[sdiy] 36dB VCF output drop while increasing emphases

[David G Dixon]

> Thank you Carsten, I think making it adjustable is the way to 
> go. I have outputs for every pole like in the 24dB Formant 
> VCF. But increasing gain with feedback needs a OTA for every 
> output. Way to much. Hmmm, thinking about making this 
> switchable as well.
> 
> Harald

I designed a 24dB filter with gain cells between every stage.  However, the
gain cells are designed to only give gains from 1 to 1.414.  There are four
gain cells.  The final gain cell goes from 0 to 1.414.  This cell sends the
feedback.  Hence, the total feedback gain goes from 0 (no resonance) to 4
(self-oscillation), while the minimum gain from cell to cell is unity.  This
is the basis of the "Quad" setting of the "Mode" switch on the Intellijel
Dr. Octature.  This filter is realized with SSM2164 quad VCAs rather than
OTAs.  I did it with LM13700 OTAs very early, but switched to 2164 VCAs,
which I believe are far superior for this kind of application.

When the Mode switch is in "Mono" mode, the first three gain cells are fixed
to unity gain, and the final (feedback) gain cell goes from 0 to 4.  This is
done with switched resistor networks.

The result of these gain cells is twofold:

1) in filtering mode, the signal maintains a robust character even as the
resonance is turned way up.  This overcomes a deficiency with Roland-style
cascaded-stage filters, which tend to sound wimpy at high resonance.

2) in self-oscillation mode, the sine waves coming from each filter stage
are amplified by 3dB, which overcomes the natural attenuation of the filter
stages, and gives the sine waves at constant amplitude from each stage.
This is useful when using the filter as a quadrature (octature) LFO.