How to understand this vactrol driver?

Hi, I have traced some schematic from a PCB (RYO optodist), because I wanted to learn how voltage controlled distortion works. I understand most of what’s happening, but I do not understand how the vactrol LEDs are driven.

If I got this correct, then the CV signal comes from the left through a 68k resistor and is then mixed with the gain pot (upper voltage divider between 12V and 0V) and a gain trimmer (2k and then a 100 ohm resistor, see lower left). This goes into a pnp common collector amplifier (correct??) which then controls then second transistor which drives the LEDs?
But doesn’t this mean that I get more gain in the first transistor (and hence brighter LEDs) when I lower the CV voltage? This seems counter intuitive! Did I make a mistake in my schematic?
I don’t really understand how the transistors work in this configuration, can anyone explain? :slight_smile:


Your schematic leaves out a 1k from the PNP base to ground. The full schematic’s available here.

I’m pretty clueless about transistors. Aided by a quick sim (with the 1k to ground, which is important!) I see as the CV increases, it lowers the current through the PNP, so the emitter voltage and hence the NPN base voltage increases, which increases the current through the NPN and hence through the LEDs. But what role the PNP actually plays (i.e. why you can’t just apply the CV to the NPN base) I don’t know.

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temperature compensation?

Oh, 5 minutes of searching spare you an hour of following traces on a pcb! XD
But I can not see the 1k to ground here:

You mean the trimmer?

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temperature compensation?

I think this is only used for things that influence pitch, because there it’s really important not to be off a little. For other things it’s a minor problem. Just guessing here :wink:

Another question: why are two vactrols used? The resistor of the vactrol sets the gain of the opamp, but couldn’t the same be done with a single vactrol and halving the feedback resistor?

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My guess would be to double the dynamic range.

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Hm… I’m not sure if I understand. The vactrols have a resistance of ~40 ohms to ~5Meg ohms (, so two of them in series have ~80 ohms to ~10Meg ohms. The feedback resistor is 68k, so the gain is 0.0068 to 850.
With a single vactrol you have 40 ohm to 5Meg ohm and then using a feedback resistor of 34k, you get the same gains. Or is it the nonlinearity of the vactrols? But since they are added it should not play a role, right?

I made a quick simulation with a single and a double vactrol and I plot the current into the vactrols and the resulting resistance at the other side. To me that looks like just the factor of two which I assumed before… so, why the two vactrols? I must be missing something!

I’m guessing it’s a feature - not a bug. The whole circuit is marketed as an “opto-distortion”. Double the opto - double the distortion?

But if I can get the same effect with only using a single vactrol? One vactrols costs approximately as much as the rest of the circuit :wink:

yes, you get the same gain, but you’ll have twice the noise floor. But, definitely try it out!

why don’t you make it yourself ?

Sorry, yeah. 2k trimmer + 100R. That’s 1k on average, right? :grinning:

And now I see you did have it — sorry, I wasn’t looking at things right…

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I suspect you’re right about getting the same thing with one vactrol and half the feedback resistance when the LEDs are switched out but that when the LEDs are in the circuit it matters. I tried doing a simulation and CircuitLab seems not to handle the LEDs very well, I get glitchy results. But it looks like changing the resistances changes the results at least somewhat.

Added: As for the driver, there’s a very similar looking thing here: . There’s a single NPN used for linear CV and then the PNP/NPN version for exponential CV.

Having re-read the first section of this Exponential converters and how they work - North Coast Synthesis Ltd. I think what’s happening here, maybe, is the resistor network is rescaling the CV; then the PNP transistor is buffering the rescaled CV, and adding a bias to it; then that buffered voltage is applied to the NPN transistor, which puts a current through the LEDs that’s an exponential function of the input CV.

I could be completely wrong.


Good point! I forgot about the LEDs! And I still don’t really understand how to simulate them in LTSpice… But what I think I have now is this:

First one is with “original” 86k and I picked 20k as my pseudo-vactrol, the second is with both of the values halved. There is a difference, but not so much… I should probably use a range of different gains and double check that I have the correct LEDs in LTspice. Oh, I also noticed that I mixed up the value, the original feedback resistor is 68k and not 86k -.-

In the end I probably need to take this to a breadboard, but I only have one “real” vactrol :wink:

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Medication caveat! I’ve used, though very differently , this type of layout before in very low voltage motor circuits. Using the two transistors and an led to force them to both trigger and more importantly stay open even when the voltage drains too low for regular operation. Think of an led as wedging the current open.
Please note I skimmed most of the other posts on this topic and if anyone gets this I’d be glad of a better explanation :relaxed:. As you were.