[sdiy] some buchla LPG questions

[lanterma at ece.gatech.edu]

On Aug 3, 2010, at 1:21 AM, matt holland wrote:

> A friend of mine is in the process of trying to make a digital
> emulation of the LPG and asked me to help him out by coming up with a
> transfer function for him (with the filter in low pass only mode).
> i've been looking at this easier to read version by peter grenader:
> http://electro-music.com/forum/phpbb-files/buchla_292_clone_191.gif
> 
> It would be great if I could ignore the 4.7M resistor and 220pf cap so
> I could map it to a standard sallen-key filter and avoid pages of
> tedious algebra, but i wanted to get some opinions on that first.
> so...

Not sure about the 4.7M. Maybe it's ensuring a DC path to ground at the input if the 10K is switched out (filter and "both" mode) and the vactrols go totally "dark."

I've never been sure about the 200 pf.

> what exactly are those components doing?
> are they vital to the character of the filter?

> -what is the significance of the 1k output resistor? i'm guessing it
> will form a 100k/(100k+1k) voltage divider like the signal in, but
> why?

It's short circuit protection. Make sure stuff does not go boom if you accidentally short the output to a grounded panel.

- Aaron