Cheap USB MIDI cable: some self assembly may be required

USB-MIDI-Cable

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…

usb-midi-disassembled

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:

usb-midi-schematic

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:

usb-midi-breadboard-experiment

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

usb-midi-extra-components-added

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

MIDI-OX

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

  1. Pingback: The perils of cheap MIDI adapters

  2. Faced to the same problem with rockband keyboard and the same MIDI adapter, I only soldered a 330 Ohm resistor between IN- and IN+ , and it did the work.
    Your solution is more complete and safe, as I understand.

  3. I also had a brief encounter with these things. It worked with my hardware but would freeze any midi communication while playing a midifile (I think it was saturating midi bandwidth when it happened)

  4. Does the last image include all your fix done? or maybe did you forget another picture with the green and black soldered. thank you, i want to fix my midi cable because it doesnt work with the rockband keytar.

    • Yes, the last image does not show the wires soldered, just follow the instructions and you should be good to go:

      • Soldered the black DIN connector wire to IN- pad
      • Soldered the green DIN connector wire to IN+ pad
    • Yours looks like fun :) I think the best option for you is to take the Didier Arenzana advice in the first comment on this blog post. It’s difficult to tell which wires go where without rough schematic.

  5. Don’t wish to be a pain, but could you give me a quick run-down on which lead came from where and where it ended up – I have exactly the same piece of kit as yours, but the green lead is not used in mine, so I suspect an ad-hoc scheme.
    Thanks

    • The second pic from the top shows you how it was connected initially and then just follow the steps I wrote in the section starting with ” It was finally time to populate the missing components:”

      • So your green was not connected to anything? Mine is cut flush. That’s why I’m querying things, don’t want to make assumptions that might come back to bite me.

  6. i can’t get it to work with an arduino,i think that the RX is the problem, because the communication led doesn’t light up and my computer can’t regconizing it, do you have any thoughts about it??

  7. I have one of these interfaces that I have been trying to get working with a Behringer Ultradyne DSP9024 without much success, Will have a go at populating the missing components. Thing I’ve noticed is that when the ‘IN’ Din plug is in the ‘OUT’ socket of the DSP9024 then the power led on the midi usb cable is illuminated even when it’s not plugged into a usb port,

  8. On further investigation with mine I have found that in the DIN Connector only 2 wires are terminated and one of them is on terminal 2. At the pcb end of the cable the spare wire has already been trimmed back to the sheath. I’ve got the opto coupler and will also have to fit a new DIN plug on at least the ‘in’ cable. Most likely will also fit a new DIN plug onto the out cable as well just to make it look symetrical. Shouldn’t really have to do this, but stimulates learning about the MIDI interface. Maplin also sells the MIDI/ USB cable for about 20 quid— and you never know that one may still be lacking the opto coupler on the in cable.

  9. ok the modification now completed and the interface reassembled. So first thing now the power led doesn’t come on when the ‘in’ plug is connected to the behringer DSP 9024. and the ‘input’ data led comes on when I do a memory dump which I can also read on the midi-ox in screen as sys data… now making some progress. The Behringer DSP 9024 windows editor still can’t see the midi ports. Next train of investigation was to load the windows editor onto a pc running windows XP and fitted with an audio card that has a midi interface- still the windows editor can’t set up the midi port so stumped for the moment and will give it some thought. The instructions for the Behriger Ultradyne are not too good and there aren’t any for the windows editor. Will update if I make any progress- one thing though is that for £3.00 and a few components this compares very favourably with other midi- usb interfaces.

  10. hello. I’ve been buying these cheap usb devices and hacking them since they first hit e-bay back in the 1990′s. I’ve done a number of things with them, my favourite being an ultra cheap midi foot pedal using a cheap three button mouse (seven possible switch combinations). These midi usb devices used to have the optoisolater but for some reason (probably cost) they started leaving it off. The reason they can do that is simple. Almost all midi keyboards take the precautionary measure of including an opto-isolater in their design. It’s very rare that any midi device on the market would ship without one because not only is it part of the official midi spec, but it’s intended to prevent ground loops in complicated setups. If you can imagine a band doing an arena concert only to delay the performance because all the midi gear from different manufacturers is humming insanely due to ground loops. So almost never need the opto in the usb device.

  11. Thanks for the instructions. My lead looks the same as yours, so I followed your instructions, but nothing worked. Then I started experimenting and got it working with the Black & White leads on the IN+ & IN-.

