<html><head></head><body>How about the programmable op amp filters based on the lm4250? <br><br><div class="gmail_quote">On August 23, 2019 9:09:48 AM PDT, Quincas Moreira <quincas@gmail.com> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<pre class="k9mail">Don’t forget the Leapfrog topology as implemented by Matthew Skala.<br><br>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><br>Sent from my iPhone<br><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">On Aug 23, 2019, at 11:57, Donald Tillman <don@till.com> wrote:<br><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;">On Aug 22, 2019, at 11:30 PM, Brian Willoughby <brianw@audiobanshee.com> wrote:<br><br>First of all, where is the specification for the Moog Ladder Filter that shows it having accurate voltage control over a defined range? Is it really that accurate? Doesn’t it require calibration periodically?<br><br>How bad is the CV accuracy for other filters? Don’t the others calibrate to the same end results?<br></blockquote><br>If a filter is tuned with a transistor's exponential base-to-collector function, then it will probably be accurate enough to throw into oscillator mode and be used to play a melody.<br><br>That's not the case with filters tuned with diodes, FETs or vactrols.<br><br>That said, if I'm buying something called a "filter", then it's not reasonable to assume that I can play a melody on it.<br><br><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;">Second, if you were to build a 4-pole SVF, would it be considered two 2-pole low-pass sections in series, or would the two dual integrators make some folks call it “four” single-pole low-pass sections in series?<br><br>Since I haven’t tried this, I’m not familiar with how global feedback might cause problems with 2 SVF in series. I do recall some nice discussions around here, though.<br></blockquote><br>There are multiple ways to build a 4-pole SVF. <br><br> -- Don<br>--<br>Donald Tillman, Palo Alto, California<br><a href="http://www.till.com">http://www.till.com</a><br><br><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;"><br>Brian<br><br><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #8ae234; padding-left: 1ex;">On Aug 22, 2019, at 11:05 PM, Donald Tillman <don@till.com> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #fcaf3e; padding-left: 1ex;">On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <tom@electricdruid.net> wrote:<br><br>I don’t like to disagree with you, Don, but I’m not sure what you’re thinking. Moog or SVF are definitely *not* the only two filter options.<br>OTA+cap-to-ground+buffer? VCA+Integrators?<br></blockquote><br>Consider a description of a filter as a sort of "taxonomy" with three layers:<br><br> Top Layer: the filter spec, number of poles, response<br><br> Second Layer: the topology that implements that filter function<br><br> Bottom Layer: implementation details, including the control element<br><br>So a Moog Ladder would be:<br><br> Top Layer: 4 pole, low-pass, with resonance<br><br> Second Layer: 4 single-pole low-pass sections in series, with feedback<br><br> Bottom Layer: the ladder circuit<br><br>And a State Variable filter would be:<br><br> Top Layer: 2 pole, multi-mode<br><br> Second Layer: 2 integrators and an inverter, in a loop<br><br> Bottom Layer: the circuit, perhaps OTAs <br><br>And so forth. <br><br>This analysis also works really well with oscillators and other functions.<br><br>Here's a Moog style VCO:<br><br> Top Layer: VCO with sine, square, triangle, sawtooth waves<br><br> Middle Layer: block diagram with exponential current source, sawtooth core, waveshapers<br><br> Bottom Layer: the circuit details<br><br>So if I dismiss the implementation details, as defined this way, it limits the number of unique filter designs.<br><br>You know I'm a big fan of implementation details. And you'd want to make sure that the implementation details didn't have a significant functional effect as you draw these lines. That's part of the craft.<br><br>But if I'm characterizing filter types, I think it's reasonable to pay attention to the implementation topology and ignore the implementation details.<br><br>-- Don<br></blockquote></blockquote><hr>Synth-diy mailing list<br>Synth-diy@synth-diy.org<br><a href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br></blockquote><hr>Synth-diy mailing list<br>Synth-diy@synth-diy.org<br><a href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br><br></pre></blockquote></div></body></html>