[sdiy] FatMan Hacking ( and I dont mean hitting me with an Axe!)

[The Old Crow]

  Using the 'free' OTA can be done as far as I can tell.  The only caveat
is that the Iabc (pin 16) has to be freed from its tie to -V and the
differential inputs be freed from their tie to GND.

  After a quick look at the schematic, one would remove C23 and use the
signal on the "+" side to feed the + input of the OTA through a 22K
resistor, with a 1K resistor also going from the +input to GND.  -input
would have a 1K to GND.  Use a diode current Rd of 15K as in the other
filter OTAs to V+.  Connect OTA output to buffer input also connect 560pF
cap from buffer input to GND.  Buffer output has 4K7 tied to -V, and the
buffer output would also be wired to the +side of a 10uF cap.  -side of
cap goes back into its hole at C23.

  I'm not sure about the resonance.  It will probably work fine left
as-is.  Otherwise, will probably need R67 pulled and the "J" side of R67
wired through a 10K resistor to the buffer output of the 2nd or new 3rd
stage, possibly with a few resistor value tweaks.

  This is all speculative, and ripe for testing...(meaning, I have not
actually done this ;)

Crow

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