[sdiy] VCO and expo converter question

[ryan]

nice tutorial. thanks!
I'm sure glad I asked here before getting the PCB made. I thought that Exponential FM would have been more typical, thats why I didn't add Linear FM. Yesterday was actually the first time I tried putting something into the FM jacks on my synth.  well, Before yesterday, I had only 1 VCO. the Idea for this VCO is to be a simple, fit on a small PCB, and take up little panel space. I have Oakley Sound VCOs for my main oscillators, but I'd like to get this one to stay in tune with those.

for the pots. I'm using 100K pots on a 100K load. arg. I already spent a bunch of $ on those fancy spectrol 100k pots. I think it'd be easier to just double the values of all the other resistors. I think that would atleast reduce the warping enough and the 10K trimmer for V/Oct would be more centered around what the value should actually be. I looked at that java applet, It looks pretty nasty the way I have the circuit now.

about the coarse adjustment, I suppose +/-5 octaves would be alot more useful. meant to change that.

thanks !
ryan


  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.
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