An integrated circuit for implementing at least a portion of a personal audio device, comprising: an audio source having an output adapted to provide source audio for playback to a listener; a combiner adapted to combine the source audio (ia) and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of a transducer (SPKR) and to generate an audio signal; an output adapted to provide the audio signal to the transducer (SPKR); a reference microphone input adapted to receive a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal (err) indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); and a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal (ref) to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit implements a coefficient control block that shapes the response of the adaptive filter by adjusting coefficients that determine the response of the adaptive filter in conformity with the error microphone signal (err) and the reference microphone signal (ref) to minimize the ambient audio sounds at an error microphone providing said error microphone signal (err); wherein the processing circuit implements a second adaptive filter having second coefficients adjusted by a second coefficient control block; characterised in that: the processing circuit is configured to detect whether or not the source audio is present at the output of the audio source, and in response to detecting that the source audio is not present, to halt adjustment of the coefficients or of the coefficients and the second coefficients while continuing to generate the anti-noise signal. an audio source having an output adapted to provide source audio for playback to a listener; a combiner adapted to combine the source audio (ia) and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of a transducer (SPKR) and to generate an audio signal; an output adapted to provide the audio signal to the transducer (SPKR); a reference microphone input adapted to receive a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal (err) indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); and a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal (ref) to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit implements a coefficient control block that shapes the response of the adaptive filter by adjusting coefficients that determine the response of the adaptive filter in conformity with the error microphone signal (err) and the reference microphone signal (ref) to minimize the ambient audio sounds at an error microphone providing said error microphone signal (err); wherein the processing circuit implements a second adaptive filter having second coefficients adjusted by a second coefficient control block; characterised in that: the processing circuit is configured to detect whether or not the source audio is present at the output of the audio source, and in response to detecting that the source audio is not present, to halt adjustment of the coefficients or of the coefficients and the second coefficients while continuing to generate the anti-noise signal.