[sdiy] TH XR2206 VCO tune pot

[Dave Manley]

Scott Gravenhorst wrote:
> Derek Holzer <derek at umatic.nl> wrote:
>   
>> If you look at the schematic:
>>
>> http://www.bugbrand.co.uk/docs/TH_XR2206VCO_BugPCB.PDF
>>
>> You'll see that the fine tune and main tune are essentially the same: 
>> 100K pots between V+ and V-. The only difference is that the wiper of 
>> the fine tune goes through a 3M3 resistor, and the main tune goes 
>> through a 100K. They are both summed after that. That's the part I'm 
>> trying to understand.
>>     
>
> The two inputs are summed by IC2B, so it's a simple summing circuit to consider. 
> Increasing R28 will limit the current that can flow through it.  If, for example, you
> double R28, you will limit the maximum current delivered to the summing node of IC2B
> by the TUNE pot.  This will reduce the top end of the range, but not the bottom, so
> the subsonic part of the range will still be there. 
>
>   
Is this true?  The coarse tune pot has +V and -V at its ends. 

The opamp gain is -R7/R28 (ignoring the other inputs, and neglecting the 
loading effect that R28 has on the pot's wiper voltage).

Try changing R28 to 200K and then 400K.  It should decrease the top and 
bottom range.

-Dave