Papareil Polivoks filter support thread

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sonicwarrior
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Papareil Polivoks filter support thread

Post by sonicwarrior » Fri Mar 16, 2012 7:19 am

Quote from viewtopic.php?p=746710#746710 :
krisp14u wrote:I built the Papareil synth labs one and if memory serves me right it needed some work to get it working right
I recently finished one but the cutoff doesn't work. Haven't had the leisure to start debugging though.
Any tips for eventually needed fixes would be highly appreciated.

Btw.: I've built it with LS776 (should be the same as the UA776) not the LM4250.

I found this http://mutable-instruments.net/forum/co ... 124&page=1 suggesting 100k resistors instead of the 110k that Papareil used.

sneakthief has made a note @ electro-music:
2. M. Bareille has Q1 (2N3904) upside-down on his PCB component layout - THIS IS INCORRECT FOR 99% of 2N3904's:
As the 2N3904 is in the cutoff path that could be my problem.
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Post by boothnavy » Fri Mar 16, 2012 11:06 am

try changing the transistor around. I think that worked for me once.
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Post by krisp14u » Fri Mar 16, 2012 11:52 am

Cheers :guinness:

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Post by sonicwarrior » Fri Mar 16, 2012 12:12 pm

Excellent! Many thanks! :yay:
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Post by Barcode » Fri Mar 16, 2012 12:40 pm

Fantastic thread! I am going to start putting my together next week and this think this saved me a few headaches....

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Post by decaying.sine » Fri Mar 16, 2012 12:51 pm

This PCB sometimes makes me :bang:
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Post by Barcode » Fri Mar 16, 2012 12:55 pm

decaying.sine wrote:This PCB sometimes makes me :bang:
I hear ya.... I'm not too happy about having to drill to use a power header.

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Post by sonicwarrior » Mon Mar 19, 2012 11:05 am

Allright, I've installed the mods compiled by julianw and it seems to work (I already used 1uF for C5/C6 and had R12 already installed).

But there is one thing I'm not sure about:
The frequency CV pot (I used 50k as the schematic says) behaves a bit oddly: I can't shut down the CV.
Can that be caused by using a 50k pot instead of the 100k?
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Post by jumunius » Mon Mar 19, 2012 2:08 pm

sonicwarrior wrote:The frequency CV pot (I used 50k as the schematic says) behaves a bit oddly: I can't shut down the CV.
You know the FM CV is bipolar yes? 12:00 should be closed (roughly).

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Post by sonicwarrior » Mon Mar 19, 2012 5:09 pm

That's what I suspected after a glance on the schematic but it doesn't close in any position of the pot. :despair:
Last edited by sonicwarrior on Mon Mar 19, 2012 5:50 pm, edited 1 time in total.
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Post by falafelbiels » Mon Mar 19, 2012 5:38 pm

The frequency pot is wired up as a voltage divider with one lug tied to ground. I connected this lug to the wiper of a trimmer that I wired up as a voltage divider tied to ground and -12V. Now I can feed the frequency pot a negative voltage and this seemingly solved the problem. However the filter does display some more odd behaviour. I can't remember what exactly because I haven't used it in a while, it had something to do with a loud hum I believe. Maybe these problems are related to my feeding negative voltages into said pot, maybe I broke the circuit, I don't know. I'm not actually smart or something.
I tried this on some MFOS VCAs that displayed the same problem and it works for them as well. Let's see how these fare in a while...

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Post by falafelbiels » Mon Mar 19, 2012 5:40 pm

Oh shit it's about the CV amount pot, sorry :hihi:
Mine just didn't close whatsoever.

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Post by jumunius » Mon Mar 19, 2012 6:18 pm

sonicwarrior wrote:That's what I suspected after a glance on the schematic but it doesn't close in any position of the pot. :despair:
What does it do to CV? Like an envelope. Does it come through as positive or negative? Scoping the wiper as you turn the pot from min to max would make it clearer.

Either way it's not your pot value.

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Post by jumunius » Mon Mar 19, 2012 6:25 pm

falafelbiels wrote:Maybe these problems are related to my feeding negative voltages into said pot, maybe I broke the circuit, I don't know. I'm not actually smart or something.
The common fix for the freq pot not closing on the Polivoks is to take the lug that goes to gnd and put it to V-. Shouldn't break anything. The hum would be something else.

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Post by Barcode » Fri Apr 06, 2012 9:14 pm

I finally broke down and built mine.... I must say this thing screams!

