Resistors
A reference I can access on my phone from my bench resistors in guitar pedal circuits. Quick reference for values and colour codes, resistors in pedals for common configurations, then background on what a resistor is and impedance. References at the bottom.
Quick Reference
Colour Code
Read the bands left to right. The first two (or three) bands are digits, the next is the multiplier, and the last is tolerance.
| Colour | Digit | Multiplier |
|---|---|---|
| Black | 0 | ×1 |
| Brown | 1 | ×10 |
| Red | 2 | ×100 |
| Orange | 3 | ×1k |
| Yellow | 4 | ×10k |
| Green | 5 | ×100k |
| Blue | 6 | ×1M |
| Violet | 7 | — |
| Grey | 8 | — |
| White | 9 | — |
| Gold | — | ×0.1 (5% tol) |
| Silver | — | ×0.01 (10% tol) |
e.g.: Brown-Green-Orange-Gold = 15 × 1k = 15kΩ, 5% tolerance.
Common Values in Pedals
| Application | Typical Values | Purpose |
|---|---|---|
| Pulldown resistor | 1MΩ | Prevent switching pops |
| Input resistor | 10k – 100kΩ | Set input impedance/level |
| Voltage divider (bias) | 2× equal values (e.g. 100kΩ) | Create 4.5V reference from 9V |
Power Ratings
Resistors come in different power ratings: the maximum wattage they safely dissipate into heat. You'll usually see ratings like 1/8W, 1/4W, 1/2W, and 1W. Guitar pedals operate at pretty low voltage and low currents, so 1/4W resistors are usually sufficient.
Resistors in Pedals
Adjusting Input/Output Level
An input resistor sets the amplitude of signal entering an amplification stage. In the Big Muff, changing the input resistor changes how much signal reaches the transistor changing the gain. In the Bazz Fuss, a current-limiting resistor limits the current available to set the bias through the diode. Increase the value for a smoother, less aggressive sound. The same principle applies at the output, where usually a variable resistor (e.g. a pot) sets the final volume.
Pulldown Resistors
When a pedal is bypassed, charge can accumulate at disconnected inputs or in capacitors (like Dubson's demo in Physics 2 on plugging into a wall outlet). Engaging the pedal can discharge this through the circuit, causing loud pops. A pulldown resistor (usu. 1MΩ) connects the signal path to ground, providing a quiet discharge path that protects the circuit.
Voltage Dividers
Two resistors in series divide the supply voltage at the junction between them:
\[ V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} \]
With equal resistors, you get half the input—essential for biasing op-amps to 4.5V (half of 9V) or setting a transistor's base voltage.
Potentiometers
A pot is a variable resistor. You can replace any fixed resistor with a pot for control over that parameter: gain (e.g. in a feedback loop), volume (pot to ground at output), tone (e.g. an RC filter).
What Is a Resistor?
A resistor opposes current flow. This relationship is Ohm's Law:
\[ V = I \times R \]
where V = voltage (Volts), I = current (Amps), R = resistance (Ohms).
Your guitar signal is an alternating voltage. When it passes through a resistor, amplitude decreases: lower amplitude is lower volume. Resistors can only decrease; to amplify, you need active devices (transistors, op-amps).
How Resistors Work
A resistor converts electrical energy into thermal energy. As electrons flow through the resistive material, they collide with atoms in the lattice structure, transferring kinetic energy. This energy radiates away as heat. The power dissipated is:
\[ P = I^2 R = \frac{V^2}{R} \]
This is why resistors have power ratings—exceed them and the resistor overheats and fails. It's also why high-power applications (like power amplifiers) use large, finned resistors that can shed heat effectively.
Physical Types
The body colour indicates composition:
Carbon Composition
Dark brown. Vintage tone, higher noise. Can scrounge from old thrift electronics.
Carbon Film
Beige/tan. Common, cheap, sufficient for most builds (esp. distortion effects).
Metal Film
Blue/cyan. Low noise, tight tolerance. You'll get these in fancy pedal kits. Preferred for audio.
Inside, a resistive film (carbon or metal) spirals around a ceramic core. Longer spiral = higher resistance.
Circuit Symbols
Two standards exist: the American zigzag and the European rectangle. I've seen the European/British symbol on some old schematics or on forums, but most use the zigzag.
Impedance
Impedance is AC resistance: it describes how a circuit opposes alternating current (but can be frequency dependent). While a resistor has the same resistance at any frequency, impedance can vary with frequency when capacitors or inductors are involved. For a pure resistor, impedance equals resistance.
Guitar signals are AC (alternating current), oscillating back and forth at audio frequencies. This makes impedance critical in pedal design.
Input Impedance
Input impedance determines how much a circuit "loads" the signal source. A high input impedance (like 1MΩ on most pedal inputs) draws very little current from your guitar's pickups, preserving the full frequency response. A low input impedance loads down the pickups, rolling off high frequencies.
Passive guitar pickups have high output impedance (typically 5k–15kΩ), so they need to see a high input impedance to sound right. Active pickups have low output impedance and are less sensitive to loading.
Output Impedance
Output impedance determines how well a circuit can drive a load. Low output impedance means the circuit can deliver current without significant voltage drop, it can drive long cables or low-impedance inputs without losing signal. Most pedal outputs are buffered to provide low output impedance (under 10kΩ, often under 1kΩ).
NOTE: output impedance should be much lower than the input impedance it's driving (at least 10:1 ratio) to avoid signal loss.