Electronic Components Explained
An overview of the parts you'll find in synthesizer circuits: what they do, how they work and when to use them. Where we stock a part you'll find a shop link; the rest is here simply to help you understand your circuits.
Want to test these parts? See Using a Multimeter & Testing Components. Reading a parts list? See BOM: The Bill of Materials.
Passive components
Resistors
What they do: limit current and set voltages. How: they resist current flow (Ohm's law: V = I × R). When: everywhere – setting op-amp gain, limiting LED current, voltage dividers, mixers and CV scaling.
- Metal film (1%) – the standard for synth audio and CV paths: low noise and accurate.
- Carbon film (5%) – cheaper and fine for LEDs and non-critical spots.
- Precision (0.1%) and tempco resistors – used in 1V/octave circuits and exponential converters, where tracking and temperature stability matter.
- Reading values: through-hole resistors use colour bands (see how to read them below); SMD resistors use a number code (e.g. 103 = 10 kΩ, 1002 = 10.0 kΩ).
How to read resistor colour codes
Through-hole resistors are too small to print a value on, so they use coloured bands instead. Once you know the system, you can read any resistor in a few seconds.
- Count the bands. 4 bands: two digits, a multiplier and a tolerance (usually 5% carbon film). 5 bands: three digits, a multiplier and a tolerance (usually 1% metal film, the standard in synth circuits).
- Find the start. The tolerance band, usually gold (5%) or brown (1%), sits a little apart from the others. Hold the resistor with that band on the right.
- Read the digits from left to right, then multiply by the multiplier band. Brown-black-orange-gold is 1, 0, × 1k = 10 kΩ ±5%.
Two more to practise: yellow-violet-red-gold = 4, 7 × 100 = 4.7 kΩ ±5%; brown-black-yellow-gold = 1, 0 × 10k = 100 kΩ ±5%.
Tip: some colours are hard to tell apart, like brown and red, or red and orange, especially on blue metal film resistors and under yellow light. When in doubt, measure the resistor with a multimeter before you solder it in.
Potentiometers & trimmers
What they do: variable resistors – the knobs on your module. How: a wiper slides along a resistive track. When: linear (B) pots for CV and most controls; logarithmic (A) pots for volume. Common Eurorack styles are the T-18 knurled shaft and the D-shaft. Trimmers are small pots for one-time calibration, such as VCO tuning or scale adjustment; multi-turn trimmers give finer control. Shop pots & trimmers · Shop knobs
Capacitors
What they do: store charge, pass changing signals and block DC. How: two plates separated by an insulator; their value is in farads (pF, nF, µF).
- Ceramic (MLCC) – small and cheap. Use 100 nF next to every IC's power pins for decoupling. C0G/NP0 types are stable enough for audio and timing.
- Film (polyester, polypropylene, polystyrene) – stable and low-distortion. The choice for VCO timing caps, filter caps and sample & hold circuits.
- Electrolytic – large values (1–1000 µF) for power filtering and AC coupling. Polarised: the stripe marks the negative leg. Choose a voltage rating above your supply (25V or more on ±12V rails).
- Tantalum – compact, polarised, used for power filtering. They can fail short if reversed or overvolted.
- Reading codes: 104 = 100 nF, 103 = 10 nF, 102 = 1 nF.
Inductors & ferrite beads
Inductors store energy in a magnetic field and resist changes in current. In Eurorack you mostly meet them as ferrite beads or small inductors on the power lines, filtering out high-frequency noise from switching power supplies. Shop ferrite beads
LDRs & vactrols
Light-dependent resistors change resistance with light. Combined with an LED in a sealed package (a vactrol), they make the smooth, natural-sounding response of low-pass gates and optical compressors.
SMD parts: sizes and footprints
More and more modules use SMD parts (surface-mount devices). Instead of legs that go through holes in the PCB, they have small metal ends that are soldered onto flat pads on the surface. They are much smaller and can be placed by machines, which is why many kits come with the SMD parts already soldered on. The flat pads they sit on are called the footprint.
