Author Topic: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages  (Read 279 times)

KarenColumbo

And yet another basic question: There's "Easy Vibe" by John Hollis. I breadboarded it, it sounds very nice, but I guess those resistance values at the optical coupling between LFO and all-pass filter stages differ between a "classic" Vactrol VTL5C3 (10 M according to data sheet) and those LDRs I bought in the www (15 k). So I guess I have to compensate the difference somehow somewhere, innit? Or not?

(btw: I just ordered some components - a timer chip and a decimal counter chip to bombard those LDR basterds with a LED chase light :) )

Here's the legendary schem:

« Last Edit: February 28, 2021, 04:59:24 AM by KarenColumbo »
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antonis

Re: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages
« Reply #1 on: February 28, 2021, 05:36:14 AM »
So I guess I have to compensate the difference somehow somewhere, innit?

If you wish to, scale respective caps values.. :icon_wink:
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ElectricDruid

Re: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages
« Reply #2 on: February 28, 2021, 10:14:15 AM »
+1 what Antonis said. Change the phase-shift stage caps until you've got a nice sweep in the audio range.

You can scale LDRs *down* by putting a resistor in parallel with them, but you can't scale them up in the same way. I note that John Hollis provides no information about what LDRs he used. He probably expected people to tweak it anyway.

PRR

Re: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages
« Reply #3 on: February 28, 2021, 01:41:32 PM »
... "classic" Vactrol VTL5C3 (10 M according to data sheet) and those LDRs I bought in the www (15 k).

When are they 10Meg/15k?? ARE they ever really 10Meg/15k?

In real life they vary from under 1k to over 10k. That's about enough for a phaser. The fact that a super-clean LDR will *eventually* go over 10Meg in total darkness is not too important, we never go there. The "15K" may be a CYA spec for a sloppy factory just making for dusk-lights. And we never too far into the low-Ohms zone. That's what the Depth knob does.
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KarenColumbo

Re: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages
« Reply #4 on: February 28, 2021, 02:29:10 PM »
Thank you guys - capacitor swapping it will be then :)  ;D
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ElectricDruid

Re: Scaling resistor values because: Vactrol vs. home-made LED/LDR sausages
« Reply #5 on: February 28, 2021, 06:08:07 PM »
When are they 10Meg/15k?? ARE they ever really 10Meg/15k?

In real life they vary from under 1k to over 10k. That's about enough for a phaser.

The Xvive VTL53Cs I've just run out of go up into multiple-MOhms pretty easily, and I'm sure they're not the only ones. Yes, they need to be in the dark, but it doesn't have to be pitch black for hours. Just "off" is fine. So reckoning on >1M max resistance is perfectly reasonable. You *will* see such a value, and often.

In the FilterFX I had to put a 220K resistor in parallel to get the range I needed. If the max resistance was anywhere near 10K, that'd have done basically nothing.

I agree with the second part though - a 10:1 ratio is enough for a decent phaser sweep, as long as you get it in the right place. That'd cover 300Hz to 3KHz (for example) nicely, which is right where we can hear it. Go up much higher and a guitar doesn't have a lot of signal to play with, go much lower and you're turning it into an effect for bass players, and they probably don't want the fundamental removed by notches either.


KarenColumbo

Generally I expected a far more spectacular effect when I bb'ed some of the vibe/phaser curcuits in recent times. Just breadboarded the legendary tri-vibe yesterday - sounds nice but I expected UGE sweeps and psychic transformances. All I got was a sweet little wobble and a bit of chorus. But as I learned by searching the web one can do quite a lot with cap swapping. And I got a couple of the new TAPLFOs from Tom, so maybe there's a bit of psychedeliciousness still in it for me.
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bluebunny

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"psychedeliciousness" - word of the week   8)

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Ohm's Law - much like Coles Law, but with less cabbage...