[sdiy] 2164 Korgasmatron VCF

[cheater cheater]

Hi David,

On Sun, Aug 8, 2010 at 21:04, 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.

On Thu, Aug 12, 2010 at 19:57, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> 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?

> 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)

> 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..

I'll make sure to listen to the demos once I'm at home :)

Really good going, keep it up!

Cheers,
D.

On Mon, Aug 30, 2010 at 08:59, David G. Dixon <dixon at interchange.ubc.ca> wrote:
> 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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