Resistors R4 and R14, along with Schottky diodes D5, D6, D7 and D9, provide input protection for the PGA2500 preamplifier when using phantom power, or when the input voltage exceeds the VA+ or VA− power supplies by more than 350 mV (the approximate turn-on voltage of the Schottky diodes).

A common fault condition is for either the hot (+) or cold (−) input of the preamplifier to be shorted to ground. With phantom voltage applied, this causes the blocking capacitors to discharge, with a large surge current presented at the PGA2500 input pins. Without the protection network, the PGA2500 would be permanently damaged by the surge current, which can reach several amperes in peak magnitude.

The Schottky diodes are forced into conduction during this fault condition, steering most of the charge away from the PGA2500 device and towards the power supplies. The series resistors can be set to a value that will help limit the input current, although care must taken to avoid adding too much resistance, since the added noise can degrade the overall performance of the preamplifier. The Schottky diodes add a nonlinear capacitance to the input circuit, which can result in additional distortion. However, with the relatively small input voltage swing present when the preamplifier is set to gains between 10 dB and 65 dB, the effect on the THD+N of the PGA2500 is small or negligible. For unity-gain applications, where the voltage swing may become large enough in magnitude to transition over a greater portion of the diodes nonlinear capacitance, the THD+N ratio may degrade by as much as 3 dB from the published typical performance specifications.