105 



it, somewhat hopefully, the SUITCASE. I shall describe it briefly as an exam- 

 ple of a rather versatile amplifier for passive listening. 



In the beginning it was intended to be sufficiently portable that one man 

 can carry it as hand baggage. For various reasons the SUITCASE has become 

 an instrument which occupies a 12 x 17 inch rack mounted chassis and requires 

 at the least a rather heavy alternating current power pack to operate it. The 

 other alternative is a heavy duty 6 volt storage battery plus seven heavy duty B 

 batteries. It is no longer regarded as one man's hand baggage. This ampli- 

 fier contains two channels which may be connected separately to two hydrophones 

 or in parallel to a single hydrophone. Both channels have a flat response with 

 a minimum of phase distortion between 10 cps and 30 kc. They are designed to 

 be fed by a low impedance line usually coming from a cathode follower contained 

 in the hydrophone. No transformers are used in the basic amplifier, thus 

 avoiding the distortion caused by them. The wide dynamic range is provided 

 by designing the basic amplifier so that it will give an undistorted output of 1 

 volt across 500 ohms for an open circuit voltage input on the hydrophone crys- 

 tal of about 0.5 X 10"^ volts compared with an equivalent self noise of about 3.0 

 X 10"° volts in a frequency band 20 kilocycles wide when used with an AX 58 hy- 

 drophone. It further provides two step attenuators, allowing between them 120 

 db of attenuation in two db steps. That is, sounds whose pressure ranges over 

 a factor of a million can be made to pass with small distortion through the am- 

 plifier. The maximum dynamic range of the suitcase at any given setting of the 

 attenuators is about 50 db when properly adjusted. Obviously such a dynamic 

 range cannot be realized for all settings and must be investigated for each use. 

 The important point in the design of this instrument is the placement of the at- 

 tenuators with reference to the stages of amplification. These attenuators are 

 so placed that with proper manipulation the only vacuum tube preceding them is 

 the cathode follower in the hydrophone, which will take about 10 volts on its grid 

 without distortion. Two interstage filter positions are provided in each chan- 

 nel. A vacuum tube voltmeter is built in with selector connections to th6 output 

 of either channel as well as to the input of a calibration circuit designed to intro- 

 duce an electrical calibration signal in series with the crystal of either hydro- 

 phone. 



The 1 volt output is adequate for feeding any of the conamon commercial 

 recorders of the disc or magnetic wire or tape variety. It is not adequate for 

 driving the pen of any of the common direct writing recorders. For this use 

 separate drivers have been designed which will feed pen recorders full wave 

 signals within their frequency response range, or rectified and averaged signals 

 for higher frequencies. 



For recording ambient noise, animal sounds and complex sounds in gen- 

 eral the magnetic tape recorders have proven to be the most satisfactory sea 

 going instruments having adequately broad frequency response. This does not 

 mean that disc recorders, magnetic wire recorders or any of the photographic 

 recorders may not be used, but they have severe limitations in one direction or 

 another. Disc recorders require very close attention and do not operate too 

 satisfactorily on a rolling ship. Wire recorders have a restricted frequency 

 response, and the photographic recorders, while excellent for many purposes 

 require considerably more technical skill and attention than the tape recorders, 

 thus reserving their use for special problems. This general class of recorder 

 stores the data so that it may readily be played back, providing a more or less 

 accurate electrical copy of the original sound pressure fluctuations. We now 

 feel that this sort of record should be made of most underwater sound observa- 

 tions. However, other presentations are required for various types of analy- 

 sis. 



