Variable sweep offset for Ross phaser?

[Mark Hammer]

The first major modulation effect I ever owned was a PAiA Phlanger.  So, my mental template of what a "proper" modulation pedal is includes not only regen, speed, width and wet-dry balance, but sweep offset (manual, initial, start, etc.), and I judge all others by that template as "complete" or "incomplete".

I like the Ross/Ropez OTA-based 4-stage phaser, but I always wished the sweep range could be moved around.  Looking at the circuit for the LFO, you can see that one of the OTAs in it has a bias voltage fed to pin 3 via a divider network formed by a 270k and 100k resistor pair, with a .05uf cap to ground to decouple the bias voltage.  I thought about it for a bit and wondered if there was any advantage to be gained by moving that bias voltage around.  So, I figured "What the hell" and tried it out this evening.  I replaced the 100k with a 75k and the 270k with a 240k, and wired up a 50k pot so that the wiper went to pin 3 and the outside lugs went to the ends of the 75k and 240k that would normally connect to pin 3.  In this way I could vary from a biasing pair that went from 290k/75k at one extreme to 240k/125k at the other. 

To my delight, I found this did move the sweep range up and down a bit.  Not as much as I would have liked, but audibly and without problem.  The only side-effect I seemed to notice was what seemed like a slight increase in speed when adjusting the range "upwards".  Clearly, some more experimentation is warranted to narrow down what the optimum component values are.  It may be that a 100k pot is better, or that 50k is fine but the values of the two side resistors need changing.  What I did like was that moving the range a little upwads made the phaser sound a little "chewier" and more suitable for rhythm work.

I cordially invite others to experiment and report back.  Given that this does not require substantial revision of the basic design, it seems like the sort of mod that Francisco Pena can easily incorporate into yet another revision of the Ropez layout.  It only needs two trace breaks and a few extra pads.  That will give us speed, regen, and offset.  Variable mix is easily done.  Now all we need to make it a "proper" phaser is variable width.

[jrc4558]

If you asked about the EZ Vibe - I could answer that. But for an OTA-based stages - I don't really know...

[Fp-www.Tonepad.com]

Mark, I agree about the sweep in the ross phaser being a little 'short' as stock. I have yet to try your variable width mod, and now this is more stuff to chew on.

The speed of the phasing in relation to the width is obvious, as the shorter sweep will give a faster modulation. I seem to prefer slow phasing and i've raised the 3.3uF cap in my ropez builds. If the width is kept constant and it's just the range being moved up and down (by the addition of the bias pot), by moving it up wouldn't it cycle quicker on the higher frequencies due to their logarithmic nature?



It may also be possible that moving the bias too much can make the oscillator clip somehow (I mean operate in an asymmetrical way).

I really wish more people built the ross phaser, it's one of those pedals with a lot of potential for mods and a lot of fun playing too.

[jrc4558]

I really wish more people built the ross phaser, it's one of those pedals with a lot of potential for mods and a lot of fun playing too.

First off - THANKS THANKS THANKS for the layout! Ross is one of the best phasers out there!
Second - I'm in the process of building amulti-stage one right now. Many a mod forthcoming.