[sdiy] Secrets of Dan Brown's "The Buchla Code" Revealed

[Grant Richter]

>
> Grant - thank you SO MUCH! This makes a lot more sense now. The 220  
> isn't just an odd throwaway like I thought it was.
>
>> 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.
>
> Oh shoot - I said something incorrect to my class in the Spring.
>
> I drew the LPG on the board, and we did some analysis... I got the  
> "frequency" part and the "amplitude" part right, but I didn't  
> interpret the "both" part right... in "both" the way you describe  
> it, both the 15K or 10K or whatever _and_ the feedback cap are  
> switched _out_. I had somehow been under the impression that in  
> "both" moth both the 15K and 10K or whatever and the feedback cap  
> were switched _in_.

I spent a great deal of time simulating that circuit under Spice.

The only behavior I could match to the real world was as described.

If you consider the switch as a center off, it makes more sense.
I know there was discussion of about a special version of the switch,  
this may have been used in some version.
Based on ohm meter readings of the switches in the actual modules  
they are just three position center off DPDT.

So center position has no terminals connected.

Amplitude side switches in resistive divider.
Frequency side switches in the 4.7 nF, and a 3 dB attenuator at the  
input stage for the earliest design.

See if you can find the 192 schematic, it is pretty much the same  
thing but used two VTL2C3 metal can Vactrols.

Also part of the filter for the Korg Polyphonic Ensemble keyboard was  
lifted whole heartedly from the Buchla design (pretty sure the Korg  
drawings have latter dates).

"Amateurs borrow, professionals steal" - Anon.