[sdiy] Is the "Both" mode in Buchla LPG really a Butterworth?

[Aaron Lanterman]

  Grant Richter <grichter at asapnet.net>
To: Aaron Lanterman <lanterma at ece.gatech.edu>,
     Synth DIY <synth-diy at dropmix.xs4all.nl>

On Thu, 18 May 2006, Grant Richter wrote:

> It is a 220 pF ceramic disk. When the 4.7 nF is switched out in the "Both" 
> mode, the circuit forms a classic passive Butterworth 12 dB / Oct two pole. 
> The reason it is smaller is because of the passive loading of the 910 pF cap. 
> If the sections were buffered, the 220 cap value would be 910 pF.

Are you sure about the "Butterworth" part?

If the feedback cap is switched out, it's an unbuffered cascade of two R-C 
sections. A Butterworth filter needs some complex-valued poles (I think 
with a Q of 1/sqrt(2), I dont't recall for sure for now).

I just analyzed it with the 220 pf and 910 pf caps, and came up with a Q 
of 0.2193 if both the feedback cap and the "drop-in" resistor are switched 
out...

- Aaron