This op-amp all-pass filter is reported to introduce a 90° phase shift at the cutoff frequency of the RC network placed at the + input.
(The FET is used as a controlled variable resistor to move the phase shifted frequency "up and down.")
In phasers, this signal is then added to the original one to have a cancellation (notch) at that particular frequency.
The famous MXR PHASE 90 phaser, instead, creates two notches to enhance the effect: it does so cascading four elementary phase shifting units like this:
How does that work?
I would expect the series of four identical units to create a 360° phase shift at the RC cutoff frequency, not two 180° shifts at two different frequencies.
It is also reported that in the phase 90 the phase shift is distributed like this (among the elementary units)
How can you have a 45° phase shift per unit using an all-pass filter that introduces a 90° shift?
It all seems very confusing to me.
I would greatly appreciate your help.