base, has compression fittings for passing the leads, and the opposite 

 piece has a small capillary tube for pressure equalization. The whole 

 assembly is covered with a rubber boot and the interior filled with 

 silicone oil. Capacitance is 0.051 yf. 



Sensitivity versus frequency is shown in Figure 3, where 

 sensitivity is expressed as the output of the hydrophone in db below 

 one volt for 1 ubar of applied pressure. Frequency response measure- 

 ments were made in a pressure tank to a frequency of 100 cps . The mean 

 of this curve was extrapolated out to 2000 cps to provide a basis for 

 system response calculations. 



(b) Preamplifiers 



(1) The transistorized preamplifiers are mounted as an integral 

 part of the hydrophone assembly to provide proper impedance matching from 

 the hydrophone to the cable and shore termination. The preamplifier gain 

 is a fixed +40 db with a flat response from 30 to 2,000 cps and within 



1" 1 db from 20 to 5,000 cps. The preamplifier has an extremely low cur- 

 rent requirement of 1.2 ma which is supplied from shore via a single 

 pair of leads which simultaneously carry the hydrophone preamplifier out- 

 put signals. 



The preamplifiers are expected to have a useful life of two 

 to three years under the present environmental conditions, with the 

 possibility of lasting much longer. Transistors were selected for uniform- 

 ity and stability, and prototype amplifiers were operated continuously for 

 two months without exhibiting any noticeable deterioration of performance. 



(2) Circuits. The circuit diagram is contained in Figure 4. 

 Two 2N167 transistors provide two gain producing stages and the 2N123 

 serves as an emitter follower output stage providing low output impedance. 

 High input impedance is maintained by applying a.c. feedback to the emit- 

 ter of the input stage and d.c. feedback through the input stage bias 

 resistor and the feedback loop. Large emitter resistors with bypass 

 capacitors provide stabilization. The 0.005 yf capacitor in the collector 

 circuit of the input stage suppresses high frequency parasitic oscillations 

 which may occur with open input . 



The preamplifier has a nominal rating of 1.2 ma at 20 vdc, but 

 will function satisfactorily using supply voltages from 8 to 35 vdc. The 

 supply voltage of course, affects the maximum signal handling capability. 



The preamplifiers require 150 ms to recover from a 30 db over- 

 load and 700 ms to recover from interrupted power. 



(3) Characteristics. Preamplifier self noise, which is plot- 

 ted in Figure 5, was measured at the amplifier output, with the input 

 shunted by a capacitance equal to that of the hydrophones . It is express- 

 ed as the equivalent per cycle voltage at the input, in db from one volt. 

 The expected minimum ambient sea noise, correspondingly expressed as an 

 equivalent input voltage, shown in Figure 5, indicates that below 10 



