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I wouldn't worry too much about the loading effects on the coarse tune
pots; it just means you'll get a bit more change per % rotation near
the end of the rotation than at the beginning. We usually tune by ear,
so the fact that it's not perfectly "calibrated" doesn't matter too
much. A lot of people do use 100K pots with 100K loads.<br>
Harry says exponential FM is more common too. It's funny - most of all
the circuit schematics I've seen had a linear FM input, and that's how
I've built all mine. Exponential FM is just putting an attenuator on a
1v/octave input, which you already had.<br>
I'll have to play around and compare the differences in sound between
the linear and exponential FM now.<br>
<br>
<br>
ryan wrote:<br>
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<div><font face="Arial" size="2">nice tutorial. thanks!</font></div>
<div><font face="Arial" size="2">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.</font></div>
<div> </div>
<div><font face="Arial" size="2">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.</font></div>
<div> </div>
<div><font face="Arial" size="2">about the coarse adjustment, I
suppose +/-5 octaves would be alot more useful. meant to change that.</font></div>
<div> </div>
<div><font face="Arial" size="2">thanks !</font></div>
<div><font face="Arial" size="2">ryan</font></div>
<div><br>
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<blockquote dir="ltr"
style="border-left: 2px solid rgb(0, 0, 0); padding-right: 0px; padding-left: 5px; margin-left: 5px; margin-right: 0px;">I've
got a tutorial on pots at <a class="moz-txt-link-freetext"
href="http://home.comcast.net/%7Esbernardi/elec/og2/partsub_pots.html">http://home.comcast.net/~sbernardi/elec/og2/partsub_pots.html</a>.
<br>
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.<br>
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.<br>
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.<br>
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.<br>
<br>
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.<br>
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<pre class="moz-signature" cols="72">--
Scott Bernardi
<a class="moz-txt-link-abbreviated" href="mailto:sbernardi@comcast.net">sbernardi@comcast.net</a></pre>
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