Phaser: The All-Pass Secret
An all-pass filter passes all frequencies at equal volume but shifts their phase. Stack several, sweep with an LFO, and phase cancellations create moving notches, the classic phaser sound.
All-Pass: The Invisible Filter
In Part 102 we saw how a delay line creates effects by mixing a delayed copy with the original. A phaser works on a similar principle, but with a twist. Instead of delaying the entire signal by a fixed time, an all-pass filter delays different frequencies by different amounts. The amplitude stays flat (every frequency comes through at the same volume) but the phase of each frequency gets shifted by a different amount.
When you mix this phase-shifted signal with the dry original, frequencies where the phase shift happens to be 180 degrees cancel out, creating a notch. One all-pass stage creates one notch. Two stages create one controllable notch. Four stages create two notches. The more stages you chain, the more notches appear in the spectrum.
Sweeping the Notches
A static phaser is just a peculiar EQ. The magic happens when you modulate the all-pass center frequency with an LFO. The notches slide up and down the spectrum, creating the characteristic swirling, swooshing sound. Add feedback (routing the output back to the input) and the notches become sharper peaks and valleys, almost like a resonant filter sweep.
Classic phasers like the MXR Phase 90 use 4 stages (2 notches), while the Small Stone uses 4 or 6. Twelve stages give you 6 notches and a dense, complex sound. The demo below lets you hear the difference.
Try it: start with 2 stages and listen to the single notch sweeping. Increase to 8 or 12 stages and hear how the phaser becomes richer. Push the feedback to hear the notches ring.