This situation is mostly encountered in the interface between an audio amplifier and a loudspeaker. In such cases, the impedance of the loudspeaker is fixed (a typical value being 8Ω), so to deliver the maximum power to the speaker, the output impedance of the amplifier should be as small as possible (ideally zero). Again, this is for a case where "Z"L cannot be changed while "Z"S can be freely changed. In tube amplifiers where "Z"S is high by nature, maximum power delivery to the speaker (a much lower impedance) is achieved through a transformer that matches the high output impedance of the tube amp to the lower impedance of the speaker.