[sdiy] FatMan Hacking

[Don Tillman]

   From: jhaible at t-online.de
   Date: Mon, 29 Oct 2001 14:52:38 +0100 (MET)

   The low end drops on both, transistor and diode ladder filters when
   resonance is increased. But the amount of bass drop is bigger on
   diode filters: This has to do with the different phase & amplitude
   values for the self oscillation point of a straight 4pole vs. a
   5pole filter which approximates the 24dB/Oct slope.

Yes...

   So in this circuit there is a dual ganged pot, one for feedback,
   and one for the filter input level.
   To compensate the bass loss ?
   Yes, but (big surprise!) *only* to the level of a straight Moog
   ladder! It's funny to see this in a circuit simulation. Rather than
   bringing the bass end up to unity gain, they only bring it up
   to the Moog filter's level. 

I didn't know that.  That's very funny.

But also very practical.  Without the gain compensation, you lose too
much low end to really describe it as a low pass filter.  And with
full gain compensation there would be noise and overload issues.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com