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Building a MIDI Thru Box Part 1

Pete Brown - 25 May 2011

I'm in the mood to build something. What I really want to build is a big old analog synth, but I haven't worked my skills up to that yet. This kit from Doepfer (which this beautiful and awesome but somewhat weak-sounding synth is based on) is definitely an option to get me started, and is now sitting in my "to do" pile here. (Incidentally, if I started with that kit and wanted to make a fat synth, I'd want to use two of them plus add some additional filters and effects. I'd also want to build it in a way that would allow me to ditch those in the future and build something even more complex in the same case/framework. It'll let me do some nice woodworking at the same time.) (BTW, looking for more DIY synth inspiration, check out this DIY digital - there are three videos to watch: video 1, video 2, video 3)

I've been looking for a relatively easy electronics project, and just happened to have a need with my synthesizers. I have several synths in a rack, and depending on my mood, I only have a couple of them powered up at any given time. However, because they're all legacy MIDI rather than USB or OSC, I have them daisy chained together. That means that if I want to use the last synth in the chain (my Virus), all the other ones (M-VS1, Morpheus, JP-8080) have to be powered on in order for the MIDI signal to make it through. Not only is that a waste of electricity, it's also personally annoying and adds to a very small (I can't really notice it) amount of lag in the MIDI signal.

I previously built a MIDI interface for my Netduino, so I'm familiar with the general design. However, I stumbled across a circuit design for a MIDI through box that does exactly what I want (I'd prefer a couple more midi outs, but I can definitely live with 5 as I have 4 MIDI synths in the rack. If I wanted more outs, I would add another inverter and discrete components as necessary. I may even do that with the final version of this board.) To that design, I added a power LED, and once I can figure out how, I want to add a MIDI activity LED.

Parts List

Here are the parts used to make the board, along with links to Mouser to purchase them.


  • Rack-mount case (I'll cover this below)
  • Standoffs
  • Screws

For the power supply, I wussed out and used and external brick power supply. I wasn't sure that I could build a 1U power supply and keep it electrically quiet, so as not to interfere with the audio equipment in the same rack. I also didn't want to deal with power supplies on this first real project. I'll deal with building power supplies in a future project. Note that you can get a cheap 9v power supply from your local store. It only needs to handle around 100mA +/-.


I assumed the schematic was correct, but figured I'd prototype it in any case. This would also allow me to experiment with the power LED and activity LEDs.


The image above shows the working MIDI Thru box circuit. I have one input and five outputs. I tested all five outputs and they worked on first go (amazing!). The power LED on the bottom seems to be running a little hot, so I'll probably pop a larger resister in there (currently it's a 220 ohm). I don't need a super bright power LED in any case. I may even try a smaller LED. The hex inverter is the one at the top. The opto-isolator is at the bottom, facing the opposite direction. Slightly covered by the power cord is the 5v regulator. There's no power switch in this prototype, but there will be in the final version.

Rack Case

I want the final version of this to sit in my equipment rack. Loose units banging around my desk annoy me, and get in the way of all my projects. I tolerate them if there are no other choices (my new DSI Tetr4, for example), but try and avoid loose modules whenever possible. The 1U rack case I bought for this project is from par-metal.com and is the 10-19082B (B is black anodized front and painted black body). The case has aluminum front and back, which means I can drill it with normal tools, and steel otherwise. It's a 1U rack case 8" deep. From their description:

Front panel is 0.125" aluminum alloy (5052-H32) with brushed finish in black or clear anodize. Other panels are 0.05" thick steel, painted in black or gray. Rear panel is 0.06" thick aluminum alloy (5052-H32) painted. Aluminum handles and hardware are included.

All screw holes are installed with 6-32 steel press-nuts to increase strength and reliability.

Standard vent holes are punched on top. Two rows of round holes are punched on side panels for mounting channels or for use as vent holes. Add suffix-x to indicate your choice of color or finish:

-B black anodize front & painted black
-G clear anodize front & painted gray
-E painted in beige


I have to say that I'm impressed with these guys. I ordered the case late Tuesday night, paid by PayPal on Wednesday afternoon, and received the case via UPS on Thursday afternoon.

