[sdiy] TH XR2206 VCO tune pot

[Scott Gravenhorst]

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. 

>D.
>
>Scott Gravenhorst wrote:
>> Derek Holzer <derek at umatic.nl> wrote:
>>> Hi Scott,
>>>
>>> thanks for your reply. I did think of this, but since the range of 
>>> another pot in the schemo is restricted by following it with a larger 
>>> resistor, rather than putting resistors between V+/V- and the pot, I 
>>> thought that I would try to understand how that worked.
>>>
>>> best!
>>> Derek
>> 
>> I can understand how that could modify one end of the range, 
>but not both. I'll be interested to > hear how that works. > >> 
>Scott Gravenhorst wrote: >> >>> 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. >> -- >> ::: derek holzer ::: 
>http://blog.myspace.com/macumbista ::: >> 
>http://www.vimeo.com/macumbista ::: >> ---Oblique Strategy # 101: 
>>> "It is simply a matter of work" >> 
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>ScottG > 
>________________________________________________________________________ 
>> -- Scott Gravenhorst > -- FPGA MIDI Synthesizer Information: 
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>-- 
>::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
>http://www.vimeo.com/macumbista :::
>---Oblique Strategy # 152:
>"Tape your mouth"
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.