[sdiy] Polivoks filter

cheater cheater cheater00 at gmail.com
Tue Mar 5 14:09:31 CET 2013


Hi Dave, thanks a lot! Those are really nicely made. At some points
there's quite a bit of distortion especially in the LP demo. Is that
when you turn up the resonance, or is there some sort of gain
parameter you're playing with?

Cheers,
D.

On Sun, Mar 3, 2013 at 1:27 AM, Dave Brown <davebr at earthlink.net> wrote:
> I put a couple of wave files up on the site.  Nothing fancy but I go through all the controls. - Dave
>
> -----Original Message-----
> From: cheater cheater [mailto:cheater00 at gmail.com]
> Sent: Thursday, February 28, 2013 5:01 AM
> To: Dave Brown
> Cc: synth-diy
> Subject: Re: [sdiy] Polivoks filter
>
> 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.
>
> Cheers,
> D.
>
> 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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