Audio

Reference:    NAVSHIPS 91875(A), para 2j

The audio assembly is identical for the three receiver types.  However the connections between this assembly, and associated assemblies and parts, differ in the three receivers.  These differences are mentioned where applicable throughout this paragraph.

(1) AUDIO INPUT CIRCUITS.  Input to the audio assembly is determined by the setting of S1101A which is controlled by the RECEPTION control.  A simplified schematic of the audio input circuits is shown in figure 2.4.   When the RECEPTION control is set at FSK, A1 BROAD, A1 SHARP, output from the BFO-mixer, V1004, is applied directly to the first audio amplifier, V1102.  But when the RECEPTION control is set at A2, A3 SHARP, A3 BROAD (the A3 positions are used only on AN/SRR-12 and -13 receivers), output from the diode detector V1005, is applied to the first amplifier, V1102, through a noise peak limiter diode, V1101, which functions as follows:

The d-c component of the rectified signal from the diode detector circuit developed across R1027 as described in paragraph 2h(5) is applied to the cathode of the noise peak limiter V1101 through R1104, R1105, and C1011 which filters the a-c component of the signal.  A portion of the rectified signal voltage is applied to the plate of the noise peak limiter V1101 by means of a voltage divider R1102, R1103.  Under conditions of normal signal, the diode V1101 conducts and feeds the first audio amplifier, V1102, but when a noise pulse which exceeds normal modulation percentage is detected, it causes an instantaneous decrease in the potential on the plate of the noise limiter diode V1101.  Voltage to the cathode, however, remains constant because the filtering action of C1101 and R1104.  Thus for a short time interval, the plate is more negative than the cathode, and during this time the diode acts as an open circuit.  Therefore the output noise pulse is limited to a value corresponding to the maximum normal modulation percentage as determined by the relative values of R1102 and R1103.  In these receivers the noise limiter is set to limit impuleses to a level corresponding to 40% modulation.

The B section of the OUTPUT control, R257 of AN/SRR-11, R457 of AN/SRR-12, or R657 of AN/SRR-13 is shunted across the grid circuit of V1102, when S1101A is set at FSK, A3 SHARP, or A3 BROAD to provide a means of adjusting the signal level before it is applied to first audio amplifier V1102.  This control is inoperative when S1101A is set at A1 BROAD, A1 SHARP, or A2 (in these positions the signal input is applied across the entire resistor, and the output is taken from the same point).

(2) SILENCER CIRCUIT (AN/SRR-12 and -13 only).-- See figure 2-5.  A silencer circuit, V1003 and V1104, is active in the audio assembly when the RECEPTION control is in the A3 SHARP or A3 BROAD position to prevent noise form being further amplified by succeeding stages in the absence of signals.

The silencer diode, V1103, is connected in series between the first and second audio amplifiers, V1102 and V1105.  Plate potential of the diode is determined by the amount of plate current flow through the d-c amplifier, V1104.  The cathode potential is determined by voltage divider R1115 and R1116.  Grid voltage of the d-c amplifier (pin 1 of V1104), supplied by B+, is determined by the setting of the SILENCER potentiometer R456 of AN/SRR-12 or R656 of AN/SRR-13, which is mounted on the front panel of the receiver, and by the average d-c level of the detector output.  The potentiometer R456 or R656 is connected between B+ and ground, and thus provides an adjustable positive voltage.

When the grid voltage of the d-c amplifier V1104 is positive, the plate of V1103 becomes negative relative to the cathode, and V1103 cannot conduct.  When the grid voltage of V1104 is reduced (as by the presence of a strong carrier) the plate of V1103 becomes positive relative to the cathode, thus causing V1103 to conduct.

When the RECEPTION control is set to any other position but A3 SHARP or A3 BROAD, the SILENCER control is inoperative because switch S1101A returns the grid circuit of V1104 to ground instead of to the silencer control.

(3) SECOND AUDIO AMPLIFIER (V1105).-- The output of the silencer diode, V1103, is taken from the its cathode and coupled to the grid (pin 1) of the second audio amplifier, V1105 through C1105.  The amplified output appears at the plate (pin 8) of V1105 and is applied to the cathode of the first diode limiter, V1106, through capacitor C1107.

(4) DIODE LIMITERS.-- Two series diode limiters, V1106 and V1107, are combined to clip equially on positive and negative portions of the audio cycle, when S1101A on the RECEPTION control is set for A1 or A2.  Plate potential on the limiters, and therefore, the level at which the tubes will become non-conductive, is set by the A section of the OUTPUT control R257 of AN/SRR-11, R457 of AN/SRR-12, or R657 of AN/SRR-13.  When the audio output from V1105 is applied to the limiter diodes (cathode of V1106), diode V1106 limits the positive portions of the input while V1107 limits the negative portions and the output control is adjustable to provide a range of 40 db.  In those positions of the RECEPTION control where limiting is not used, A3 and FSK, the plate potential on the diodes is fixed by returning the plate to B+ through R1112 at such a value that the diodes remain conductive beyond the point that produces maximum power output from the output stage.

(5) AUDIO OUTPUT STAGES.-- Signal voltages are taken from the cathode of the second limiter, V1107, and applied to the grid (pin 1) of the driver stage, V1108 through C1108.  They are amplified and applied to the control grid (pin 1) of the beam power pentode, V1109 through R1134 and C1109.  A negative feedback voltage is applied from the plate (pin 5) of V1109 to the cathode (pin 5) of V1108 via resistor R1132 and R1134.  The feedback is provided to maintain a nearly constant output voltage under changing load conditions.  (Such as due to connecting two headphones in parallel or connecting various loads to the receiver audio output terminals.)

The pentode, V1109, feeds the primary of output transformer T1101; and signals are taken from a balanced secondary winding to feed a 600-ohm line, and an unbalanced winding (with one side grounded) for phone jack connections.

The phone LEVEL control, R258, R458, or R658 (depending on receiver type) regulates the output which is applied to the two PHONE jacks J265, J266 or J465, J466 or J665, J666 connected in parallel.  The same audio signal that feed the PHONE jacks also goes to the OUTPUT meter M252 of AN/SRR-11, M452 of AN/SRR-12, or M652 of AN/SRR-13 through the appropriate multiplier resistors as selected by the ADD DECIBEL switch S255, S455, S655.  The OUTPUT meter is connected directly to the output winding to prevent the LEVEL control and phones from affecting the meter readings.

(k) OUTPUT METER.-- The signal to the OUTPUT meter, M252 of AN/SRR-11, M452 of AN/SRR-12, and M652 of AN/SRR-13 is attenuated in four steps to permit the meter to indicate, in decibel, a wide range of output levels. Zero db of the meter circuit corresponds to a power level of six milliwatts into a 600-ohm load.  Multiplier resistors provide the appropriate amount of attenuation.  These multipliers are selected by the ADD DECIBEL switch S255 of AN/SRR-11, S455 of AN/SRR-12, or S655 of AN/SRR-13, for the +20 db, +10 db, 0 db, and -10 db positions.  There is no attenuation provided when the switch is in the -10 db position inorder that the low output levels can be read.  The -10 db position of the ADD DECIBEL switch is spring loaded, and the switch will fall back to the 0 db position of the switch arm is not held in the -10 db position.

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