DIY Eurorack Pedal IO

A guitar effects pedal IO module serves as a bridge between a Eurorack modular synthesizer and a universe of external guitar effects pedals. It enables the incorporation of your favourite stompbox effects into your signal chain, opening up the new sonic territory of massively distorted basses, lush chorused pads, dubby delayed blips and bops, phasers, flangers, or just an inline chromatic tuner pedal to get those oscillators singing in harmony.

In operation, a Eurorack guitar pedal interface simply reduces the output voltage from the synth to a guitar-like level to send to the stompbox and then amplifies the return signal back to a healthy 10V peak-to-peak Eurorack audio level. Our design incorporates trimmers for both the send and return levels and a handy wet/dry mixer. 

In this tutorial, we’ll build a DIY effects pedal IO module for Eurorack using the N8 Synth Eurorack breadboard paired with a 4HP Eurorack Control Deck. The breadboard and Control Deck, pre-drilled panel, and pin headers can be purchased together as 4HP 1×6 Eurorack Prototype kit.

Photo of the completed synth to guitar pedal interface

Synth pedal IO schematic & circuit layout

Eurorack stomp box adapter Schematic - click to expand
Diagram of the synth pedal I/O module on Eurorack Breadboard
Eurorack stomp box adapter layout - click to expand

Generally, Eurorack modules have a signal level somewhere around 10 volts peak to peak, while guitars output something between 250mV and 1V peak to peak. Given the range vast range of guitar pickups available, even this range is approximate. Either way, our synth’s output is too hot for most stompboxes. R1 and R2 form a potential divider, reducing Eurorack’s 10Vpp to ~0.45mVpp. The input trimmer RV1 lets us reduce this further as needed. C1 & R3 act as a high-pass filter, removing any DC offset from the signal. J2 is a stereo jack with our FX send on the tip and return on the ring.

The return signal is buffered by U1B with C2 and R5 forming a highpass filter, while C3 rolls off some of the high-end frequencies and noise. RV2 controls the mix between our synth input ( dry ) and affected return signal ( wet ). The mixed signal can then be reduced by the output trimmer, RV3, if required. U1A amplifies the signal, bringing it back to 10Vpp. R7 and R1 set the gain of the amplifier stage with C4 stabilising the op-amp. Finally, C5 filters any DC offset that has been added to the signal before it is output via R1.  R1’s purpose in life is to protect U1A from any signal accidentally patched into the output jack J3.

Bill of Materials

We’ll build the Eurorack guitar pedal interface using a 4HP Eurorack Prototype kit. The kit contains a Eurorack Solderable Breadboard and Eurorack Control Deck, a pre-drilled Eurorack panel, and the pin headers used to connect them. The Euroack breadboard provides power to our module, and the Eurorack Control Deck makes mounting our controls and I/O a breeze.

The 4HP Eurorack Prototype kit, jacks, and pots required for this module are available from the N8 Synth store. The remaining components are widely available and relatively inexpensive. If you are just getting started building modular synths, stock up on these components, as they are ubiquitous in the schematics you’ll find online.

CategoryRefValueNotes
4HP 1x6 Eurorack Prototype Kit--Includes Eurorack Breadboard, 4HP Control Deck, 4HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J3.

Add to basket

PotentiometerRV1, RV3B100K9mm vertical trimmer potentiometer

Add to basket

PotentiometerRV2B10K9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

Add to basket

HardwareJ1, J3PJ 3001F3.5mm vertical mounting jack socket

Add to basket

HardwareJ2PJ-366ST3.5mm vertical mounting stereo jack socket

Add to basket

Hardware--1x Davies 1900H style knob, white

Add to basket

Hardware--8 pin DIP IC socket for U1
ICU2TL072TL072 Op amp
CapacitorC3,C410pfCeramic 50V
CapacitorC6,C7,C8,C9100nFCeramic 50V
CapacitorC1,C21uFCeramic 50V
CapacitorC5,C10,C1110uFElectrolytic. 50V, 5mmx11mm
ResistorR2,R6,R81K1/8W or 1/4W 1% metal film
ResistorR1, R710K1/8W or 1/4W 1% metal film
ResistorR3100K1/8W or 1/4W 1% metal film
ResistorR5150K1/8W or 1/4W 1% metal film
ResistorR41M1/8W or 1/4W 1% metal film

Constructing the Eurorack Pedal IO Module

Building the breadboard Pedal IO

Our Pedal IO circuit is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should be right at home, as the layout is basically the same. If you need a quick overview of the design and features of the N8 Eurorack breadboard, the product page provides a diagram of the layout and connections.

As with any PCB construction, we want to build low to high. Starting with the wires and then adding components in reverse order of height. Beginning with the lower profile components makes our life easier, giving us more room to manoeuvre without the taller parts getting in the way.

1. First up, we add the wires. Strip around 3mm of insulation from each wire end and feed the conductor through the appropriate holes. It can be helpful to work in sections and use masking tape to hold several wires in place before flipping the board for soldering.

You may want to leave the two long wires connected from rows 16 and 17 of the edge connector for later so they aren’t in the way when soldering in the pin header.

2. Next, we solder in our resistors, starting with those that lie flat on the board – R5 and R8. Again, holding components in place with masking tape can be helpful or bending the pins slightly to stop the component sliding out of the holes.

3. Add the IC socket, taking care to orient it correctly – the notch aligns with row 28. Solder pins 1 and 8 of the socket, check it is square to the board, adjust if needed, and then solder the remaining pins.

4. Capacitors. Solder in the small ones and then the big ones! Make sure to check the polarity of the three electrolytic caps, C5,C10 C11.

5. Solder in the 2×8 pin header (J4), our power connector. The power connector can be mounted on either side of the breadboard, but for a 4HP module like this, mount the power connector on the component side. 

