Help with adding a switch to change number of stages in MXR Commande Phaser

[Rambozo96]

Been racking my brain on this one. Far as I can tell this is a 4 stage phaser so I toyed with the idea of adding a switch to change it from a 4 stage to a 2 stage but I'm not sure how to do it. My guess would be adding a switch at the output of the second opamp or "second stage" in front of the 100k resistor or "R20" but would I have to do something with the 10k resistors running in series above the opamps? I guess a CMOS chip in this circuit is doing the same job as the JFET's in a Phase 90. Im guessing I can leave the connections to the chip alone since there wouldn't be any signal going to the other 2 stages but I could be wrong. Thanks in advance!

[Kevin Mitchell]

You've got the right idea - mostly.

Don't worry about that 100k on top (R20). No need to play with the feedback resistor.

Add a SPDT switch - center lug gets R21 (the side that WAS hitting the opamp output of the 4th stage)
The outer lugs of the switch will go to the output of the 2nd stage and 4th stage.

It's as simple as that. Though I'm not sure if you're going to want to do something with the feedback from the - input of the 2nd stage. I'm not used to seeing ties like that in a phaser. But I'd neglect it seeing how the desire for 2 phases would be for subtly.

Tinker and conquer

[Mark Hammer]

The U6 switches are being pulse-width modulated.  That is the amount of time they spend being "on" and "off" is varied.  If it was happening slowly, you'd notice it.  But since it is happening at a fairly high frequency, we hear as the "average" value of the 6k2 plus the switch-resistance gradually changing, not unlike what might happen if there was an LDR or a FET in the same location.

I would agree with most of what Kevin posted but for the problem of R14+R16 that forms the feedback path.  Since the suggested switch only taps the signal at two different locations, it does not cancel that feedback, which IS added to the phase-shift signal available at U1-pin 7.  If the objective is to be able to flip from a Phase 90 structure to a Phase 45 structure, then one leg of R14 needs to be lifted to remove the feedback from the 2-stage sound.  That would involve a DPDT toggle, rather than a SPDT as Kevin proposed.  Wire it up so that when you revert from 2 to 4 stages, R14 is reconnected.

Given the design and how the bypass switching is done, I will also put in a plug for installing a SPST toggle that lifts/reconnects one end of R21.  This will cancel the dry signal, providing a pleasing vibrato.  The pitch wobble from 2 stages will be gentler than that from 4 stages.  I'll suggest that, before you do more to the enclosure, just unsolder one end of that resistor, see if you like the sound, and only proceed with the mod if you like it.  Note that, with high feedback settings (in the 4-stage mode) it begins to sound like a modulated bandpass filter.

[Kevin Mitchell]

Good points! So without considering that feedback tie your two stage mod would sound more like a vibrato and less like a phaser if you go with my simple idea.

You'd think I would know better after building a dozen easyvibes 

[Rambozo96]

Thank you guys for the help and suggestions. Will keep you posted on the end result.

[Mark Hammer]

Good points! So without considering that feedback tie your two stage mod would sound more like a vibrato and less like a phaser if you go with my simple idea.

You'd think I would know better after building a dozen easyvibes 

I have often been "the opinion that launched a thousand facepalms". 

[Rambozo96]

Oh and I was going to change C3 to .015uf and C5 to .22uf like they do on the Phase 90's to get close to the Univibe sound. The .005 difference on C3 maybe negligible though if I did decide to change it over so I may leave C3 alone.

[Mark Hammer]

I'd say leave well enough alone.  Do the suggested modes and and hold up at second bass.  The plastic Commande Series enclosures are obviously easy to machine, but there isn't unlimited space in there, and the traces won't hold up to repeated applications of heat.  Do the simple stuff, and then make yourself a proper Uni-Vibe or functional equivalent.  Besides, a musical Uni-Vibe sweep relies on a different waveshape, and also uses a different range of speeds (slow sweep of Vibes is largely inaudible, compared to slowly-swept phasers).  Do justice to each category of effect, and you'll be better for it.