to junction 51 base 69 is connected from junction 51 feedback to the amplifier is provided through a capacitor 71 and a capacitor 72 capacitor 71 is connected between junctions 51 and 52 capacitor 72 is connected between junctions 50 and 53 a conventional darlington circuit 73 is provided including a transistor 74 and a transistor 75 transistor 74 has a collector 76 connected from junction 52 transistor 74 has a base 77 connected to junction 53 transistor 74 has an emitter 78 connected to a base 79 of transistor 75 transistor 75 also includes a collector 80 connected from junction 52 and an emitter 81 a resistor 82 is connected from emitter 81 to junction 21 a resistor 83 is connected between junctions 49 and 52 resistors 84 and 84 39 are connected between junctions 53 53 39 and 53 39 54 respectively a resistor 85 is connected between junctions 31 39 and 55 a zener diode 86 is connected between junctions 22 and 55 a capacitor 87 is connected between junctions 56 and 49 junctions 55 56 and 57 are connected together a resistor 122 is connected between junctions 53 and 23 a resistor 128 is connected between junctions 126 and 127 junctions 51 and 126 being connected together and to base 69 a resistor 89 is connected between junctions 57 and 60 a resistor 90 is connected between junctions 24 and 60 a capacitor 91 is connected between junctions 25 and 60 a silicon controlled rectifier scr is provided at 92 including an anode 93 a cathode 94 and a gate 95.