MidiThru is an active MIDI splitter useful for distributing MIDI clock and powering MIDI devices through MIDI connector.

There are two variants available: original 1 to 5 and MidiThru x2 which is two 1 to 5 MidiThru combined.

MidiThru_v02_built (small)


Both are easy to build, inexpensive and very useful MIDI gadgets. A must have for any non trivial MIDI setup.

63 thoughts on “MidiThru

  1. Pingback: MidiThru | Futuredraht

  2. Hey Peter,
    I love this kit, but do you have a solution for a midi merge situation? It appears that this is a 1 to 5 out but I would love to construct a 4, or 5 into 1 output format. I can’t seem to locate any projects online, maybe you know of a source for a DIY kit? thanks!

  3. This is a nice and compact solution, good work! Will it be difficult for a beginner to build this with just 3 MIDI outs? Will I just have to drop 2 of the MIDI out connectors and their corresponding resistors and inv Hex Schmitt Triggers? Or would I have to change the supply voltage or something?

  4. Thanks for your instant reply! Excuse my slurry wording, sure the triggers are part of the IC which I just have to leave unconnected…

  5. Hey, would this work for note data? I’d like to utilize more midi synths with out buying equally more midi keyboards (I already have one nice one I paid to much for to begin with). I’m already planning on making one for my drum machines, but I’d love to make 2 if it works for note data as well!

  6. …to anyone considering ordering this board and building the midi thru — the components are easy to find and inexpensive, its easy to build, and the finished unit works flawlessly. Highly recommended!

    • Agreed! This was my first build where I just bought the PCB and sourced all the parts myself so I was a bit out of my comfort zone but I had absolutely no problem getting this up and running in about an hour tonight! Thanks Peter! Excellent work!

      • Any plans to produce this case? I’d love to build this but I’m terrible with cases and don’t have access to a laser cutter for the acrylic. The cases on your other kits look great! Thanks!

      • At this point i don’t have a case for MidiThru. Even if i’ll come up with one, it’s not going to be a full box case. The plan is to use a “sandwich” case with only top and bottom plates.

  7. Do you think the existing regulator and the first trigger could drive 20 outs of I just continued to attach triggers in parallel? I only ask because you might have the network values worked out already, save me some time and brain power 🙂

    • I did not try this, however it should be possible, because connecting 4 inputs to 74LS output is not a problem, and 78L05 has enough current capability to power 5 ICs.

  8. This midithru will be my first attempt at building a real through hole project, I’ve practiced my soldering technique on a handful of beginner soldering kits. Any suggestion on which components to install first? I plan in building Midirex after this .

  9. Hey,
    Just wondering, why would you put SN74LS14N (which is an inverter) to revert midi in and then revert it again afterward?
    I don’t really get this part logicwise, eventhough I guess it’s about electronical matter that I don’t really understand. Could we only use an AOP in all midi input ?

      • I guess AOP is the french word for operational amplifier. I do understand why do you need the schmitt trigger, I was thinking that it was only an inverter.
        Anyway thanks for the schematic, I think I will have the use for it very soon :p!

  10. Hi,
    I’m interested into building a thrubox kit but I would better have it 1->8 minimum.
    Ive read I could use 2x hexinverter To do so.
    Could I somehow make it with your pcb ?
    Do you have any schematics or pcb files to make it happen ?


  11. Hi!

    I’m trying to build this midi thru schematic as a 1×10 thru 19″ rack, using an extra schmitt trigger to repeat the output ports. I’ve collected all the parts and built everything up on a breadboard to test, but I can’t seem to get it to work. So far no midi has come from the input to the output, and the LED has been turning on and off irregularly, also changing in light intensity.
    I’ve checked all the connections multiple times, they should be fine. My electronics knowledge is very basic, I try to learn a lot along the way. What might be the issue here?

    I left out the voltage regulator and the three capacitors and two diodes within that part of the circuit, and used a generic USB power supply, a 5V 1A iPhone charger. Could that be the problem?

    Also, I’ve left the remaining in and out ports on the inverter unconnected, just to test one output first. But that should be fine right?

    Also, what parts break easily, and how can I test them? I’ve already replaced the optocoupler because I thought that was the problem, but it did not make any difference.

    I hope you can help me out!

    • It’s not possible to guess the problem you are having. You need to get an oscilloscope and and probe the signal from the beginning to the end and see where it breaks.

      Fist check the optocoupler output and verify that the short negative pulses are present there when MIDI events are received. Then follow to Smidtt triggers inputs and outputs.

      You can leave power supply parts out, however, it’s highly recommended to have a 100n across trigger IC volteage supply pins.

  12. I recreated the project on breadboard using the schematics provided. It didn’t work untill i looked up the 74LS14N data sheet and found out the IC needs vcc to pin 14 and gnd on pin 7. When i added this, everything suddenly worked like a charm 😀 Is that part missing from the schematics, or am i missing something?

    Thanks for supplying the schematics for free by the way! My next step is transfering everything to prototyping board, 3D printing an enclosure, lasercutting a 19inch rack plate, and having everything mounted in the studio rack as part of a large midi patchbay. Loving this project!

    • Modern schematic capture CAD tools typically provide IC power connection automatically. This is why you don’t see it explicitly specified on the schematics.

  13. Hi,
    What is the “negligible” propagation delay about ? Are we talking about something like 1 ms or something like nothing like in analog world ?
    If I use the 2x version and connect one of the first row output in the second row input, how much delay would be added ?

  14. I’d bought the pcbs for this a long time ago and I thought I’d source the parts and build it up. I may have a different version or something as there’s one part I’m not sure about. The unit is not powering so I’m thinking it could be the power switch but it’s only a 4 pin rectangle on the pcb. Any info would be great or a build doc?


  15. Just wanted to say thank you, this is a great piece of gear, and it was a very straightforward build, even for a beginner like me!

  16. Just a note in case anyone else is having the same problem as me:

    The 100n across power pins (7 & 14) of the schmitt trigger IC is indeed critical. I hand wired this circuit and had problems with the module spitting out random/unpredictable MIDI data. After some (too much) trouble shooting I realized the obvious: I forgot the 100n cap. Works flawlessly now, awesome utility.

    Thanks for sharing this schematic!

  17. For those looking at powering this with a modular PSU:
    The circuit draws about 4mA on the +5V rail using the components spec’d in the BOM. Tested in-circuit under normal MIDI through usage.

  18. On the topic of an appropriate power supply, this one meets all the needs and has the added benefit of interchangeable blades for international compatibility.
    Part info: CUI SMI12-9-VW-P5R
    Digikey part #: 102-SMI12-9-VW-P5R-ND
    Link: https://www.digikey.com/en/products/detail/cui-inc/SMI12-9-VW-P5R/12720807

    Slightly cheaper North America only adapter is as follows
    Part info: CUI SWI15-9-N-P5
    Digikey part #: 102-4671-ND
    Link: https://www.digikey.com/en/products/detail/cui-inc/SWI15-9-N-P5/7399207

  19. 74LS14N or 74HC14N?
    Schematic says 74LS14N the bom says 74HC14N. Does this matter?
    Sorry if this is something obvious, i usualy only put toghether kits.

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