Hydrophone is similar with microphone, except that it is used in underwater environment. Off course a hydrophone can be used to pick up sound in air, but it will be less sensitive since it’s mechanical-acoustical properties is design to match the acoustical impedance of water.
The schematic shows the circuit of a low noise hydrophone amplifier with DC-servo. One half of LT1113 (A) is used in non-inverting configuration to amplify the voltage signal from hydrophone, and the other half (B) null the current and voltage offsets error of amplifier A. This DC-servo B also nulls out the DC errors of hydrophone output. The C1 value should match with the hydrophone capacitance, usually between 200 pF and 8000 pF. The servo should have time constant that is larger than the time constant of hydrophone capacitance and the 100M source resistance to prevent canceling the low frequency signal acquired by hydrophone. [Circuit schematic diagram source: Linear Technology Application Notes]
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