StringTheory 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 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 StringTheory, we hope you make great music with it!


To complete this build you will need

  • Soldering iron
  • Solder
  • Wire cutters
  • A voltmeter or multimeter
  • 8mm and 10mm sockets, spanners or pliers

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

Build Instructions

Putting StringTheory 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 and D2 – small signal diodes

stbg1_thumbWe’ll start on the “front” of the PCB. This is the side that doesn’t have the words “STRING THEORY” printed on it. Locate the positions for D1 and D2 (you’ll find they’re right next to each other). 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 diodes into place, check them, then flip the board over and solder them into place. 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).

R1 to R5 – resistors

stbg2_thumbAll of our resistors are in a group adjacent to the diodes that you’ve just soldered, on the same side of the PCB. Refer to the Bill of Materials to see which values go where (if you’ve bought one of our kits, you’ll have found that all of the parts are clearly labelled). Make sure that you put the correct values in the right places – check them against an identification chart or with a meter if you’re unsure. Solder all of the resistors into place once you’re certain you know what goes where. It doesn’t matter which way round they go as long as they’re in the right location.

X1 – 16MHz crystal

stbg3_thumbStill staying on the front of the PCB, locate and position the 16MHz crystal, then solder it into place. You’ll notice on the back of the PCB that another component will go back-to-back with this later, so trim the leads down as short as you can to ensure that everything fits nicely when to get to that step. This component can be oriented any way round.

C3 and C4 – 100nF ceramic capacitors

stbg4_thumbContinuing to populate the “front” of the PCB, locate the two ceramic capacitors C3 & C4 (they’re near your diodes), position them and solder them into place. Again, there’s no “right way round” for this type of component.

C1 and C2 – 22pF ceramic disc capacitors

stbg5_thumbYou’ll find one of these at each end of your 16MHz crystal. These capacitors can be positioned either way round. Position them and solder them into place. Again, we’ll be positioning another component behind these later so snip the leads nice and short.

SW1 – tactile switch

stbg6_thumbStill on the same side of the PCB, locate and position the 6mm tactile switch. You’ll find that the pins are spaced to ensure that you get the correct orientation. Snap it into place, then solder it.

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

U1 and U2 – IC Sockets

stbg7_thumbWe don’t solder Integrated Circuits (“chips”) directly to the board – the heat would damage them. Instead, we use these IC sockets. Make sure that you line up the notch at one end of the component with the notch marked on the PCB silkscreen, as this helps us identify the pins for testing and ensures that 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 and J2 – 2-pin headers

stbg8_thumbStill 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.

J3 – 16-pin box header

stbg9_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.

P1, P2 and P3 – 10K linear potentiometers

stbg10_thumbSnap all three of the potentiometers (“pots”) into place. All three are the same value. Each one will only fit the way its intended to.

Drop the panel into place and ensure that, when snugged down onto the pots, it’s not twisted or misaligned – you want the sides of the panel to be nicely parallel with the sides of the PCB. Make any adjustments that you need to then solder the pots into place. Remove the panel again.

JK1, JK2, JK3 and LED1

For this step, we’ll position all of the remaining parts and then “sandwich” them between the PCB and panel before soldering them.

Locate and position JK1 (if you bought our kit, this will be the GREEN jack). When positioned correctly the pins will all locate and the body of the jack will match up with the printed outline.

Locate and position JK2 and JK3 (if you bought our kit, these will be the BLACK jacks). When positioned correctly the pins will all locate and the bodies of the jacks will match up with the printed outline.

Locate and position LED1. The long lead goes through the hole marked “+”. Allow the LED to sit against the PCB, then widen the leads slightly so that it stays there while we work on other parts.

stbg11_thumbWith all of these components in place, place the panel over the top of them and spin a washer and nut onto P3. Don’t tighten it right up, we just want to make sure that our parts don’t move around too much while we solder them.

Check that everything is nicely positioned, then turn the whole assembly over and solder JK1, JK2 and JK3 into place. Then position the LED (if you’re using our recommended flat-top LED set it up to be flush with the face of the panel) and solder that into place too.

stbg12_thumbThat’s all of the soldering done!

Fit all of the remaining washers and nuts and snug everything up. You’re almost there – but first we need to give the build a quick test to check that it’s good to connect to power and that it’s OK to fit the ICs.


Before we connect the StringTheory 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 J3.


Next, ensure that there is NO continuity between pins 7 and 8 of the U1 IC socket.


