Another MIDI Splitter V2

Some time ago, I presented a MIDI splitter project that has been working reliably for several years. Unfortunately, it was originally designed using components I happened to have at home. When I tried to build another unit, I had difficulty sourcing some of the parts at a reasonable cost. Moreover, the DIN5 connectors were wired using cables, which made the assembly process quite inconvenient.

For these reasons, and based on that experience, I decided to design a new version that fits entirely on a single PCB.

Design goals:

1. 1 input, 8 outputs
2. USB-C power supply
3. Clean, minimalist enclosure
4. Easy assembly

The complete project, along with documentation, is also available on my GitHub repository, while the STL files can be found on GrabCAD.

For this project, I used components that are readily available from online stores. Since the splitter is intended to distribute a signal, it primarily requires DIN-5 connectors for input/output, as well as a power connector—in this case, USB Type-C. The enclosure will be 3D-printed.

Electronic components:

1. Resistors / capacitors
2. USB-C connector
3. DIN-5 connectors
4. H11L1 optocoupler
5. 74HC14 inverters (Schmitt-trigger)
6. LED (optional)

Mechanical components:

1. M3 screws
2. M3 metal standoffs (12 mm)

Tools:

1. 3D printer
2. Soldering iron
3. PCB fabrication capability (or ordering PCBs online)

MIDI is a very interesting interface. From a hardware perspective, one of its key differences is that it uses opto-isolation on the input. This isolates connected devices and prevents so-called “ground loops.” Data transmission is achieved using current rather than voltage, which helps reduce susceptibility to noise. Additionally, each output is protected by a resistor, so even a short circuit during operation should not damage the driver.

MIDI input and output schematics can be found on many websites; I used a SparkFun article as a reference. It provides a lot of valuable information about the interface and its hardware implementation. With that covered, it’s time to move on to the schematic.

The design is divided into three sections: INPUT, OUTPUTS, and POWER.

1. POWER – 5V supply, decoupling capacitors, and configuration of the USB CC lines
2. INPUT – the MIDI input stage, including an H11L1 optocoupler that isolates the signal and converts it into a voltage level. This signal then drives both the LED and the buffer stages for the outputs
3. OUTPUTS – eight MIDI outputs driven by inverters

As you can see, the entire schematic fits on a single A4 page. After assigning components to the PCB, the next step is to start designing the board layout.

For this project, I mainly used surface-mount components to save space. However, for those who prefer traditional soldering, I also designed a through-hole version—although I haven’t tested it yet. If you manage to test it, please let me know.

I aimed to create a compact device that could accommodate all outputs within minimal dimensions. Unfortunately, the size of the connectors was the limiting factor. As a result, all output connectors are placed along the longer edges of the PCB, while the shorter edges host the MIDI input on one side and the power input on the other. The board measures 65 × 98 mm and includes mounting holes spaced 10 mm inward from the edges. KiCad project is avaliable on GitHub.

To save time, I placed an order for the PCB and components while working in parallel on the enclosure design.

Designing the enclosure was the most time-consuming part, but also a very engaging one. Even with a finished PCB design and a 3D model, I had to go through multiple iterations to properly fit the assembled board and mounting features together. As a result, I ended up with a pile of test enclosures that are no longer useful.

The enclosure is divided into two parts. They overlap at a defined interface, and metal standoffs are screwed into the top part to hold the entire assembly together. The first prototypes were printed “blind,” without the electronics, just to verify the fit—hence the collection of test cases.

The initial version of the enclosure was split along the bottom, which caused the USB connector to be divided into two parts. At the time, I thought it was the only way to slightly extend the connector outward. Another challenge was the DIN connectors, which I wanted to partially cover. This required carving rounded cutouts in the bottom part to match the connector shapes. It was a lot of work, but I eventually reached a version that looked quite decent—one of the photos shows this early design.

Later, I realized that splitting the enclosure along the DIN connectors was a much better approach. This allowed for clean circular openings and resulted in a more aesthetic design.

Another issue was how to mount the PCB inside the enclosure. Initially, I considered using heat-set threaded inserts, but I wasn’t fully confident in their durability. Instead, I chose to glue threaded standoffs directly to the enclosure, which should provide a sufficiently strong connection.

Finally, I included space for LEDs. The enclosure supports two indicators: PWR and MIDI IN. The PWR LED can be either THT or SMD, depending on preference. The design also allows for either a through-hole opening or dual-color printing. As shown in the photos, the light diffuser works very well, softening the LED output and preventing harsh light. The enclosure was printed using ASA, while the LED diffuser was made from white PLA.

The final version of the enclosure is available on my GrabCAD profile and GitHub.

I started soldering with the most challenging components. First the USB connector, then the resistors and capacitors, followed by the integrated circuits, and finally the DIN-5 connectors. Initially, I soldered only two connectors to test the splitter’s functionality. After verifying that everything worked correctly, I proceeded with the remaining connectors and could observe the final result. Once soldering was complete, I checked all connections to make sure nothing had been missed.

Assembly process:

1. Screw the threaded standoffs into the top part of the enclosure. It is recommended to add a small amount of epoxy adhesive, as the threads do not always fit perfectly.
2. Place the PCB onto the metal standoffs and cover it with the bottom part of the enclosure.
3. Fasten everything together using screws.

The splitter works—I managed to assemble five units. In my opinion, they look quite clean and aesthetically pleasing, especially the LED section. I have already tested them with my instruments, and everything works as expected. Since I have several devices powered from a 5V rail, using USB-C for power turned out to be a good choice.

An important part of this project was also creating complete documentation. I believe this goal has been achieved, as all project files are available on GitHub.

I hope you found this project interesting. I encourage you to build your own unit or create your own version :)