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

[JH.]

 > Is the "Both" mode in Buchla LPG really a Butterworth?

No. It's been a long time since I simulted this, but it was a mix of VCA and
6dB/8ve
filter function.

JH.


>
>   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
>
>