6. Solder the single-row, 90-degree, 40-pin male header to the left edge connector. We are using a 4HP Eurorack Control Deck, so the pin header is mounted on the bottom of the breadboard.

7. If you omitted the two long wires connected from rows 16 and 17 of the edge connector in Step 1, don’t forget to add these now.

8. Insert the TL072 op-amp (U1) into its socket, taking care to orient it correctly – pin 1 aligns with row 28.

Photo of the completed breadboard layout of the synth stompbox IO circuit
Eurorack Pedal I/O breadboard layout - click to expand

Top Tip: It can be tough to strip the insulation from the short wires, like the ones connecting the power rails to the TL072 op-amp. Instead of stripping 3mm of insulation from each end, remove 6mm from one end of the wire and then cut the wire to length. You should then be able to slide the short piece of insulation along the conductor, leaving 3mm at each end.

The Pedal IO Control Deck

N8 Eurorack Control Decks have logical front and rear sides. The front has screen-printed boxes indicating where pots, jacks, and switches are mounted. The rear doesn’t have these boxes. Components can be mounted on either the front or the rear.

For this module, we mount our jack sockets and pots on the front of the Control Deck, the side with screen-printed boxes.

Check out these guides for further details on mounting components on your Control Deck.

4HP Eurorack Prototype Kit - click to expand

1. On the rear of the Control Deck, solder the short wires connecting the “d-bus” to the spare horizontal C pad below each position where the jack sockets (J1, J2 J3) will be mounted, cells JSP2, JSP4, JSP6

We are using the d-bus to create a common ground on the control deck so that we don’t need to add a ground connection for each socket on the breadboard. This saves space on the breadboard for our core circuit.

2. Our schematic shows that pin one of our input and output trimmer potentiometers ( RV1 & RV3 ) are connected to the ground. We can make these connections on the control deck, saving space on the breadboard. On the rear of the control deck, in cells JSP1 & JSP5, connect short wires from the unused vertical A pad to the “d-bus”.

3. Dry-fit the potentiometers on the front side of the Control Deck. If the pots have metal support tabs on their top and bottom edges, they should be tucked under the pot’s body, as illustrated here and shown in the photographs of the module on this page.

4. Dry fit the jacks on the front side of the Control Deck. If in doubt, check out this guide for correctly positioning jacks on the Control Deck.

5. Attach the pre-drilled Eurorack panel using the hex nuts and washers provided with the jacks and pots, checking that the jacks are centred in the holes and that each component is seated on the Control Deck. 

6. Leaving the panel attached, flip the Control Deck and solder the pots and jacks into position. The input jack (J1) needs its switch pin connected to ground. We achieve this by soldering its switch pin (B) to its ground pin (C) on the rear of the Control Deck. This is illustrated in the pictures on this page.

7. Position the 40-pin female header on the rear and solder it into place from the front. Hold it in place with masking tape if needed, solder a pin at either end and check it is perpendicular before soldering the other pins.

8. Connect the finished Control Deck to the breadboard using the pin headers.

Connectivity

Pedal IO send and return lead - click to expand

Our module uses a 3.5mm stereo jack to both send and return the signal to our guitar effects pedals. To achieve this, we use a standard 3.5mm to 6.35mm y-splitter cable. The module sends to our stompboxes via the tip of the 3.5mm jack with the return on its ring.

 

Testing, Testing, check one two.

Before you power up your pedal IO synth module for the first time is good to do some basic tests. While not extensive, these help keep the magic smoke in the components where it belongs.

Visual inspection

  • Compare your module to the diagrams and schematic on this page. Do all the components and wires look like they are in the right place? Anything missing?
  • Inspect the solder side of the Eurorack Breadboard and Control Deck.
    • Are any of the pads shorted by solder splashes or untrimmed component leads?
    • Are all the component leads soldered? 
    • Have solder bridges indicated on the diagrams been made?

Continuity

We want to ensure there is no continuity between +12v, ground and -12v rails. We’ll do this using a multimeter.

Put your multimeter in continuity test mode, then, with your module unpowered, check the continuity between the following points on the circuit:

  • Connect one of the multimeter’s test leads to the +12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to +12V. There should be no continuity

Power Up

If your new module passed the continuity tests and visual inspection, it’s time to power it up. If you have a bench power supply, it is good practice to use this for the first power-up of a DIY synth module so that it is a minimum safe distance from your other modules.

The Eurorack power connector format is sadly a little open to interpretation, and many a module has lost its life to the specification’s vagueries.

N8 Eurorack prototype boards follow the most common convention. A white stripe is printed next to the -12V end of the power connector. Typically this is where the red stripe of the power cable should be aligned. BUT not every manufacturer follows this convention, and this is a DIY synth tutorial, so chances are you made your cables, right?

Always check that your power supply is supplying -12V at the red stripe before connecting power to your synth module and that the red stripe is connected to the -12V pin on the module.

Making your module look beautiful

The final step is to make your shiny new DIY Pedal Interface look the part next to those commercial Eurorack modules. The good news is you can do this with nothing fancier than an inkjet printer and some sticky-back plastic.

We have a complete guide to making labels for Euroack panels here. If bright green is your thing, you can download our finished label, ready for printing.

Download the Pedal IO label template.

We’d love to see your build. Share your pix with us on Facebook and Instagram.

Photo of the completed Eurorack stompbox interface module

10%

off, especially for you 🎁

Join the N8 Synth mailing list to receive an exclusive discount and keep up to date on our latest products, tutorials & offers!!

We don’t spam! Read our privacy policy for more info.T&Cs apply.

Shopping cart
Sign in

No account yet?

Start typing to see products you are looking for.
Shop
0 items Cart
My account