[sdiy] LT spice runs of the Buchla 191 highpass core

[Aaron Lanterman]

I tried a simple version of the Buchla 191 highpass filter. This looks  
like a regular RC highpass filter followed by a buffer, where the R is  
replaced by two diodes, one going to a positive control voltage and  
one going to a negative control voltage. (Well, really they are diode  
connected transistors.) As you vary the CV, it changes the dynamic  
resistance of the diodes.

When I use the basic diode model that comes with LT Spice and I look  
for -3 db cutoff points, I get cv (in volts) to cutoff (in Hertz) data  
like this:

0.3 28.6
0.325 74
0.35 200
0.375 518
0.4 1340
0.425 3600
0.45 9500

When I instead use the built in basic NPN model, I get data like this:

0.4 13
0.425 36
0.45 95
0.475 256
0.5 662
0.525 1690
0.55 4557
0.575 12000

Note the diode connected NPN in LT spice seems to need an additional  
0.125 volts or so before its starts doing its thing.

Both graphs are nicely exponential (plot against the log of the data  
on the right, and you get a straight line), and seem to have a 0.18  
volt per octave control characteristic.

Interesting...

- Aaron