Easy Exponential CV for the XR-2206

[Thomas Henry]

Hello all,

As promised, here's another freebie VCO chip hint. The schem shows how to easily add an exponential CV to the XR-2206. This is a quick-and-dirty alternative to the more precise version (Figure 76) in the VCO Chip Cookbook.

It's not supposed to be dead-on 1V/Octave, but is more useful for drums and analog sequencers. Nonetheless, even with off the shelf resistors it gives a legit exponential response. You might want to experiment replacing R1 and R3 with some sort of trimmer network to see if you could get more precise results.

I've labeled it Figure 76(b) in case you want to glue it into your copy of the book for an alternative to what's there.

And in the credit-where-it's-due department: the XR-2206 expects to withdraw current from pin 7. What I did was reverse the topology of a design by Terry Mikulic from some thirty years ago. So, really, this is an obvious idea for the XR-2206 that's been waiting all these years to be found.

When I ran it on the bench, it covered a dozen octaves easily. The usable CV range spans about -6V to +4V.

Thomas Henry

Edit: I just realized that I used the designation C2 twice---just relabel one C3 if you want.

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Easy Expo.pdf
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[goodrevdoc]

Thanks Thomas! I'll have to give this a go. You've managed to turn my dusty old Radio Shack relic into the little breadboard that could.(sometimes)
-justin

[bugbrand]

Nice work as always Mr.TH

Thanks for keeping me damn busy!
I've got the VCO book on order from sMs now -- whoop whoop!

[Thomas Henry]

Thanks for the thanks, guys...that's makes it even more rewarding to contribute some more!

Thomas Henry

[State Machine]

Nice idea and cheap to implement. I have been collecting the extra figures you have posted Tom in these threads and I am including them with every shipment of the VCO Chip handbook.

I have never really looked at the XR2206 as close I have now until I read the books section on this IC. I did use the chip about 10 years ago when I built a data communications link for a rocket project but never considered it for musical applications. I am impressed with the chips fidelity and frequency range!

Bill

[The Alison Project]

Great, thank you. Throwing it in my copy now

Thomas, can we expect extra pages free for Magic Smokes book if you come up with anything?

[Thomas Henry]

Certainly! It seems that every time I finish a book, no matter the topic, I always learn some new things I could have written about. In other words, these are typically ongoing projects. When I hear great feedback from others who are interested in the stuff, I'm always ready to keep the conversation going.

Thanks for the interest,

Thomas Henry

[xpmtl]

Seems that the schematic's been lost after the october crash.
May we have a repost ?

cheers,

X

[drapdap]

Someone have this one maybe? Could you be so kind to post it again?

thank you!