If your StringTheory 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


Connect your StringTheory to your modular power supply by connecting the supply to the 16-pin IDC header J3.

Touch your meters BLACK probe to pin 8 of the U1 IC socket.

Touch the RED probe to pin 7. 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 StringTheory

  • 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 StringTheory

U1 and U2 – ICs


If you bought your ATMega328P-PU IC from us, it will already have the latest firmware on it. If you’ve obtained your own, you’ll have to program it yourself before you can use it, so do this before proceeding any further.

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 U1 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 U2 IC will have a solid printed dot in the corner that holds pin 1. Align the pin 1 end of the IC with the notched end 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.

J2 – gate bus jumper

stbg14_thumbWe use a jumper on header J2 to specify whether we want to send the gate signal from StringTheory to the Eurorack gate bus. With the header in place, the signal is sent to the bus and any modules that read a gate signal from the same bus will detect when StringTheory sends a gate. With the jumper off, no gate signal is sent. You can connect the jumper, leave it off, or just connect it to one pin so that, while it won’t send a gate to the bus, you’ll at least know where the jumper is in case you change your mind.


stbg15_thumbFinally, fit you knobs to the potentiometer shafts. If you’ve use our specified pots and knobs (or if you’ve bought one of our kits), these will be simple push-on D-shafts. With the knobs on, rotate all of the pots fully counter-clockwise and then set the end-caps with the pointers pointing in the right direction, then snap them into place.

That’s it! Your StringTheory 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.


The initial version of this BOM had an error (we’d copied the wrong part no/link from Mouser for the 28-pin IC socket). The corrected value is now in the table below.

If you’ve ordered the wrong component due to our error, please get in touch and we’ll send you a replacement part from our stock free of charge.

Reference Value Description Qty Supplier &  Part Notes
C1, C2 22pF Ceramic disc capacitor 2 Mouser – 75-1C10C0G220J050B Closest match at Mouser; 2.5mm lead spacing plain ceramic disc will do fine
C3, C4 100nF Multilayer Ceramic Capacitor, 50V 2 Mouser – 81-RDEF51H104Z0K1H3B
D1, D2 1N4148 Small signal diode 2 Mouser – 512-1N4148
J1, J2 2-pin, male IDC header 2 Mouser – 538-87891-0206
J3 16-way male IDC box header, straight 1 Mouser – 710-61201621621
JK1 3.5mm female Stereo vertical jack socket 1 Thonk – “Thonkiconn” PJ366ST PCB designed for this exact part; DON’T FORGET TO BUY WASHERS & NUTS!
JK2, JK3 3.5mm female Mono switched vertical jack socket 2 Thonk – “Thonkiconn” PJ301M-12 PCB designed for this exact part; DON’T FORGET TO BUY WASHERS & NUTS!
LED1 3mm red LED 1 Mouser – 604-WP424SRDT Recommended
P1, P2, P3 10K LIN Potentiometer, 9mm , vertical 3 Thonk – Alpha 9mm B10K Recommended; for others, ensure anti-rotation tab is removed
R1 10K Resistor, metal film, 0.25W, 1% 1 Mouser – 603-MFR-25FRF5210K
R2 220R Resistor, metal film, 0.25W, 1% 1 Mouser – 603-MFR-25FRF52-220R
R3, R5 1K Resistor, metal film, 0.25W, 1% 2 Mouser – 603-MFR-25FRF521K
R4 330R Resistor, metal film, 0.25W, 1% 1 Mouser – 603-MFR-25FRF52-330R
SW1 6mm x 6mm Tactile 4.3mm Switch 100g 1 Mouser – 688-SKHHAJ
U1 ATMega328P-PU 8-bit 32k 28-pin Atmel AVR microcontroller 1 Mouser – 556-ATMEGA328P-PU Recommended
U1 28-pin DIP IC socket 1 Mouser – 571-1-2199298-9
U2 6N137 Opto-isolator 8-pin DIL 1 Mouser – 859-6N137
U2 8-pin DIP IC socket 1 Mouser – 571-1-2199298-2
X1 16 MHz Low-profile U4 Crystal 1 Mouser – 449-LFXTAL027945BULK
6mm D-shaft Sifam knob small w/skirt 3 Thonk – Sifam/Selco style knobs
6mm D-shaft Sifam knob centre cap 3 Thonk – Sifam/Selco style knobs
2-pin, female Jumper 1 Mouser – 151-8010-E