[sdiy] 2164 Korgasmatron VCF

[cheater cheater]

Oh.. so you used HP and BP in parallel? I thought you wanted HP and LP
for notch response.

Of course this highpass-notch thing is interesting too. You can go on
with combinations like that forever, like in the Xpander.

Cheers,
D.

On Wed, Sep 1, 2010 at 08:59, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> > 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
>
>
> Sorry.  I misspoke.  It's actually the sum of 12db HP and 12dB BP.  This is
> just math, actually.  The transfer function is:
>
>    s^2 + s              s^2                 s
> ---------------- = ---------------- + ----------------
> s^2 + (2-G)s + 1   s^2 + (2-G)s + 1   s^2 + (2-G)s + 1
>
> where G is the gain of the resonance amplifier.  The first term is 12dB HP
> and the second term is 12db BP.  When G = 0, this reduces to:
>
>  s
> -----
> s + 1
>
> which is simply 6dB HP.  Of course, when G = 2, the denominator becomes zero
> at the cutoff frequency (remember: s = jw and w = 1 at the cutoff frequency)
> and the filter oscillates.
>
>