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<DIV dir=ltr align=left><SPAN class=624165604-05102018><FONT color=#0000ff
size=2 face=Arial>I did have one other question, though...</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=624165604-05102018><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=624165604-05102018><FONT color=#0000ff
size=2 face=Arial>What is the significance of "V_CH" which seems to play the
role of ground almost everywhere in the schematic? Is this some specific
voltage which is not 0V?</FONT></SPAN></DIV><BR>
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<FONT size=2 face=Tahoma><B>From:</B> Donald Tillman [mailto:don@till.com]
<BR><B>Sent:</B> Thursday, October 04, 2018 7:08 PM<BR><B>To:</B> David G
Dixon<BR><B>Cc:</B> synth-diy@synth-diy.org<BR><B>Subject:</B> Re: [sdiy]
Polymoog resonator question<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>Hey Dave,</DIV>
<DIV><BR></DIV>
<DIV>The inverter stage in those filters attenuates, reducing the loop gain by
a factor of 10, lowering the tuned frequency by a factor of the square root of
10. </DIV>
<DIV><BR></DIV>
<DIV>On a more philosophical note, this is because a state variable filter
isn't R's and C's; it's two integrators and an inversion in a loop. It
doesn't matter how those parts are implemented, the overall equation is the
same. </DIV>
<DIV><BR></DIV>
<DIV>In this case, the attenuation in the inverter stage is equivalent to each
of the integrators tuned down by sqrt(10).</DIV>
<DIV><BR></DIV>
<DIV> -- Don</DIV>
<DIV>
<DIV>
<DIV>--<BR>Donald Tillman, Palo Alto, California<BR><A
href="http://www.till.com">http://www.till.com</A></DIV>
<DIV><BR></DIV></DIV></DIV>
<DIV><BR></DIV><BR>
<DIV>
<BLOCKQUOTE type="cite">
<DIV>On Oct 4, 2018, at 1:00 AM, David G Dixon <<A
href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</A>> wrote:</DIV><BR
class=Apple-interchange-newline>
<DIV>
<DIV>Hey Team,<BR><BR>I have been looking at the Polymoog resonator
schematic (the one from the<BR>synthfool site), and I'm a little confused.
It's just three state-variable<BR>filters, and the RC networks have Rs
which range from 2.4k to 12.4k (a 2.4k<BR>resistor and a 10k pot), and the
Cs are 68nF, 12nF, and 2.7nF.<BR><BR>The stated frequency ranges are
60-300Hz, 300-1500Hz, and 1500-7500Hz.<BR>However, when I calculate the
frequencies one should get from these<BR>component values (assuming the
formula f = 1/(2*pi*RC), I get frequencies<BR>which are about 3.3 times
higher than the targets. By my calculations, for<BR>the same
resistance ranges, the correct capacitors would be roughly 220nF,<BR>47nF,
and 8.2nF.<BR><BR>Is the design wrong, or am I stupid, or am I missing
something? If the<BR>stated component values are the actual ones used
in the Polymoog, then one<BR>cannot argue with the fact that it sounds
fantastic, even if the stated<BR>frequencies are all wrong. However,
I'd just like to know that I'm not<BR>smoking crack
here.<BR><BR>Cheers,<BR>Dave Dixon</DIV></DIV></BLOCKQUOTE></DIV>
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