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Post by sonicwarrior » Sat Apr 07, 2012 7:03 am

Barcode wrote:I finally broke down and built mine.... I must say this thing screams!
Does your CV pot close completely in the middle position?
[Will have a closer look at that when I have build most of my backlog]
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Post by Barcode » Sat Apr 07, 2012 10:59 am

sonicwarrior wrote:Does your CV pot close completely in the middle position?
[Will have a closer look at that when I have build most of my backlog]
Indeed it does. I built mine to the exact specs from the Electro-Music link above.

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Polivoks filter

Post by davebr » Mon Feb 04, 2013 10:35 pm

I just finished building a Polivoks filter on a breadboard.

I used NTE888M op amps and found the CV range and cutoff to work quite well using the stock resistor values. I did significantly increase the input resistor to get close to unity gain through the filter.

What I didn't like was the resonance just being rail to rail square waves. They totally swamped out any input to the filter. I played around with various circuitry changes trying to limit the gain of the first op amp and finally leveraged the current limit feature of the part with clamping diodes right at the output. That lowered the output from the rails but the waveform was more of a rounded square. I found that some series resistance improved the waveshape some but running the clamping diodes to the resonance control really cleaned the waveforms up to a nice sine wave.

There is plenty of resonance range and it mixes nicely with the input signal. I added a switch to disconnect the limiting if desired but I think I like it better with

I don't know how the NTE888M compares with a uA776 but would suspect it is similar. The data sheets are nearly exact.

I have scope images of some waveforms on my Polivoks filter page.

Dave

Resonance oscillation without modification: BP and LP outputs
Image

Resonance oscillation with diode modifications: BP and LP outputs
Image
Last edited by davebr on Sat Feb 16, 2013 12:43 pm, edited 1 time in total.

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Re: Polivoks filter

Post by sonicwarrior » Tue Feb 05, 2013 8:28 am

A drawing of the mods would be nice for dummies like me. I have no clue what I should change to implement this mod. :oops:
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Post by russma » Tue Feb 05, 2013 11:39 am

I ordered a Polivoks PCB from Marc Bareille last week. I'm really looking forward to building this filter.

:party:
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Re: Polivoks filter

Post by davebr » Tue Feb 05, 2013 12:05 pm

sonicwarrior wrote:A drawing of the mods would be nice for dummies like me. I have no clue what I should change to implement this mod. :oops:
I have a schematic with all the mods included on my page. I didn't specifically highlight them out so you have to compare with the original.

[EDIT - I have outlined the modifications I made on the schematics so they are easier to follow]

Dave
Last edited by davebr on Sun Oct 06, 2013 4:47 pm, edited 1 time in total.

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Re: Polivoks filter

Post by keninverse » Tue Feb 05, 2013 2:37 pm

Hey Dave!
I noticed you worked on a board with the original russian chips. Did you hear or see any differences in your build vs these ICs?

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Re: Polivoks filter

Post by davebr » Tue Feb 05, 2013 9:20 pm

keninverse wrote:Hey Dave!
I noticed you worked on a board with the original russian chips. Did you hear or see any differences in your build vs these ICs?
I took some frequency plots with my scope on that board with the Russian ICs and posted those below that picture. Both the bandpass and the lowpass look very good. [EDIT - I just got some &#1059;&#1044;1208 Russian parts and did frequency plot comparisons between the NTE888M and &#1059;&#1044;1208 parts. I updated all the scope images for comparison.]

I took the same measurements with mine with the NTE888M parts and the bandpass doesn't look nearly as good. I am not sure why yet. [EDIT - setup and measurement error. I have updated the website with new scope images.]

Also when the filter is below cutoff for both BP and LP the DC levels shifts around a bit depending on the CV. That's not very good either. I'm trying to get some older uA776 parts to compare with. [EDIT - same setup and measurement error as above. The DC shift at high frequencies was due to my VCO at high frequencies. I used a different VCO and updated the scope images.]

[EDIT - There is a slight frequency and output amplitude difference between the &#1059;&#1044;1208 Russian parts and the NTE888M parts but otherwise they look very similar. Scope images have been added to my website for comparsion. In this image the &#1059;&#1044;1208 output is the cyan and yellow traces and the NTE888M output is the red and blue traces. You can see the slight center frequency and output amplitude shift between the parts. More detail and comparisons are on my Polivoks filter page.]

Image

Dave
Last edited by davebr on Sat Feb 16, 2013 12:59 pm, edited 4 times in total.

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Re: Polivoks filter

Post by sonicwarrior » Wed Feb 06, 2013 1:46 pm

Overlooked the link to that schematic. :oops:

If you need a UA776: I think I still have some metal can ones. But they were not cheap (6 € per piece).
It may be cheaper to get the Russian original.
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Post by falafelbiels » Thu Feb 07, 2013 7:55 am

OK I support this filter, it has become my favourite for groansome action now that I have come to accept its' controlquirks.

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