Macronix): TME - LCSC - I also recommend looking for "MX25L1005" on AliExpress, you can get them really cheap there.
74HCT164 (or 74LS164, NOT 74HC164): TME - Mouser - LCSC
The BOM for assembling the board is available here. Otherwise, there is little special to keep in mind, just solder the SMD components first, and use pin sockets for the nanoCH. It needs to be removed from the synthesizer to be re-flashed.
The 5-pin header at the bottom of the board is where MIDI input is. You will need to wire in a broken-out MIDI connector here. Only two need to actually be wired up. The 2nd pin from the left wires to the Current Sink on the MIDI connector and the 4th pin from the left to Current Source.
This step changes depending on if you’re running Windows or Linux, and if you have bought the WCHLinkE. In any case, you will need the minichlink utility.
If you are running Linux, it is part of a repo here and can be built using the makefile there. Also read the readme there for instructions on installing the required udev rules.
If you are running windows, download the pre-built executable files here and here (you need both files).
A pre-built binary of the synthesizer’s firmware is here. To use a WCH-LinkE to flash it, wire up the single-wire IO pin of the programmer (labeled "SWDIO" or similar) to the SWIO pin of the nanoCH (the middle of the three-pin header on the back). To then use it to flash the firmware file, the command-line minichlink(.exe) -w synth.bin flash -b should work on either OS, and you are already done.
If you did not buy the WCHLinkE, but have an Arduino Uno laying around, you can use Ardulink. It is available on its own repo here. Follow the instructions there to set it up. Note that it requires a Linux install or WSL to build, but can be used from either OS. Additionally, the repo mentions connecting the nanoCH’s power to an Arduino pin, but I’ve found that to be really unstable and it still works fine with the nanoCH powered continuously. You only need to wire up SWIO to the Arduino (its the middle pin of the three-pin header on top).
On Windows: minichlink.exe -c COMx -w synth.bin flash -b where "COMx" needs to be substituted with whichever COM port your Arduino uses.
On Linux: minichlink -c /dev/ttyACMx -w synth.bin flash -b where "ttyACMx" needs to be substituted with whichever tty port your Arduino uses.
A set of patches for all standard MIDI instruments must be uploaded to the synthesizer at least once. A public-domain file containing patches is included in this repo: GENMIDI.op2. You can edit this to your liking using the OPL3 Bank Editor (scroll down and use one of the "Fresh dev builds", the latest GitHub release is years old and has issues).
A custom tool I made can then be used to send this data to the synthesizer over the MIDI port. PatchUploader.jar (requires Java 11 or higher). If your system does not already associate .jar files with Java, you may need to run it through a command line with java -jar PatchUploader.jar
The UI should be straight forward. After selecting the the .op2 file and the MIDI port of the synthesizer, click the upload button and you should see activity on the synthesizer in the form of a flashing blue LED on the nanoCH. It should flash multiple times and the synth should then reset and play a jingle. If nothing happens, check if the MIDI port is wired up correctly and if any software or hardware in your setup is potentially blocking the sysex messages used by the uploader.
The uploader software releases the MIDI port when it is done, meaning you can keep it open and use it multiple times by just clicking the button again. The file is also re-loaded each time and does not need to be selected again.
The synthesizer can operate in three distinct modes.
Mono: both sound chips are combined into a single output. This allows their voices to be used together, 18 in total. This is the most basic and default mode.
Stereo: every note is played on both chips simultaniously, but its volume on either chip is adjusted according to the MIDI channel’s stereo pan, giving the illusion of stereo audio. This, however, reduced the amount of usable voices back to 9.
Independent: each chip acts as a separate synthesizer. Each responds only to events on a specific range of MIDI channels and their outputs can be individually obtained on the same output as the stereo audio.
MIDI sysex messages are used to switch between these. The codes are as follows:
Switch to Mono: 0xF0 0x7E 0x56 0x72 0xF7
Switch to Stereo: 0xF0 0x7E 0x56 0x76 0xF7
Switch to Independent: 0xF0 0x7E 0x56 0x75 0xF7
By default, the synthesizer listens to all channels in Mono and Stereo modes and in Independent mode, channels 1 - 8 will go to the first chip and 9 - 16 to the second. An alternative firmware image is available here where the synthesizer only listens to channel 15 in Mono and Stereo and in Independent, channel 14 goes to the first and channel 15 to the second chip.
To fully customize this behavior, you will need to build the firmware yourself, after editing synth.c and changing the values in the accepted_channels array for Mono and Stereo and accepted_channels_first and accepted_channels_second for Independent. These are right at the beginning of the file and are just lists of MIDI channels (warning: 0-indexed).
If you want to build the firmware from source, it has do be done on a native Linux install, or in Windows Subsystem for Linux. You will need to install the RISC-V GNU Toolchain first, though this step varies between distros. I recommend cloning the repo and building it from source, as that is what worked for me.
Then, clone this repo and execute the script to set up the environment: sh clone_subrepos_and_patch.sh
Then, you can enter the RV_Firmware directory and use the makefile there. Simply make will both build the firmware and then flash the output using minichlink.