[sdiy] Polivoks filter

cheater cheater cheater00 at gmail.com
Thu Feb 28 14:01:23 CET 2013

Hi Dave,
thanks a lot for the demo, but it's a bit difficult to appreciate the
filter's sound in the setup you've used. If I could ask you, would you
redo the demo, recording to line-in, and try to tune the filter an
octave or two lower? Due to reverb and other acoustic properties the
filter kind of sounds like any old filter :) and then it's generally
difficult to figure out what a filter is doing if the input signal is
pitched way up. The typical test is a sawtooth wave in the lower
registers, going through a filter with a quick attack and long (0.5 -
1 second) decay and a lot of positive env mod.


On Thu, Feb 28, 2013 at 7:49 AM, Dave Brown <davebr at earthlink.net> wrote:
> I received my front panel and finished building my Polivoks filter.  I updated the schematics and highlighted my few mods.  I put a 4 minute video showing some of the various modes and sounds.  I do like this filter a lot and I like having both the soft and hard resonance.  I like the soft resonance for traditional filter sound and the hard resonance for adding somewhat uncontrolled edge to the waveforms.  The tracking is pretty good although I haven't done a complete test of the range of tracking.  The beginning of the video demonstrates a sequencer driving both a VCO and the filter in self-oscillation which tracks pretty well.  Since there is no fine control the tuning is a bit tricky.
> http://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm
> Dave
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dave Brown
> Sent: Saturday, February 16, 2013 12:25 PM
> To: 'cheater cheater'
> Cc: 'synth-diy'
> Subject: Re: [sdiy] Polivoks filter
> 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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