Tonepad Ross Phaser Mods...Direction Needed

[pryde]

Hello,

First, hello to everyone!

I am preparing to build a Tonepad/Ross Phaser, or "Ropez" and have the pcb and parts in hand. I really am only interested in 2 mods: Depth (LFO Sweep) and Univibe

I have read endless posts on here dating back to 2003 from Mark Hammer and others doing various mods with the Ropez and my head hurts bad 

It seems like there are different ways to acheive these 2 mods but still really don't understand what works best or how to actually do them on the pcb. Here is what I know:

Univibe: Need .015, .22, 470pf, .0047 caps and two- 2pdt switches which I have. How does one add the caps and the switches to the existing .0033 caps to switch between phaser and univibe modes? Also, does anything else need done for this mod?

Depth: Replace the 270k resistor above IC4 and run a 500kb pot and 180k resistor in series. Is this correct? Is this the best way or is there another, better way?

I am not interested in any other mods besides these two. Thanks for any advice and help with this.

[Mark Hammer]

Personally, I would recommend building the unit twice: once as a phaser, and once as a Uni-Vibe.  The reasoning is that:
a) Vibing wants a narrower range of speeds than phasing does;
b) Phasing wants feedback/resonance, but vibing doesn't want ANY;
c) The sweep waveform used in the Ross/Ropez/Small Stone is great for slow dramatic phasing, but triangular is often better for faster sweeps;
d) Changing 4 cap values simultaneously involves a LOT of wiring, begetting lots of risk, not to mention a rats nest of wiring and audible popping when switching over unless you're in bypass mode;
e) It's basically another $8 or so worth of parts to make another unit that has the right speed range, the right caps, the right sweep waveform, and few headaches.  The switches required to convert back and forth would amount to about the same expenditure.

If you only bought one board, I understand your dilemma.  But if you made a board, I encourage you to crank out another, and make both effects properly.  You'll be glad you did.

Having said that, a Vibed Ropez can probably do just fine with a 50k-100k speed pot instead of 500k.  The speed control should be reverse log.  Alternatively, use log and wire it opposite so that the resistance gets larger as you rotate clockwise, and the speed slows down.

You can safely eliminate the Regen pot, and the pair of 27k resistors and 1uf caps accompanying the Regen pot.

The sweep range, sweep waveform, and depth are influenced by the divided-down voltage at pin 3 of IC4, and the current from V+ arriving at pin 1.  The way the circuit is now, there are those two 270k resistors that each connect to V+.  The trick used by the first issue Small Stone, and re-used here, is to replace the 270k resistor connected between V+ and pin 3 with a 180k and 100k in series.  IN fact, imagine you have three resistors in series from pin 3 to pin 1: 180k, 100k, and 270k.

If the junction of 100k and 270k goes to V+, then you have replicated (within a few kilohms) what you see in the Tonepad drawing.  If the junction of the 180k and 100k goes to V+ instead, then you have 180k on one "side" of that connection, and 100k+270k on the other.  That's equivalent to having a 370k resistor between V+ and pin 1 (instead of 270k), and a voltage divider made from 180k/100k feeding pin 3.  That results in less current going to pin 1, and a higher bias voltage going to pin 3, resulting in a more triangular waveshape, slightly faster sweep rate, and reduced sweep width more suited to faster sweeps.  If you want to subtle-ize the sweep even more, you can simply increase the value of the 10k resistor between IC4 pin 9 and "LFO".  You could do that via a toggle, a pot, or simply a selected value.  You probably don't want the total resistance to go much below 9k (for max sweep) and maybe 20k for min sweep.

As per usual, vibrato would be achieved by lifting one end of the 27k resistor between pins 1 and 6 of IC1.

Finally, I can recommend replacing the 15k feedback resistor on IC1b with a larger value, like 33k, to increase the output level.  Then, exchange the 150k terminating resistor on the output for a 100k log pot, with a 47k-51k fixed resistor between the pot and ground.  That will let you dial in volume levels a bit louder than unity, and a bit lower than unity, so that the pedal becomes better suited to solos and rhythm work.

[pryde]

Thank you Mark for the detailed explaination. Unfortunatly I bought only one board so I will skip the Univibe stuff all together and just stick with the Phaser.

So, would you recommend doing any worthwhile mods with just the Phaser itself?

Is the LFO sweep worth doing, either as a swtich via SS or as an adjustable pot?

Would it be best to just build it bone stock and be happy 

Thank you

[Mark Hammer]

I have a 5-knobber that I made, that has speed, regen, wet level, sweep width, and volume, plus a dry-lift (vibrato switch).  I've described the volume control in the earlier post.

I used a 9k1 resistor instead of the 10k as the current limiting resistor feeding the Iabc pins of the 4 OTAs.  The sweep is just crazy wide.  It just keeps going up and up and up.  The width control is simply a 10k variable resistor in series with that.  Its effect is to reduce the highest portion of the sweep without having any impact on the lowest part of the sweep.  It can be useful.

At one point I inserted 2 fixed stages of phase shift to supplement the existing 4.  Gotta say, I thought I'd like it, but I didn't.  Didn't reaslly seem to sweep as high to my ears.  Not exactly sure why, but I removed it and went back to just 4.

I also ran a 10uf  cap from the "LFO" point to ground to replicate something that EHX did with the first (6-chip) Small Stone.  That cap serves to reduce the hypertriangularity of the LFO at faster speeds, like an automatic LFO waveform adjustment.  Makes fastest speeds more Leslie-like and less boing-ey.  Useful addition.

Finally, there is the Phase-filter mod, which adds more in tandem with a vibrato switch.  You can hear the difference when wet is mixed with dry, but the filtering and quasi tremolo effect becomes more pronounced when the clean/dry signal is eliminated.  A simple DPDT toggle switches one end of the 3300pf caps in the last two stages from the input of those stages, to ground.

[pryde]

OK. Sorry I am not really comprehending some of the suggestions.

I think I would maybe like to just try the LFO sweep mod (i.e. Depth or Width I think others have called it unless I am mistaken)

EDIT:

Of the 2 options for modifying LFO sweep, which one do you recommend: Modifying the 270k at the beginning, or modifiying the 10k at the end with series variable resistors?

[Mark Hammer]

Depends what appeals to you more.  Varying the current-imiting on the LFO output has no impact on the LFO speed, and does not change the LFO waveform...at least in the same way.  Implementing something like what EHX does (see schem) will change the speed range, waveshape, and sweep width simultaneously.  There is an absence of dialability but a certain convenience to it as well.

[asatbluesboy]

Mine is heavily modded and would be a pain to open unless I'm sure of a reward, so I thought I should ask/hijack: what should I expect if I change all the filter caps to the Small Stone values? I find it way too deep sometimes--lose a lot of highs.

[Mark Hammer]

I've made about 6-8 of these, and while I like it, I find that the sweep doesn't come out consistently across builds.  Some have a pleasing sweep, and others are just sort of "lumpy".  I gather this has something to do with the tolerances of the two 270k resistors, and the 100k that forms a voltage divider with one of those 270k units.

This is the roundabout way of saying that you may have to futz around with other things, once the cap change is done, before you get a sweep you like.  A simple Ross-to-Vibe cap change will NOT work instant magic....if that's the "reward" you're looking for.

[asatbluesboy]

Yup, that's precisely the reward I'm looking for. Mine is a real black ROSS, so I guess I'll just breadboard another one and mess with values 'till I find something I like. Thank you as always, Mark.