What arrived is a flat pack version of the case, complete with hardware and those little handles. The front panel and the little handles are all wrapped in plastic, so they're unlikely to get scratched. The pieces were all separated by the packing paper in any case.


For a more complex layout, like a synthesizer full of pots, buttons, and encoders, I'd probably use something like Front Panel Express to do the panel work (or build my own larger CNC machine. muah hahahaa!). However, in this case, I simply have a few holes to drill in the rear of the unit and one rectangular cutout for the power switch in the front, so no worries. The steel pieces all have nuts attached so you don't have to think about fitting your own, or dealing with shallow stripped threaded holes. Perfect!

The aluminum front panel was nicely anodized (which will make it slightly harder on my drills, but it's worth it) and the rest is painted black. I'm not sure if the rear aluminum is anodized or painted. The paint was nice and even, both inside and out, without annoying drips, runs, or missed spots. There were no sharp edges. Including shipping this case came to $57.50, which I think was an excellent deal.

Here's a photo of it assembled, but with the plastic still on the front panel. Note that if you want to use the little handles (I haven't decided, but likely won't) you need to orient the sides so the threaded end faces the back rather than the front.


Other than the front being aluminum instead of punched steel, and this case being actually painted on the inside, there is little to no quality difference between this case and the ones that hold my other synthesizer rack modules. That's good. You'll want to use aluminum fronts and backs rather than steel, if you intend to cut the openings using normal tools.

Next Steps

Now that I have a working prototype on a breadboard, I've started designing the circuit board in FreePCB. This is my first time designing a board, so I fully expect to botch it somehow. However, I'm doing this as much for the PCB experience as for the actual utility of the project. The next blog post will go through how I designed the board and, assuming I follow through with getting it fabricated, I'll post a follow up with the results of that as well (it can take several weeks to get a board depending on how much you're willing to spend).

posted by Pete Brown on Wednesday, May 25, 2011
filed under:      

6 comments for “Building a MIDI Thru Box Part 1”

  1. Mario Vernarisays:
    Hi Pete!
    I would suggest to use the 74(AC)14 (as the original circuit suggests) instead of the 7404, because it embeds a Schmitt-trigger input stage that rejects much better the noise. If you have a long tripping cable from MIDI out, a good noise rejection would be surely a good choice.
    MIDI stuffs exciting me a lot: I am waiting for the complete article!
  2. Colinsays:
    Hi Pete,
    Any idea about MIDI support in Silverlight, is there any libraries out there you know that can play (or even receive) MIDI in realtime? (As opposed to just playing a file)
  3. Petesays:

    That's great info. Looks like I'll have another Mouser order before I finalize this build. Thanks :)


    No MIDI support in Silverlight yet. However, it may be possible with Silverlight 5 Trusted applications, as they'll support p/invoke when SL5 is released. The MIDI libraries are just standard DLLs, so should be relatively straight-forward.

  4. Mark Lindellsays:
    Pete, it looks great!

    I know you started your post with "I'm in the mood to build something" but you could get a lot of functionality by just buying a legacy piece of hardware. I own the JL Cooper midi patch bay. Here's someone selling one on Craigslist for $25 http://wichita.craigslist.org/msg/2395722211.html

    This single space rack mount unit has 64 memory slots to program routing, merge and accept midi patch changes messages to configure it on the fly. I've used it for years.

    So you spent $48 on the case alone! Yikes. I guess the experience is worth the money spent, huh?

  5. Petesays:

    All good points. I do buy a fair bit of used equipment, but this one was specifically chosen for the project. I know it'll cost me way more than used equipment - especially when I go and have a board made :)

    I'm doing this simple project to gear me up for a far more complex project. I'm going through most of the same steps, but at a smaller scale.

  6. Svensays:
    I tried my self as well and I think you are right. That sucks big time. Because having MIDI out is a must and it would be splliaecy useful with the smart instruments with an external DAW. I have noticed or at least I do not know how, that the smart instruments do not offer chords different to the regular ones you find on the major and minor modes. So no 7ths, 6th or anything beyond that. That sucks too. For what I am seeing now it looks a product good with ideas but half cooked.

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