Twin-T Passive Notch Calculator
Design a passive twin-T notch network. Precise component matching produces a deep null at f₀ = 1/(2πRC). Simple 6-component hum reject.
How to Use
- Enter the desired notch frequency.
- Pick a convenient capacitor (typical 0.01-1 µF for audio).
- Tool computes R (equal pair), R/2 (shunt), and 2C (shunt).
Show Work
Component Values
History
The twin-T network was invented by Edwin M. Chapin at Bell Labs around 1936 for telephone-line equalization. Chapin\'s 1938 patent demonstrated the symmetric T-of-resistors / T-of-capacitors structure, showing that with R1=R2=R, R3=R/2, C1=C2=C, C3=2C, a precise notch appears at f₀ = 1/(2πRC).
The network became a staple of analog instrumentation from the 1940s onwards, especially in vibration analyzers and seismographs where rejecting specific mechanical resonances or mains-frequency noise was critical. Hewlett-Packard\'s 1950s distortion analyzers used twin-T notches to reject the fundamental before measuring harmonic distortion.
The twin-T\'s simplicity and passive operation kept it popular in guitar amp and audio hobbyist circles even as active filters became standard — a matched-component twin-T is cheaper and quieter than a buffered op-amp notch, and needs no power supply at all.
About This Calculator
Enter the desired notch frequency and a convenient capacitor value. The tool computes R = 1/(2π·f₀·C), then outputs R (×2 for matched arms), R/2 (shunt), and 2C (shunt). Use ±1% metal-film resistors and COG/NP0 ceramic capacitors for best matching — notch depth is limited by component tolerance.
For deeper, narrower notch, follow the twin-T with an op-amp buffer and add positive feedback from the output to the shunt junction. Everything runs client-side.
About the Twin-T Passive Notch Calculator
Working on electronics and circuit design? The Twin-T Passive Notch Calculator is a free browser tool that gives you the answer in seconds. Design a passive twin-T notch network. Precise component matching produces a deep null at f₀ = 1/(2πRC). Simple 6-component hum reject.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Why passive?
No power required. Ideal for battery-operated instruments, audio guitar circuits, or any application where an op-amp would be overkill. Passive Q ≈ 0.25 but deep notch depth (30-40 dB) is achievable with matched components.
Component matching?
Critical. Use ±1% metal-film resistors for R (2x) and R/2, plus COG/NP0 or polystyrene caps for C (2x) and 2C. Mismatched parts give shallow or off-tuned notches.
Need higher Q?
Add an op-amp buffer with positive feedback — see the active notch-filter calculator. Passive twin-T with buffer alone gives Q up to ~2; with positive feedback, Q > 50.
How do I use the Twin-T Passive Notch Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Guitar Pickup Hum Kill
60 Hz twin-T between pickup and amp eliminates single-coil hum without op-amp power.
Power Supply Ripple
120 Hz notch on a rectified + regulated supply rail to kill 2×line-frequency ripple.
Audio Test Generator
Reject leakage of 1 kHz test tone for THD measurement down to -60 dB.
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