    The keyboard used is a 49 key Commodore MK10 with battery power. Playing into an iPad 2.

  12. Hello, Thank you for the info.
    I design midi controllers and I always go out of the MIDI standards. One of those standards is that there is always an opto coupler at the input of a midi device.
    This Chinese crap does not follow these standards. Yet these MIDI USB cable work with eg Yamaha equipment, but apparently not with MIDI outs buffered with a 74HCT04.
    Or are the midi standards changed now?

    Regards Nico

  13. hey guys.. i bought two of these thinking the 1st was duff.. 2nd was exactly the same.

    works with :
    yamaha psr 330

    doesnt work:
    yamaha sy85
    yamaha pss-580
    roland pc-200 mk II

    grant – i would of thought something like the sy85 would have the optoisolater you talk about.. this is a strange one..

    il try and have a go at fixing but go knows what im doing hah.. i do abit of soldering on laptops etc but i cant get my head round this for some reason. im sure its pretty straight forward eh?!

    cheers

    • You can try this modification yourself but it does require a little nerve (and a good eye) as you have to trim and solder the optisolator into place and also getting the resistor pins bent just right and making sure the leads don’t touch any other components. then you have to cut the wires that go into the midi usb device and then using a pair of pliers you need to draw the whole cable through about 3/4 of an inch because you need to get the hidden green wire thats been trimmed at the factory. It needs to be long enough to reach the circuit board. You need the green and black wire and then trim the red down the way the green one was trimmed down. All in all, it’s an annoying fix as the wires are all over the place and it’s hard to get the circuit board positioned and god help you if you drag it out for ages and you strain the wires that come from the USB side and they break off. it can make you mad and frustrated if you’re not an electrical engineer (they’ve allready gone mad years ago so it’s easy for them). Also an old soldering iron with a blunt worn round tip will make it a pain if not impossible. Use good solder with rosin core (ideally very thin .6mm). But if you’re up for the challenge and if you have a maplin nearby ask them for optocoupler CY94. Clean the solder pads before you solder! Get them nice and bright. Solder the isolator the right way round! Don’t kill it with a lot of heat. Just a tiny bit of solder will do. Look at the pictures he provided to see which way it goes on the board because if you solder it backwards, getting it off the circuit board is harder than putting it on plus you’re likely to peel off the traces. And sadly even when done correctly it doesn’t always work properly. I’ve had it work “somewhat” at times. Midi notes go through the isolator but what comes out are the wrong notes with arbitrary midi channels (always something like, midi channel 1 “note on” and midi chanel 8 “note off”). Thats one for the bin. I salvage the midi and usb cables though. The midi standard was created to leave an aweful lot of options open and even if you do get it working properly it still might not work with all your keyboards. These things obviously skirt around the midi standard something terrible (to save money obviously) plus I don’t think the engineers who made it really knew what they were doing. And last but not least, if it doesn’t work out and you wreck it, don’t beat yourself up too badly It happens. At least you gave it a go. I think musicians in general follow the philosophy of, “it’s better to have loved and lost than to have never loved at all.” Good luck!

      • yes that is my way of thinking aswell :)

        i do have a maplin close by.. i will go in and ask..

        im not opposed to fiddly things.. and i do like messing around with stuff.. i just keep thinking im going to hook something up the wrong way (ground prob) and then electrocute myself haha

        il give it a go anyways and if it doest work il just invest in a slightly higher price one for now.. thanks mate :)

  14. Pingback: Problem beim MIDI umschalten

  15. Has anyone got this thing working with a Roland ED PC180a or a similar Roland controller of the era? No midi being received by the interface part of the cable, but Reaper and Kontakt are picking up the generic cable name.

    Please help!
    John

  16. I bought one off amazon… £3.00.
    So far it works note on off with iPad Air & apple CCK with old Radium 49 keyboard ( Maudio) & Axiom 49 2nd gen… Ok pitch bend works with some apps…and the controller seems to only send change messages from the middle position, but for the money…can’t complain, although I might have a go at the opto coupler addition..see if it smooth it out any.

  17. Thanks a lot for sharing this valuable information. I made the recommended modification using a 4N35 optocoupler (I had a spare one in my drawer, but it has 6 pins so it is a little more difficult to solder on the board) and now my MIDI keyboard works fine.

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