Run a signal through a chain of all-pass filters and mix the result back with the original. All-pass filters change phase without changing level, so nothing is heard until the two paths meet, at which point they cancel wherever they are half a cycle apart.
Notches, not a comb
Each pair of all-pass stages produces one notch, and the stages are spread across the sweep rather than evenly across the spectrum. A four-stage phaser has two notches, an eight-stage has four.
That is the difference from a flanger. A flanger’s notches are harmonically related, which is why it sounds metallic. A phaser’s are not, so the sweep reads as a softer swirl that sits under a source without competing with its pitch.
Rate and depth
Rate is the sweep speed, from 0.05 Hz to 5 Hz. Depth sets the top of the sweep: the notches always start around 200 Hz and travel up to roughly 4 kHz at full depth.
The sweep is logarithmic, so the notches move by musical interval rather than by hertz, and the movement sounds even from bottom to top instead of rushing through the high end.
Stages
Stages change how much of the spectrum is affected at once. Two is subtle enough for a background part. Four is the familiar pedal sound. Six and eight get dense, and on full-range material they start to thin the source.
Changing the count while the preview is playing crossfades the extra stages rather than switching them in, so the setting can be auditioned without a click.
Feedback
Feedback sends the end of the chain back to its input, which sharpens the peaks between the notches into resonances that ring as the sweep passes them.
Below 30% it adds definition. Above 60% the resonance becomes the main event and the level rises with it, so watch the meters before exporting.
Mix
Cancellation needs both paths, so 50% is where the notches are deepest. Fully wet removes the dry path and with it most of the effect, since an all-pass chain on its own barely changes what you hear.
Example: an electric piano part
Rate 0.3 Hz, depth 70%, 4 stages, feedback 25%, mix 50%.
One sweep takes a little over three seconds, which moves slowly enough to be felt across a phrase rather than heard as an effect on each note.