<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="">On Aug 23, 2019, at 9:09 AM, Quincas Moreira <<a href="mailto:quincas@gmail.com" class="">quincas@gmail.com</a>> wrote:<br class=""><div><blockquote type="cite" class=""><br class="Apple-interchange-newline"><div class=""><div class="">Don’t forget the Leapfrog topology as implemented by Matthew Skala.<br class=""></div></div></blockquote><div><br class=""></div><div>I don't know it. 'Gotta schematic?</div><div><br class=""></div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class="">Also the Steiner Parker, the Wasp filter. The EMS diode ladde and Arp 2600 filters maybe closely related to the Moog but there are differences. Still cool to see a young mind tackle this analysis<br class=""></div></div></blockquote><div><br class=""></div><div>The Steiner Parker is a rare exception. It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors.</div><div><br class=""></div><div>The Wasp filter is a State Variable with 4069 inverters replacing the inverting opamp in the integrators. Certainly the overdrive characteristics of the 4069 inverter are different than an opamp, but it's in a local feedback loop, and in a global feedback loop, and I think the OTA overdrive will predominate anyway.</div><div><br class=""></div><div>The EMS diode ladder is the same as the Roland diode ladder, and they're both Moog Ladder knockoffs, knocked off sufficiently to get around the patent.</div><div><br class=""></div><div>The original ARP2600 filter is a clone of the Moog Ladder. </div><div><br class=""></div><div>The later ARP2600 filter has the same filter topology, but implemented inelegantly to get around the patent. </div><div><br class=""></div><div>So the last three are, in fact, Moog Ladders with legal issues. :-)</div><div><br class=""></div><div> -- Don</div></div><div class=""><div style="color: rgb(0, 0, 0); font-family: Menlo; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;">--<br class="">Donald Tillman, Palo Alto, California<br class=""><a href="http://www.till.com" class="">http://www.till.com</a></div>
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