Thanks to Scott Stites, Luka, Mark Verbos, and others involved in bringing this design to DIY and of course to Don Buchla for creating it.
The Verbos design:
http://www.simple-answer.com/291.pdf
The electro-music.com thread presents a few variations on the Verbos design:
http://electro-music.com/forum/viewtopic.php?t=14380
And Scott goes even further, discussing voltage-controlled resonance, etc.:
http://mypeoplepc.com/members/scottnoanh/slsdiy/id11.html
Marjan Urekar posts a clarified version of the original schematic with interesting notes:
http://urekarm.tripod.com/synth/download.html
Everyone gets rid of the selected 2N3958 dual FET, which seems to go for about $7.00 each on eBay as of this writing. I can't say if that effects the sonics of the module or not.
The original uses the 2N3566 transistor, kin to the 2N3565 found across the years in many places in Buchla designs. The 2N version is long out-of-production, and unless they get more, Mouser have just obsoleted the PN version of the 3566. I got a few...and it turns out that Mouser still have a lead-free version for 0.64 each:
http://www.mouser.com/Search/Refine.aspx?Keyword=pn3566
One builder in the EM thread states that he first used the 2n390X and later switched to the 3566, finding that the 3566 is a bit smoother to his ears. If so, it would be good to have modules using each type of transistor!
So the overall news. I've built Luka's version, tested and enjoyed it, then gone ahead and began replacing things to more closely approach the original component list:
-Carbon Comp resistors (not all, don't have -every- type in stock
-PN3566 transistors
-Silver Mica capacitors for the .022 pair in the original schematic
I haven't compared this to one with all modern components but am seemingly aware of a slight change in the overall character; can't say if it's for the better or worse. Will have to do a side-by-side.
For those who have read "Blink" and validated its concepts, the "feel" of the 291 did indeed change with the substitution of carbon composition resistors, toward something less clearly defined. Will be interesting to see if this is the case sonically.
Again, why pursue slight changes away from modern standards? Because I've owned the original and like it just like that, and want to re-place exactly that.
What is it like?
This filter, like many Buchla designs, mis-behaves if you're expecting utter predictability. This is "character". Using the original BPF mode, you will find that the amplitude in the bass region is something like 12dB higher than the upper octaves. A sweep down into the bass will WHUMPH you up if you're not expecting it. And as you'd expect with vactrols, it's plucky and "acoustic". Although it doesn't close completely (30Hz being listed on the front panel as the minimum setting and the 12dB response means some pass-through even then), in HPF mode it will indeed go up and out to silence (the clever engineers at Korg in the early workstations with multimode digital filters simulated an HPF fully opening by allowing the Fc to raise to the point of instability/nyquist and then lowering the volume after that; sounds like an HPF opening all the way. Kudos!).
The BPF mode makes detuned-sawtooth synth strings very simple. The wide bandwidth cuts the bass but doesn't eliminate what you need in the timbre.
A very wide-ranging, great-sounding filter. You should have a pair or more in your setup.
Changes to the modified design
The modified design offers outputs for lowpass and highpass responses, and the luka layout adds voltage processing from a CGS design. It takes up nearly the same amount of space as the Verbos dual unmodified design, which is the size of an original Buchla PCB. I'm not a fan of stacking boards if not needed...
