Ross Phaser Stereo Mod

[jmasciswannabe]

Ran across this the other day while searching for stereo phaser:
http://www.youtube.com/watch?v=q6fURkOPl5Q

Pretty cool! I want to to do that but to my ross phaser built from tonepad:
http://www.tonepad.com/getFile.asp?id=99

Would it essentially be the same deal as lifting the 10k resistor in the lfo path and running the slave lfo to it? Am I correct in thinking that the IC 4 operates as the "clock" for the LFO. I am not interested in mamby pamby pseudo stereo, wet one side, dry other side trickery. I need the good stuff!

Hmm....I wonder what running two of those digitech phasers that can do the ramp phase (up or down) would sound like in stereo with one going up and one going down. 

[Mark Hammer]

There are many different versions of "stereo".  In the linked-to video, theintent is to process a signal that is already stereo, such that the identical phasing is applied equally to each channel.  As the video illustrates, that simply involves using the LFO of one of the units to drive all 4 phase-shift stages.  You could do that with any form of phase-shifter whether OTA-based, like the Ross/Small Stone, or FET-based, or switch/PWM-based.  The key thing is to assure that there is enough...whatever...to drive 8 instead of 4 stages.  In the case of the Ross, that generally means reducing the value of the current-limiting resistor from the LFO so that it can provide enough current to sweep all 8 stages; which is the 10k resistor that is labelled "LFO".  How much lower to make it, is something you'll have to confer with others about, since the chips can be blown if too much current is applied to pins 1 and 16.  I know that changing the 10k out for a 9k1 on one of my Ropez builds yields a remarkably wide sweep.  I can do Steve Hillage cosmic sweeps, no problem.  But going below 9k1 is likely to yield risk....at least for 4 stages.  I'm imagining that 8k2 to drive 8 stages might be safe, but don't rely on my judgment.

Keep in mind that if you are processing a mono guitar signal. then the two ganged phasers will yield no particular magic, since they'll be doing the same thing to the same signal.  That's not stereo; that's just redundant.  You'd get far more mileage running them in series with different LFO rates to achieve an aperiodic sweep.  I've done it plenty of times and its awesome.

The trickier thing is getting the units to countersweep, such that one sweeps up while the other sweeps down.  How one would provide complementary/anti-phase LFO outputs from that type of LFO is beyond me.  I'm sure it can be done.  I just don't know how.

A third possibility is to swap out the 3300pf caps for other values (e.g., 3900pf, 4300pf), so that the notch locations are staggered for channel A and B.

[jmasciswannabe]

Mark, I was hoping you would post!! I've already got a tricked out Ross with all the various mods you've suggested over the years. I was also reading a post that you had made regarding stereo and notches but was way over my head. Let's see here.....here it is:

http://www.diystompboxes.com/smfforum/index.php?topic=58892.0

Anyway, I'll keep in mind the 10k resistor. Was going to use in conjunction with the stereo panneur, ping pong delay, etc....Yeah, the countersweep was what I was thinking of with the ramp up on one side and ramp down on the other...that would be neat. Looks like this build might have a few sockets

[Gurner]

The trickier thing is getting the units to countersweep, such that one sweeps up while the other sweeps down.  How one would provide complementary/anti-phase LFO outputs from that type of LFO is beyond me.  I'm sure it can be done.  I just don't know how.

I'm not sure which specific LFO you are referring to, but since an LFO is just a low frequency AC signal...why not run the LFO into an inverting opamp & you've then got a complimentary LFO (or am I missing something fundamental here?!)

[frequencycentral]

 

The trickier thing is getting the units to countersweep, such that one sweeps up while the other sweeps down.  How one would provide complementary/anti-phase LFO outputs from that type of LFO is beyond me.  I'm sure it can be done.  I just don't know how.

I'm not sure which specific LFO you are referring to, but since an LFO is just a low frequency AC signal...why not run the LFO into an inverting opamp & you've then got a complimentary LFO (or am I missing something fundamental here?!)

That's how I do it: http://www.frequencycentral.co.uk/?page_id=280

[jmasciswannabe]

That's how I do it: http://www.frequencycentral.co.uk/?page_id=280

Very cool. I had seen your Sonic Death Ray but not this one.

[jmasciswannabe]

Rick, I read about the issue with the Gemini design attempting to be sold out from under you. Understandable, if you don't want to share details, maybe you could just point me in the right direction?

I get where the switch would go for the lfo sync, would the inverter switch go in the same place off the end of the "slave" lfo and just switch between running through the inverter and bypassing it?

I'm already going to be messing with the amz mosfet boost as an inverter for a stereo attempt on a flanger, might try it here and see what happens.

[frequencycentral]

Here's the LFO section of Gemini:

https://dl.dropbox.com/u/967492/Gemini/hidded/Gemini%20Project%20Publish/Gemini%20LFOs%20Schematic.JPG

Two identical LFOs. LFO 1 is also routed through an inverter, so phase core 2 can be modulated by LFO 2 or by an inverted version of LFO 1.

The LFOs are routed to the Iabc inputs of the OTAs via 4K7 resistors (to answer Mark's question).

[Mark Hammer]

More or less what I figured.  The Gemini/Causality LFO circuit is fashioned after the Phase 90's, which is easier to invert.  That's not to take anything away from it, but if the task is to provideganging to an inverted copy of the same OTA-based LFO circuit used in the Ross and Small Stone, it requires a different solution, as near as I can figure.