Concept behind step modes on Phasers?

[joelap]

Hey all, whats the concept behind how a step mode works on phasers like the Boss PH-3 and Ibanez PM-7?  I was wondering if it'd be possible to mod a PH-2 to also have a step function.  Whats the theory behind it?

[Mark Hammer]

Not familiar with the PH-7, but the PH-3 is digital.  Heck, if they tried to cram all the stuff neded for an analog Barber-pole phaser effect inthere, they'd likely need several stacked SMD boards.

That being said, "stepped" phasing can be done with FETs the same way stepped filtering is done in the Maestro FSH-1.  You have some source of periodic or aperiodic (random) voltage changes, and you use a clocking circuit to "block" those changes off in some manner so that the voltage dictating the FET resistance is held or unchanged for some discrete period of time. 

In the FSH-1, a clock is used to block the random voltages coming off a noise source, via a sample & hold circuit.  If one wants more control than a S & H offers, what is often used is a counter circuit like a 4017, where a simple clock steps the counter through its various outputs.  Each output is set, often via sliders, to a different output level (essentially a variably attenuated V+) Those variable output votages then drive the FETs to achieve fixed resistance changes which change as fast as the master clock steps the counter circuit.  RG has some relevant circuits over at geofex.

[trevize]

search for the pseudo random generator in the geofex site.

you could also look for sample and hold circuits in the diy synth pages.