[sdiy] TH XR2206 VCO tune pot

[Derek Holzer]

Thanks! Voltage dividers I've got no problem, inverting summers are the 
ones I'm working on ;-)

D.

David G. Dixon wrote:
>> Oscar asked if I knew the 0V frequency of the oscillator--in fact I
>> don't because I haven't started the tuning process yet. I wanted to get
>> the range under control first.
> 
> You "should" be able to calculate the 0V frequency of the VCO.  I'm not sure
> how to do this with XR2206, but it's fairly straightforward with a typical
> saw-core VCO.  You simply need to know the reference expo current, the raw
> ramp voltage, and the timing cap size.
> 
>> So why would having control over the 0V frequency be important
>> otherwise? So that your pot starts in the middle of the audible range....?
> 
> It's not a matter of "controlling" the 0V frequency; just knowing what it
> is.  Disconnect all the tuning pots (if you can), alligator-clip the
> 1V/octave input to ground, and measure the frequency.
> 
>> In option b), the 100K summing resistor is *replaced* by the 300K
>> resistor, correct?
> 
> Yes.  This reduces the current by a factor of three, which reduces the
> summing amp gain by a factor of three.  Hence, a 30V tuning range (pot
> connected directly to rails) with a 300k input resistor is equivalent to a
> 10V tuning range (pot with 200k of extra resistance on the rails) with a
> 100k input resistor.
> 
> If you understand how voltage dividers work, and how inverting summers work,
> you can design these sorts of changes yourself with absolute confidence!
> 
> 

-- 
::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
http://www.vimeo.com/macumbista :::
---Oblique Strategy # 60:
"Don't be afraid of things because they're easy to do"