[sdiy] TH XR2206 VCO tune pot

[Oscar Salas]

Hello eveyrone! Now that I achieved to send mails to this very good elist (I was sending the mails in html format, Thanks Derek) I add here the message that I i tried to send yesterday, maybe things already said but here it is:

Hello,

If we input 1V in the 1/V Octave Input, that it has a 100K resistor and the reference resistor of the inverting summer is 2K, we can know the Voltage in the inverting summer for a octave span.

If for an inverting amplifier Vout=Vin(Rref/Rin)

Vout=1V(2K/100K) ;  Vout = 0.02V

So, 0.02V is what we have in the inverting sumer output when we input 1V and is related to an octave span.

If for example we want that the coarse potentiometer responds over a span of 10 octaves, that seems useful, the inverting summer output must be +0.1V to -0.1V.

If the Vin is +/-15V and Vref=2K:

0.1V=15V(2K/Rin) ; 0.1V/15V=2K/Rin ; Rin=2K/(0.1V/15V) ; Rin=300K

And following those steps for a Fine potentiometer with an octave span, that seems useful, +/-15V, Rref=2K, Rin=3M

Also you could feed the potentiometers with reference voltage +/-5V so for the Coarse Rin would be =100K and for the Fine=1M.

About the 10 octaves span, forexample that it would be 16hz to16384hz, the center would be 512hz. That it must be with voltage input=0. What center frequency do you have now with Vin=0?

You can try diferent values for the timming capacitor, that for the audio range is C7 in BugBrand schematics.

Also if I understood correctly in the book "making music with the 3080 OTA" by Thomas Henry, R43 (in BugBrand schematics) is related with the reference current so with the VCO center frequency.

Oscar.

--- On Sat, 10/10/09, Derek Holzer <derek at umatic.nl> wrote:

> From: Derek Holzer <derek at umatic.nl>
> Subject: [sdiy] TH XR2206 VCO tune pot
> To: "synth-diy" <synth-diy at dropmix.xs4all.nl>
> Date: Saturday, October 10, 2009, 6:52 PM
> 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
> 
> -- ::: derek holzer ::: http://blog.myspace.com/macumbista ::: http://www.vimeo.com/macumbista :::
> ---Oblique Strategy # 59:
> "Don't avoid what is easy"
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