[sdiy] usage of 3080 in micromoog

[Antti Huovilainen]

On Fri, 6 Oct 2006, Dave Manley wrote:

> pair.  So I'm not sure it's that much simpler.  Is there some other 
> characteristic of the 3080 they are exploiting that an opamp wouldn't have 
> provided?

I can see two possible reasons: They like the sound of the OTA distortion 
(which is certainly case for the VCA) and it might have been cheap. I'd 
favor the first - the OTA distortion in VCA at least is critical for the 
Moog sound.

> Also was the motivation for the ladder simplicity or that at the time there 
> really weren't a lot of choices for making a voltage dependent resistance?

Considering the time when Moog designer the ladder filter, the only 
options for gm stage were differential pair, exploiting diode nonlinearity 
to vary the gain with bias voltage, using JFETs as voltage controlled 
resistors and using vactrols.

The differential pair is clearly the winner from engineering point of view 
(good matching, DC rejection) and the Moog ladder is nothing but 5 
differential pairs placed in series. Very elegant design that eliminates 
biasing and buffering circuitry from all but first/last stage and, most 
importantly, does it without requiring a single opamp.

Antti - Who has yet to actually build a Moog ladder filter

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