If you wanted a top-echelon audio power amplifier fifteen years ago, it would be big, heavy, power sucking, and hot. That's because Class A amplifiers generate tremendous amounts of heat during operation. Over 75% of the energy that goes in goes out as heat, not sound. Class A/B amplifiers are only slightly better since they operate as class A amps until certain point when they slide into more efficient class B operation where only 50% of consumed energy becomes heat. To dissipate all this heat these power amplifiers also need large heat sinks, which take up space and add weight.