A programmed data processor obtains a number of input voltage measurements for a number of speaker drivers, respectively, and a sensed shared current being a measure of current in a single power supply rail that is feeding power to each of a number of audio amplifiers while the audio amplifiers are driving the speaker drivers in accordance with a number of audio channel test signals, respectively. The programmed data processor computes an estimate of electrical input impedance of each of the speaker drivers using the input voltage measurement for the speaker driver and using the sensed shared current. Other embodiments are also described and claimed.