# Help with Exponential Convertor in VCO

Discussion in 'The Projects Forum' started by tyvokka, Oct 19, 2014.

1. ### tyvokka Thread Starter New Member

Oct 19, 2014
2
0
I'm currently trying the DIY route for a modular synthesiszer which is going towards my degree in Uni. I'm using the circuit schematic provided by Ray Wilson at MFOS for the VCO (attached), and before physically constructing the circuit I plan to place individual components of each module circuit through electronic simulations using TINA. This is for the sake of the final dissertation which will obviously need to contain the justification of my knowledge of the circuits involved. So i've made and simulated the control voltage summer, have a justification for the component values and the required 18mV (per volt of input) at the output of the summer. The issue i've been having is with the next stage of the VCO: the expo converter (which I have highlighted with a red rectangle in the attachment).

I understand the basic principle of the circuitry, it's just trying to understand how the individual components interact together. My understanding thus far of the functionality of the expo converter comes down to knowing that linear voltage at the input is converted into an exponential scale that we can hear. I know that by having matching pairs of transistors the difference in temperature between each of them during operation will be small, and therefore, easier to compensate for with a tempo resistor (R24) when in direct contact as a heat sink. So the NPN pair has a combined reaction to temperature which the tempo then cancels out, which in turn will decrease the frequency drift / adjust the scale of the incoming CV.

Beyond this I am under the impression that the op amp acts as a current source around one of the transistors which in turn fixes a value of current through the other transistor, and that R17 being fed back to to the emitter junctions determines the boundaries of the maximum current value able to pass through the exponentiator. So i'm sorry to be a pest, but could somebody with the required understanding of this circuitry give me a run through on exactly how each of the components in the highlighted area of the schematic interact?

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2. ### rogs Active Member

Aug 28, 2009
279
37
In it's simplest description, it's a linear to log converter. A linear change in the base emitter voltage is converted into a doubling of collector current. The 'standard' musical synth use of 1 volt per octave effectively translates as 83mV per semitone. Allows all the CV resistors to be the same value.
By far the biggest problem with this type of circuit is the sensitivity to temperature variation. These notes may be of some interest to you?:

http://schmitzbits.de/expo_tutorial/index.html

I'm afraid I'm not academic enough to go through your circuit at the component level, but I can tell you from the various musical VCO circuits I've encountered over the years that the effectiveness of that particular circuit will be highly dependent on the quality of the temperature stabilisation. I'm not sure how you verify this without actually building one. I can't see how TINA can simulate tempco, without knowing the actual physical layout? ....Which is critical in this case.

I preferred the 'real world' option to solving the problem taken by Moog over 30 year ago. By using a 3046 transistor array, they would deliberately use high current in another device in the package to heat the whole package, including the 'matched' pair, so that the controlled temperature range was not affected much by ambient changes. Worked very well!
There's a copy of the original circuit here: http://dl.lojinx.com/analoghell/MoogProdigy-TechnicalServiceManual.pdf ). The temperature calibration is the first item described.

If you're committed to your circuit as presented, then I hope someone can help with the academic run through you've asked for. If you're open to other options, do have a look at Moog's take on the problem.
And believe me, it can be a real problem!!

Last edited: Oct 19, 2014
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3. ### tyvokka Thread Starter New Member

Oct 19, 2014
2
0
Thank you very much for a great response! I'm will be testing this physically as well as implementing it onto a PCB in due time, but my supervisor for my project wants me to chop up the VCO into its basic building blocks and then test them accordingly to present some form of understanding. So for the likes of the summer, comparators, integrators etc I should be able to, but with this particular piece of circuitry I don't haven't the slightest clue where to start in simulation terms, because as you said, it's temperature dependant...

I will read through the notes by René Schmitz more thoroughly and perhaps backtrack to the traces left by Moog also!

Thank you again for the response