[sdiy] VCO and expo converter question

[Scott Bernardi]

I've got a tutorial on pots at 
http://home.comcast.net/~sbernardi/elec/og2/partsub_pots.html.
In general to avoid loading effects, you want your voltage divider pots 
to be 5 to 10 times less than the resistance that loads them. So for a 
100K input resistor, use a 10K up to 50K pot. To see the effect of 
loading, I have a link to Chris List's java applet that plots loading 
effects.
You might want to decrease R2 a bit to give yourself more Coarse freq 
adj range. As you have it, you'll only get 6 octaves. Using a 300K will 
give you a 10 octave tuning range.
The FM input you have is feeding the exponential input - linear FM is 
more typical. To get linear FM, move the end of R28 to pin 6 of U7B. 
Also, you'll want to change the value of  R28 to something above 1M.
I would also have a second 1v/octave input - duplicate the R27 input.  I 
also find having a front panel "LFO switch" useful. This would sum a 
large negative voltage into your input summer, which would switch the 
frequency way down. -15v through 300K into the summing amp with switch 
you down 5 octaves.

The 100pF compensates for the extra phase shift running  IC1A in the 
feedback loop of the opamp, and it is to prevent oscillation. I also use 
100pF because everybody else does.

ryan wrote:

> hi,
>  
> I've been studying expo converters for about 2 or 3 weeks now because 
> I wanted to adapt an active CV summing stage to René Schmitz's 4069 
> VCO. It also is going to use a PTC tempco resistor instead of the NTC 
> resistors.  What I've come up with seems like it should work, atleast 
> mathematically, but I'm not sure about a few components which I've 
> seen on other expo converters.
>  
> here is the schematic for the 4069 VCO with a modified expo converter. 
> Everything after the MAT-02 was copied from the original 4069 VCO
> http://www.sdiy.org/destrukto/4069_VCO.png
> this seems to be the common way of making an expo converter using the 
> MAT-02 as the differential pair. The part I'm unsure about is the 
> capacitor in the feedback part of the 2nd opamp (the opa2277). I 
> picked a 100pF capacitor here although I have no idea what it should 
> be, or how to calculate what it should be.  I've been told that it's 
> purpose is to prevent the opamp from oscillation but I just put it 
> there because everyone else does...
>  
> so, does anyone have any comments, suggestions, explanations about 
> this capacitor? I've seen it in many of your designs although the 
> value seems to range from 1pF to 680pF in different designs. 
>  
> one more question. I've chosen 100K pots for all the pots.because I'm 
> unsure how to pick the values of pots in a voltage divider.  Is there 
> some rule of thumb to follow when choosing pot values? It seems like 
> the pot will affect the voltage seen at the input of the opamp so this 
> could cause a problem?
>  
> thanks...
> ryan


-- 
Scott Bernardi
sbernardi at comcast.net

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