Systems and methods for detecting the impedance of an output load coupled to a digital amplifier and compensating for changes in the response of the amplifier. One embodiment of the invention is implemented in a Class D pulse width modulated (PWM) amplifier. In this embodiment, a digital PCM test signal is generated. This test signal is processed by the amplifier to produce a corresponding analog audio output signal that is used to drive a speaker. A sense resistor placed in series with the speaker is used to generate a test voltage that is compared to a reference voltage. When the test voltage reaches the reference voltage, the current through the sense resistor (hence the speaker) is at a known level, so the value of the digital test signal is noted. The impedance of the speaker is then determined from the test signal value and the speaker current.