An Easy Introduction To Building DIY Eurorack Modules By Nonlinearcircuits (Part 1)

For Perth-based designer Andrew Fitch, who started building synthesizers in 1998, the Eurorack brand Nonlinearcircuits grew out of a long-running workshop series hosted by West Australian Modular. While Nonlinearcircuits modules are neither the easiest DIY modules to start with nor the most typical, there are several reasons why Nonlinearcircuits is an unusually compelling choice for people who are new to Eurorack or new to DIY electronics (or both).

Nonlinearcircuits’ earliest modules grew out of the particular constraints of the West Australian Modular workshop series. Andrew has said that he often designed one or more modules each month to give participants fresh and exciting projects to build. Designs were optimized for simplicity and accessibility: components needed to be easy to source and assembly needed to be manageable within a workshop session.

What sets Nonlinearcircuits apart is its focus on DIY. Whether you're new to surface-mount technology (SMT) or have some soldering experience, NLC modules are designed with builders in mind. Andrew Fitch’s design philosophy revolves around simplicity: standardized components like 100k resistors, minimal use of rare or specialty chips, and straightforward circuit layouts make these modules ideal for DIY enthusiasts.

Because many workshop participants were attending workshops as a way to build up a complete modular synthesizer, over the course of Nonlinearcircuits’ long history, Andrew has designed modules that cater to a wide variety of needs within a modular system. From utility modules to effects (lots of phasors), oscillators, filters, and even sequencers and a spring reverb, NLC’s catalog is one of the most diverse of any Eurorack brand. Importantly, these modules often have a unique and experimental edge. They’re not “toy” projects. There’s always something in Nonlinearcircuits’ designs to interest advanced users, while simultaneously often being fairly accessible to beginners.

Building NLC modules is more than a technical exercise; it’s an opportunity to join a global community of synth enthusiasts who value creativity, collaboration, and hands-on exploration. Andrew has stated that he was influenced by the DIY online communities he experienced living in Japan in the 1990s:

This communalist philosophy of DIY is very much a part of the NLC builders community today. Through the Nonlinearcircuits Builders Guild on Facebook and an associated Discord channel, new builders and those experienced in the DIY community share tips and encourage one another, troubleshooting issues, offering build advice, and celebrating completed projects. Many first-time builders find these projects to be an approachable gateway into the world of SMT soldering and modular synthesis, thanks to clear instructions and a philosophy that celebrates imperfection and discovery.

Experimental Eurorack Modules by Nonlinearcircuits

Perhaps the most compelling reason to build Nonlinearcircuits modules is that they are like no other modules in the Eurorack world. Although influenced by Serge designs, Andrew has a particularly experimental and whimsical approach to design. Many Nonlinearcircuits modules incorporate chaos as a generative element (perhaps reflecting the maven’s Ph.D. research on chaos circuits with Professor Herbert H. C. Lu at the University of Western Australia). Other circuits are based on obscure research papers (Ming Rod, Bi-Di Choppers, Let's Get Fenestrated) or unique twists on speculative DIY circuits (Beat Freq). Some circuits appear to be influenced by simulation of biological processes (Squid Axon, Cellular Automata), and a few are based on classic synthesizers (1050 Mixer Sequencer, Choral Generator, MUN).

Even in Andrew’s simpler designs—simple utility modules, mixers, or other voltage processors—there’s often a creative or experimental twist that sets them apart. For example Let's Get Fenestrated, a triple window comparator, features three distinct comparator designs, each optimized for different types of signal processing and modulation. Even something seemingly simple like a mixer becomes a unique part of a custom synthesizer as reimagined by Andrew. The Router module, developed for the cellF neural synthesizer at MoNA’s Mofo exhibition, is a versatile signal-routing module capable of multiple configurations: 1-to-4, 4-to-1, 4-to-4, and 2x2-to-2. With a simple PCB modification, it also supports 2x1-to-2 routing. Pots act as manual switches (0 = off, 10 = on) or as CV/gate thresholds for dynamic control.

How to Start Building Nonlinearcircuits Modules: SMT vs. Through Hole

Before diving into building Nonlinearcircuits (NLC) modules, it’s important to understand the two main types of components you’ll encounter: surface-mount technology (SMT) and through-hole (TH) components. Each has a unique role in electronics assembly and requires different soldering techniques.

What is Through-Hole Technology?

