Audio amplifier with volume limitation in dependence on spectral content or identity of source

The high frequency components in a wide band audio source tend to mask the distortion experienced by the Listener. This means that more distortion can be tolerated from an FM tuner, CD player or tape player signal (wide band) than from an AM tuner signal (narrow band), except for cases when the wide band source is providing a narrow band signal (eg piano). A first embodiment of the invention provides a distortion threshold which is selectable in dependence on the identity of the source. A second embodiment of the invention provides a distortion threshold which is selectable in dependence on spectral content.