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

[karl dalen]

Add in memory moog, instrumentation amp coupling with LF363 as ladder
buffers driving two CA3080.One as VCA and one as reso feedback VCA.

KD

--- Dave Manley <dlmanley at sonic.net> skrev:

> Your comments about the Mini-D made me wonder what circuits
> Moog has used over the years in their ladder filter.  In short, they
> used a different circuit in every product.  A summary below:
> 
> Modular 904a - discrete transistor buffer
> Minimoog D - discrete transistor buffer
> Taurus - dc coupled 3080 (Iabc=30 uA) directly connected to ladder
> Micro - dual fet, 3080 (Iabc=450uA)
> Prodigy - two darlington configured 3046, to 3080 for differential to 
> single ended conversion and which is also the vca;  a diff-amp 
> configured 4558 provides the resonance feedback
> Liberation - apparently the same as Prodigy, I don't have schematics, 
> just circuit description
> Rogue - as in the Prodigy, but the two darlington 3046 is replaced with 
> two LF353's,
> Source - dual fet, plus two 3080's one for  vca, one for resonance feedback
> 
> -Dave
> > 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
> >>
> >>     
> >
> >
> >
> >   
> 
>