SMD resistors and ceramic capacitors come in standard rectangular sizes with a four-digit code. The first two digits are the length and the last two the width, in hundredths of an inch: 0603 = 0.06 × 0.03 inch = 1.6 × 0.8 mm.
| Size (inch code) | Metric code | Size in mm | Typical resistor power | Hand soldering |
|---|---|---|---|---|
| 0201 | 0603M | 0.6 × 0.3 | 1/20 W | Not for hand soldering: microscope work |
| 0402 | 1005M | 1.0 × 0.5 | 1/16 W | Hard: magnifier, very fine tweezers and a steady hand |
| 0603 | 1608M | 1.6 × 0.8 | 1/10 W | Very doable with tweezers and a fine tip; very common in Eurorack (e.g. Mutable Instruments) |
| 0805 | 2012M | 2.0 × 1.25 | 1/8 W | Easy: the best size for your first SMD build |
| 1206 | 3216M | 3.2 × 1.6 | 1/4 W | Very easy, but takes up more space on the board |
Watch out: inch or metric? The codes in this table are the usual inch-based codes. Some manufacturers (mostly in Japan) use metric codes, and the numbers overlap: a metric 0603 is the tiny 0.6 × 0.3 mm part, which is an inch-code 0201! Always check the size in millimetres in the datasheet before you order.
Why smaller gets harder
- You can hardly see or hold them. A 0402 part is about the size of a grain of sand. You need a magnifier or microscope, sharp precision tweezers and a fine soldering tip.
- They disappear. One sneeze or a slip of the tweezers and a part is gone. Work over a light, plain surface like a silicone soldering mat and always order spares.
- No markings. 0603 and larger resistors usually have a value code printed on them; 0402 resistors and almost all ceramic capacitors have none. Once a part leaves its paper tape, you can only find its value by measuring. Keep the tape and open one strip at a time.
- Tombstoning and bridges. Small parts easily stand up on one end (“tombstoning”) when one pad melts before the other, and solder bridges form quickly between close pads. Use flux, little solder, and solder wick to clean up.
- Lower ratings. Smaller resistors handle less power, and smaller ceramic capacitors often have a lower voltage rating. Ceramic capacitors in small sizes also lose more of their capacitance under DC voltage, so a small 10 µF capacitor may only give a few µF on a 12 V rail.
Can you swap sizes?
The footprint on the PCB is designed for one size, but there is some room to improvise:
- One size smaller usually works: an 0603 part on 0805 pads, or an 0402 part on 0603 pads. The part bridges the gap between the pads; just centre it well.
- One size bigger is tight: an 0805 part on 0603 pads can be done, but the solder joints are small and the part may touch nearby parts. Avoid it on crowded boards.
- Two sizes difference doesn’t work: a 0402 part simply can’t reach both 0805 pads.
- Check the ratings: when you go smaller, make sure the power rating (resistors) and voltage rating (capacitors) are still high enough. In most Eurorack signal circuits a 1/16 W resistor is plenty, but on power lines it may not be.
- Value, tolerance and type stay the same: a 10 kΩ 1% resistor stays 10 kΩ 1%, and a C0G capacitor should be replaced with a C0G capacitor.
- Chips can’t be swapped: IC and transistor packages such as SOT23, SOT223, SOIC and TQFP have a fixed pin layout. The same chip in another package needs a different footprint.
Tip: if you design your own boards, 0805 or 0603 is a great choice for hand soldering. And if a kit offers the SMD parts pre-assembled, it saves you a lot of time and squinting.
SMD IC packages
Chips come in many shapes. The pin pitch (the distance between pin centres) decides how hard they are to hand-solder.
| Package | Pins | Pitch | Hand-soldering | Typical use |
|---|---|---|---|---|
| SOT-23 | 3–6 | 0.95 mm | Easy | Single transistors, small regulators |
| SOT-223 / DPAK (TO-252) | 3 + tab | 2.3 mm | Easy, needs heat for the tab | Voltage regulators, such as our LD1117DT50CTR (DPAK) |
| SOIC | 8–16 | 1.27 mm | Easy | Op-amps (TL072 in SOIC-8), logic chips |
| TSSOP / MSOP | 8–28 | 0.65 mm (0.5 mm MSOP) | Moderate: use flux and drag soldering | DACs, small digital chips |
| LQFP / TQFP | 32–144 | 0.5 mm | Moderate to hard | Microcontrollers, such as our STM32F405RGT6 (LQFP-64) |
| QFN | 16–64 | 0.4–0.65 mm | Hard: pads are under the chip; hot air or paste | Codecs, modern microcontrollers |
| BGA | many | balls under the chip | Not by hand | Leave these to the factory |
See How to Solder for the drag-soldering method.
