fig1 illustrates a block diagram of the present invention . the present invention 10 is connected to a tip and a ring line of a telephone line by a coupling unit 12 which is of conventional construction that may include a pair of clips for selective connection to the tip and ring lines . a protection unit 14 is provided for overvoltage protection and thermal protection . the protection unit 14 includes a thermistor 16 and a surgector 18 , manufactured by rca corp . and others , which respectively provide thermal protection for long term overcurrent conditions and protection against high voltage surges . a fullwave - rectifier 20 is provided for providing ground and positive potentials for application to signalling circuits 22 . a switch 24 permits the selective choice of the talk mode , which is identified by the letter &# 34 ; t &# 34 ;, and the monitor mode , which is identified by the letter &# 34 ; m &# 34 ;. in the talk mode , the present invention activates the transmitter and receiver 26 and 28 . in the monitor mode , the speech network 38 couples signals on the telephone line to the receiver 28 to permit monitoring of the telephone line without disturbing the line in the manner known in the art . alternatively , in the monitor mode , the amplifier 30 and loudspeaker 32 may be activated as described below to provide an audible reproduction of the line signals on the line being monitored by the loudspeaker including voice , synthesized voice , dialing and signalling signals appearing on the telephone line . in the monitor mode , the transmitter 26 is decoupled by the speech network 38 from the telephone line . in the monitor mode when the amplifier 30 is activated , the receiver 28 is deactivated by the control circuit discussed below in conjunction with fig2 a - c . the series network switch 34 is opened in response to the activation of the signalling circuits 22 to disconnect the transmitter 26 and receiver 28 from the telephone line during the activation of the signalling circuits . the continuity polarity indicator 36 is a pair of oppositely poled light emitting diodes which are respectively colored with different colors to permit the telephone line technician to visually determine the polarity of the individual lines of the telephone line to which the individual connectors of the coupling unit 12 are connected . the speech network 38 includes a passive hybrid network of a construction similar to that commonly used in ordinary telephone sets which is useful for both the monitor and talk modes . an attenuator 40 is provided for controlling the gain level of the output signal from the amplifier 30 . the combination of a pushbutton mode switch 42 and a mode latch 44 permits the telephone line technician to selectively operate the amplifier 30 and loudspeaker 32 to produce an audible representation of the signals appearing on a telephone line and to selectively choose a first or a second gain level when the amplifier 30 is activated . the mode latch 44 functions to count and hold three separate conditions sequentially in response to the sequential closing of the pushbutton mode switch 42 . a power timer 46 is provided to disable the operation of the amplifier 30 after an elapsed time has occurred in which no signals have appeared on the telephone line for a period between 5 and 8 minutes . the power timer 46 turns off the amplifier 30 if no signals have been received above a certain threshold after 5 to 8 minutes . if the amplified audio signal exceeds the threshold level in the level detector of the power timer 46 , the timer is reset allowing continued operation of the amplifier 30 . the power timer 46 functions to conserve the power supply provided by a 9 - volt battery 48 . a mute switch 50 is provided for controlling the transmitter 26 and receiver 28 in a conventional manner . mode switch 52 is selectively opened and closed under the control of the mode latch 44 to control the application of power to the amplifier 30 . a keypad assembly 54 is connected to the signalling circuits 22 to provide inputs for selectively activating the generation of dtmf tones . the signalling circuits 22 include power supply circuits , a tone modulator , load control circuit and logic , timing and dtmf tone - generating circuit of conventional construction . fig6 of ser . no . 813 , 156 illustrates a suitable keypad assembly and circuits for implementing the function of the signalling circuits 22 . fig2 a - c illustrate a detailed electrical schematic of the preferred embodiment of the present invention . like parts in fig1 and 2a - c are identified by the same reference numerals . parts which are identified in a labelled block in fig1 are identified by a block closed by a dotted line in fig2 a - c . integrated circuits are identified by their convention part numbers . the overall function of the amplifier mode latch 44 and control circuit 56 is to permit the user of the telephone test unit to selectively control the activation of the amplifier 30 and , if the amplifier 30 is activated , to select either a first gain or a second gain to control the volume of the signal audibly produced on the loudspeaker 32 . additionally , during operation of the amplifier 30 the control circuit 56 automatically electronically mutes the transmitter 26 and receiver 28 to prevent audio feedback into the amplifier . during muting , the control circuit 56 short circuits the receiving unit 26 and open circuits the transmitting unit 28 . the mode pushbutton 42 determines the operation of the amplifier 30 to select the off mode , low gain amplification mode and high gain amplification mode as described below . the telephone line technician presses the mode pushbutton 42 located in the telephone test set to advance to the next operating mode as defined