[sdiy] 2164 Korgasmatron VCF

cheater cheater cheater00 at gmail.com
Wed Sep 1 08:19:01 CEST 2010


On Tue, Aug 31, 2010 at 20:11, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> You mean the wiper?
>
> Yes.  I meant the wiper.  I'm not always "down" with the "lingo".

Ahh ok :) I'm asking because I'm not good enough with electronics to
tell for sure that you couldn't do something like that with the centre
tap as well..

> Yes, it is close to an all-pass.  Mathematically, it's a series combination
> of a 12dB bandpass and a 12dB highpass.

Wait.. I thought you coupled LP and HP in parallel.. that would mean
passing every frequency fed into it... how does that compare to HP->BP
or BP->HP? Not sure where the bandpass bit came from

How was Saskatoon?

Cheers,
D.

On Tue, Aug 31, 2010 at 20:11, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> > In simulation, if I feed the input into both the HP and LP destinations
>> on a
>> > single filter, I do indeed get a kind of gentle notch.  The minimum
>> > amplitude at the cutoff frequency (with zero resonance) is 6db (i.e.,
>> 50% of
>> > the input amplitude).  It's not a sharp notch like one would expect from
>> a
>> > proper notch filter, but more of a smooth dip in (log) amplitude at the
>> > cutoff frequency.  Is that going to sound like much?
>>
>> I think the phase response is much more important than the amplitude
>> response here! You have not created a notch, what you have is an
>> all-pass.
>
> Yes, it is close to an all-pass.  Mathematically, it's a series combination
> of a 12dB bandpass and a 12dB highpass.
>
>> >> I don't know if you posted any details before... but what is your
>> >> "dial in a zener voltage" method. Is it just adding series resistance
>> >> or something more complex ?
>> >
>> > Yeah, basically.  In this case, the zeners are connected on one end to
>> the
>> > negative input of the opamp, and on the other end to the centre tap of a
>> > 100k pot.  One end tap of the pot is connected to the opamp output, and
>> the
>> > other to a 30k resistor which is grounded.  That gives me full zener
>> range
>> > from 3.3V to the rail.  The pot can be a trimmer as well.
>>
>> You mean the wiper?
>
> Yes.  I meant the wiper.  I'm not always "down" with the "lingo".
>
>> > One thing, though: I can't seem to reproduce the "formant" sound with
>> the
>> > notch setting at high resonance like that soundclip on the MOTM 420
>> webpage.
>> > All the other clips I think I can emulate.  I'd like to know exactly
>> what
>> > was done to get that sound, to see if my filter can do it as well.  I
>> think
>> > they might be using two filters.  When I get the other half wired up to
>> the
>> > panel, then I should be able to get all kinds of interesting timbres
>> with
>> > both filters in series.  However, I won't be able to do that until I
>> return
>> > from Saskatoon.
>>
>> From what I understand, to create a notch you need to use separate
>> cutoffs for the LP and HP.. dependin on the spread, you can get
>> formant style sounds too (about mid resonance, and you should use a
>> nice difference of maybe one octave or two. the LP band should also
>> not track as much as the higher band does. for best results the high
>> band is a BP)
>
> I've got it sorted now.  It requires two filters to get proper formants
> (although you can squeeze something like formants out of a single filter,
> with some doing).  You can hear some of the results in my soundclips on
> Muffwiggler.
>
>> > However, I'll definitely want to normal filter A's CV
>> > inputs to filter B, and also normal filter A's output to one of filter
>> B's
>> > inputs.
>>
>> The typical topology with two filters that gives the best results is
>> LP->HP with a soft clipper inbetween. This is because the LP output
>> still maintains most of the amplitude and can give the clipper a go.
>> If you use LP as the second filter, it'll sound weedier. Of course if
>> you normal to HP on the second one you can still have BP and HP in the
>> first one which is nice too. Some settings of high pass can give nice
>> big amplitude to (e.g. slightly self-resonant HP at a low freq
>> setting)
>>
>> BTW, can you send me the final schems you have used? :) Thanks..
>
> Sure.
>
>> I'll make sure to listen to the demos once I'm at home :)
>>
>> Really good going, keep it up!
>
> Thanks.
>
>> > I've finally got some soundclips of the Korgasmatron up on Muffwiggler.
>> > Three of them also show off my redesigned CGS analog shift register.  Go
>> > here:
>> >
>> > http://www.muffwiggler.com/forum/viewtopic.php?t=21058
>> >
>> > If you can't access the soundclip files, let me know and I'll email them
>> to
>> > you privately.
>
>



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