Im need to convert the output of a microphone ( balanced XLR ) to a
input of a standard soundcard ( unbalanced 3.5mm plug or 1/4 inch jack
- doesnt matter ).

This is for a room equilization project, so the quality of the sound
information must be retained as accurately as possible. ( otherwise
the entire room correction process will suffer in accuracy )

If anyone has links to schematics of such circuits or can tell me how
to build such a cable/converter, I would really appreciate it.

Thanks,
Witek.