Univibing the Ropez

[Aharon]

I did some searches and came up with a few mods to the ropez to turn it into a Univibe of sorts.
I already built one Ropez as is.
I just want to check if I got the latest correct mods.
1)Cap change to Univibe spec.
2)Add vibrato sw.
3)Input current mod,50KB pot in series with 10K resistor wired a variable resistor.
4)LFO mod,change cap from 3.3uF to 2.2uF and change rate pot to 100KB.
5)Output level mod,change gain resistor to 22K and add 100KA pot for output level wired as voltage divider.

Thanks for any help.
Aharon

[Mark Hammer]

That's what I did, and I couldn't keep my hands off the damn thing with those changes in place.

Note as well that none of the circuitry for the regen is needed, since regeneration does nothing for Univibe-like circuits.  Using this layout as a reference - http://www.tonepad.com/getFile.asp?id=47 - that would include the 500k regen pot, the 1uf caps going into and out of the regen pot, and the 27k resistor between input and wiper and the one just after the second 1uf cap.

The 50k pot in series with the 10k resistor in #3 in your list limits the current from the LFO to the phase shift stages to reduce the amount of sweep.  You can use a 100k pot as well, although 250k (generally the next standard value up from there) may be too much.  On the other hand, you can always stick fixed resistors in parallel with the added pot to get whatever range of sweep width adjustment you want.

Univibes do relatively little at slow sweep rates, so the 3.3uf cap is reduced to 2.2uf to increase the overall LFO speed range a bit.  The actual speed is set by the combined resistance of the 500k speed pot and the 4k7 resistor in series with it.  Again, because the slow speeds don't really provide anything useful, the faster part of the range is "carved out" by ignoring the other 400k of resistance change and using a 100k pot.  Of course, one could always just stick a 150k fixed resistor in parallel with the existing 500k pot I suppose.  The key thing is you want to be able to use as much of the pot rotation to get subtle differences in faster speeds as you can.

[Aharon]

Finished!!!!!!!!!!
Thanks Mark....AGAIN..........
Thanks to Francisco for his generosity with the layouts,always great.

I built it and
performed the mods as decribed by  Mark and itemized by me above and the "vibe" sounds amazing , it all fit in a hammond box (the next up from the small,what is it 1590?).
So.... I got Level,Depth and Speed plus a vib/phase switch.
Not bad for 2 days' work split in 3 hours each sitting.............
Thanks again guys.
Aharon

[Mark Hammer]

I swapped notes with Steve Daniels yesterday and lining up some SIP versions of dual op-amps and the JRC13600 is on the stack for later in the year, though how MUCH later will depend on a number of things.  A SIP version of the Ropez (whether Univibed or not) would easily fit in a 1590B.  I'm keeping my fingers crossed.

I'm assuming the "Level" control is for adding some gain to the output/mixing stage and the option to have the effect louder/softer or same-level as the bypass signal?

[Aharon]

I used the next size up from the small Hammond box,I cant read the # now,the board is there tight.....

Yes the LEVEL is just that,I put a 22K in the gain loop and a 100KA pot as a voltage divider,that seems to give me just a tad more volume than the bypassed signal when maxed and of course lower down to zero.

There's a glitch in an otherwise perfect build (no such a thing?),there's a thump whenever the bypass sw is activated ,either in or out,I never boxed the other Ropez I built without mods so I don't know if they both do it.
How can I fix that,what is causing it?
Thanks
Aharon

[Aharon]

mmmmm?
The LEVEL introduces noise at any position except max.
MAybe should wire it as a variable resistor to ground?
Aharon

[Mark Hammer]

Hmmm.  Maybe the solution is to leave the 100k fixed resistor in place, and simply vary the gain of the mixing stage.  Since that last op-amp is used in inverting mode, you can set the gain below unity as well as above.

Gain is determined by, of course, the ratio of the feedback resistance to the input resistance.  Stock, you see a pair of 27k input resistors plus a 15k feedback resistor.  Although this sets the gain at a little above 0.55, it is mixing two copies of the same signal, so you double the input sum, halve the gain, and you end up with close to original signal level.  Using an 8k2 or 6k8 fixed resistor in series with a 25k variable resistor, would give an output level ranging from about half the bypass level to a little more than double.  As near as I can tell, that should have no impact on the output impedance (which may be the source of your noise issues).

[Aharon]

Thanks MArk for the tip.
I had to undo the LEVEL mod and stick the 150K fixed resistor to ground at the output.

The funny thing is the LEVEL pot worked for at least 5 min in the beginning and then it deteriorated till it quit working.
If I ever feel like taking the board out and adding that 25K level pot at the gain loop I will keep your suggestion in mind but for now I will enjoy the vibe as is for a while,the 22K resistor gives it more or less unity gain.

What about a fix for the on/off thump?
Thanks Mark.
Aharon

[Mark Hammer]

Sadly, that I have no clue about.  Pops, I understand, thumps...not so much.

[Aharon]

Yes ...it's a pop rather than a thump..........gotta  describe the event  correctly.
Is there a fix?
Thanks
Aharon

[Mark Hammer]

I guess you could consider sticking a 1M-2M2 resistor in parallel with that 100pf cap on the input.  What you're hearing is the rapid drain of the .01uf input cap.  You need to provide a way for it to bleed off when in bypass.

[Aharon]

Ok,I'll try that.
Thanks
Aharon

[Aharon]

As if by magic when I undid the Level mod the pop when switching the effect on/off disapeared,I guess the caps could be discharging through that 150K output resistor.
Anyway,if any of you are planning to mod the Ropez be aware of that little detail,might be better to use Marks suggestion and get level control at the OP's gain loop as he explained.
The Ropez univibed sounds very nice indeed.
I guess I should tackle the real thing next...............

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