ECE4450 L24: State Variable Filters and the Oberheim SEM VCF (Analog Circuits for Music Synthesis) 

Definitely!

@@Lantertronics I'm using digital implementation of SV filter in my FPGA based synth and I would like to implement similar nonlinearity in the feedback path. It looks like the diodes are acting as exponential function and are somehow combined with the linear path in variable ratio. Interesting variation on the SV filter is also the Novation design: zeninstruments.blogspot.com/2017/09/novation-bass-station-schematic.html

I'm working with the Powers that Be at my employer to coordinate a mechanism by which you can donate to Georgia Tech and have that donation earmarked to support this kind of work. Details coming soon.

I finally got it worked out! And it really helps with showing my superiors that there is a hunger for this kind of material. Do a RU-vid search on "lantertronics donation" (without the quotes.) I'll post the link a a separate comment since RU-vid seems to delete links sometimes.

Here is the link for the video with the details about donating: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-VBu-LST1p9c.html

@@Lantertronics Awesome!

Huh... I honestly have no intuition about that 10 pF cap. At really high frequencies it's basically bypassing the 100K, which means you're getting *more* second order feedback, and it seems like that would potentially increase oscillation at high frequencies. Of course there may be some phase shift weirdness going on. So... beats me! :)

@@Lantertronics Ah, that makes sense. Maybe just a minor phase correction then. Thanks for the reply!

Yup... these videos were primarily created for students in my class at Georgia Tech, which are mostly seniors in EE. So they do assume a EE background -- and the videos generally assume you've seen previous ones in the series: ru-vid.com/group/PLOunECWxELQS5bMdWo9VhmZtsCjhjYNcV

I feel like there's a lack of advanced engineering content on RU-vid, and I hope to fill that gap. If anyone is looking for a more tutorial introduction, I highly recommend checking out channels by "The AudioPhool" and "Moritz Klein."

​@@Lantertronics I am no EE but I absolutely learned something from listening to your video twice. Beginners should absolutely expose themselves to indecipherable information, eventually they will learn through context and then later on when they have a better understanding, go back to what they found confusing before and it will make sense.

I'm thinking it's mostly just from the OTAs themselves. That's just a guess though. One could breadboard it with JFETs and then try swapping in regular op amp buffers and compare.

Thanks!

I don't script my lectures ahead of time (although I edit them heavily), so occasionally I surprise myself upon playback. :) Actually, I'm working on a mechanism with my employer by which people can donate to Georgia Tech and earmark the donation to help fund more of this sort of work with my students and create more of this sort of content -- although I won't be able to buy a beer with it. ;) Details will be forthcoming in a future video. (Since I'm creating these lectures as part of my duties as a professor, I can't set up set up something like a personal Patreon and accept money directly -- Georgia Tech, like most universities, has policies about these things.)

Not a beer, but you can now support this channel a targeted donation to Georgia Tech earmarked for my work with my students. Do a search on "lantertronics donation" on RU-vid (without the quotes.)

Here's the link to the video with instructions for donating: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-VBu-LST1p9c.html Any amount appreciated, even the price of a beer. :)

Also, if the op amps are a bit slow, you would want to avoid to go through them for the feedback loop going through the second integrator, since the phase loss would tend to destabilize the loop, more so at higher cutoff frequencies. That should be less of a concern for the feedback loop from the first integrator, since the the phase around that loop is nominally -90 degrees, so far from instability (and it stabilizes the loop, if it's active), unlike the nominal -180 degrees of the second loop. But I don't know if the phase loss is a concern at all.

@@possible-realities That's a good theory! The 741 is certainly a slow op amp.

And I like your theory about loading the JFET...

Cool! Where are you teaching?

@@Lantertronics I teach at the University of Illinois at Chicago.

@@Lantertronics I sent you a "thank you " note via email.

Negative feedback to the rescue! :)

@@Lantertronics Ah true! And the feedback path that isn't the adjustable Q path is DC coupled, so it'll be fine no matter how the Q is adjusted. Thanks :D

Whoops! Thanks for catching that -- it was an oversight on my part. I just added it to the playlist.

I'd suggest checking out my ECE Design Fundamentals series: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-A720hgdE45M.html