What is this opamp circuit with diode?
It's an acitve high pass filter (Sallen-Key topology), the diode is to clip large negative swings as Dave mentions.  

Note for your circuit R1 = R124, R2 = R108, C1 = C69, C2 = C59 in the equations below.

The undamped natural frequency Fo is given by:



and the Q is given by:



So for this filter Fo is 27.43Hz, and the Q is 0.88.
This means the the output will be 20 * log10(0.88) = 1.1dB down at Fo.