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

[Grant Richter]

>
>> 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?

Nope, I was wrong. I forgot that Butterworth, Tchebyschev, etc. are  
same topology with different resistor values.

I was just using it as shorthand for a chain of resistive lowpass  
sections.

My apologies.

One fun thing to look at is the lowpass filter used in the ARP 2600  
envelope follower section. To save the cost of per stage buffers,  
they made each section a high and higher resistor value with a  
smaller and smaller cap value. Then a single buffer at the end.

If you simulate it, the frequency response has quite a sharp corner  
for a mostly passive filter. Must have taken a fair amount of time  
with the slide rule (slip stick) to figure that circuit out.