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.