Receptive fields in primary auditory cortex (A1) can be rapidly and adaptively reshaped to enhance responses to salient frequency cues when using single tones as targets. To explore receptive field changes to more complex spectral patterns, we trained ferrets to detect variable, multi-tone targets in the context of background, rippled noise. Recordings from A1 of behaving ferrets showed a consistent pattern of plasticity, at the single neuron and population level, with enhancement for each component tone frequency, and suppression for inter-tone frequencies. Plasticity was strongest near neuronal best frequency, rapid in onset, and slow to fade. While attention may trigger cortical plasticity, the receptive field changes persisted after the behavioral task was completed. The observed comb filter plasticity is an example of an adaptive contrast matched filter, which may generally improve discriminability between foreground and background sounds, and we conjecture, may predict A1 cortical plasticity for any complex spectral target.