Polivox VCF layout

LFLab · Posted: Mon Jan 13, 2014 3:33 am Post subject:

Busy troubleshooting this, I built one, but it doesn't work.
I am suspecting the programmable opamps I bought on ebay, but found a couple of other things as well:

The Q pot is actually cutoff, and cutoff is Q

Don't use 741's for the single opamp, since there is no compensation or supply bypassing, it oscillated in my build. TL071 is ok (haven't checked the scope yet, but it doesn't get hot, contrary to the 741 I used).

R8 is marked as 82k but in marc bareille's schematic this is 162k.

Checked all component designators against Marc Bareille's schematic, and they are the same, R2 and R3 are swapped, but both 100k. C3 and C4 are C5 and C6, but that's clear as well.

The trimpot is a cutoff range adjuster.

I am going to get ua776 opamps and retry (or, if I cannot get those, I'll plonk in LM4250's and change R10 and R11).

LFLab · Posted: Thu Jan 30, 2014 4:10 am Post subject:

So, plugging in ua776 it works somewhat, the resonance control seems to work, but cutoff range doesn't do a whole lot, and the FM knob also isn't functioning correctly.
All in all, not sure what I'll do, better roll my own, methinks.

There are some people who have built the VCF from this layout correctly, but I don't feel like fiddling with it anymore, especially since there are some issues with it, and it is a simple enough schematic to plonk it into Eagle myself.
From what I did hear, playing with the resonance knob, it can properly scream, so I'll certainly build a couple of these.

airfrankenstein · Joined: Jan 10, 2010 Posts: 62 Location: france

thanks for the update. I was on the point of building this but perhaps will go back back to bareille's version or hackster's (which seems to be the blue lantern version).

LFLab · Posted: Mon Mar 10, 2014 5:22 am Post subject:

There's also a PCB by Girts on Muffwiggler, seems to work well (and you can order the original semicons to go with it (transistor, programmable gain opamps).

airfrankenstein · Joined: Jan 10, 2010 Posts: 62 Location: france

yeah i know looks very nice tight budget after euro excess spending

vladosh · Posted: Tue Mar 11, 2014 5:51 am Post subject:

this is how this polivoks sounds ,the lpf is useless so this is the bpf ,built two of these ,it needs alot attenuation on all inputs and outputs as i read somewhere else too,so a bit cleaner version of this filter would be great

https://soundcloud.com/synthacks/polivoks-vcf-demo

airfrankenstein · Joined: Jan 10, 2010 Posts: 62 Location: france

Thanks Vladosh! Most useful.

Snaper · Joined: Feb 28, 2014 Posts: 217 Location: Hungary

tokyomatik · Posted: Sat May 09, 2015 5:21 am Post subject:

Finally after long time i went back to this layout, etched the board and i can confirm it works
pretty aggressive filter, but ....now my only question is about the trimmer
what is the best procedure for calibration?
resonance pot during the calibratio?

LFLab · Posted: Mon May 11, 2015 7:04 am Post subject:

You got it to work?
Fascinating, since tracing the connections on the board seems to indicate that this is quite different from the schematics floating about.
Did you do any component substitutions? What IC's, transistors did you use?

In the meantime, I built two of the Girts Polivoks VCF's (Ericasynths) and they are very nice. Pretty aggressive indeed (I'd call them "full of character" Very Happy

).

tokyomatik · Posted: Mon May 11, 2015 7:42 am Post subject:

well, from the various recordings i heard of it, i would say that goes very close
i followed some tips around the forums and some mods that i found here:

http://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm
i implemented the soft resonance switch and the frequency pot goes to -15v instead of GND with a 47k resistor in between

for the ICs I used a couple of MC1776CP

somebody told me also that C3 & C4 are kind of critic
better to use red Wima type caps instead of ceramics, from what I understood, with ceramics is not gonna work

the layout can be a bit confusing(cutoff & resonance inverted) i'm not sure wich one is the bandpass or lowpass but doesn't go "deep" from my point of view

the trimmer should help to find the range for cutoff, but should i have the resonance in selfoscillation during the calibration?

tokyomatik · Posted: Sat May 16, 2015 4:30 am Post subject:

I'm also curious about the Erica version of this board.....maybe I will give it a try
but debugging this one is part of the learning process and since I feel not so far from the result I'm not gonna give up just because someone else made a better design, I'm also not the only one that etched this board and make it work so its not hopeless to try

isak · Posted: Thu Nov 19, 2015 1:35 am Post subject:

Hi guys

Just wanna say I'm following your post.
The vcf dosnt work for me to, so I hope your new findings will solve all the issues.
BTW, is there any new update for the pcb?

LFLab · Posted: Thu Nov 19, 2015 5:11 am Post subject:

I wonder, as mentioned above, I've tried to get it to work, but when I started to trace the PCB it just was so different from the schematic that I gave up.
Built two of the Ericasynth Polivoks VCF's, and they sound great, pretty easy build.

isak · Posted: Thu Nov 19, 2015 7:32 am Post subject:

Is it etched pcb?

LFLab · Posted: Thu Nov 19, 2015 8:49 am Post subject:

Yes, self-etched board from the layout N.D. re-posted on 30th of jan 2012. I'd go with something else personally (and did).

isak · Posted: Thu Nov 19, 2015 11:11 am Post subject:

Sorry man, I a bit confused

Didn't we said that this board is not working?

LFLab · Posted: Thu Nov 19, 2015 12:14 pm Post subject:

The layout posted in this thread does not work, at least mine didn't.
When I traced that layout, it turned out to be quite different from the polivoks vcf schematics that are around.
I tossed that self etched layout in the bin, and built two Ericasynths polivoks vcf's which are working well and sound excellent. These pcbs are professionally made, and in my case also came with silkscreened frontpanels, and original russian semiconductors.

The one Tokyomatic built was also a self etched board from the layout in this thread, and in his case, as well as mine, it didnt work.
I think by now he's tired of debugging Smile

tokyomatik · Posted: Thu Nov 19, 2015 11:18 pm Post subject:

somebody told that the caps shoud be poly Wima type, with ceramic it's not gonna work....i used wimas on mine...and no problems

LFLab · Posted: Thu Nov 19, 2015 11:27 pm Post subject:

So, you did get the layout posted earlier in this thread to work? Fascinating!

isak · Posted: Fri Nov 20, 2015 1:39 am Post subject:

Ok, got you.
Thanks for the info Smile

Said that, is there any self etching polivoks vcf that work out there?

tokyomatik · Posted: Fri Nov 20, 2015 4:28 am Post subject:

yes i used the layout found here.

isak · Posted: Fri Nov 20, 2015 8:34 am Post subject:

Thanks, can you please reload it just for me to be sure that is the one that work?