[sdiy] doepfer a107 cem3372 oberheim expander filter

[Antti Huovilainen]

On Fri, 29 Jul 2005, Stefan wrote:

> I first wanted to clone the oberheim matrix\expander filter and then 
> found out that it was apparently possible to make even more filter types 
> out of the cem3372. Since i'm not very knowledgeble regarding filter 
> design, i need some help to find out how.

You can use the same trick with pretty much any 4pole filter that provides 
single output per stage (ladder filters are out, OTA & SSM2040 should 
work).

The idea is to sum the individual taps with different weights. The XPander 
service manual provides the needed equations to calculate the weighs (you 
do have to do some math though).

XPander provides 8 different modes (8*4 precision resistors!) plus an 
extra 8 modes by bypassing the first stage of the filter. If you wanted 
more flexibility, you could use 5 resistors per mode and also tap the 
input (not done in XPander because tap outputs are scaled).

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova