[sdiy] VCO and expo converter question

[Scott Bernardi]

Really? Almost every VCO circuit I have seen has a linear FM input. I 
guess you just would use a regular 1v/octave input to do exponential FM, 
just attenuate the signal.

harrybissell wrote:

>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.
>>
>>    
>>
>
>
>  
>

-- 
Scott Bernardi
sbernardi at comcast.net

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