[sdiy] HPFs and ADSRs

[Antti Huovilainen]

On Tue, 25 May 2010, David G. Dixon wrote:

>> Be aware that MS-20 highpass filter is quite different from SVF. Without
>> resonance, it's 1-pole highpass and increasing resonance shifts it
>> somewhat towards 2-pole.
>
> Yes.  I was looking at Rene Schmitz's clone, but it's a pretty weird LP/HP
> topology, with the second stage fixed in lowpass mode, and the resonance not
> seeming to be hooked up to anything in highpass mode (is this a mistake on
> the schematic?).  I wonder how different it would sound from an SVF, or even
> a simple cascaded-OTA (or cascaded-VCA) 2P HPF?  Any thoughts?

The resonance in second version of MS-20 filters works by feeding back the 
output to bandpass input in the filter. Hence the connection is correct 
for highpass mode too, although it results in slightly strange 1-2-pole 
hybrid. As for sound, I'd say it differs quite a bit from SVF when fed 
with high amplitude signals or using resonance near self-oscillation. You 
can't make 2-pole resonant filter by cascading RC stages (as you can for 
3-pole and higher).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
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