MidiAccess Build Instructions

Congratulations! You’re a few simple steps away from owning a great little synth module that will serve you for years and that you’ll have the satisfaction of having built yourself.

Building this project takes about half-an-hour if you’ve done some DIY electronics before, complete beginners should expect to take a little longer. The instructions assume that you can identify components, solder and do simple voltage and continuity tests with a multi-meter. There are lots of resources available to help you learn these skills – just hit Google or YouTube and you’ll soon be ready to start!

But before you do… we highly recommend that you

  • Take the time to read through this guide a couple of times
  • Check your parts against the Bill of Materials at the bottom of this page to ensure that you have everything that you need

Enjoy building your MidiAccess, we hope you make great music with it!


To complete this build you will need

  • Soldering iron
  • Solder
  • Wire cutters
  • A voltmeter or multimeter
  • 2mm allen key (all of our machine screws are hex-headed; you might need a screwdriver instead if you’re obtaining your own hardware)
  • Spanners, sockets or pliers for tightening M3 nuts

You’ll also need a clean, well-lit workspace with a heat-resistant surface to work on.

Build Instructions

Putting MidiAccess together is relatively straightforward; the parts count is fairly low and the parts are easy to identify.

Components go on both sides of the PCB – these instructions will highlight this as you go along but as a general rule – parts go on the side of the PCB that has a marking for them.

Solder parts into the marked positions; where a part has to go a specific way round there will be markings on the PCB to guide you, and a clear note in the instructions.

Typically we add parts to the board in height order, starting with the shortest parts (this makes the PCB easier to manage and handle during the build). We’ll only deviate from this approach occasionally where it’s sensible to do so.

D1 – small signal diode

_MG_7414_thumbWe’ll start on the “front” of the PCB. This is the side that doesn’t have the word “MidiAccess” printed on it. Locate the position for D1. Diodes have to be inserted the correct way round – so make sure that the black bar at one end of the diode matches the bar on the printed symbol. Set the diode into place, check it, then flip the board over and solder it. You can tidy the wires now with a pair of cutters or do them later (I tend to do mine as I go along as it makes the PCB easier to photograph).

R2 – 10K resistor

_MG_7415_thumbNext, locate R2 – it’s just across the board from the diode you just soldered. Resistors don’t care which way round you put them but you do have to be sure you’re using the correct value – either check it with a multi-meter or refer to this chart to be sure you have the correct resistor. Then put it in place, flip the board over, solder it, tidy if you want to.

R1 and R3 to R12 – 220R resistors

_MG_7416_thumbAll of the remaining resistors on the board are 220 Ohms; go ahead and put them all in place. You can give yourself extra art points for keeping the colour bands running the same way right across the PCB (but it doesn’t make any difference to the circuit if you don’t do this, so don’t sweat it).

C1 – 100nF ceramic capacitor

_MG_7417_thumbContinuing to populate the “front” of the PCB, locate the ceramic capacitor C1 (it’s adjacent to the largest cluster of resistors), position it and solder it into place. Again, there’s no “right way round” for this type of component.

With that done, we’re finished with this side of the PCB for now – the next few components go on the other side.

U1 and U2 – IC sockets

_MG_7419_thumbWe don’t solder Integrated Circuits (“chips”) directly to the board – the heat would damage them. Instead, we use these IC sockets. Although these don’t really have a “correct” way round, it’s sensible to line up the notch at one end of the component with the notch marked on the PCB silkscreen, as this helps us make sure we put the ICs in the right way later.

Holding these in place whilst turning the PCB over can be tricky – I usually locate the socket in the PCB, put a piece of card over it, then turn the PCB and card over at the same time – this holds the socket in place. Slip the card out from under before you start soldering, though.

Install one at a time and make sure that you don’t miss any pins when soldering.

J1 to J4 – 2-pin headers


Still on the back of the PCB and working with one at a time, locate the pin headers in their holes, flip the PCB over and solder into place.

You might find it useful to just solder one pin, then check that the header is straight, making any adjustments before soldering the remaining pin.

J5 – 16-pin box header

_MG_7422_thumbThe Eurorack power header has to be correctly oriented in order to function correctly and not damage your module or your power supply, so take great care to ensure that the gap in the header shroud matches the gap in the printed outline on the PCB silkscreen.

Position the header, then lay a piece of thin card over it to help you to hold it in place while you flip the board over. Slip the card out from under the header then solder one pin into place.

Before you continue to solder the remaining pins, flip back over and check once more that all of the headers are correctly oriented.

If you are happy that everything is laid out correctly, continue to solder all of the remaining pins.

The back of the PCB is now complete; it’s time to flip over and finish the front.

