[sdiy] Polivoks filter

[Dave Brown]

I will.  Right now I think I will finish building it in a module before using or recording.  It will be a lot easier and I don't risk shorts with wires and cables flying everywhere.  A couple of times now the weight of all the cables has pulled my PCB to the floor ...

Dave

-----Original Message-----
From: cheater cheater [mailto:cheater00 at gmail.com] 
Sent: Saturday, February 16, 2013 11:00 AM
To: Dave Brown
Cc: synth-diy
Subject: Re: [sdiy] Polivoks filter

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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