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From car audio’s fledgling beginnings in the 1930s as a simple AM in-dash noise source to today’s fully immersive, multi-speaker, hundreds (even thousands) of watt theater-like environment, it’s been a remarkable journey.
Early high-end automotive audio was a niche market driven by a handful of audiophiles seeking to bring a much better sound experience to the wheeled environment. Eventually, as audio technology advanced, the niche audio market quickly adopted it. Today, nearly entry-level vehicles boast some form of premium audio environment.
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However, the journey was not without its challenges. As system power ramped up, audio amplifier footprints became unwieldy. Class-AB amplifiers demanded obscene amounts of vehicle power and increasingly larger heat-management solutions. So, designers started looking for new directions that would reel in the power monster, especially for electric vehicles, where every watt is precious.
Some components, such as RF front ends and preamps, are highly integrated and have relatively small footprints and power requirements. They don’t particularly challenge space or power constraints. However, high-power Class-AB audio amplifiers do. They’re relatively inefficient devices and the largest audio system components (other than speakers) are the primary challenge to more efficient audio system designs.
A second challenge is the cabin environment. Along with these high-power systems came a mandate to improve the cabin with better acoustic properties.
Applying physical acoustic solutions had its limitations. Physical materials only add weight and take up space, which is impractical. Therefore, designers again looked to technology for solutions.
And they found them in the way of Class-D amplifiers and highly integrated microcontrollers. Let’s drill down a bit on how designers evolved car audio into today’s immersive systems.
First and Foremost, the Audio Amplifier
The staple, for many years, was the Class-AB audio amplifier. Its analog output design allows the device to connect directly to speakers with nothing in the circuit except maybe a capacitor. However, as power demands ramped up, Class AB became unwieldy. The cooling and supply power requirements for high-wattage systems made for a large footprint that required robust heat management, i.e., large heatsinks, good ventilation, and lots of available power–all precious commodities in many wheeled environments.
Designers decided to replace the well-entrenched, moderately efficient (typically 50% to 70%), Class-AB circuitry with Class-D circuitry. Class D was an ideal solution. It’s a switching platform and much more efficient (over 90%). Class D’s efficiency meant the amplifier uses less power and generates less heat, resulting in a smaller, less power-hungry footprint.
There was, however, a gotcha (of course). While Class-D circuits have overcome early sound-quality issues, their digital signal can’t be connected directly to speakers. Moreover, the signal contains an unacceptable amount of ripple. That meant additional components were required between the output of the amplifier and the speakers. A simple, cost-effective solution was to couple the speakers via an L/C filter. This filter provides the proper output signal and filtering.
However, the high power output from the amplifier made the L/C components large, particularly the series inductor. And each audio channel requires two inductors. A better solution was needed.
The design came in the form of a new Class-D audio amplifier with a slower switching speed (2.1 MHz) and proprietary single inductor (1L) modulation technology. By eliminating one inductor and redesigning the remaining inductor, this innovative design allows for the use of a smaller, more cost-effective circuit (Fig. 1). This met the high power demands of premium audio, optimizing space and cost.
