[sdiy] Polivoks filter

[cheater cheater]

Hi Dave,
that looks great! Would you like to record some demos and post wav's somewhere?

Thanks,
D.

On Sat, Feb 16, 2013 at 7:11 PM, Dave Brown <davebr at earthlink.net> wrote:
> 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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