[sdiy] Multimode filter topology

[Dave Manley]

Antti Huovilainen wrote:
> On Mon, 17 Aug 2009, David G. Dixon wrote:
> 
>>  By permanently wiring the filter as a 4-pole LP (which resonates 
>> easily), the stagewise outputs can be added and subtracted in various 
>> ways to arrive at all possible LP, HP and BP filter modes, and I'm 
>> assuming they will all resonate (although I haven't tested this).
> 
> Congrats, you essentially rediscovered what Oberheim did in Matrix 12 ;)
>
The schematics and diagrams shown on the referenced web page are from 
the Oberheim service manual, so the filter designations are Oberhiem's, 
not Grants.

Also note the quality of the result is very sensitive to the resistors 
used in combining the various filter stages.  I ran some spice 
simulations some time ago, when I first stumbled across this method and 
was thinking about making a morphing version.  You need precise control 
for good results (Oberheim used 1% resistors).  I could try to find the 
simulation results if interested - or try looking in the archives.

BTW, this is a good excuse to look up "sensitivity analysis" in a filter 
design book.

-D