Phase 90
Four all-pass stages sweeping two notches, and exactly one knob.
What the circuit does
Four all-pass filter stages shift phase without changing level, and the shifted signal is summed back with the dry one. Where the two cancel, a notch appears, and four stages give two of them. A low-frequency oscillator moves the stages, so the notches sweep.
What to listen for
A narrow, dry swirl rather than a liquid one, because there is no resonance feeding back into the sweep. One control means it is voiced rather than adjustable, which is a large part of why it is so recognisable.
Where it goes
The modulation slot
After the gain and before the delay, so the delay repeats a modulated signal rather than modulating an already repeated one. Reversed, the movement gets applied to the echoes as a block and the effect smears instead of swirling.
Controls
- Speed