Through-hole (TH) technology has been a staple in DIY audio projects for decades, from the classic Doepfer A-100 modules to countless DIY guitar stompboxes. This method uses components with long leads inserted into pre-drilled PCB holes, prioritizing durability and ease of assembly. Larger and sturdier, through-hole parts like jacks, knobs, and LEDs are ideal for user-facing controls that endure frequent use. Even in SMT designs, through-hole components remain essential for robust mechanical connections, making them a beginner-friendly choice for DIY Eurorack modules and stompboxes.

What is SMT?

Several tiny surface-mount devices (SMDs) rest on a fingertip, illustrating the range of sizes used in SMT components.

Surface-mount technology (SMT) is a method where electronic components are soldered directly onto the surface of a printed circuit board (PCB) rather than requiring holes for their leads. SMT components come in a variety of size standards, from tiny 0201 packages—barely larger than a grain of sand—to the relatively larger 0805 size, which is commonly used in Nonlinearcircuits (NLC) modules. These size standards reflect the physical dimensions of the components and play a key role in determining how challenging they are to work with.

In the world of Eurorack synthesizers, space is at a premium. Modules need to pack a lot of functionality into a small form factor to fit within a standard rack. SMT components, especially compact sizes like 0805, are perfect for this because they allow for dense, efficient circuit designs. Smaller components free up room on the PCB for more complex circuits or additional features, enabling the creation of cutting-edge synthesizers without increasing their physical size.

For DIY synth hobbyists, this means learning to work with SMT is becoming essential. As the industry increasingly relies on SMT components to meet the demands for smaller, more powerful designs, being comfortable with SMT soldering ensures you can build and repair modern synthesizer modules.

While the smallest SMT components, such as 0201 or 0402, can be difficult to see and handle without specialized equipment, 0805 components offer a great balance for beginners. They are large enough to solder with a standard fine-tip soldering iron and tweezers, making them accessible even if you're just starting out. Most NLC modules use 0805 components precisely because they are approachable while still leveraging the space-saving advantages of SMT.

Although SMT soldering may seem intimidating at first, starting with 0805 components provides an ideal learning curve. With a bit of practice and the right tools, you'll quickly gain the confidence to tackle increasingly complex builds and fully immerse yourself in the world of synthesizer DIY.

How to Start Building DIY Nonlinearcircuits Modules

If you've never held a soldering iron before, you may want to start by practicing tinning—a simple technique where you apply a thin coat of solder to the tip of a wire or soldering iron. This will help you get comfortable with the way solder flows and how to control your soldering iron. A good first exercise is soldering two wires together. Strip about 1/4 inch of insulation from each wire, twist the exposed ends together, and apply solder to create a solid connection. This basic skill will give you the confidence to move on to soldering components onto a PCB.

Although solder isn’t particularly dangerous to handle, the process of soldering can present some hazards. Flux fumes, while not acutely toxic, can irritate your eyes and respiratory system, so you always want to solder in a well-ventilated space. I’ve never come close to getting solder to spatter, but cutting wires with flush-cutters can propel them at high speed in unpredictable directions, so it’s also a good idea to wear safety goggles to protect your eyes, just in case. Additionally, washing your hands after handling solder or soldering components is a simple way to minimize exposure to any residual materials.

The temperature you should solder at depends on several factors, including the melting point of your solder, the size and shape of the tip on your soldering iron, the type of soldering you are doing (such as SMT or through-hole), and even your level of experience and personal preference. The type of solder you use plays a key role, as lead-free solder typically melts at a higher temperature (around 217°C for SAC305) compared to leaded solder (183°C for 63/37 alloy). However, the soldering iron’s tip size and mass also affect heat transfer; a larger tip can deliver heat more efficiently to the joint, while a smaller tip may require a slightly higher temperature to maintain good thermal performance.

In most cases, you’ll want to solder at a temperature significantly higher than the melting point of your solder. This is because the solder needs to remain fluid and workable long enough to flow properly across the joint and form a strong connection. For leaded solder, a typical temperature range is 315°C–350°C, while lead-free solder often requires a range of 350°C–375°C. The added heat helps compensate for heat loss to the component, PCB, and surrounding air, ensuring the joint reaches the appropriate temperature quickly without prolonged exposure that could damage sensitive parts.

