Useful Ropez tip

[Mark Hammer]

I spent some time this weekend trying to troubleshoot and resurrect builds that were 95% of the way there but never made fully operational.  One of them was a dual board with a pair of "Ropez" Ross Phaser clones, one of them dedicated to Univibe tone (4 different caps, vibrato/phasing switch), and the other allocated to phase-shifter/phase-filter sounds (4 identical caps).

If you look at the the Ross/Ropez schematic in the document at Tonepad, you will see there is a 10k fixed resistor on the output of the LFO.  The current applied to each of the 4 OTAs is limited by this resistor.  In effect, it is your preset "sweep width" control.  I replaced it with a 6k8 fixed resistor in series with a 50k pot set up as variable resistor.  This gives it a nice range of usable sweep widths, from a little more than stock, to much less than stock (though still audible).  Larger resistances equal reduced sweep.  As with any LFO-modulated effect, being able to turn the width down for faster LFO settings provides more usable tones.  Just don't drop the resistance value down too far (probably not below 3k9 or so) because OTAs don't like too much current - it can fry them.

One of the things I found interesting about the Univibed Ross is that lower sweep width settings produced an almost tremolo-like sound.  Nice!

If you plan to convert a Ross/Ropez into a dedicated Univibe pedal, note that the LFO speed is partially governed by the value of the capacitor shown as 3.3uf, and the resistor between pins 9 and 16 shown as 4k7.  The cap sets the overall range of LFO speeds, and the 4k7 resistor sets the maximum speed.  Very slow Univibing is not nearly as audible an effect as very slow phasing, largely because the notches produced are much shallower and broader and increased regen does nothing to alter this.  For this reason, it may be more suitable for a Uni-vibe-optimized version of the circuit to leave out the components associated with regen, and to use a 2.2uf cap (or smaller) for the LFO, along with an adjusted component in place of the 4k7 resistor.  This should get you a range of speeds that makes all parts of the speed pot's rotation useful.

[ddpawel]

Did you use exact Univibe cap values or that used by lynx.bc.ca site? Do you know how to change the cap values to have regularly distance notches like Flanger. Can I try 33n, 15n, 12n, 8.2 n, 6.8nf, 5.6 n, 4.7n, 3.9n. Is there any way to have uniform nothes in phaser?