If the M-150's input impedance is 200kohm, a 0.008uF or 0.01uF cap
as a high pass filter
will work.
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