It’s important to strike the right balance—too low a temperature will result in cold joints that are brittle and unreliable, while too high a temperature risks damaging components or lifting PCB pads. Practicing on scrap boards or components is a great way to experiment and find what works best for your soldering style.

Best first DIY Nonlinearcircuits Modules

Traditionally people who are new to DIY electronics are encouraged to start with larger through-hole projects for their first soldering project. Although the vast majority of NLC projects use the 0805 SMT standard, there are a small number of through-hole NLC projects as well. This includes Timbre!, Sloth Chaos (Single, 4hp), Neuron, Mixer, Low Pass Gate, Dual OTA VCA, Dual LFO, ADSR312, De-Escalate, and (perhaps the most classic first project of all) a passive multiple.

For those ready to dive into SMT soldering but seeking a gentle introduction, NLC also offers a range of hybrid modules that combine 0805 SMT resistors and capacitors with through-hole IC chips and connectors. This approach provides the best of both worlds: you’ll gain experience working with SMT components, but the more complex or heat-sensitive integrated circuits (ICs) are through-hole, making placement and soldering easier. Great first projects in this category include bong0 and BOOLs.

One of the best modules that I like to recommend for people to start with is the 4hp Mix module. With just 14 components to solder, it's one of the simplest modules in the NLC catalog, but it’s more than just an easy project; it’s a genuinely useful module which you would probably want multiples of in any large NLC-focused rack. As the name suggests, it’s a four-input mixer that handles both audio and CV (control voltage) signals. Its pots are intuitive, with a zero point at the center and gain control in both directions. Turn a pot to the right, and you amplify the signal (1x gain); turn it to the left, and you invert it (-1x gain). The fourth pot has a neat trick up its sleeve: if nothing is patched into input 4, it acts as a voltage offset, providing a range of -5V to +5V. That’s a lot of functionality packed into a slim 4HP module.

Another great place to start is with Nonlinearcircuit’s 1U modules. These are small modules which don’t typically use knobs, so they make great fast projects which can mostly be completed in an evening. The 1U Sloth Chaos is a scaled-down version of the larger Sloth Chaos modules, offering slowly evolving chaotic voltage outputs perfect for adding subtle modulation or randomness to your patches. With only a handful of components and no potentiometers, it’s an easy and rewarding build.

Slightly larger, the 1U PiLLs is a scaled-down version of a circuit from the Mobius PiLL. It uses two cross-coupled PLL ICs to generate chaotic, noise-driven soundscapes. The module includes vactrols for organic modulation and offers five inputs and four outputs for complex interactions between audio and CV signals. This makes it an excellent choice for those who enjoy creating experimental textures or working with chaotic behavior.

The 1U XOR and 1U Signum are both utility-focused modules with creative twists. The XOR acts as a passive logic module or pseudo ring modulator, working with audio, CV, or gate signals to produce unpredictable and interesting results. With just five component types, the XOR is also one of the smallest NLC builds. Another small build, the Signum is a three-state switch, processes CV in unique ways, offering outputs based on whether the input is positive, negative, or near zero.

All 1U NLC modules are compact, easy to build, and bring useful tools to your rack with just a few hours of work at most.

How to Read a Nonlinearcircuits BOM

Nonlinearcircuits (NLC) Bills of Materials (BOMs) follow a consistent format, but interpreting them effectively requires some familiarity with DIY electronics terminology. Each BOM lists the components required to assemble a module, including values, quantities, and package types. Below are key elements to understand when reading an NLC BOM:

Nonlinearcircuits BOM Organizational Structure

NLC BOMs are usually in PDF format and include some discussion of the module and the circuit's origins and structure. There’s usually a schematic, which uses standard electronics symbols to represent the module’s circuit structure. Schematics are a valuable resource for understanding how the components work together and troubleshooting potential issues during the build process. Most importantly there’s a list of components, their quantities and locations on the circuit board.

