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2nd Ross Phaser - schematic

11/29/2002 12:40 AM
B Tremblay
2nd Ross Phaser - schematic
Rob Strand has completed the schematic of the Ross Phaser from the photos at my site. This is the second of the three known versions, produced between the two models that Kluson Guy mentions.  
Many thanks to Rob for contributing his time and skill.  
Ross Phaser -
12/3/2002 10:21 PM
Mark Hammer The Ross and the rest
I was looking at the op-amp 97C11 schematic last night and it has some interesting features.  
As many phaser afficionados know, when feedback is inserted into a phaser, it needs to be inserted one less than the total number of stages foir reasons I won't go into (there's a clever way of avoiding what I can't explain well...DOH!).  
Normally this would involve feeding back the allpass output to stage 2. Of course what this means is that the feedback signal only passes through stages in a 4-stager, or 5 in a 6-stager.  
The Ross adopts a very different approach. It is essentially a 4-stage phaser, but it inserts an 5th allpass section in the feedback loop. It may not look like it to the naked eye since we are all so accustomed to FET-based designs where the variable resistance goes to ground and the cap goes between the output of the previous stage and the non-inverting input, but allpasses can work in either manner.  
This 5th section allows the recirculated signal to pass through5 phase-shift stages without needing to be a 6-stage phase shifter to do so. Clever. Note that it is a fixed allpass stage, such as found in the MXR Phase 100.  
What is also clever is the insertion of a cap in the 5th stage feedback loop to keep noise and oscillation under control.  
I'm liking this thread. Ross products never really took off out here so this is a strain of pedal I'd never known much about.
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