The amount of voltage gain required for the hydro- 

 phone preamplifier is determined by the gain necessary to 

 insure that the voltage levels corresponding to the lowest 

 expected ambient noise levels would be above the input noise 

 level of the remainder of the system. The desirability of 

 maintaining as large a dynamic range as possible for the 

 hydrophone -preamplifier requires that the minimum amount 

 of gain necessary to accomplish the above purpose be speci- 

 fied. In this case the specified voltage gain is a nominal 

 30 dB ±1 dB (28 dB as measured). 



The output stage of the preamplifier was especially 

 designed to prevent high frequency oscillation when used 

 with long cables. This required the addition of two output 

 transistors to the basic preamplifier and necessitated at 

 least 12 mA of current at about 12 volts. The output imped- 

 ance is less than 50 ohms, with a maximum output voltage 

 not less than 2 volts rms. 



The hydrophone and preamplifier unit with connec- 

 tions to the Marsh and Marine connector is shown in the 

 simplified schematic of figure 6. 



ELEMENT 



POWER 



SIGNAL 



COMMON 



-o 3 



-o 1 



-o 6 



-o 2 



-o 5 



CAL 



Figure 6. Block diagram of Wilcoxon Research Co. hydrophone M-H90-A with 

 18-inch cable and connections to a Marsh and Marine cable connector. 



10 



