Building a DIY Eurorack Mixer

Building an active Eurorack mixer is a great first project for someone interested in building a DIY modular synthesizer from scratch or adding to their existing Eurorack synth. The simple circuit and low component count make this mixer ideal for beginners to hone their soldering and synth module construction skills.

While not the most glamorous of modules, an audio mixer module is essential for mixing signals from multiple Eurorack oscillators or creating a sub-mix for synth drum modules. Need a CV mixer? No problem remove a few capacitors and swap out some pots as detailed below.

We’ll be using the N8 Synth Eurorack breadboard, paired with a 6HP Eurorack Control Deck, to build the DIY mixer module. The breadboard and Control Deck, along with a pre-drilled panel and pin headers, can be purchased together as 6HP 2×6 Eurorack Prototype kit. The great thing about building a module with a solderable breadboard is that you can change or add to it as your needs grow. Want some soft clipping? Add a couple of diodes and resistors. Need a CV mixer? Replace a few caps with wires, and you’ve got one. 

Photo of the completed Penta Mix Audio, 5 channel Eurorack mixer

5 Channel Mixer Schematic

Circuit diagram for a 5 channel Eurorack mixer
Eurorack mixer schematic - click to expand
Breadboard layout for a Eurorack mixer module
Eurorack mixer layout - click to expand

Looking at the mixer schematic, we see that the circuit is relatively simple. The two operational amplifiers of the TL072 (U1) are configured as inverting buffers. U1B sums the five inputs creating the mixer, and U1A buffers the output.

Audio inputs arrive via the 3.5mm sockets J1-J5 and are attenuated by the potentiometers RV1-RV5. The capacitors C7-C11 act as high-pass filters to block any DC offset in our audio signals. These capacitors can be omitted to use the circuit as a CV mixer.

RV1-RV5 are 100K ohm potentiometers with a logarithmic/audio taper ( A100K ) as we are dealing with audio signals. Alternatively, you could substitute these for 1M ohm linear potentiometers allowing the effect of the relatively lower value, 100K resistors R1-R5, to give an approximately logarithmic response. RV6 attenuates the mixed signals before the op-amp U1A buffers the output.

So what about C12 and C13, you may ask. These stabilising capacitors stop our op-amps from “ringing” or oscillating. Operational Amplifiers can become unstable and “ring” when their input is driven by a fast-changing signal (for instance, a square wave) and they are driving a capacitive load  (for instance, a long patch cable). Very, very simplistically, these act as lowpass filters to reduce the gain of high-frequency signals beyond the audio range. A detailed explanation of this is beyond the scope of this article ( and its author’s ability to explain! ), but for the smarter students in the class, here is a real explanation. Techniques to avoid instability in Capacitive Loading

Finally, at the top right of the schematic, we see our standard Eurorack filter capacitors ( C10-C13 ), 16pin power connector and C8 & C9, which act as decoupling capacitors for the TL072.

Bill of Materials

We’ll build the audio mixer module using a 6HP 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 mixer, and the Eurorack Control Deck makes mounting our controls a breeze.

The 6HP Eurorack Prototype kit and the pots and jacks 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
6HP 2x6 Eurorack Prototype Kit--Includes Eurorack Breadboard, 6HP Control Deck, 6HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J7.

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PotentiometerRV1-RV6A100K9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

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HardwareJ1-J6PJ-3001F3.5mm vertical mounting jack socket

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

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Hardware--5x Davies 1900H style knobs, black

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ICU1TL072TL072 Op amp
Hardware--8 pin DIP IC socket for U1
CapacitorC1-C52.2uFCeramic 50V
CapacitorC64.7pFCeramic 50V
CapacitorC710pFCeramic 50V
CapacitorC8-C11100nFCeramic 50V
CapacitorC12-C1310uFElectrolytic. 50V, 5mmx11mm
Hardware--16 pin DIP IC socket for U2
ResistorR1-R8100K1/8W or 1/4W 1% metal film
ResistorR91K1/8W or 1/4W 1% metal film

Constructing the Eurorack Mixer

Building the breadboard mixer circuit

Our mixer 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 Eurorack breadboard, the product page provides an overview 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 end of the wire 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.

 Ensure that the five lengths of wire on the edge connector have sufficient slack to allow capacitors C1-C5 to be mounted. Or break the rules and mount those capacitors first.

2. Next, we solder in our resistors, starting with those that lie flat on the board. In this case, we’ll add a chip holder before R6 and R8. Again it can be helpful to hold components in place with masking tape.

3. Capacitors. Solder in the small ones and then the big ones!

4. Solder in the 2×8 pin header (J7) that is our power connector. This can be mounted on either side of the breadboard.

5. Finally, solder the single-row, 90-degree, 40-pin male header to the left edge connector. Because we are using 6HP Eurorack Control Deck, the pin header is mounted on the top of the breadboard.

Simple breadboarded Eurorack mixer circuit

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 Control Deck

Our mixer has six 3.5mm jack sockets for our audio signals and six potentiometers to set the level of those signals. We used 9mm Alpha RD901F-40 style pots and Davies 1900h style knobs, but if you’d like a little more finger room, swap them out for 9mm trimmers.

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. The side of the Control Deck a pot is mounted on determines the order its pin connections arrive at the edge connector, on the front 1-2-3 on the rear 3-2-1. 

In this mixer design, we mount our jack sockets and pots on the rear of the board, the side without screen printed boxes.

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

6HP Eurorack Prototype Kit ( click to expand )

1. On the front of the Control Deck, solder the short wires connecting the “d-bus” to the spare C pad of each cell. We use the lower vertical C pad for the cells that will host potentiometers, and for the cells that will host a 3.5mm jack socket, we use the horizontal C pad.

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 pot or jack on the breadboard. This saves space on the breadboard for our core circuit.

2. Position the 40-pin female header and solder into place. Hold it in place with masking tape if needed, solder a pad at either end and check it is perpendicular before soldering the other pads.

3. Dry-fit the potentiometers on the rear side of the board. 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.

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

5. Attach the pre-drilled Eurorack panel using the hex nuts, checking that the jacks are centred in the holes and that each component is seated on the control deck.

Important: the side of the Control Deck pots are mounted on changes the order in which their connections arrive at the edge connector. On the front, 1-2-3. On the rear, 3-2-1. Make sure you mount them on the correct side of your Control Deck.

6. Leaving the panel attached, flip the control deck and solder the pots and jacks into position. The input jacks in positions J1, J2, J3, J4, and J5 need their switch pin connected to ground. We achieve this by soldering the two pins together on the Control Deck.

7. Connect the finished control deck to the breadboard using the pin headers.

Testing, Testing, check one two.

Before you power up your mixer 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 in this article. 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 mixer look beautiful

The final step is to make your mixer 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 acid house yellow is your thing, you can download our finished label, ready for printing.

Download the Penta Mix Audio label template.

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

Front panel view of complete Penta Mix Audio DIY Eurorack mixer

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