A little more personality in your Ropez/Ross phaser

[Mark Hammer]

Now that I've been able to get my several problematic Ropez buolds working, I was able to carry out a few experiments.  Here are some things you can do if your tastes run to a more extreme sounding phaser.

• The regen circuit is set to mix in a regen signal level that is more or less the same level as the phase shifted signal arriving at pin 13 of IC2B.  In fact the 27k resistor after the Regen pot and the existing 27k resistor between pins 8 and 13 of IC2 are mixing resistors, combining two signals at pin 13.  If the value of the 27k regen resistor (the one in the corner of the board, right beside the wire link near the Regen pot) is reduced, then the recirculated signal can be made to verge on oscillation.  I reduced mine to 24k, though you could probably go as low as 22k without damaging anything.  The worst I can see is that you can't turn the Regen pot up ALL the way, but you should be able to get a more pronounced emphasis.
• There is a 10k resistor between IC4 (the LFO) and IC2 on the board that is perpendicular to a 1k resistor.  This is a current limiting resistor between the LFO and the Iabc (current control) pins on the phase shift stages.  It is essentially a preset sweep width control in a way.  Values lower than 10k will permit a larger control current to be sent to the phase shift sections, essentially increasing the sweep in the high end (i.e., it keeps going even higher in its sweep).  Values larger than 10k will reduce the sweep width.  On one of mine, I replaced that 10k resistor with a 2k2 fixed resistor and a pot wired as variable resistor.  My sense is that a 25k pot will likely be just about right.  I only had a 50k handy, so I paralleled a fixed resistor with it to set maximum resistance.  The total resistance range goes from 2k2 to about 18k.  I still found the highest resistance yielded too much sweep width at the fastest speeds, which is why I recommend a 25k pot.  If I understood and remember Mike Irwin correctly, you do NOT want to go below 1k or the OTAs will fry.  I think you'll find 2k2 severe enough.

The wider sweep, coupled with greater resonance produces a more extreme sweep.  Think A/DA Flanger.  Obviously it's NOT and A/DA Flanger, but the sweep (at min resistance) will be more in the zone of "it just keeps on going and going".  As is true of anything producing notches that sweep up high, sticking a fuzz before this sounds wonderful.  I don't have any 8-stage setups at the moment, but I suspect it would sound pretty damn good if you had 8 stages to sweep that far.

At the other extreme, being able to reduce the sweep width makes vibrato created by lifting the dry signal more palatable.

If you wanted to just dip your foot in the water without ripping your Ropez apart, solder a 120k resistor in parallel with the 27k regen resistor on the copper side, and a 4.7k resistor in parallel with the 10k current limiting resistor, for a taste of what the extreme sweep can sound like.

Enjoy!  I know I do.

[nelson]

Excellent Mark!

Just so happens I have an etched board for the ross phaser sitting on my workbench just now.

This gives me the motivation to drill the board, has been sitting there for over a month.