Reading component markings
SMD resistors
| Code | How to read it | Value |
|---|---|---|
103 |
3 digits: two digits, then the number of zeros | 10 000 Ω = 10 kΩ |
4R7 |
R marks the decimal point | 4.7 Ω |
1002 |
4 digits (1% parts): three digits, then the number of zeros | 10 000 Ω = 10 kΩ |
01C |
EIA-96 (1% parts): a code for the digits (01 = 100), a letter for the multiplier (C = ×100) | 10 kΩ |
000 or 0
|
Zero-ohm jumper | 0 Ω |
Capacitors
-
Through-hole ceramic and film: the same 3-digit code in picofarads.
104= 100 000 pF = 100 nF,222= 2.2 nF. A letter after it gives the tolerance (J = 5%, K = 10%). - SMD ceramic (MLCC): usually not marked at all. Keep them in their labelled tape until you place them, or measure with a meter that has a capacitance range.
-
Electrolytic and tantalum: value and voltage printed in full, for example
10µF 25V; small SMD tantalums use a code such as106(10 µF).
Through-hole resistors
Colour bands: 4 bands (2 digits + multiplier + tolerance) or 5 bands (3 digits + multiplier + tolerance). Brown-black-orange = 10 kΩ. When in doubt, measure it with a multimeter.
Polarity and voltage ratings
Polarised parts only work one way round, and some fail dramatically when reversed. Check these before you solder:
| Part | How to find the orientation |
|---|---|
| Electrolytic capacitor (through-hole) | The stripe marks the negative leg; the longer leg is positive. On many PCBs the shaded half of the circle is negative. |
| Electrolytic capacitor (SMD can) | The black-marked part of the top marks the negative side. |
| Tantalum capacitor | Careful: here the stripe marks the positive side. |
| Diode | The band marks the cathode, matching the line in the PCB symbol. |
| LED | The longer leg is the anode (+); the flat side of the rim is the cathode. |
| IC | The notch or dot marks pin 1; match it with the notch on the PCB silkscreen. |
Voltage rating: choose capacitors rated well above the voltage they see. On ±12 V Eurorack rails, 25 V or more is a safe choice (like our 10 µF 25 V SMD capacitors); 16 V parts on a 12 V rail work, but with little margin. A higher voltage rating than the BOM is always fine, as long as the part still fits the footprint.
Semiconductors
Diodes
What they do: let current flow in one direction only. A silicon diode drops about 0.6–0.7V when conducting.
- Signal diodes (1N4148) – clipping, wavefolding, envelope generators, rectifiers and logic.
- Schottky diodes (1N5817 / 1N5819) – low voltage drop; used on module power inputs to protect against reversed ribbon cables.
- Rectifier diodes (1N4001–1N4007) – for higher currents in power circuits.
- Zener diodes – conduct backwards at a fixed voltage; used as simple voltage references and for protection (clamping CV inputs).
- Precision voltage references (LM4040, LM336) – accurate, stable voltages for offsets and tuning.
LEDs
Light-emitting diodes show signal activity or power. Always use a series resistor (typically 1–10 kΩ in Eurorack) to limit the current; the long leg is the anode (+). Flat-head LEDs (like our red and green ones) sit flush behind a panel. Bi-colour LEDs show positive and negative voltages in different colours. Shop LEDs
Transistors
What they do: amplify or switch signals. A small base current controls a larger collector current.
- NPN / PNP (BC547/BC557, 2N3904/2N3906) – VCO cores, gate and trigger buffers, LED drivers.
- Matched pairs (e.g. BCM847, or hand-matched BC547s) – the heart of exponential converters for 1V/octave tracking, and of ladder filters.
- JFETs (J111, J201, 2N5457) – used in reset/sync circuits, analog switches and some filters.
- MOSFETs (2N7000, BS170) – simple switching, for example driving LEDs or relays from logic.
Voltage regulators
Chips such as the 7805 or 78L05 (+5V) and 79L05 (−5V) create a stable local voltage from the ±12V rails. Use them when a circuit or a microcontroller needs its own clean supply. LDO (low-dropout) regulators such as the LD1117DT50 (5V) or the AMS1117-3.3 (3.3V) supply digital chips.
Integrated circuits
Op-amps
The workhorses of analog synths: buffers, mixers, amplifiers, comparators, integrators and active filters. TL072 (dual) and TL074 (quad) are the classic Eurorack choice. NE5532 is low-noise for audio; LM358 / LM324 can work close to the negative rail; MCP6002-type rail-to-rail op-amps suit 3.3V/5V digital front ends. Always fit IC sockets so you can swap chips easily: 8-pin for a TL072, 14-pin for a TL074 and 16-pin for chips like the AS3340. Shop ICs & sockets
OTAs (operational transconductance amplifiers)
Chips like the LM13700 and CA3080 turn a control current into a gain – ideal for VCAs, voltage-controlled filters and resonance control.
