<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class="">Hey Dave,</div><div class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">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 class=""><br class=""></div><div class="">In this case, the attenuation in the inverter stage is equivalent to each of the integrators tuned down by sqrt(10).</div><div class=""><br class=""></div><div class=""> -- Don</div><div class=""><div class=""><div>--<br class="">Donald Tillman, Palo Alto, California<br class=""><a href="http://www.till.com" class="">http://www.till.com</a></div><div class=""><br class=""></div></div></div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Oct 4, 2018, at 1:00 AM, David G Dixon <<a href="mailto:dixon@mail.ubc.ca" class="">dixon@mail.ubc.ca</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">Hey Team,<br class=""><br class="">I have been looking at the Polymoog resonator schematic (the one from the<br class="">synthfool site), and I'm a little confused. It's just three state-variable<br class="">filters, and the RC networks have Rs which range from 2.4k to 12.4k (a 2.4k<br class="">resistor and a 10k pot), and the Cs are 68nF, 12nF, and 2.7nF.<br class=""><br class="">The stated frequency ranges are 60-300Hz, 300-1500Hz, and 1500-7500Hz.<br class="">However, when I calculate the frequencies one should get from these<br class="">component values (assuming the formula f = 1/(2*pi*RC), I get frequencies<br class="">which are about 3.3 times higher than the targets. By my calculations, for<br class="">the same resistance ranges, the correct capacitors would be roughly 220nF,<br class="">47nF, and 8.2nF.<br class=""><br class="">Is the design wrong, or am I stupid, or am I missing something? If the<br class="">stated component values are the actual ones used in the Polymoog, then one<br class="">cannot argue with the fact that it sounds fantastic, even if the stated<br class="">frequencies are all wrong. However, I'd just like to know that I'm not<br class="">smoking crack here.<br class=""><br class="">Cheers,<br class="">Dave Dixon</div></div></blockquote></div><div class="">
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