Phasers

[Sean Costello]

Budweiser [FTS] wrote:
> 
> What is the ultimate analogue phaser design?
> 
> I am working on a signal processing modular set up
> (that I mentioned a couple of days ago), and the main
> module i am interested in is a dual phaser. Anyone
> have any designs?

I have some C code for a Csound phaser module! Ok, not analog, but still enough to discover some
interesting things:

- Beyond 16 stages or so, the sound of phasing becomes closer to flanging (although with several
thousand stages, very weird, frequency dependent echoes occur...).
- The stage from which the feedback is tapped has far more of an impact on the sound than the stage
from which the output is tapped.
- Audio rate modulation of a phase shifter is a FANTASTIC sound. You may want to have a provision
for linear modulation of your phaser, for incorporation of FM. The more phase stages, the more
dramatic the effect.

As far as phaser design, an OTA-based approach seems like a good idea. You could use an
LM13600/13700, and cascade as many stages as you wish. The tap points for feedback and output should
be seperate, and selectable (maybe rotary switches, where the output is selected from the outputs of
even stages). A combined linear/exponential modulation input would be cool, as linear modulation is
nice for FM, while exponential modulation is better for the characteristic sweeping sound.

Tom G, as always, has a relevant circuit:

http://www.mindspring.com/~vco/cbook/pics/psm1a_sd.gif

Barry Klein also discusses using OTA's in his book, as well as describing several other phaser
topologies.

Sean Costello