[sdiy] Random bits on the Buchla LPG

[jhaible at debitel.net]

> Anyway, between the feedback cap and where it ties between the vactrol
> resistors, there's this 220[pF] cap to
> ground, and I can't figure out its purpose.

When the 4n7 feedback cap is switched off, you have the 220pF to GND.
With feedback switched on, you'd probably only want the 4n7. But leaving
the the 220p in place doesn't change much:
You have a capacitive voltage divider of 4700/4920 = (almost) 1,
your effective feedback cap is 4.92nF (seen from the vactrol side),
wich is almost the same as 4n7.
The only catch is that the opamp must drive the 220p*4n7/(4n7 + 220p)
= 210pF load. (Which is sort of ok.)

> Any clues what the big resistor and the 220 pF are meant to do, or how one
> would come up with putting them in? 

The "big resistor" will provide a DC path to the opamp input, providing
its (ever so small) bias current, when the Vactrols are completely turned off.

JH.





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