<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=ISO-8859-1">
<title></title>
</head>
<body text="#000000" bgcolor="#ffffff">
I've got a tutorial on pots at
<a class="moz-txt-link-freetext" href="http://home.comcast.net/~sbernardi/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>
<br>
ryan wrote:<br>
<blockquote type="cite"
cite="mid00e201c3b216$27f07300$46ff6244@supercomputer">
<meta http-equiv="Content-Type" content="text/html; ">
<meta content="MSHTML 6.00.2800.1106" name="GENERATOR">
<style></style>
<div><font face="Arial" size="2">hi,</font></div>
<div> </div>
<div><font face="Arial" size="2">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.</font></div>
<div> </div>
<div><font face="Arial" size="2">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</font></div>
<div><font face="Arial" size="2"><a
href="http://www.sdiy.org/destrukto/4069_VCO.png">http://www.sdiy.org/destrukto/4069_VCO.png</a></font></div>
<div><font face="Arial" size="2">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...</font></div>
<div> </div>
<div><font face="Arial" size="2">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. </font></div>
<div> </div>
<div><font face="Arial" size="2">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?</font></div>
<div> </div>
<div><font face="Arial" size="2">thanks...</font></div>
<div><font face="Arial" size="2">ryan</font></div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Scott Bernardi
<a class="moz-txt-link-abbreviated" href="mailto:sbernardi@comcast.net">sbernardi@comcast.net</a></pre>
</body>
</html>