The diode protects the input to the op amp, in the event that fast negative going input is applied. You should not need the diode for all amps, just some that are susceptible.
this is in a PA amplifier. wouldn't half the waveform always be shorted anytime the signal level gets above .7V?? as this is the first stage of the amp, I would think the signal level to be above .7v but I am not sure?
there is no voltage difference between input and output of that unity gain buffer, hence the diode doesn't conduct under normal operating conditions.
regards
OH YEA! yea yea I wasn't thinking of it that way. ok i get it now. I am so used to seeing diodes across op-amps in guitar distortion boxes that I wasn't thinking about it.