[sdiy] Polivoks filter

[Tim Ressel]

For what its worth I tried this on my eurorack Polyvoks filters. I saw no DC offset. 


Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




>________________________________
> From: Dave Brown <davebr at earthlink.net>
>To: synth-diy at dropmix.xs4all.nl 
>Sent: Saturday, February 16, 2013 10:11 AM
>Subject: [sdiy] Polivoks filter
> 
>I just finished hand building a Polivoks filter using the Marc Bareille
>schematics with the reversing attenuator.  I ended up making a couple of
>minor changes to reduce the filter to near unity gain and softening the
>self-resonance output level so it would not clip rail to rail.
>
>I built this filter using NTE888M programmable op-amps since they are
>readily available.  I did some frequency plots with my scope using XY mode
>and was disappointed with the DC level shift I saw on the low pass output.
>I got some of the УД1208 Russian parts to compare and discovered the same DC
>shift.  I had repaired a Polivoks filter previously and the frequency plots
>did not show any DC level shift.  I eventually figured out the DC shift was
>occurring on the VCO output as I swept to high frequencies.  I tried
>different VCOs and repeated the measurements with a VCO that had a constant
>amplitude output with no DC shift over the frequency range of the
>measurement.  Both the NTE888M and УД1208 Russian parts looked good with no
>DC shift in the low pass output.
>
>I saw only two minor differences between the NTE888M parts and the УД1208
>Russian parts.  The NTE888M parts have slightly higher gain and the cutoff
>or center frequencies was slightly higher at the same control settings and
>input CV.  I assume the frequency shift is due to a slightly different Iset
>response curve or a different Vbe bias point for the Iset control pin.
>
>I have some uA776 parts on order and will also do the same comparison
>(assuming the parts are genuine).
>
>Now I need to finish this filter with a front panel and play with it some.
>Details, photos, and scope images are on my site on my Polivoks Filter page.
>http://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm
>
>David J. Brown
>Email davebr at earthlink.net
>Website http://modularsynthesis.com
>
>
>
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