Polivoks substitutions (UA776? KR140UD12?)

regarding the build page note :"UA776, a few adjustments will be necessary in R15…R18."
what are those recommended adjustments?
well, more specifically - i have two KR140UD12 from an original polivoks. so i sub two KR140UD12 for the two LM4250 - what values do i need to futz with to get this to jive with the new circuit? im going to put the chips in on sockets obviously, but want to get the resistors in the ballpark on the first try if possible so i dont have to rework.
thanks so much! very excited about this project

No complete datasheet online, no Spice model available, no real unit in my drawers => I don’t know.

@mutante
Maybe original schematics and datasheet will help you.

  1. CV
  2. Audio IN
  3. RES CV
  4. Audio OUT

КР140УД8Б=UA740TC
КТ315Г=2SC388, 2SC380

The Datasheet is very valuable schrab ! :wink:

Fell free to ask if google translation won’t help.

only real difference i can see then is the 100k vs. 110k resistors in the original schematic? +/-12.5 volts - am i going to be driving these KR140UD12 with good juice? 110k and 12.5 seems so arbitrary

This may be some help…it is Marc Bareille’s adaptation of the original Polivoks filter posted above and includes his ideas on how to modify the current resistor between the LM4250 and ua776 parts. Bear in mind Pichenettes has chosen a different value for his board.

http://m.bareille.free.fr/modular1/vcf_polivoks/polivoks_schem.pdf

PS ua776 and russian part are meant to be very similar in performance…

baraille suggests using something upwards of 1meg for the current resistors on his adaptation if you use a 4250, the shruthi polivoks uses a 100k with the 4250. baraille uses a 110k with a 776, and the original uses a 110k with KR140UD12. im just going to run with a 100k and see how it sounds. are there any online samples of how the mutable filter board is “supposed” to sound so i can A/B compare? ive used real polivoks before, but do not have access to one at the moment to compare. i recall the filter struck me as similar to the synthacon at the edge of resonance, or a misbehaving MS20.

I have found that when it comes to cutoff range, the resistors in the exponential current source have a much bigger impact than the 100k series resistor to the current pin of the LM4250. Another thing to mess with is the 1k/47k divider in the filter core. You can get different trade-offs in terms of maximum cutoff frequency / distortion by changing those (33k gives a higher cutoff but more distortion).

ah, excellent tips. ill probably have the board done tonight and will update with sound clips of lm4250 vs. the soviet chips.

just wanted to chime back in and let the forum know that the original soviet chips work perfectly in this filter implementation! built the thing, fired it right up, worked perfectly - and i can confirm from having played several polivoks over the years that the filter is just as gnarly and weird as the original! thanks so much, getting excited about sounds like these is what drives me to make music!

Rally old thing, but what can I say, it is actual for me right now.
I’ve ordered some LM4250, still waiting for them as well as for the boards too, but I was wondering that have you replaced the resistors when you’ve used original soviet chips?
I’ve built a few Polivoks filters from Ericasynth kits, so I have a few chips left.