Typical Column Structure in NLC BOMs

Andrew Fitch, the designer behind Nonlinearcircuits, organizes BOMs in a way that balances clarity and detail. The BOM may have either three or four columns, depending on the module's complexity and the amount of information provided. These columns typically include:

1. Component: Specifies the type and value of the component, such as "Resistor 100k (0805)" or "Capacitor 10µF."
2. Quantity: Indicates the number of each component required for the build. For instance, "Qty: 10" means 10 identical resistors are needed.
3. Designators (Occasional): Lists the specific locations on the PCB where each component is placed, using labels like "R1, R2, R3" for resistors or "C1, C2" for capacitors. Some component’s location may be simply specified with a number or letter. Look on the PCB to find where the component is supposed to go. This column may be combined with the quantity column in some BOMs.
4. Notes: Provides additional details, such as sourcing tips, specific installation instructions, or alternate component options. For example, "Super-bright LED.” Often this column specifies a suggested part number from Tayda or Mouser. Many components can typically work for a certain BOM item, and BOM Squad is a great resource to find other components people are using for a certain build.

Substitutable Component Values in NLC BOMs

One of the unique aspects of Nonlinearcircuits BOMs is their frequent inclusion of substitutable component values, allowing builders to tweak certain aspects of a module’s behavior. These substitutions are often documented with multiple columns in the list of components or with a separate table showing substitutions, which is cross referencing in the notes section of the main component list.

On BOM Squad we help clarify these substitutions by creating separate versions of the BOM which you can toggle between. This allows you to export the entire parts list for whatever your preferred flavor of BOM happens to be. (Occasionally individual parts may be substituted in an ad hoc manner. This is common with, for example, LEDs, where you may have a choice of color based on personal preference. In this case, we make a note of this potential for substitution in the component dropdown of the virtual BOM for that module.)

An excerpt from the bill of materials (BOM) for the Nonlinearcircuits (NLC) ADSR312 module. The table lists key electronic components, including an RL, which are used to control the brightness of LEDs by adjusting the amount of current flowing through them. Capacitors in the BOM are labeled using standard industry notation, such as "104" which represents a capacitance of 100nF (0.1µF).

Notes on Resistor and Capacitor Notation (e.g., 2k5, 2u2)

In electronics, shorthand notation is often used to indicate values of resistors, capacitors, and other components. This system is compact, easy to read, and avoids potential confusion caused by symbols like decimal points, which can be misread or obscured in technical documents. Here's an explanation of how to interpret these notations:

Resistor Notation

Resistor values are typically given in ohms (Ω), kilohms (kΩ), or megohms (MΩ). Instead of using a decimal point, the multiplier (e.g., "k" for kilo) is often placed where the decimal point would be.

Examples:

• 2k5 = 2.5 kΩ = 2,500 Ω
• 4M7 = 4.7 MΩ = 4,700,000 Ω
• 100R = 100 Ω (the "R" stands for "resistor" and is used instead of a decimal point for values below 1 kΩ).

This notation is particularly useful in schematics and BOMs, as it avoids misinterpretation due to poor print quality or formatting.

Capacitor Notation

Capacitor values are given in farads (F), but because most capacitors have very small capacitances, the common units are microfarads (µF), nanofarads (nF), and picofarads (pF). Shorthand notation replaces the decimal point with the unit prefix.

Examples:

• 2u2 = 2.2 µF (microfarads)
• 4n7 = 4.7 nF (nanofarads)
• 100p = 100 pF (picofarads)

This avoids confusion between units. For instance, "2u2" makes it clear that the value is in microfarads, while "2.2" could be ambiguous.

Another common way to denote capacitor values is through numerical codes printed directly on the components, such as "104." These codes follow a standardized system where the first two digits represent the significant figures of the capacitance, and the third digit indicates the number of zeros to append in picofarads (pF). For example, "104" means 10 followed by 4 zeros, or 100,000 pF, which is equivalent to 100 nF or 0.1 µF.

These numerical codes are particularly useful for small components like ceramic capacitors, where space constraints make it impractical to print full values. Additionally, variations such as "104 (small)" and "104 (big)" may be used in bills of materials (BOMs) to distinguish between physical sizes, voltage ratings, or other specifications, even though the capacitance remains the same. The "small" and "big" designations help builders ensure proper fit and functionality in specific circuit layouts.

If you’re new to electronics, this notation can feel overwhelming at first. It’s not always clear why certain components are labeled the way they are, or how to interpret size and specification differences. Rest assured, though, that with practice, these patterns become easier to understand. It might help to keep a quick-reference chart or guide handy while you’re learning. Many beginners also find it useful to compare physical components to their BOM descriptions to get a feel for how size and markings correlate in real-world projects.

