KB variable phase knob help

[Aentons]

I was looking a the VFE Klein Bottle schematic and trying to see how the variable phase shift knob works because you don't see those a lot and I guess I don't understand how it works. Is this it? I think I read somewhere that it centers on 1k Hz. Any enlightenment would be greatly appreciated?

[Scruffie]

It's an all-pass filter as you'd find in any phaser pedal but the variable resistor (which would normally be a vactrol, J-FET etc.) has been replaced by a pot.

[Aentons]

It's an all-pass filter as you'd find in any phaser pedal but the variable resistor (which would normally be a vactrol, J-FET etc.) has been replaced by a pot.

Thanks, that makes a perfect sense now that you say it. I am familiar with filtering concepts but didn't make the connection. I'm relatively new at this stuff so I guess that comes with experience.

One other related question:

For phase inversion, there is normally a switch which toggles between inverted and non-inverted opamp inputs. Are there pros/cons to one method over the other? Like for instance, is the variable method more frequency dependant or bandwidth limited(not sure if that's the right term) since it uses an all-pass filter?

[Scruffie]

Yes the phase shift is frequency dependant hence the "centred at 1KhZ" which will be 90 degrees.

A phase inverter switch does just that and switches the phase from 0 to 180 degrees for all frequencies.

If you get rid of the two 22k resistors in that snippet and have the op amp as just a non-inverting buffer, what does it look like then? You could also swap the pot and capacitors position.