Hydrophone is a tool like microphone, but this tool is used in the underwater environment. When used to pick up a sound on the air then this tool becomes less sensitive because it’s mechanical-acoustical properties is designed to match the Acoustical impedance of water.
Circuit shown in this article is the hydrophones of a low noise amplifier with DC-servo. To amplify the voltage signal from hydrophone, this circuit uses one half of LT1113 (A). It uses a non-inverting configuration. The other half (B) is used to null the voltage and current offsets error of amplifier A. To nulls out the DC errors of hydrophone output,this circuit also uses DC-servo B. To prevent canceling the low frequency signal acquired by hydrophone this circuit uses 100M source resistance. The value of C1 usually between 200pF and 8000 pF which will match with the hydrophone capacitance. The time constant of hydrophone capacitance must be smaller than the time constant of servo. [Circuit schematic diagram source: Linear Technology Application Notes]