Building a Eurorack Attenuverter
An Attenuverter is a versatile control voltage (CV) utility module that combines the complementary functions of an attenuator and inverter in a single module.
Attenuation refers to reducing the amplitude of a control voltage, while inversion flips its polarity. An Attenuverter combines these two operations behind a single knob allowing the incoming control voltage to be fully attenuated at the mid position, fully inverted when the knob is rotated fully anticlockwise or untouched with the knob in the fully clockwise position. Of course, in between the extremes, we can vary the amount of attenuation. Our Attenuverter module includes two outputs, which are 180 degrees out of phase with each other for further flexibility.
Along with attenuation and inversion, adding a DC offset to a control voltage is often helpful to shift the signal up or down, for instance, offsetting an LFO so that its output swings between 0V-10V rather than -5V and 5V. Our Attenuverter includes a third output and offset control, which allows +/- 5V to be summed with the attenuated and inverted control voltage.
In this guide, we’ll build a Eurorack Attenuverter using the N8 Synth Eurorack breadboard paired with a 4HP Eurorack Control Deck. and 4HP panel. You could can easily build variations by combining basic building blocks of the Attenuverter circuit in different combinations. For instance, a dual Attenuverter module could be constructed using a 6HP Control Deck. If the offset and additional output aren’t important in your setup, our 10HP Control Deck could host six Attenuverters in a 10HP module.
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Attenuverter Schematic & Layout
Attenuverter circuit overview
A control voltage ( CV ) enters the module at J1. RV1, R8, and R11 are configured as a voltage divider. Varying the rotation of RV1 sets the voltage at the non-inverting input of U1A. R7 & R6 form a voltage divider across UA1’s inverting input. When Rv1 is in its mid position, 50% of the incoming CV is seen at the non-inverting input, and due to R7 and R6 being of equal value, 50% of the CV is also seen at the inverting input. With equal voltages at both inputs, the op-amp’s output will be 0V, so the CV appears fully attenuated at the module’s outputs.
As RV1 is turned anticlockwise, the voltage at the non-inverting input of U1A increases, causing the UA1 to increase its output to match. With RV1 fully anticlockwise, the output UA1 will equal the CV input, e.g. the output is neither attenuated nor inverted. Conversely, turning RV1 fully clockwise causes the voltage at the non-inverting input of U1A to drop toward zero. In response, U1A’s output will drop to match the inverse of the incoming CV so that its non-inverting input also sees 0V. With RV1 fully clockwise, the output of U1A will be the control voltage fully inverted.
R8 and R11 act to adjust the taper of RV1 so that the linear pot appears to have a log(ish) response. This makes the Attenuverter control more precise as it approaches the midpoint. To adjust the taper R8 and R11 can be replaced with values in the range of 10K-50K. Larger values of R8 and R11 will make the response more linear.
U2B is configured as an inverting buffer driving output 1 (J3), and U2A is configured as a non-inverting buffer driving output 2 (J4). When RV1 is clockwise, output 1 will be in phase with the CV input, while output 2 will be 180 degrees out of phase.
RV2 provides the offset voltage, which U1B sums with the attenuated and inverted CV from the output of U1A. U1B is configured as an inverting summing amplifier, so the Offset Output ( J2 ) will also be in phase with the CV input when RV1 is clockwise. Again, a non-linear response from RV2 helps us dial in 0V offset more easily by making the pot less sensitive to changes around the cross-over from positive to negative. In this case, setting R4 to a value smaller than RV2 is enough to give us a log(ish) response. Also, note that R4 sets the offset range to approx. +/- 5V, reducing its value will expand the offset range; for instance, 6.8K gives us approx. +/- 8V.
