[sdiy] Pole-mixing, Highpass filters, and switching the first stage off

[Tom Wiltshire]

Hi All,

 I've got a question about pole-mixing filters of the Oberheim Xpander/Matrix type. Olivier Gillet gives a nice explanation of the concept:

	http://mutable-instruments.net/static/documentation/pole_mixing.pdf

and the schematic..

	http://mutable-instruments.net/static/schematics/Shruthi-Analog-PoleMixing-v02.pdf

Olivier's filter does the same as the original Xpander filter and switches out the first stage for some of the responses.

The point of this seems to be to allow access to the input signal directly, since now the output of the first stage is just the input (or the input inverted, anyway) which is required for the high pass filters.

The question is this - given that we're just adding poles into a mixer, why not just add a resistor directly from the input, instead of going to all this trouble of switching the first stage out?
What am I missing? Is my assumption about the purpose of the switch wrong?

Thanks,
Tom