above . if the amplifier 30 is operating in either of the two amplification modes , the power timer 46 shuts off the power supply 5 to 8 minutes after the last signal is detected on the leads of the coupling unit 12 . the mode latch 44 controls and retains the mode set by the mode pushbutton 42 . in the &# 34 ; off &# 34 ; mode , as set by the mode pushbutton 42 or the power timer 46 , the mode latch 44 turns off the power supply and enables normal operation of the test set transmitter 26 and receiver 28 . in the low gain mode , the amplifier 30 is turned on , and the audio input is set to the low gain ( attenuated ) path through the attenuator 40 to the input of the amplifier 30 . in the high gain mode , the amplifier 30 remains on , but its input is connected directly to the source of audio without being shunted by the attenuator 40 . in both modes of amplification , the transmitter 26 and receiver 28 are muted to prevent acoustic feedback ( howling and squealing ) from the speaker 32 . it was discovered in conjunction with the assignee &# 39 ; s prior amplifier used with the ts21 test set , that the receiver acts as a microphone which fed audio signals into the amplifier . the relatively high gain of the amplifier produced unacceptable acoustic coupling to the loudspeaker associated with the amplifier . control of the amplifier 30 as a function of the operation of the mode pushbutton switch 42 and mode latch is as follows . pressing of the mode pushbutton switch 42 causes a ground to appear on the line 58 which discharges the debounce capacitor 60 through resistor 62 . after a 10 millisecond discharge to a logic zero on capacitor 60 , the logic level on the output of inverter 64 goes high , clocking the mode latch 44 to advance to the next state . if the mode latch 44 was previously in the reset state , the output q0 will go high . upon release of the pushbutton mode switch 42 , capacitor 60 charges back up to vdd through resistor 62 . upon the recharging of capacitor 60 , the output of inverter 64 goes low with no change in the state of the mode latch 44 . when the output terminal q0 of the mode latch 44 goes high , the amplifier power switch transistor 70 is turned on as a result of the high logic level conducted by diode 72 at nor gate 74 which activates the battery timer 46 for the initiation of the time - out period . the high logic level from terminal q0 of the mode latch 44 powers the timer reset transistor 76 . when no signal is amplified by the amplifier 30 and with the output level q0 set to a high logic level , inverted by inverter 77 , the timer 46 reset input at pin 15 of counter 78 is inactive at a low logic level . as a result of the output q0 of the mode latch 44 being at a high logic level , field effect transistor 80 is turned on grounding the 6 db input signal attenuator resistor 82 setting the low gain amplifier mode state . if the pushbutton mode switch 42 is pressed again , another clock signal is generated at the input of inverter 64 , as described above , advancing the mode latch 44 to the next state at which the output q0 is equal to a low logic level and the output q1 is equal to a high logic level . the power switch transistor 70 , the power timer oscillator including transistor 76 , inverter 77 , resistors 84 , 86 , 88 and 90 and capacitor 92 continue operation via diode 94 . the oscillator cyclically produces pulses which are counted by counter 78 . the time interval of the timer 46 is determined by counting a predetermined number of pulses . the low logic level on the output q0 of the mode latch 44 causes the field effect transistor 80 to turn off which removes the ground from the attenuator resistor 82 . in this second mode , the amplifier 30 is in the high gain state and remains there until the power timer 46 times out or the pushbutton mode switch 42 is pressed a third time . the mode latch 44 is reset to the off state by one of two methods . the first , as just described , is when the mode pushbutton switch 42 is pressed a third time . in this state , the mode latch 44 is clocked , and both the outputs q0 and q1 go high . the resultant high logic level state signals are inverted by inverters 96 and 98 , anded by nor gate 100 outputting a high logic level . the resultant high logic level at the output of nor gate 100 is nored by nor gate 102 to provide a low logic level logic through a 10 millisecond rc delay produced by resistor 104 and capacitor 106 which is applied to inverter 108 . this rc delay , which time period is not critical , is required to guarantee a minimum reset signal at the reset terminal of the mode latch 44 and prevents malfunctions caused by coincidence of the mode latch 44 outputs from inadvertently resetting the mode latch as it is clocked from low gain to high gain modes . the low logic level state at the input to inverter 108 results in a high logic level at the reset pin 7 of the mode latch 44 causing the resetting of the mode latch turning both the outputs q0 and q1 to a low logic level logic and turning the amplifier 30 and related circuitry off . the logic reset pulse returns to a low logic level after 10 milliseconds since the q0 and q1 outputs of the mode latch 44 have been reset , and the mode latch 44 is again ready to be activated by the pushbutton mode switch 42 . the other means of resetting the mode latch 44 is for the power timer 46 to time - out resulting in a high logic level to appear at the output q3 of the counter 78 which causes the nor gate 102 to output a low level and the mode latch 44 to be reset as described above . as stated above , the mode latch 44 and control unit 56 function to mute the transmitter 26 and receiver 28 when the amplifier 30 is operative in both the monitor and talk modes . the talk mode is when the switch 24 is positioned