Building a DIY Buffered Multiple

Building an active buffered mult 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 active multiple perfect for beginners to hone their soldering and synth module construction skills.

While not the most glamorous of modules, a buffered mult module is an essential utility for any modular synth. A multiple simply takes one CV signal at its input and provides one or more copies at its outputs. The buffered (or active) bit of the title means that electronics in the mult compensate for that one CV input being connected to several outputs, ensuring they are exact copies.

If we just split our control voltage several ways, we’d see an overall drop in voltage as the source module tries to drive multiple destinations. This splitting is what a passive mult does. In some cases, that’s fine, but if we want to control the pitch of a couple of oscillators, for example, we wouldn’t want our control voltage dropping and messing with our tuning.

We’ll be using the N8 Synth Eurorack breadboard, paired with a 4HP Eurorack Control Deck, to build the DIY Eurorack buffered multiple module. The breadboard and Control Deck, along with a pre-drilled panel and pin headers, can be purchased together as 4HP 1×6 Eurorack Prototype kit.

Completed DIY buffered multiple Eurorack module with summing inputs

Eurorack Buffered Multiple Schematic

Circuit diagram for a Eurorack buffered multiple with summing or averaging inputs
Eurorack buffered mult schematic - click to expand
Eurorack breadboard and control deck layout for a buffered multiple with summing or averaging input
Eurorack buffered multiple layout - click to expand

The buffer multiple circuit centres around a single TL074 IC, which provides the four op-amps that do the buffering. U1D is the input buffer, and U1A, U2B, and U2C are the three output buffers. The op-amps for each output buffer are configured as a unity gain inverting amplifier. For accuracy, we want to ensure the input and feedback resistor pairs ( R5 & R8, R6 & R9, R7 & R10 ) are closely matched in value. For example, if you measured R5 as being 9.8K ohms rather than 10K ohms, you should find a matching 9.8K ohm resistor for R8. The 1K ohm resistors, R11-R13, protect the op-amps from a short circuit across their outputs.

Our inputs enter via J1 & J2. But wait, why are there two inputs? Doesn’t a mult take one input and provide several copies of it? Yes, it does, and if you connect just one input, that will happen. However, we’ve added a second input to get a bit more bang for our horizontal pitch. How this second input interacts with its sibling is set by the switch SW1. The switch allows the inputs to be summed or averaged. 

U1D is configured as a summing inverting amplifier. By doubling or halving the feedback resistance, SW1 changes the gain of the amplifier, giving either the sum or average of the two inputs. Again we want R1-R4 to be closely matched for accuracy.

Finally, those capacitors. C1 stabilises the op-amp and stops a fast-changing signal from overshooting. C2-C3 are decoupling capacitors for the TL074, and C4-C7 are the standard Eurorack filter capacitors.

 

Bill of Materials

We’ll build the Eurorack buffered multiple module 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 buffered mult, and the Eurorack Control Deck makes mounting our controls a breeze.

The 4HP Eurorack Prototype kit jacks and switch 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.

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HardwareJ1 - J5PJ-3001F3.5mm vertical mounting jack socket

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SwitchSW1SPDTOn-On, sub-miniature SPDT switch with pin spacing of 1.27mm and pin ( or lug ) width of up to 1.52mm

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CapacitorC110pFCeramic 50V
CapacitorC2 - C5100nFCeramic 50V
CapacitorC6, C710uFElectrolytic. 50V, 5mmx11mm
ResistorR1-R4100K1/8W or 1/4W 1% metal film
ResistorR5-R1010K1/8W or 1/4W 1% metal film
ResistorR11-R131K1/8W or 1/4W 1% metal film
ICU1TL074High speed J-FET input quad operational amplifier
Hardware--14 pin DIP IC socket for U1

Constructing the Eurorack Buffered Multiple

Building the breadboard buffered mult circuit

Our buffered mult is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should feel 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.

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 R4, R5, R9 and R10. 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 (J6), which is 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. 

5. Finally, solder the single-row, 90-degree, 40-pin male header to the left edge connector. Because we are using 4HP Eurorack Control Deck, the pin header is mounted on the bottom of the breadboard.

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

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.

In this buffered mult design, we mount our jack sockets and switch 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 front of the Control Deck, solder the short wires connecting the “d-bus” to the spare horizontal C pad below each position where the 3.5mm jack sockets (J1-J5) will be mounted. 

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

2. Dry-fit the switch on the front side of the board, aligning its middle/common pin in the upper of the to two oval C – pads as illustrated here 

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

4. Attach the pre-drilled Eurorack panel using the hex nuts provided with the jacks and switch, checking that the jacks and switch are centred in the holes and that each component is seated on the control deck. Note the body of the switch should not contact the control deck.

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

6. Position the 40-pin female header on the rear and solder it 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.

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

Testing, Testing, check one two.

Before you power up your mult 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 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 multiple look beautiful

The final step is to make your shiny new buffered mult 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 Sum & Mult label template.

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

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