Moog Filter!

[Rene Schmitz]

Hello Tadej and DIYers!

As was pointed out some time ago, this is the normal behaviour of the moog
filter. There is a fixed hi-pass in series with the filter. The decoupling
caps from the ladder to the differential output stage. The effect is that
the resonance loop is only effective for frequencies above this fixed
frequency.
The decreasing passband gain is also normal for this filter. It is said
that the highpass in the res. loop give the mini its characteristic sound,
haveing more bass at the end of a decaying envelope sweep than at the
start, compared to a DC-coupled resonance loop. This also explains why the
self oscillation stops below some frequency. (Because the (loops) gain
falls off below this frequency.)

Bye
 René


At 14:40 26.09.99 +0200, Tadej Bozicko wrote:
>    Hi all!   I have one quick question! While trying to simulate moog
>ladder filter (lowpass) in  spice, I noticed some weird behaviour and am
>now wondering if thats normal or  what. The facts are: - as control current
>(from exp. converter) is decreased, so is  output level (at frequencies
>that are below cutoff). Output changes less with  higher value off
>currents, but that gives range from 1kHz to 100kHz with  original values
>from Minimoog filter schematic. Weird... - as resonance is increased, the
>level at middle frequencies  (below cutoff) decreases. - there is always
>some bass-boost around 10Hz (in respect to  mid frequency level)   What I
>would like to know is, if this behaviour is like it  should be, or am I
>doing something wrong. The scheme is copied right since I've  checked it
>hunded times. I've used component values as in original Minimoog  filter
>circuit.   Tadej 

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