Why couldn't they use TI's hybrid digital technology to improve efficiency? I have done it for home audio:
Amanda Harris: the Prius of home audio - EcoRenovator
Quote:
At this time, NO in-dash car head unit has a DC-DC power supply providing rail voltage to its amplifier IC. (some units, such as a discontinued Panasonic model and some specific Alpine and Kenwood units could be fitted with an outboard switching DC-DC unit to double rail voltage)
What this means is, for all practical purposes 100% of all in-dash car head units you can buy are working with 0-12 (or 14.4 under ideal conditions) volts and the vast majority use BTL configured output transitors with a 0.5/1 bias.
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Except in hybrid and electric cars that have high voltage electrical systems. No chip amplifier will handle 200+v (discretes can be designed for it - much to the joy of those who build loud car stereos!) so a DC/DC converter will be used to reduce it. Then the design is more like that of a home audio system. In fact, most universal input power supplies for home electronics will run from the high voltage system in a hybrid or electric car without any modifications.
I remember a friend of mine in high school who turned up the (aftermarket) stereo so loud that the lights were dimming... and then complained that gas is too expensive!