with the movable contact in contact with the terminal &# 34 ; t &# 34 ;, as illustrated in fig2 and the monitor mode is with the movable contact in contact with the contact &# 34 ; m &# 34 ;. the control circuit 56 mutes the transmitter and receiver 26 and 28 which eliminates acoustic feedback to the amplifier 30 and loudspeaker 32 through the transmitter 26 and receiver 28 . as explained above , experiments with the assignee &# 39 ; s prior art amplifier and ts21 test unit have shown that unexpectedly the receiver 28 coupled audio back into the amplifier 30 and loudspeaker 32 to cause unacceptable howling and squealing . the control circuit 56 for the transmitter 26 does not interfere with the operation of the continuous polarity indicator 36 . as stated above , the control circuit 56 short circuits the receiver 26 and open circuits the transmitter 28 during the operation of the amplifier 30 . the operation of the control circuit 56 in muting the transmitter and receiver 26 and 28 is as follows . when the &# 34 ; ring &# 34 ; line is negative with respect to the &# 34 ; tip &# 34 ; line , and the amplifier 30 is off , transistor 110 receives base drive from the tip side of the telephone line via the series network switch 34 , which is described in detail below , resistor 112 , schottky diode 114 , and resistor 116 . it should be noted that if the loop current is high , transistor 110 receives additional base drive from diode 118 . while in the on state , transistor 110 provides continuity from the series network switch 34 through terminal 120 of the transmitter 26 through transistor 110 to ground . if the amplifier 30 is switched on , a potential of roughly 7 . 8 volts from the amplifier is applied to the gate of field effect transistor 122 to cause it to turn on to short the receiver 28 . in addition to turning on the field effect transistor 122 , the drive from amplifier 30 also turns on transistor 124 via signal applied through resistor 126 . the turning on of transistor 124 shunts the loop current and the drive current for transistor 110 causing it to be turned off and disconnecting the transmitter 26 . when the &# 34 ; ring &# 34 ; side is positive with respect to the &# 34 ; tip &# 34 ; side and the amplifier 30 is off , continuity from the speech network 38 to the switched terminal 120 of the transmitter 26 is supplied through transistor 128 . transistor 128 receives its base drive from the &# 34 ; tip &# 34 ; side of the telephone line via the series network switch 34 , resistor 112 , schottky diode 134 , and resistor 132 . if loop current is high , additional base drive is applied through diode 130 . if the amplifier 30 is switched on , transistors 122 and 124 are turned on to mute the receiver 28 . the on state of transistor 124 is not important in the operation of the muting circuit in this condition . when the amplifier 30 is turned on , optoisolator 135 turns on which turns on transistor 133 . the turning on of transistor 133 shunts the loop current and the drive current for transistor 128 . the input to the amplifier 30 during the monitor mode is via the winding 136 which is an auxiliary winding of the network transformer having four additional windings of conventional construction . the function of winding 136 is specifically for coupling the desired level of line signals during monitor mode to the input of the amplifier 30 and is not found in a conventional network transformer having four windings 138 &# 39 ;-- 138 &# 39 ;&# 39 ;&# 39 ;&# 39 ;. in the talk mode with the switch 24 as illustrated in fig2 the winding 136 inductively picks up both the output signals from the transmitter 26 and the received signals . in the monitor mode with the switch 24 in the position where the movable contact is in contact with the contact identified by &# 34 ; m &# 34 ;, capacitor 140 couples tip line of the phone line to the amplifier 30 to provide an input which has a high impedance , such as 500k ohms , to cause negligible loading upon the operation of the audio network of the telephone line to which the coupling device 12 is coupled . the input to the amplifier 30 consists of dc blocking capacitor 142 , and resistors 144 and 146 . the resistors 144 and 146 are provided to cause a minimum voltage breakdown under transient surge conditions . since the transmitter 26 is decoupled by the speech network 38 during the monitor mode , the control circuit 56 deactivates only the receiver 28 by short circuiting it as described above . the series network switch 34 functions to disconnect the transmitter and receiver 26 and 28 from the telephone line during signalling so that direct current does not flow through the speech network during pulse signalling and during tone signalling the speech network does not load down the outgoing tone levels nor consume dc power which is needed by the tone signalling circuits 22 . the function of the series network switch is described in detail in the aforementioned ser . no . 813 , 156 wherein it is referred to as the &# 34 ; speech mute switch &# 34 ;. the mosfets 148 and 150 are normally fully biased into conduction except when the optocoupler 152 is biased into conduction by the presence of a high level command mutesw from the signalling circuits 22 . dc blocking capacitor 154 and resistor 156 shunt the mosfets 148 and 150 with a relatively high impedance path compared with the impedance of the mosfets 148 and 150 in the on condition to provide a small amount of ac leakage around the series network switch 34 so that during signalling the pulses or tones may be audible through the loudspeaker 32 or receiver 28 . while the invention has been described in terms of its preferred embodiment , it should be understood that numerous modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims .