[sdiy] CEM3328 seems to only have buffers at the end

[Antti Huovilainen]

On Mon, 20 Feb 2006, Mike wrote:

> reduce distortion at the OTA input pair.  Each stage is independent of the
> other stages. In contrast, the Moog filter and diode ladder filters do not
> have this "closed-loop linearization per stage", perhaps this is part of the
> reason for the different sound of the filters when they are over-driven?
> Different distortion behavior as the filter is over-driven?

Having done some analysis of the filters 
(see http://dafx04.na.infn.it/WebProc/Proc/P_061.pdf), I'd say "rich open 
loop design" is somewhere between marketing speak and misunderstanding.

A short and simplified summary:
Moog: y(n) = y(n-1) + g*(tanh(x(n)) - tanh(y(n-1))
OTA:  y(n) = y(n-1) + g*tanh(x(n) - y(n-1))

Both filters have feedback (you need feedback to have filter). The 
difference is that Moog distorts signal and feedback separately while OTA 
distorts the combination. OTA filters are thus quite a bit more linear 
(this doesn't apply to VCAs as they don't have feedback).

SSM2040 falls to OTA camp while SSM2044 goes to Moog (roughly - both 
contain their own special tweaks). CEM patents don't provide enough 
information to say one thing or the other. Does anyone have internal 
schematics for CEM filters?

Antti

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