Before we build pedals on breadboards, we need to read the language they're written in: schematics.
Today you'll learn to identify passive component symbols, then use three online tools to draw and simulate simple RC filters.
Resources & Tools
Symbol reference: AllAboutCircuits passive component symbols allaboutcircuits.com/technical-articles/schematic-symbols-electronic-components-passives...
What it does: _____________________________________________________________
Name: ___________________________
What it means: ____________________________________________________________
Name: ___________________________
What it means: ____________________________________________________________
Part 2
Draw RC Filters
CircuitCanvas
Using circuitcanvas.com, draw each filter circuit described below. Then sketch it here by hand. Use proper schematic symbols. Label each component with its value.
A.
Low-pass filter (treble cut): R = 22 kΩ in series, C = 10 nF (103) to ground. Output taken between R and C.
Low-pass schematic
B.
High-pass filter (bass cut): C = 10 nF (103) in series, R = 22 kΩ to ground. Output taken between C and R.
High-pass schematic
C.
Look at the two drawings. What is the only difference between a low-pass and a high-pass RC filter?
Answer
CSCM 373-01 — Fuzzbox Physics
Schematics & Passive Filters (continued)
Part 3
Simulate & Observe
EveryCircuit
Open the EveryCircuit passive low-pass filter example. Watch the animation, then experiment.
Set the frequency source to a low frequency (e.g. 100 Hz). Observe the output amplitude relative to the input. Now increase the frequency to 2000 Hz. What happens?
Observation
2.
Try to find the cutoff frequency — the frequency where the output drops to about 70% of the input. Record it here:
R value
C value
Measured fc (from sim)
Calculated fc = 1/(2πRC)
Match? (Y/N)
Default values
Default values
3.
Now modify the circuit: swap the positions of R and C (capacitor in series, resistor to ground). What kind of filter is it now? Verify by sweeping the frequency.
Answer
Part 4
Build & Measure in Falstad
Falstad Circuit Simulator
Open falstad.com/circuit. Build an RC low-pass filter using R = 22 kΩ and C = 10 nF (0.01 μF).
Add a voltage source set to sine wave. Right-click the output node and select "View in New Scope" to see the oscilloscope.
Falstad tips: Right-click a component to change its value. Use Draw → Inputs and Sources → Add Voltage Source. Use Draw → Passive Components for R and C. Use "t" key to slow/speed up the simulation.
1.
Sweep the source frequency from 100 Hz to 5000 Hz. Fill in the output level at each frequency (estimate from the scope — full = 1.0, half = 0.5, etc.):
Frequency
100 Hz
300 Hz
500 Hz
723 Hz
1 kHz
2 kHz
5 kHz
Output level
2.
At 723 Hz (the calculated cutoff), the output should be approximately 0.707 (−3 dB) of the input. Is yours close? Why or why not?
Answer
3.
Now rewire the circuit as a high-pass filter (swap R and C). Sweep the same frequencies. What changed?
Answer
Part 5
Calculate Cutoff Frequencies
Pencil & Paper
Use the formula fc = 1 / (2πRC) to calculate the cutoff frequency for each combination. Show your work.
R
C
Calculation
fc (Hz)
Guitar context
22 kΩ
10 nF (103)
Tone knob (typical)
10 kΩ
47 nF (473)
Bass-heavy tone
100 kΩ
1 nF (102)
Bright treble
1 MΩ
100 pF
Guitar cable capacitance
Q.
A long guitar cable has about 100 pF of capacitance per foot. With a 20-foot cable (2 nF total) and a guitar pickup impedance of ~500 kΩ, what is the cutoff frequency? What does this mean for your tone?
Answer
Big Question
Every pedal circuit you will build contains at least one RC filter — whether it's a coupling cap (high-pass), a tone control (low-pass), or a feedback filter. In your own words, explain:
Why does swapping the positions of R and C change a low-pass filter into a high-pass filter?
Use the concept of the voltage divider and the capacitor's frequency-dependent impedance in your answer.