Cheap USB MIDI cable: some self assembly may be required


I had recently bought a Rock Band 3 Wii keyboard which has come down in price to around £10 with an intention to connect it to a cheap Chinese USB to MIDI interface cable for around £3 pictured above to do some MIDI experiments on my computer. Videos online demonstrated that connecting the IN connector to the keyboard and USB cable to the PC was all that was needed to use it, but I just couldn’t get it working. The Rock Band keyboard detected a MIDI cable and switched to MIDI mode, PC properly detected USB MIDI device, but there were no messages in the MIDI-OX application. That’s when I decided to open up the plastic casing to find a few surprises…


First thing I noticed, were the missing components on the board pictured in the red square. After further investigations, it turned out that the wiring was done completely different to the MIDI specifications. Here is a very rough schematic of how it’s done and how it should have been done:


The MIDI specification requires an opto-isolator to completely isolate MIDI connected circuits. The USB cable did not have these parts populated and used a common ground for signal transfers. Rock Band keyboard probably was not expecting this. No wonder it did not work!

So, I went to Maplin and purchased a low-current opto-coupler SFH618-2 for £1.39. A quick assembly on the breadboard and a bit of soldering for ground, 5V and data lines to get to this:


And it worked! It was finally time to populate the missing components:

  • Got rid of the R11
  • Soldered opto-coupler in U1 and it was a perfect fit for the pinout. I only had to shorten the pins to mount it as close to the board as possible.
  • Soldered a 220 Ohm resistor in R5
  • Removed the red DIN connector wire completely from GND connection and trimmed it
  • Soldered the black DIN connector wire to IN- pad
  • Soldered the green DIN connector wire to IN+ pad


The case fit back perfectly, though required some glue to hold in place and MIDI-OX finally came to life!


This USB MIDI cable might be a cheap and quick way to add USB enabled MIDI for Arduino, but I’m really wondering if it would have worked with any MIDI device at all due to incorrect wiring. You get what you pay for: some self assembly may be required.

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85 thoughts on “Cheap USB MIDI cable: some self assembly may be required”

  1. There are a lot of model of this cheap usb 2 midi converter, I bought nearly every models and try, only one model can work, finally I decide to make my own converter using the ch345chip, no more missing component, should work fine.

  2. Hi! Did you try it with Apple devices? I tried to connect it to iPad via lightning cable, but I got a message that says ” inte false is not supported “

  3. I was pulling my hair out, opened my device like yours and found like yours no opto coupler fitted, fitted like you on breadboard all working. i used a 6N137 because that’s all i had in stock.

    1. Hi, VERY USEFULL tech solution!
      Also my cheap USB2MIDI cable (from China) has the same problem.
      But also here I have only the opto-isolator 6N137 that has got 8 pins.
      Can anybody help me understand if the following connections are correct?
      Looking to the image in the post:
      Upper Left –> pin2 [6N137] pin7 –> Upper Right
      Lower Left –> pin3 [6N137] pin6 –> Lower Right
      In fact the optoisolator 6N137 has 8 pin (4 per side).
      Please help me…

  4. Hi, awesome article.
    I have pretty much the same device, but I notice that mine is missing another component. I think it might be a capacitor, the component is usualy found on the back side of the circuit board and is marked C3. Can anyone please possibly tell me what cap I’m supposed to use there? Thanks in advance.

      1. Thanks a lot for checking… I figured it out on my own after a while of testing. The device is working as it should now, but it still can’t do the things I bought it for. I’ll buy one of those fancy “Uno” midi to usb cables some day when I have cash, appearently it will work…
        Thanks anyway for the help.

  5. Thank you! Have just ordered the Maplin component and will have a bash at this. Thank you very much for posting both the maplin link and the photos – very useful. I seem to spend my life soldering tiny things these days!!

  6. To add to my last comment, yes, you need to bridge R11. If you don’t you get all sorts of note-off messages which are on different octaves and sometimes different notes / channels. I think one person mentioned this. Anyway, if you have this issue, just bridge R11 as well as doing the other stuff.

  7. Hi,

    I recently bought two of these adapter cables, both labelled differently, but using the same PCB with no opto-isolator fitted.

    Before I go to the trouble of ‘upgrading’ them, can anyone tell me why the none of the USB ports on my new HP desktop don’t fall back gracefully to work on these and many other USB 2 devices which I own?

    Even the two ports which are labelled USB2 suffer the same problem.

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