Class D audio amplifiers like the MAX9704 offer almost 90% efficiency, but they need 10 V to operate. For battery-operated systems, that would require a large number of batteries wired in series. You can avoid bulky battery packs by incorporating a switched-mode boost converter like the MAX1771, which generates 12 V from inputs as low as 2 V and delivers output currents up to 2 A. A battery-operated class D amplifier can then run off a pair of AA cells.
You can create such a system using evaluation boards (EVkits) available for both the class D amplifier and the boost converter. Simply wire them together and connect two AA alkaline cells, an MP3 player for the audio source, and a pair (in this example) of Celestion Ditton 44 loudspeakers (see the figure). The output power is limited by the MAX1771 EVkit, but the class D amplifier uses the total available power, estimated at 6 W, very efficiently. The volume from this simple circuit is excellent, and the two AA cells have an estimated battery life of over four hours.
The main limitation on power output is the MAX1771 EVkit, whose maximum output current at 6 V is 0.5 A. The kit can be modified to deliver as much as 2 A, which boosts the power output (at 6 V) to 24 W. Two AA batteries can support this power output as before, but perhaps not for four hours.
This technique can also provide uninterrupted audio in an automobile, allowing entertainment to continue at full volume while the engine is being cranked. That capability will be of increasing interest if a proposed law takes affect requiring car engines to stop at traffic lights and then restart when the accelerator is depressed. The voltage from a 12-V battery dips to as low as 6 V when the car is cranked during cold conditions, and that drop in supply voltage can have a substantial effect on the audio amplifier.
Rear-seat applications can also benefit from this circuit, because DVD players initiate a relatively long reset period when their power supply is interrupted. DVDbased navigation systems need protection from supply dropouts as well.