2nd generation VCFs, not to mention this patent!!

[Bjorn Julin]

Not to mention the new filed patent no:
US05912587

BJ


>From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
>To: tomg <efm3 at mediaone.net>, synth-diy at node12b53.a2000.nl
>Subject: RE: Re(2): 2nd generation VCFs (was: Xpander VCF)
>Date: Thu, 26 Oct 2000 14:40:33 +0200
>
>	>The patent
>	>calls for n,p,n,p current mirrors. n,n,n,n also works but has about
>	>1/2 the gain.
>
>How could that be ? Current mirror means reverting the direction, so
>how could it possibly work with npn only ??
>
>	> It's very much like it! I'm surprised they got a patent. I can't
>argue that
>	> it's not a improved version but it's very close to the original.
>
>I like it better the longer I look at it.
>
>In my previous mail, I had already typed a paragraph to say that I can't 
>see
>where the improovement in terms of CV feedthru rejection would be, but
>then I saw my error and deleted that remark.
>In the Moog ladder, there would be CV feedthru from different Betas
>in one row (subtracting a tiny amount of current from each side of
>the ladder, and with beta mismatch subtracting more from one side
>than the other), and with the 3-transistor mirrors that effect is smaller !
>(At least that's how I understood it.)
>
>Which makes it a real improovement over the Moog cascade, not just
>a workaround.
>
>Back to the Moog ladder, if anybody wants it ultra low CV feedthru,
>one should match the transistors (and all of them in pairs) for
>Beta as well ! (And the top and bottom ones for Vbe, of course.)
>
>But it's not really necessary for normal synth VCFs. Remember the
>SSM2044 applications comprise the use of a VCF *after* the VCA,
>and more general audio applications, where thumping would be more
>critical.
>
>JH.
>

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