Moog Filter!

[Harry Bissell]

Happy to  help... I have simulated also...

I dont get low freq boost (I get cut due to DC blocking caps at the
filter input and the ladder buffer (.22uF ???)

Spice gets some crazy results... Did you model the minimoog "transistor
only" ladder... the one with discrete ladder amp... It runs better.  I
like the original "model D" schematic with 150 ohm resistors in the
ladder bias.

The roll off below cutoff with increasing resonance is REAL !!! it
really does that.

You need some minimum value for collector current, or the filter becomes
a VCA (completely off)... Watch out for the drive circuit to the expo...
its tricky. I've tried a lot of things to get good results from the
sim...

Run the filter at very high number of time points per step... I use 2000
- 5000 with Electronic Workbench...

I've seen the outrageous high end response (100KHz)... Obviously the
models aren't perfect... If you are using generic models try using real
transistors... the sim is slow but more accurate...

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