Twin-T Passive Notch Calculator

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.

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.