A DC power supply control apparatus monitors respective terminals supplying positive and negative DC supply voltages to a utility device. The voltage at the negative terminal is compared with a target value for that voltage to produce an offset current representative of any voltage differential therebetween. This offset current is used to produce an offset voltage, that is added to or subtracted from a target value of the positive supply voltage to produce an error amplifier reference voltage. This reference voltage is compared with the voltage at the monitored positive terminal, to produce an error voltage, in response to which the power supply's modulator loop adjusts the power supply's positive output voltage, as necessary, to maintain the intended DC voltage differential between the positive and negative DC supply voltages.