XO-1.75 microphone socket

[John Watlington]

In Tony's defense, he was reminding us to fix what was
an acknowledged omission in XO-1 (the C3 motherboard
which fixed it was never put into production).   We never
reflected the change in the 1.5 specs, causing further confusion.

I apologize for accepting an 0402 for the current limiting
resistor on 1.5 --- I requested 1206, but anything larger
than an 0402 would have required quite a bit of board
change (due to separate ground planes under the analog
section).

The spec'ed continuous rating of the 1.5 microphone input
is +9V/-6V.  Exceeding this is likely to damage the laptop.

Yes, personally I recommend building sensors to use the
internal 2V biasing supply.   It uses the built-in rechargeable
battery!

Cheers,
wad