SV: Re: [sdiy] usage of 3080 in micromoog

[karl dalen]

--- Harry Bissell Jr <harrybissell at prodigy.net> skrev:

> 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...

I tried some years ago with a LM13700 and had severe
problems with noise! And i couldent  remedy it! strange!

KD

> 
> 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
> > 
> 
>