Whether an audio output amplifier of an electronic device is connected through an accessory connector to a headphone or a self powered audio device is determined by sensing the signature of the input power signal in an input power line to a voltage regulator circuit that ultimately powers the amplifier. The signature is sensed flowing through a sense circuit that is in series in the input power line while the amplifier outputs a high power, inaudible signal to the connected device. The signature is output to a controller. The controller indirectly determines from the signature, whether the impedance of the connected device is that of (1) a headphone (speaker) for which a headphone amplifier is selected to provide more power for a lower impedance load, or (2) a self powered output device (“line in” input) for which a “line out” amplifier and isolation resistor are selected to drive a high capacitive load.