The Modern Device LM4889 audio amplifier breakout board uses one of National Semiconductor’s “Boomer” amps. It uses what in the industry is referred to as a “bridge-tied load”. This is an efficient system that increases the total amount of power available to the speaker. It also eliminates the need for an output capacitor (like in the LM386), which tends to be bulky and can limit the low-end response. It has a shutdown pin to put the chip to sleep – just in case you’re trying to build a super low-power microcontroller application, that spends most of its time sleeping.