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?
> 
>  

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