This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be assembled easily, since all the components are common. In the figure we have the basic configuration for a filter that rejects the frequency determined by the double-T feedback network. See the math section of the website for formulas for calculating frequency. With the indicated values, this frequency is 1kHz. The supply must be symmetrical from 6 to 12V. The input signal, not indicated in the original diagram, is applied to the inverter input.
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