<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On Nov 22, 2022, at 10:23 AM, Donald Tillman <<a href="mailto:don@till.com" class="">don@till.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class="">On Nov 22, 2022, at 2:01 AM, Lanterman, Aaron D via Synth-diy <<a href="mailto:synth-diy@synth-diy.org" class="">synth-diy@synth-diy.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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Does anyone recognize the topology Don is using here in the bandpass sections?
<div class=""><a href="http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg" class="">http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg</a></div>
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<div class="">The low and high bands are 3-pole Sallen-Keys, but I’m not sure what’s going on in the middle filters.</div></div></div></blockquote><br class=""></div></div><div class=""><br class=""></div><div class=""><div class="">Hey Aaron,</div><div class=""><br class=""></div>It's a twin-tee filter, but connected in an unusual way. </div><div class=""><br class=""></div><div class="">The input is directed into the twin-tee's "ground", and the twin-tee's normal notch filtering action is the opamp's feedback loop.</div></div></div></blockquote><br class=""></div><div>Next question; how does that work?</div><div><br class=""></div><div>The opamp with a twin-tee notch in the feedback loop becomes a sharp peak filter.</div><div><br class=""></div><div>The input signal comes into the underside of the twin-tee (which would normally be grounded) which doesn't seem to make sense. But as far as the circuit goes, there's no ground reference other than the opamp's + input. So the opamp + input is moving wrt to the input signal.</div><div><br class=""></div><div>But why do that?</div><div><br class=""></div><div>Well, if you had a standard inverting opamp configuration, the zero resistance summing node would load down the twin-tee.</div><div><br class=""></div><div>And if you had a standard non-inverting opamp configuration, the gain could never go below unity.</div><div><br class=""></div><div>So this is a way to get the non-inverting opamp configuration, and no load on the twin-tee, but with the + input still grounded, and thus no minimum gain of unity.</div><div><br class=""></div><div> -- Don<br class="">--<br class="">Donald Tillman, Palo Alto, California<br class=""><a href="https://www.till.com" class="">https://www.till.com</a></div></body></html>