The dynamic range of the section for signals pre-processing in the underwater acoustic unit is conditioned by hydrophones sensitivity of the receiving antenna and by the internal noise level of operative assemblies composing the section. When we use a receiving antenna with interferometric fiber-optic hydrophones in that complement, optical and digital components are parts of the section. Our paper focuses on the calculation technique to get tolerable internal noise level reduced to the hydrophone input value. The technique is concerned with the underwater-acoustic unit containing receiving antenna with interferometric fiber-optic hydrophones.