36 the compression point adjustment circuit 36 provides a signal to the photo electronic gain control circuit 32 which in turn provides gain control to amplifier 30 an output transformer 38 receives a signal from the output loss pre compensation circuit 34 and provides a signal to an output 40 0026 fig2 illustrates a power supply circuit 44 for use with the circuit 20 of fig1 a battery voltage input 46 is connected to a diode 48 a positive voltage regulator 50 converts the switched battery voltage which would normally be 12 volts to a voltage suitable for operating circuit 20 an output 54 permits connection of the regulated positive voltage to circuit 20 a negative voltage converter 52 receives the signal from the positive voltage regulator 50 and provides a negative voltage as needed to output 56 for operation of circuit 20 the diode 48 connected between input 46 and positive voltage regulator 50 provides reverse polarity protection for the circuit 44 referring now to fig3 the isolation transformer 24 may be a 300 600 ohm audio transformer such as the model 42tl023 audio transformer distributed by mouser electronics output transformer 38 may be a 500 16 ohm audio transformer such as the mouser electronics model 42tl026 audio transformer the function of the transformers.