Component Notation Varies but A Few Clues Can Help

To identify components in a BOM or schematic, look at their value notations and context. Resistors are marked with R, k, or M (e.g., 100R, 4k7, 1M) and are either small rectangular SMT components with printed numbers or cylindrical through-hole components with color bands. Capacitors use u, n, or p to indicate microfarads, nanofarads, or picofarads (e.g., 2u2, 4n7, 100p). SMT capacitors resemble resistors but lack markings, while through-hole versions may be ceramic discs or cylindrical electrolytics with polarity markings.

Keep in mind that capacitors are also sometimes simply represented with their numerical codes, so if you see a number in a BOM without notation microfarads, nanofarads, or picofarads, that's likely referring to the cap's numerical code.

Potentiometers (pots) are adjustable resistors, often labeled in BOMs with a value and the term "pot," such as 100k pot. However, the BOM may not always specify critical details like taper type (linear or logarithmic) or physical features (e.g., knurled or smooth shaft, panel mount, or trimmer style). When these details are not explicitly mentioned, it’s essential to check the notes in the BOM or accompanying documentation for clarification.

If the BOM lacks sufficient information, tools like BOM Squad can be invaluable for cross-referencing common parts used in similar modules and determining the appropriate taper and type. For example, a 100k pot used for mixing or attenuation is often linear (Lin), while one used for volume or frequency controls is typically logarithmic (Log). The physical format of the pot, such as whether it has a knurled shaft for knob mounting or is a trimmer for PCB use, may also depend on context or explicit guidance in the build notes. When in doubt, consulting the schematic, module photos, or builder community can provide the clarification needed to select the correct potentiometer for your project.

Nonlinearcircuits BOM-List Additional Notes

Nonlinearcircuits BOMs frequently include several footnotes which are often grouped in a section called “Additional Notes.” These notes provide useful and occasionally vital context for a build. There are a few common themes that are frequently found in Additional Notes:

• One common feature is specific guidance on component selection and substitutions. For example, all passive components are typically 0805-sized, and capacitors are recommended to have a voltage rating of at least 25V, preferably 50V, except for 10µF capacitors, which are generally only available at 25V. Notes often suggest alternate component values for specific cases, such as replacing 10µF decoupling capacitors near ICs with 100nF if preferred, while emphasizing the importance of keeping 10µF capacitors near power connectors and PCB edges.
• Customization options are frequently highlighted, giving builders the opportunity to tweak resistor or capacitor values to adjust the module's performance. For example, resistors marked with an asterisk (*) can often be modified to optimize functionality, such as adjusting RL values to control LED brightness or altering specific resistors to fine-tune signal balance or mix intensity. These adjustments often come with warnings about potential trade-offs, like increased sensitivity to specific settings or altered behavior that might require recalibration.
• Another common theme is assembly advice tailored to the module's design. This can include tips for ensuring proper placement and alignment of components, such as dragging solder across vias for specific resistors or ensuring that bipolar LEDs are positioned against panel windows before soldering. Special considerations for through-hole components like tempco resistors may also be detailed, such as ensuring they rest on associated transistor pairs for proper thermal coupling.
• Design and schematic clarifications are often provided to address potential confusion. For example, builders are frequently reminded to ignore unused components on specific PCB versions. Numbering anomalies in diodes or other components are also noted, with reassurance that they won’t impact assembly as long as the specified parts (e.g., 16 LL4148 diodes and 2 S1JL power diodes) are placed correctly.
• NLC notes often include practical sourcing recommendations, such as using Tayda for resistors, capacitors, and ICs, while Mouser, Farnell, or E14 are recommended for specialized components like Schottky diodes. Occasionally, part numbers are included to simplify ordering. Builders are also encouraged to seek community support through the Nonlinearcircuits Builders Guild on Facebook or by email for troubleshooting or advice.
• Finally, troubleshooting guidance and assembly warnings are frequently embedded in the notes. These can range from ensuring proper diode orientation (e.g., checking cathode markings) to explaining how certain adjustments might affect the module’s behavior, such as the potential for locking up under specific settings, which can be resolved by tweaking a range pot. Collectively, these notes provide a comprehensive roadmap for both novice and experienced builders, ensuring smooth assembly and opportunities for experimentation.

Conclusion

In part 1 we covered the history and key features of Nonlinearcircuits designs and learned about some common features found in NLC BOMs. In part 2 we'll look at examples of NLC BOMs and also look at some of the common symbols found on NLC circuit boards.