Adjustable Notch Filter

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Opening my old notes, I found a circuit that I used to use often: a 50 Hz (or 60 Hz, or other frequency) notch filter. This circuit was designed by Peter Lefferson (circa 60-70s) from Milton Roy Co, St. Petersburg FL. This notch circuit is quite creative and flexible because its frequency, bandwidth, and depth can be adjusted with a trimpot. Although nowadays filters with this kind of character are very easy to realize with DSP, this circuit is still useful if you want to build an analog-based audio system. In general, audiophiles often prefer totally analog circuits.



The values I listed can be changed, but still use the limitations as stated in the image. Of course, you should make sure by testing it either with an oscilloscope or spectrum analyzer if available. Specifically for $VR_1$, the coefficient a in the formula, is the part of $VR_1$ that is active (not shorted). In the formula for setting FREQUENCY, the value of $R_2$ is $R_2$ plus the value of the resistance on the left side of $VR_2$ and the value of $R_3$ is $R_3$ plus the value of the resistance on the right side of $VR_2$. Hopefully useful and TABIK.



(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same topic )

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