Question on Ross Phaser Mark Hammer univibe Mod

[jimbob]

Im looking at the Tonepad Ross Phaser and thought about doing the easyvibe mod but I cant figure out this- According to the schematic would I use 4 spdt switches to switch between those 4 caps choices and is the vibrato 27k resistor change (spst ?) used for the easyvibe mod as well? Or is there another choice that will switch everything at once assuming thats what is needed to go into easyvibe mode.

[jimbob]

Anyone? I know I can be the only one curious

[Mark Hammer]

Personally, I'm a sucker for things where you flick a switch here or there and your change the entire topography of the design.  The sort of switcherama you're describing, however, is the sort that I try to avoid.  Vibing a Ross Phaser requires a number of changes to optimize it for that function:

1) You have to change the caps. (already noted)
2) You have to change the speed-range setting cap (from 3.3UF to 2.2UF) and resistor/pot (100k is probably about right for the pot and the 4k7 can probably be increased a bit).
3) You have to cut the regen path out of the circuit.

It's also a good idea to replace the 10k fixed resistor on the output of the LFO with something like an 8k2 fixed resistor in series with a 100k variable resistor to adjust sweep width.

That's a whole heap of changes to my way of thinking.  I simply made a dual board with two PCBs side by side, setup one for phasing and the other for vibing.  You have to buy the extra chips and passive components, and you need a larger case, but then you don't have to buy all those toggles.

[jimbob]

I think Ill just make another board and make those changes to that board. How is the phaser after all those changes? Worthwhile?

[Mark Hammer]

It makes both a very nice phaser and a very nice Univibe.  If you replace the 27k resistor in the Regen loop after the 1uf cap with a 24k or even 22k unit, you'll be able to get near oscillating ultra-sharp feedback sounds.  Drop the 10k resistor on the output of the LFO to 6k8 or 8k2 for ultra-wide sweeps.  Slow wide sweeps with lotsa regen sound quite dramatic IMHO.  For narrower sweep, you can either stick a 100k variable resistor in series with the reduced fixed resistor to get different current-limiting resistances, or just use a toggle to select wide/narrow sweeps (like the SS) with 2 different resistances.