The output spectrum of a low-pass amplifier that has a limited linear region and is overloaded by the addition of a large sinewave to its input signals of interest, is determined from a simplified but realistic model. The validity of the model depends on both the signals of interest and the background noise being small compared with the linear range of the amplifier, and thus small compared with the sinewave that causes overloading. This enables the effect on these small components to be considered as that of a periodic gain modulation of the amplifier at twice the frequency of the interfering sinewave. The degradation in detectability of small signals is found to reach a plateau as the interfering wave increases. The degradation may be zero if the interference is at the high-frequency end of the input band and increases without limit as the interference approaches dc.