and 34 ring 34 the center taps of the primary and secondary windings of the transformer are interconnected by an electrical path 31 which includes a constant current diode 32 the center tap of the primary winding of the transformer is connected to the positive side of the line as shown in the figure the multivibrator circuit is operative in the manner of conventional multivibrators the circuit differing primarily in that the base current of the two transistors is determined by constant current diode 32 in the feedback path 31 and is switched between the bases by the induced voltage in the secondary winding 27 the multivibrator circuit organization is such as to permit a variation in voltage at the primary center tap the arrangement results in a constant base supply regardless of conditions on the telephone loop the output of the multivibrator is supplied to the regulated circuits 23 24 and 25 through secondaries of transformer 28 and the diodes of the regulated circuits the regulator circuit 22 comprises diodes 40 and 41 zener diodes 42 and 43 and enhancement mode field effect transistor t1a diodes 40 and 41 are connected between the base electrodes of transistors 21a and 21b respectively in order to isolate the base regions of those transistors and transistor t1a the regulated capacitor circuit 23 comprises diodes 50 51 52 and 53 connected between opposite ends of a transformer secondary 54 and across diode 42 as shown in fig1 a capacitor 56 is connected between the common line and diode 42 diode 42 at regulation voltage turns on transistor t1a and shunts the base drive from transistors 21a and 21b thus reducing the oscillation level of the multivibrator circuit to a point where.