[sdiy] New VCF schematics

[Osamu (Sam) HOSHUYAMA]

Hello Rene,

René Schmitz wrote:
> Hello Osamu and List!
> 
> At 13:38 10.11.02 +0900, Osamu (Sam) HOSHUYAMA wrote:
> >* SVF using cheap OTAs and CMOS inverters. (WASP style?)
> 
> This has an interesting twist to the usual SVF structure.
> I'm still not sure how it works though. Could you please elaborate?

If you consider the two summing amplifiers as one, you'll understand
that it's
an equivalent of the usual SVF.  HPF, BPF, and LPF will be obvious.

The notch filter output may be the most difficult part to understand.
Roughly... The BPF output signal is subtracted at the first summing
amplifier,
the output of the summing amplifier has a notch type response.

Actually, the unusual SVF structure which the VCF is based on was
published
in a Japanese electronics magazine in 80's.  I myself couldn't
understand it
at first.  I thought in those days "Why the notch frequency signal is
recovered 
at the other outputs?"  But I can answer now "That's a magic of negative 
feedback. It's very similar to virtual short."  (This means from point
of view 
of noise, it may not be a good circuit.)  I understood how it works by 
calculating the transfer functions.  If you need derivation of the
transfer 
functions, I'll try to describe it.


> >* 4 pole VCF with intentional distortion. An attempt to simulate SSM2040
> >for cheap.
> 
> Hmm, could we also use an asymmetric clamp over the cap? A zehner perhaps.
> Shouldn't that yield similar results?

I guess so.  I used the transistor saturation because I believe SSM2040
uses it,
and the saturation level can be controlled by one trim pot.

Instead of a zener diode, clamping with a low leakage diode to a low 
impedance voltage source is another choice.  These modifications are
easier than 
following my schematic.  But, please be careful about the signal level.
The purpose of my attempt is basically introducing soft clipping into
the VCF.

Regards,  Sam Hoshuyama
(Any comments to improve my English would be appreciated. Off list
please. ;-)
-- - ---  - -- ------ - -------- -- - ---- -
Osamu (Sam) HOSHUYAMA      houshu at rogers.com
                              Ottawa, CANADA
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