[sdiy] usage of 3080 in micromoog

[Harry Bissell Jr]

I'd give these reasons...

The input impedance of the 3080 is a function of the
bias current... run at a very low bias current and the
input will be a very high impedance... all in a single
stage.

Excellent input common mode range allows you to
eliminate coupling caps and still have a ground
referenced output

If you look at the original Minimoog D filter ane
compare with the 3080 internal circuit... they
are somewhat similar. The 3080 can be considered a
well
matched differential pair... with the load resistors
replaced with the internal current mirrors...

Pretty easy and it works well... and its way cheaper
than the three opamp config or two (matched) FETs
and one opamp.

I've tried different ways, the 3080 is "good enough"
for rock and roll as they say...

H^) harry

--- Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:

> 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
>