Need a notch filter calculator.

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a notch filter is a "band reject filter" -- sort of like a the hole in a donut -- you sum a high pass and low pass filter -- use resistors to lower the Q -- it can be done with passive devices if you have a lot of patience.

best to get a CAD program like SwitcherCad (free from linear tech) and play around with the RLC values.

you can also make some valiant assumptions with respect to the input impedance of the amplifier and use an RC notch filter, or just fix the input value of the amplifier with a resistor and plug and play.

an active notch filter can have the Q continuously adjusted. most often people use active notch filters in applications like hum removal or to notch out the fundamental frequency for analysis of distortion residuals.
 
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