[sdiy] tau ladder filter

[Dave Manley]

I thought I'd seen every configuration of ladder filter, but I stumbled 
on this Tau 1010 Ladder Filter as transcribed by Jim Patchell.

http://www.freeinfosociety.com/electronics/schematics/audio/tau1010ladderfilter.pdf

Is there a reason why using PNP's in the ladder is of any benefit or was 
this just some form of patent infringement avoidance? (I have to admit I 
haven't tried to look at the dates of the circuit vs the patent, vs the 
claims, etc).

-Dave