January 29, 2017
I have uploaded my derivation for the VA Diode Ladder Filter on the App Notes page, complete with sample RackAFX project and analysis:
The filter works as expected (matches the theoretical response) and self-oscillates nicely. I created this Topic for any questions or comments you may have; feel free to post them here.
August 27, 2018
Great stuff, Will.
Any clues as to how you would form the highpass of this filter?
PS: when testing this filter I noticed the cutoff freq was "off" by about half an octave. For me, multiplying the small g by 1.5 corrected this, but that seemed like kind of a hack. Not sure, but perhaps it is just my code.
January 29, 2017
The error in the cutoff is known and is present in the analog design too, AFAIK. So it is part of the "character" of the filter...
There isn't a way to make a HPF from the structure that I know of. But, there is a connection with the Moog ladder filter as the diode version was supposedly designed to circumvent Moog's patent, and it does also consist of series 1st order LPF blocks. Oberheim derived the "variations" for the other versions of it from a global H(s) transfer function (aka the Oberheim Variations in the Synth book). But the leapfrog structure with the crossed feedback paths would complicate any derivation that I could see. (let me know if you figure it out)
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