[sdiy] some buchla LPG questions

[matt holland]

On Mon, Aug 2, 2010 at 11:57 PM,  <lanterma at ece.gatech.edu> wrote:
>
> 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 suppose the point of that would be to ensure the input of the output
opamp is pulled to ground so that there's nothing at the output in the
absence of an input signal?

>
> 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.

ahh right! my initial hunch was "current limiting for some reason".

>
> - Aaron
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