[sdiy] TH XR2206 VCO tune pot
David G. Dixon
dixon at interchange.ubc.ca
Sat Oct 10 23:08:04 CEST 2009
Derek,
Thomas Henry's designs always allow the full range of voltage on the coarse
tuning pot. It's a philosophical choice. I think he assumes the builder
will customize the range to suit his own needs. For example, some may wish
to use the VCO as an LFO, while others may only want to use the VCO within
the audio range.
You can add resistors to restrict the range to whatever you desire. The way
to figure this out is as follows:
If you are using a +/-15V PSU, then with no resistors you will get the full
30V range out of the pot. At 1V/octave, that's a range of 30 octaves! If
you want to reduce the range to, say, 10 octaves, and assuming you're using
a 100k pot with a 100k input resistor to the summing node, then you must
either
a) put 200k of additional resistance between the rails and the pot, thus
making the total rail-to-rail resistance equal to 300k, or
b) put a 300k resistor between the pot and the summing node.
The disadvantage of replacing the input resistor is that you cannot select
the centre frequency; it will always be whatever the VCO generates at 0V.
By putting resistors between the rails and the pot, you can select not only
the number of octaves, but also the range of those octaves.
Let's assume that the frequency at 0V is 1kHz. If you put 100k resistors on
each rail, the total rail-to-rail resistance will be 100k + 100k (pot) +
100k = 300k. The centre voltage will still be 0V, but the two end voltages
will now be -5V and +5V. This will restrict the range to 10 octaves centred
on 1kHz, or 31.25Hz to 32kHz.
This is still a little bit high, perhaps, so what if we redistribute those
200k slightly differently? Let's try 90k on the -15k rail, and 110k on the
+15V rail. The total resistance between the rails is still 300k = 90k +
100k + 110k. However, the lowest voltage available on the pot is now 90/300
of the distance between -15V and +15V, or -15 + (90/300)*30 = -15 + 9 = -6V.
The highest voltage available on the pot is (90 + 100)/300 = 190/300 of the
distance between -15V and +15V, or -15 + (190/300)*30 = -15 + 19 = +4V.
This drops the frequency range by one octave from before, so that it will
now be from 15.625Hz to 16kHz; just about exactly in the audio range. Of
course, you'd probably use a 91k resistor, as this is the standard value.
You can divide up the 30V range anyway you like to get whatever range of
frequencies you desire. Each volt of the restricted range will give one
octave of frequency. You just have to know what frequency your VCO
generates at 0V of CV, and place the range relative to this frequency.
Clear as mud?
> Actually it's the main tune pot I want to change, it's too wide.
> Recommendations?
> D.
>
> Tom Bugs wrote:
> > Yep - try changing it to 4M7 (I like it at 3M3 but I hear what you're
> > saying)
> > xb
> >
> > Derek Holzer 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.
> >>
> >> 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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> >>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >>>>
> >>>
> >>> -- 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.
> >>>
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>
> --
> ::: derek holzer ::: http://blog.myspace.com/macumbista :::
> http://www.vimeo.com/macumbista :::
> ---Oblique Strategy # 169:
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