Fit the standoffs and front panel

_MG_7423_thumbThe final components will require that we fit the front panel first so that they align neatly and sit as flush as possible to the panel surface. The first thing we need to do, therefore, is put the mounting posts in place so that our front panel has something to attach to.

Attach a standoff post into each of the four M3 holes and secure it in place with a nyloc-style nut. Make sure they’re nice and snug but don’t overtighten – you don’t want to crush the PCB.

_MG_7424_thumbWith the standoffs in place, you can now fit the front panel. Align the holes in the panel with the holes in the standoffs, then attach the panel using four M3x6mm machine screws. You can tighten these up as we won’t be removing the front panel again, but once more be careful not to over-tighten.

JK1 to JK3 – DIN sockets

_MG_7425_thumbGently drop JK1 into place (you might need to give it a bit of a jiggle or manipulate the pins a little to get it to seat properly). When it’s in place, you should find that the metal rim of the jack sits less than a millimetre proud of the top panel.

We need to flip this over and keep it in place and keep it pushed right into the PCB – we’ve found that using a small coin (a UK penny is perfect) as a “shim” under the flipped-over jack is perfect. Solder one or two pins and then flip it back and check it’s still where you want it to be. De-solder and adjust if necessary, then flip back over and solder the rest of the pins.

Repeat this procedure for JK2 and JK3.

Once this is completed, you’re done with soldering! It’s now time to test your MidiAccess!


Before we connect the MidiAccess module to a power supply, we’ll make a quick test for short-circuits. You’ll need your multi-meter for this step.

With your meter set to detect continuity

Using your meter, ensure that there is NO continuity between the pins shown on J5.


Next, ensure that there is NO continuity between pins 5 and 8 of the U1 IC socket.


If your MidiAccess passes this test, it’s safe to connect it to your power supply and continue with the next set of tests. If it fails this test, take a good look over your PCB for shorts between soldered connections. Rectify and try again.

With your meter set to measure voltage

8-pin-IC-volt-testConnect your MidiAccess to your modular power supply by connecting the supply to the 16-pin IDC header J5.

Touch your meters BLACK probe to pin 5 of the U1 IC socket.

Touch the RED probe to pin 8. You should measure approximately +5V here (a small amount of drift is acceptable). +5V here means that your power is going to the right places and you’re good to finish up.

IMPORTANT NOTE: These tests are designed to ensure that MidiAccess

  • is safe to use
  • will not damage your power supply
  • will not damage your Eurorack modules

Skipping these tests, carrying them out improperly or ignoring the results can carry serious risks and consequences for you and your equipment. In proceeding to connect StringTheory to your equipment, you are assuming liability for any consequential loss or damage.

Finishing MidiAccess

U1 and U2 – ICs

_MG_7426_thumbDisconnect from the power supply and insert the ICs U1 and U2 into their sockets. These have to be properly aligned – this is where making sure the sockets were installed correctly pays off.

The U2 IC will have a notch in one end. Match this to the notch in the socket, then gently but firmly insert the IC into place. Note that you might need to narrow the splay of the legs a little with some gentle finger pressure first.

The U1 IC will have a solid printed dot in the corner that holds pin 1. Align this with the top left corner of the socket, then gently but firmly insert the IC into place. Again, you might need to adjust the legs of the IC a little first.

That’s it! Your MidiAccess is ready to connect up and use; take a look at the owner’s manual to see what you can do with it next!

Bill of Materials

In addition to your PCB and panel, you’ll need parts. The full list of parts for the project is given below, with Mouser part numbers to aid you in identifying compatible parts. Although we’ve used all Mouser parts in the example build above, you may be able to obtain cheaper compatible parts by shopping around.


Reference Value Description Qty Mouser Part Notes
C1 0.1uF / 100nF Multilayer Ceramic Capacitor, 50V 1 81-RDEF51H104Z0K1H3B
D1 1N4148 Small signal diode 1 512-1N4148
J1, J2, J3, J4 2-pin, male IDC header 4 538-87891-0206
J5 16-way male IDC box header, straight 1 Mouser – 710-61201621621
JK1, JK2, JK3 5-pin, female DIN socket, PCB mount 3 490-SD-50BV
R1, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12 220R Metal film resistor, 0.25W, 1% 11 603-MFR-25FTE52-220R
R2 10K Metal film resistor, 0.25W, 1% 1 603-MFR-25FTE52-10K
U1 6N137 Opto-isolator 1 859-6N137
U1 DIP8 8-pin IC socket 1 575-199308
U2 74HC14N Hex Schmitt Trigger 1 595-SN74HC14N
U2 DIP14 14-pin IC socket 1 575-199314
M3, 14mm Standoffs 4 855-R30-3001402
M3 x 6mm Machine screws 4 Use eBay or local hardware store
M3 “Nyloc” nuts 4 Use eBay or local hardware store