I use that site all the time but forgot to look there for a twin-T.Twin T calculator: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm
The twin-T notch filter I published earlier in this thread will likely be what I layout on the PCB.
It's from Jansek.
The component sensitivity hasn't been much of an issue largely due to the fact Mouser stock 10 nF/polypropylene in 1%.
With Mouser now having 200 piece minimums on 1% resistors ($4/200) it also becomes practical to hand-select values.
The notch depth varies somewhat with temperature and power on warm-up drift of Fc but it's not significant WRT A/D overload or FFT measurement of HD2-10.
I want two filter networks so I can do simultaneous Left/RIght measurements AND/OR (and that's a big "AND/OR") balanced and floating measurements.
Balanced and floating with a 40 dB INA and CMRR stage isn't that difficult but making it do both balanced and single-ended (dual channel) may be a real challenge.
Switching the tuning is one issue. Not only the number of contacts but the contact resistance, contact resistance effect on notch depth and introduction of distortion above -150 dBc as the contacts age.
If I make it do both balanced and SE the switching may have to be with jumper links or have three filters: Two SE and one balanced.
With three filters I can have two sets of input connectors and switch their outputs.
The notch filter will also need to have a switch for Generator Monitor and Input connector.