[sdiy] TH XR2206 VCO tune pot

[Scott Gravenhorst]

Derek Holzer <derek at umatic.nl> wrote:
>I've been building several of the very XR2206 VCOs designed by Thomas 
>Henry, with a PCB layout by Tom Bugs:
>
>http://www.bugbrand.co.uk/docs/TH_XR2206VCO_BugPCB.PDF
>
>However, I find that the main tuning pot seems more than a bit too wide.
>
>The bottom third is subsonic, the middle third is within hearing range 
>and the upper third is ultrasonic. The fine tune, on the other hand, is 
>quite subtle--as it should be. But I'd prefer that most of the main pot 
>was audible, so what can I do?
>
>One thing I noticed is that the fine tune pot has a 3M3 resistor after 
>it (R46), while all the other inputs have 100K. If I wanted to reduce 
>the range of the main tune pot, I could increase the value of the 
>resistor after that pot (R28), right?
>
>So rather than trial and error, is there any predictable way to find a 
>good range? How would that be calculated? Is it a linear thing, where 
>twice the value (i.e. 200K) would give me half the range (which would be 
>fine with me)? Or is it more complicated than that?
>
>Thx+best!
>Derek

You can restrict the range of the pot this way:

     v+
     |
     |
     Ra
     |                                            
     |                                       
     P
     O<----- ...
     T
     |
     |
     Rb
     |
     |                                  
     V-     

Where POT is the original tuning pot and Ra and Rb are resistors you will add.  The
values of these resistors will have to be trimmed or calculated by you.  If Ra and Rb
are the same resistance as the pot, then the new range will begin at about a third of
where it was without Ra and Rb and it will end about 1/3 before it gets to the end.  I
can't guess what frequencies this will give...  Ra and Rb can also be trimmers so that
you can adjust the exact range you get.

-- ScottG
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-- Scott Gravenhorst
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