Moog Filter Description

[tomg]

> > Hi,
> > I am new to the mailing list and have been working on building my own modualr for a while now.
> > 
> > Can anyone explain how the Minimoog transistor (diode) ladder curcuit works?  I have seen the
> schematic, but I also need to know which are the CV voltage inputs (Freq and Resonance).  Also,
are
> two of these put into series for the 4 pole filter, or is the circuit already 4 poles?
> > 
> >  
> 


I forgot the disclaimer.....I do not know what I'm talking about. This is
some good information and a little guessing. I think I'm close.

-tg 


> The easy part first...it's already 4 poles.
> 
> Depending on the design cutoff and res cv can be many places. Usually the res cv
> goes through some sort of expo-converter into an ota. The cutoff cv usually goes
> through a mixer pnp transistor connected to a expo-npn that's connected to the bottom 
> npn pair on the ladder.
> 
> The cv voltages are summed by a emitter follower pnp then applied to a npn expo-transistor
> The voltage at the expo-tranny base applies the collector current to the ladder.The ladder is 
> biased in an increasingly positive potential by the divider string of resistors. The bottom pair
> of transistors split the expo-current evenly across the ladder with no signal applied.
> 
> When a signal is applied to the input, the current on the right and left side of the ladder is 
> modulated inversely. This modulation is non-linear but this factor is reversed by a pair of 
> current-to-voltage trannys at the top of the ladder. This voltage is a log of the ladder current.
> 
> As the standing current is increased by adding voltage to the cufoff mix The collector impedance
> in the ladder decreases. Less of the signal goes to the inverse side of the ladder so the cufoff 
> frequency increases. The push-pull voltages at the top of the ladder are applied to a diff-amp
and 
> then the output. The output signal is returned to the bottom of the ladder for the resonance
loop.
> 
> -tg