Duo Phase

[Nostradoomus]

Update: Added pull down resistors, changed the LEDs to yellow. Much better!

 

[Chuck D. Bones]

I'm still learning -- does this commutative property follow from the assumption of our allpass network being an LTI system?

Those are some big words! I would say "yes" because it is linear. That is, at reasonable drive levels it's linear. All bets are off if you overdrive a phase shifter. It is not "time invariant," but that part doesn't matter. We get the same phase / pitch shift for any order of the stages. The FEEDBACK tap is between the 1st & 2nd stages; for FEEDBACK settings above zero, the order will make a difference. Uni-Vibe doesn't use feedback. Some derivatives, such as the EVH, do use feedback. Some simulations and experimentation are in order.

 

[Dreamlands]

Those are some big words! I would say "yes" because it is linear. That is, at reasonable drive levels it's linear. All bets are off if you overdrive a phase shifter. It is not "time invariant," but that part doesn't matter. We get the same phase / pitch shift for any order of the stages. The FEEDBACK tap is between the 1st & 2nd stages; for FEEDBACK settings above zero, the order will make a difference. Uni-Vibe doesn't use feedback. Some derivatives, such as the EVH, do use feedback. Some simulations and experimentation are in order.

Interesting! Makes sense at "reasonable" drive levels, yes -- and I want to better understand your point about feedback, so I'll dig into that, thank you.
Among other ideas for phaser related experiments, I was wondering about being able to emulate a Univibe style response by keeping the capacitor values the same, and instead independently varying the illumination of each LDR rather than having them ganged to a single LED (eg use a microcontroller to drive the LED/LDR pairs).