[sdiy] Thomas Henry XR-2206 VCO Page Up

[Scott Stites]

>Why not just use a larger cap? That should shift the nonlinear part 
>further down, and maybe free up something at the high end.
>
>Cheers,
>  René

We tried that, and it made some difference - there was a slight bit of improvement over the original value Tom started out with, which was .022 uF.  Beyond that, it didn't seem to really help the scaling.  We then went the route of lowering the value of R43 to allow more current through, but that really messed with the scaling.  It just seemed we'd hit some internal limit on the XR2206.

My thought was to actually approach it from the angle of introducing a bit of non-linearity into the CV itself at the lower range.  I was thinking something along the lines of an ideal diode with a breakpoint set for around the CV associated with 60Hz or thereabouts (inspired by Jim Patchell's writings about ideal diodes as a reference for this idea).  Then, when the voltage dropped below the breakpoint, run the ideal diode voltage through a 'low end' trimmer that one could use to tweak the last step of the bottom octave for accuracy.  Tom never mentioned an opinion on if I was even in the ballpark (most likely out of kindness), but he did point out, justifiably, that at that point one is making a simple thing more complicated (diminishing returns) and if one were to go to those lengths, starting with a standard sawtooth or triangle core design in the first place would be more sensible.  In that case, one could use an external multiplier and get the VC skew as well.

Now, if there is a simple solution, it wouldn't surprise me at all if it came from this list.  That could be one reason why Tom decided to put it out to the public (he really didn't mention that).  The main reason is as it sits now, it's a very simple yet useful, quite fun project.  And, I think he was ready to move on (the only time he's really got to play with this stuff is when there's a break from his teaching gig).

Cheers,
Scott




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