Alfa AS33xx synth chips
Alfa RPAR in Riga, Latvia makes modern versions of the classic Curtis (CEM) and RCA synth chips, giving you a complete building block on a single IC:
- AS3340 / AS3340A – voltage-controlled oscillator (CEM3340 version): temperature compensated, triangle, saw and pulse outputs with voltage-controlled pulse width, linear and exponential FM, hard and soft sync. The AS3340A is the improved version. Check the datasheet for the negative supply resistor when running on −12V.
- AS3320 – 4-pole voltage-controlled filter with voltage-controlled resonance (CEM3320 version).
- AS3310 – voltage-controlled ADSR envelope generator (CEM3310 version).
- AS3360 – dual VCA / transconductor for amplifiers and filters (CEM3360 version).
- AS3330 – dual VCA with both linear and exponential control inputs.
- AS3280 / AS3080 – OTAs (CA3280 / CA3080 versions) for VCAs and filters.
- AS2164 – quad VCA (SSM2164 version), great for mixers and multi-channel VCAs.
When to use them: when you want a stable, well-tested synth voice with few parts – they're behind many classic polysynth sounds and modern Eurorack VCOs and filters. AS3340 datasheet (PDF)
Logic & timing chips
- CD40106 – six Schmitt-trigger inverters: simple oscillators, LFOs and clean gate signals.
- CD4017 – decade counter for simple sequencers and clock dividers.
- CD4013 / CD4040 – flip-flops and binary counters for clock division.
- CD4066 / CD4051 – analog switches and multiplexers for switching signals with CV.
- NE555 / TLC555 – timers for LFOs, pulse generators and simple oscillators.
Microcontrollers & DACs
Digital modules use microcontrollers such as STM32 (like the STM32F405RGT6 found in many Mutable Instruments designs, or the STM32F103C8T6 development board for experimenting), RP2040 or ATmega (Arduino), together with DACs (e.g. MCP4922) to output control voltages. STM32 chips are programmed with an ST-Link programmer, ATmega chips with a USBasp AVR programmer.
Electromechanical parts
Jacks
3.5 mm mono jacks (such as the WQP518MA) are the Eurorack standard for audio, CV and gates; stereo jacks are used for stereo outputs and headphones. Most have a switch contact (normalling) that connects a default signal when nothing is plugged in. Secure them with jack socket nuts. Shop jacks
Switches, buttons & encoders
Toggle switches (also as sub-mini) select modes or ranges (SPDT for two positions, ON-OFF-ON for three); momentary tactile switches with switch caps send manual gates. A rotary encoder is an endless knob used for menus on digital modules. Shop buttons & switches
Choosing hardware for a kit
Jacks, pots and switches must match the PCB footprint and the panel holes. Use this as a quick guide and always compare with the BOM.
| Part | What to check | From our shop |
|---|---|---|
| Mono jack | The Thonkiconn-style footprint is the DIY Eurorack standard | WQP518MA, Mono Jack Set |
| Stereo jack | Different footprint from mono; used for stereo and headphone outputs | WQP419GR, PJ301-CM |
| Potentiometer | Value and taper (B = linear, A = log), shaft type (D-shaft or knurled T18) and shaft length | D-shaft pots, T18 pots |
| Knob | Must match the shaft: D-shaft or T18, 6 mm | Knobs |
| Trimmer | Top or side adjust, value | Trim pots |
| Toggle switch | Size (mini or sub-mini) and type: ON-ON, ON-OFF-ON, or momentary | Mini, Sub-mini |
| Tactile switch | Footprint and cap | Tactile Switch Set |
| Encoder | Detents, push switch, shaft | Rotary encoder |
Tip: fit the jacks, pots and switches into the panel before soldering them, and tighten the nuts. Then solder. Everything lines up perfectly with the panel holes.
Power headers & pin headers
10-pin and 16-pin box headers connect your module to the bus board via a ribbon cable; the notch prevents plugging in backwards. Unshrouded versions are available as through-hole or SMD power headers. Male and female pin headers connect stacked PCBs together, and jumpers set options on a board. Shop box headers · Shop pin headers
Prototyping
Test circuits on a breadboard before soldering, then move to stripboard, perfboard or a PCB. Shop breadboards
Want to understand these parts in depth? Watch our favourite DIY educators, read one of our recommended books, or learn how to solder.
Continue Shopping
-
Jack sockets
Jack sockets for Eurorack modules
