[sdiy] VCO and expo converter question

[harrybissell]

hmmm... I'd have said that expo FM is a lot more common than linear
FM...
(especially in commercial synths)... but wht not have both in this case
?
There are advantages and disadvantages to both linear and expo fm   :^P


The nonlinearity of a 'loaded' pot is often not noticible unless you
have some
panel graphics that make it obvious...  Making the summing resistors
bigger
should be OK.

H^) harry

ryan wrote:

> 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.
>