Bill of Materials
We’ll build the Eurorack Attenuverter 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 Eurorack breadboard provides power to our Attenuverter module, and the Eurorack Control Deck makes mounting our controls 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.
| Type | Refs | Value | Qty | Notes | |
|---|---|---|---|---|---|
| 4HP 1x6 Eurorack Prototype Kit | - | - | 1 | Includes Eurorack Breadboard, 4HP Control Deck, 4HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J5. | |
| Hardware | J2, J3, J4, J1 | PJ-3001F | 4 | PJ-3001F | |
| Hardware | 2 | Davies 1900H Clone White Round Shaft | |||
| Potentiometer | RV1, RV2 | B100K | 2 | B100k 9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style ). | |
| IC | U1, U2 | TL072 | 2 | DIP8 TL072 Operational Amplifier | |
| Hardware | 2 | DIP8 IC socket for U1 and U2 | |||
| Capacitor | C9 | 30pF ±20% 50V | 1 | Multilayer Ceramic | |
| Capacitor | C7, C8 | 10uF ±20% 50V | 2 | Radial Electrolytic 5x11mm | |
| Capacitor | C2, C1, C3, C4, C5, C6 | 100nF ±20% 50V | 6 | Multilayer Ceramic | |
| Resistor | R3, R9, R14 | 1K | 3 | 1/8W or 1/4W 1% metal film | |
| Resistor | R11, R8 | 27K | 2 | 1/8W or 1/4W 1% metal film | |
| Resistor | R1, R2 | 4.7K | 2 | 1/8W or 1/4W 1% metal film | |
| Resistor | R7, R6, R4, R5, R10 | 10K | 5 | 1/8W or 1/4W 1% metal film |
Constructing the Eurorack Attenuverter
Building the breadboard Attenuverter circuit
Our Attenuverter 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. The product page provides a diagram of the layout and connections for a quick overview of the breadboard’s design and features.
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 by 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 breadboard for soldering.
2. Next, we solder in our resistors, starting with those that lie flat on the board—R4, R7, R10, and R14. Again, masking tape can help hold components in place.
3. Add the two chip holders.
4. Capacitors. Solder in the small ones and then the big ones! The two electrolytic caps, C7 and C8, are polarized, so make sure to mount them with the correct orientation.
5. Solder in the 2×8 pin header (J5), our power connector. The power connector can be mounted on either side of the breadboard, but we mount it on the component side for a 4HP module like this.
6. Finally, solder the single-row, 90-degree, 40-pin male header to the left edge connector. Because we are using a 4HP Eurorack Control Deck, the pin header is mounted on the rear 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 Attenuverter 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, while the rear doesn’t have these boxes. Components can be mounted on either the front or the rear.
For this Attenuverter 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.
1. On the rear of the Control Deck, position the short (black) wires connecting the “d-bus” to the spare horizontal C pad below each JPS cell where a 3.5mm jack sockets (J1-J4) will be mounted. Flip the control deck over to solder these wires in place from the front.
We are using the d-bus to create a common ground on the control deck, saving space on the breadboard for our core circuit by not adding a ground connection for each jack.
2. Our schematic shows that the tip (T) of J1 is connected to pin one of RV1, and pin three of RV1 is connected to ground. We can also make these connections on the control deck, saving space on the breadboard.
On the rear of the Control Deck, connect the vertical A pad of JPS1 to the horizontal A pad of JPS2 – the yellow wire in the diagram – and the vertical C pad of JPS1 to the d-bus. Flip the control deck over and solder these wires from the front.
3. Dry-fit the potentiometers on the front 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 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 from the rear. The input jack (J1) needs its switch pin connected to ground. We achieve this by soldering its switch pin to its ground pin on the rear of the Control Deck.
7. Position the 40-pin female header on the rear and solder it into place from the front. If needed, hold it in place with masking tape. Solder a pad at either end and check that it is perpendicular before soldering the other pads.
8. Connect the finished Control Deck to the breadboard using the pin headers.
9. Insert the two TL072 op amps in the DIP sockets.
Testing, Testing, check one two.
Before you power up your attenuverter for the first time, you should 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 it 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 Attenuverter look beautiful
The final step is to make your shiny new DIY Attenuverter 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 Eurorack panels here. If bright green is your thing, you can download our finished label, ready for printing.
Download the Attenuverter label template.
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