[sdiy] Polivoks filter

Wed Mar 6 15:55:58 CET 2013

Read his interesting and informative page on the topic:

	http://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm

The bit about the resonance and adding resonance limiting is about 1/4 the way down.

T.

On 6 Mar 2013, at 13:20, cheater cheater <cheater00 at gmail.com> wrote:

> interesting. why is there "hard" and "soft" resonance? are those two
> available in parallel at the same time, or are they selectable via
> switch?
> 
> Cheers,
> D.
> 
> On Tue, Mar 5, 2013 at 7:56 PM, Dave Brown <davebr at earthlink.net> wrote:
>> I'm only playing with the Polivoks filter controls.  You can hear the hard resonance break into clipping around 0:49 and again at 1:50.  I think that is what you are talking about.  The output goes from +/-10V to +/-29 volts.  I added comments about the hard resonance clipping to both the LP and BP demo.
>> 
>> Dave
>> 
>> -----Original Message-----
>> From: cheater cheater [mailto:cheater00 at gmail.com]
>> Sent: Tuesday, March 05, 2013 5:10 AM
>> To: Dave Brown; synth-diy
>> Subject: Re: [sdiy] Polivoks filter
>> 
>> 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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