(6) Special amplifiers and chart recorders for obtaining 

 graphical records of ambient noise on a continuous basis. 



(7) Two channel audio monitoring equipment. 



(8) Filters for use while recording or monitoring. 



(b) Terminal Variable Gain Amplifiers 



These amplifiers are quite similar to the preamplifiers, and 

 are assembled on phenolic printed circuit boards which plug into printed 

 circuit connectors. The circuit diagram for the amplifiers, which are 

 identical, is shown in Figure 9. 



Self noise is plotted in Figure 10. 



Maximum input voltage level and dynamic range versus frequency 

 are plotted in Figure 11. 



In general, the preamplifier description in paragraph 4b(2) 

 applies to the terminal amplifiers. 



Attenuators covering a range of to -38 db in calibrated steps 

 of 2 db are used at the input to the amplifiers. The amplifier has two 

 fixed gain selections, i.e., +20 db and +40 db, which in conjunction 

 with the attenuator, provide a variable amplifier gain from -18 db to 

 +40 db in 2 db steps. 



(c) Monitor Amplifier 



This two channel amplifier is rack mounted and consists of 

 conventional vacuum tube operated amplifier stages. There are separ- 

 ate volume controls for each channel and ganged controls for bass and 

 treble adjustment. 



The monitor reproduces the output from the record-play ampli- 

 fier (RPA) during the record or playback mode from either track or both 

 tracks simultaneously. 



(d) Two Track Recorder 



The two track recorder is mounted on the equipment rack and 

 has a 14 inch reel capacity for 12-hour continuous recording at 1 7/8 ips, 

 using 1 mil tape. Speeds of 3 3/4, 7 1/2 and 15 ips are also available 

 with proportionate reduction of recording time. 



(e) Rerecording Circuits 



The output of each tape amplifier, in record or playback mode 

 is terminated in phone jacks on the front panel. 



The RPA outputs are supplied via cathode followers which 



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