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Old 13th July 2011 | Show parent
About as easy as cake. Make up 2 all pass MFB stages. Connect them in series. There will be 2 resistors tied to ground to set the frequency. Replace those with a dual log pot. 100k works good. Now you can tune the phase with a knob.

If you need example circuits, plenty are found in Don Lancaster's classic Opamp Filter Cookbook. It's basically the same as 2 stages of an old MXR Phase 90 pedal using a tuning pot instead of jfets sweeping up and down the resistance of the all pass filter. In the Phase 90, the phase delayed signal is mixed with the dry signal to obtain the nulls.
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