Building a Eurorack State Variable Filter – Part 2
This is part two of our Eurorack LM13700 State Variable Filter build guide, where we move from theory into hands-on construction.
In part one, we explored what a State Variable Filter (SVF) is and why it’s such a useful and popular filter topology in modular synthesis. We saw how an SVF can provide low-pass, high-pass, and band-pass responses simultaneously from a single circuit, making it a powerful and flexible tool for sound design. Whether you’re gently shaping the tone of an oscillator, carving space in a mix, or adding movement and animation to a patch, the SVF is one of the most versatile filters you can have in a Eurorack synth. We also walked through how this particular filter works, explaining the role of the integrators, the feedback paths, the cutoff control, and the resonance shaping used in our design.
If you haven’t read part one, don’t worry — you don’t need a deep understanding of the theory to build the module successfully. This page focuses on the practical assembly of the filter using an N8 Synth 8HP Eurorack Prototype Kit. Where helpful, we’ll briefly reference the ideas from part one, but the emphasis here is firmly on getting components soldered in the right place and building a reliable, great-sounding module.
So grab your soldering iron, take a moment to check you have all the required parts to hand, and let’s get started building your very own LM13700-based state variable filter.
Eurorack State Variable Filter Schematic & Module Layout
Bill of Materials
We’ll build our Eurorack State Variable Filter using the 8HP Eurorack Prototype Kit, along with one additional solderable breadboard. The prototype kit includes a Eurorack solderable breadboard with standard Eurorack power headers, an 8HP Eurorack Control Deck, a pre-drilled Eurorack panel, and the required pin headers for connecting everything. For this module design, the additional breadboard provides the extra circuit space needed for the filter core. The Eurorack Control Deck keeps all panel components neatly organised and makes mounting pots and jacks straightforward.
The 8HP Eurorack Prototype Kit, along with the required jacks, pots, and chips, are available from the N8 Synth store. The remaining passive components are widely available, inexpensive, and commonly used in analogue synth circuits. If you’re just getting started with modular DIY, it’s well worth stocking up on these parts, as you’ll see them appear again and again in schematics and build guides you’ll find online.
A note about capacitors
The ceramic capacitors’ voltage rating is not critical. We’ve specified 50V, as this is common for the cheap bulk capacitor kits you’ll find on Amazon and the like, but any voltage rating greater than 25V will work. The type and tolerance are also not too critical. However, you may wish to use lower tolerance parts for C5 and C10, as these affect the filter’s cutoff frequency.
| Type | Refs | Value | Qty | Footprint | |
|---|---|---|---|---|---|
| 8HP 2x5 Eurorack Prototype Kit | LM13700 SVF CV Board, LM13700 SVF Control Deck, 8HP Eurorack Panel | 1 | Includes the powered breadboard, control deck, panel and pin headers J7, J15, J23 | ||
| Euroback Breadboard | LM13700 SVF Main Board | Bread and Butter B1 | 1 | N8 Synth BBB1 | |
| IC | U3, U2 | TL072 | 2 | DIP8 TL072 Operational Amplifier | |
| IC | U5, U4 | LM13700 | 2 | Dual Operational Transconductance Amplifier | |
| IC | U1 | TL074 | 1 | DIP14 TL074 Operational Amplifier | |
| Potentiometer | RV2, RV3 | B10K | 2 | Alpha-RV901F style -- D Shaft | |
| Potentiometer | RV1 | B100K | 1 | Alpha-RV09 style | |
| Knob | Cutoff, Res | Rubber Knob White D Shaft | 2 | Rubber Knob 11.5mm D Shaft | |
| Jack Socket | J1, J6, J4, J7, J2, J3 | PJ-3001F | 6 | PJ-3001F | |
| Hardware | - | - | 2 | 8 pin DIP IC socket for U2 and U3 | |
| Hardware | - | - | 1 | 14 pin DIP IC socket for U1 | |
| Hardware | - | - | 2 | 16 pin DIP IC socket for U4 and U5 | |
| Pin Header | J14 | 1x40 Male | 1 | 2.54mm Pin Header 1x40 Male | |
| Pin Header | J16 | 1x40 Female | 1 | 2.54mm Pin Header 1x40 Female | |
| Pin Header | J17, J18, J19, J20 | 1x2 Female | 4 | 2.54mm Pin Header 1x2 Female | |
| Pin Header | J13, J12, J11, J10 | 1x2 Male | 4 | 2.54mm Pin Header 1x2 Male | |
