of operation of led 82 a diode 102 is connected from the output of amplifier 92 to the input of amplifier 88 to switch led 82 off when led 86 is illuminated similarly a diode 104 is connected from the output of amplifier 90 to the input of amplifier 88 to turn led 82 off when led 84 is illuminated the input of amplifier 90 is connected to conductor 80 through a resistor 106 a potentiometer 108 connected to vcc 2 and ground and a resistor 110 are connected to the input of amplifier 90 and are used to vary the point of operation of led 84 which is coupled to the output of amplifier 80 through a resistor 112 a diode 114 connected from the output of amplifier 92 to the input of amplifier 90 will switch off led 82 when led 86 is illuminated the input of amplifier 92 is connected to conductor 80 through a resistor 116 a potentiometer 118 connected to vcc 2 and ground and a resistor 120 are connected to the input of amplifier 92 are used to vary the operating point of amplifier 92 whose output is connected to led 86 through a resistor 122 potentiometer 96 is adjusted so that when no change in dc voltage appears on conductor 80 indicating linear operation of the headroom extension system led 82 alone will be illuminated potentiometer 108 is adjusted so that when.