| Capacitor | C26, C27 | 10uF ±20% 50V | 2 | Radial Electrolytic 5x11mm | |
| Capacitor | C21, C22, C16, C17, C24, C25, C20, C19, C9, C8, C12, C13 | 100nF ±20% 50V | 12 | Multilayer Ceramic 4mm | |
| Capacitor | C3, C4, C2 | 1nF ±20% 50V | 3 | Multilayer Ceramic 4mm | |
| Capacitor | C1 | 1uF ±20% 50V | 1 | Multilayer Ceramic 4mm | |
| Capacitor | C5, C10 | 300pF ±20% 50V | 2 | Multilayer Ceramic 4mm | |
| Capacitor | C15, C14, C11 | 10uF ±20% 50V | 3 | Radial Electrolytic 4x8.5mm | |
| Zener Diode | D2, D1 | 1N4734 Zener Diode | 2 | DO-35 | |
| Pin Header | J8 | 1x6 Male | 1 | 2.54mm Pin Header 1x6 Male | |
| Pin Header | J9 | 1x4 Male | 1 | 2.54mm Pin Header 1x4 Male | |
| Pin Header | J22 | 1x4 Female | 1 | 2.54mm Pin Header 1x4 Female | |
| Pin Header | J21 | 1x6 Female | 1 | 2.54mm Pin Header 1x6 Female | |
| Potentiometer | RV5 | B1K | 1 | 3296 Trimmer Horizontal | |
| PNP Transistor | Q1, Q2 | 2N3906 | 2 | TO-92 | |
| Resistor | R30, R21 | 15K | 2 | Metal Film 1/4W | |
| Resistor | R17, R16, R27 | 5K1 | 3 | Metal Film 1/4W | |
| Resistor | R23 | 1M | 1 | Metal Film 1/4W | |
| Resistor | R20, R45, R11 | 30K | 3 | Metal Film 1/4W | |
| Resistor | R33, R32, R31 | 1K | 3 | Metal Film 1/4W | |
| Resistor | R3, R10 | 240K | 2 | Metal Film 1/4W | |
| Resistor | R18, R19, R29, R28 | 220R | 4 | Metal Film 1/4W | |
| Resistor | R14 | 51K | 1 | Metal Film 1/4W | |
| Resistor | R4, R5, R6, R1, R15, R26, R25, R2 | 100K | 8 | Metal Film 1/4W | |
| Resistor | R24, R22 | 680R | 2 | Metal Film 1/4W | |
| Resistor | R9, R12, R13 | 10K | 3 | Metal Film 1/4W | |
| Resistor | R8 | 1K5 | 1 | Metal Film 1/4W |
Constructing the Eurorack State Variable Filter
Our filter circuit uses five ICs, so it is split across two N8 Synth solderable breadboards. The depth of an 8HP Eurorack panel is ideal for this layout, as it allows the two boards to be neatly stacked using a small number of additional pin headers.
We use the powered N8 Synth breadboard (BBA) to host the cutoff and resonance CV circuitry, including the Eurorack power header. The unpowered breadboard (BBB) serves as the main board and hosts the filter’s core.
Building the LM13700 SVF Main Board
Our State Variable Filter’s main board is built on an N8 Synth solderable Eurorack breadboard. If you’ve used a sprung breadboard before, you should feel right at home, as the layout is essentially the same. The product page provides a diagram of the layout and connections, along with an overview of the breadboard’s design and features.
As with any PCB construction, it’s generally best to build from low to high, starting with wires and then adding components in reverse order of height. Beginning with the lower-profile components makes the build easier by giving you more room to manoeuvre without taller parts getting in the way. That said, adding the IC sockets (if you’re using them) early on can be helpful, as they provide a clear reference point for placing the wires.
Add the DIP sockets for U1, U4, and U5 to the BBB breadboard (the board without the Eurorack power header). It’s often useful to solder one or two pins first, check that the socket is sitting flat against the board, and then solder the remaining pins.
Next, add the wires according to the layout diagram. It can be helpful to print out the layout and mark off each wire as you fit it.
Using the Bill of Materials or the Synth Designer app to identify the correct component values, fit the resistors, capacitors, and the two diodes.
Important: Do not add the pin headers just yet. Set the board aside for now and move on to the CV board.
Top Tip: It can be tough to strip the insulation from the short wires, like the ones connecting the power rails to the TL074 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.
Building the LM13700 SVF CV Board
Our State Variable Filter’s CV board is built on a powered N8 Synth Eurorack breadboard. As before, the product page provides a diagram of the layout and connections, along with an overview of the breadboard’s design and features.
While our build order will still follow the usual low-to-high approach, we will start by fitting the pin headers J21 and J22. Several wires will cross over their pins later, and installing the headers first makes routing those wires much easier.
- Position the pin headers labelled J21 and J22 on the rear of the breadboard and solder them in place from the front.
- Next, add the wires, followed by the remaining components, as before.
- Solder the power header J7 in place on the front of the board.
- Flip the board over and add the remaining female pin headers J16–J20 to the rear, soldering them into place from the front.
With the CV board complete, we can now use it to align the pin headers on the Main board. In the finished module, the CV board sits on top of the Main board, so begin with the CV board component side down.
- Insert the long pins of the male pin headers J8–J13 into their corresponding female headers on the rear of the CV board.
- With the CV board face down, position the Main board onto the exposed short pins of the male headers.
- Double-check that the Main board is correctly oriented relative to the CV board.
- Holding the two boards together (masking tape can be helpful here), solder the exposed pins of the male headers on the rear of the Main board.
- Finally, separate the two breadboards and attach the angled 40-pin male header, J15, to the rear of the Main board, as shown in the layout.
Building the LM13700 SVF 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 State Variable Filter module, we mount our jack sockets and pots on the front of the Control Deck, the side with screen-printed boxes, and add several wires to the rear.
Check out these guides for further details on mounting components on your Control Deck.
Important: The wires on the rear of the Control Deck should be added first.
We will use the d-bus to create a common ground on the Control Deck, saving valuable space on the breadboards for the filter core.
- On the rear of the Control Deck, position the short (black) wires connecting the d-bus to the spare horizontal C pad of each JPS cell where a 3.5 mm jack socket will be mounted. These are JPS3, JPS4, and JPS7–JPS10. Flip the Control Deck over and solder these wires in place from the front.
- The schematic shows that pin 1 of RV2 and RV3 are also grounded. Pin 1 corresponds to the A pads of JPS1 and JPS2. Make these connections on the rear of the Control Deck by connecting a black wire from each JPS cell’s vertical A pad to the d-bus.
- Dry-fit the potentiometers on the front of the Control Deck. If the pots have metal support tabs on their top and bottom edges, tuck these under the pot body, as illustrated.
- Dry-fit the jacks on the front of the Control Deck. If in doubt, refer to the Control Deck guide for correct jack orientation and positioning.
- Attach the pre-drilled Eurorack panel using the hex nuts supplied with the jacks and pots. Check that the jacks are centred in the panel holes and that all components are seated flat against the Control Deck.
- Leaving the panel attached, flip the Control Deck over and solder the pots and jacks into position from the rear. The audio input jack (J1) and the two CV jacks (J2 and J3) require their switch pins to be connected to ground. This is achieved by adding a small solder bridge between the switch and ground pins of JPS3, JPS4, and JPS7 on the rear of the Control Deck.
- Remove the panel, position the 40-pin female header, J23, on the rear of the Control Deck, and solder it into place from the front. If necessary, hold the header in position with masking tape. Solder one pin at either end first and check that the header is perpendicular before soldering the remaining pins.
If you haven’t already inserted the ICs, add them now, double-checking that they are in the correct orientation. Then assemble the three boards using the pin headers.
Testing, Testing, check one two.
Before you power up your module 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.
Calibration
As noted in part one, the cutoff CV input is designed to have a musically useful exponential response. While this is not intended to be a precision 1 V/octave converter, it is possible—with a little care—to dial in a close approximation. A full explanation of how the exponential converter works is beyond the scope of this guide, but we cover the theory in more detail in our MS-20 filter build guide.
To approximate a 1 V/octave response, we want the output of U2B to change by approximately 18 mV for each 1 V change at its input. Because U2B is configured as an inverting amplifier, this means the output voltage decreases by 18 mV for every volt applied to the input. Achieving this scaling depends on the ratio between the input resistor R6 and the effective feedback resistance formed by R8 and RV5. The purpose of RV5 is to allow this ratio to be adjusted to compensate for component tolerances and to trim the response.
Since R8 is 100 kΩ, the total feedback resistance needs to be approximately 1.8 kΩ, as: 1800 / 100000 = 0.018
There are two practical ways to calibrate this:
- With the power disconnected, measure the series resistance of R8 and RV5 and adjust RV5 until the total resistance is close to 1.8 kΩ.
- Alternatively, if you have a voltage source capable of generating accurate 1 V steps, temporarily disconnect R5, apply a known input voltage, and measure the voltage at pin 7 of U2B relative to ground. Adjust RV5 until a 1 V change at the input produces an 18 mV change at the output.
- Once set, reconnect R5.
Making your State Variable Filter look beautiful
The final step is to make your shiny new DIY filter look the part next to those expensive 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 red is your thing, you can download our finished label, ready for printing.
Download our State Variable Filter label template.
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