External Amplifiers for Automotive Use: Why and How

To enhance the driving and passenger experience, today's vehicles, regardless of size or price, offer more and more entertainment and information features. Today's factory-installed infotainment systems typically combine entertainment, multimedia, and driver information in a single module. They offer AM/FM and satellite radio, a CD/DVD player for music and video, a navigation system, data and multimedia ports (USB, Bluetooth®, line-in, line-out, video-in), and general and vehicle status information.

Even the best factory-installed systems offer a subpar listening experience for discerning users. When users demanded the very best in car audio quality, upgrading factory-installed systems with parts of significantly higher quality was once a necessity.

Today, premium original equipment manufacturer (OEM) products in high-end vehicles match or even exceed comparable aftermarket systems in almost every respect. With the exception of the head unit infotainment system, these high-end systems include powerful factory-installed external amplifiers separate from the head unit for better sound quality.

For low- to mid-range vehicles with a basic head unit and no external amplifier, adding an external aftermarket amplifier greatly improves the sound quality; see Figure 1.

Figure 1: Low-end automotive amplifier: discrete preamplifier and amplifier

In a typical system, a power amplifier has three parts: the pre-amplifier, where the signal goes into the amplifier and where components such as crossovers and equalizers are located; the power supply, which powers the amplifier; and the amplifier itself, which drives the speakers.

The pre-amplifier section of the amplifier may have a gain control stage; it controls the amplification of the head unit signal in order to drive the amplifier to its maximum power level. Depending on the amplifier topology, this control stage can be implemented with discrete operational amplifiers and discrete components, as well as a fully integrated analog-to-digital converter (ADC) with an analog front end or CODEC solution.

The preamplifier stage of higher-end amplifiers may also perform some level of signal processing to improve the audio output (such as crossover or equalization); this signal processing occurs in the digital domain using a processor or digital signal processor (DSP). Fully integrated audio codecs with embedded miniDSP convert the input audio analog signal to digital information, perform audio signal enhancement, and convert it back to analog, as shown in the system block diagram in Figure 2. Other alternative topologies include discrete ADCs or digital-to-analog converter (DAC) combinations that can increase system flexibility; see Figure 3.

Figure 2: Mid-range automotive amplifier: Integrated preamplifier with gain control and EQ

For the actual amplifier stage, by using Class D audio amplifiers, the system power efficiency can exceed 90% while maintaining top line audio fidelity. This efficiency improvement significantly reduces size, weight and heat dissipation; in addition, while Class D audio amplifiers require external inductors, new lower cost automotive grade inductors enable system designers to cut system costs and achieve similar functionality to Class AB based audio systems in a smaller external size.

Figure 3: High-performance automotive amplifier: integrated preamplifier with analog gain control, crossover, and EQ

TI's Class-D automotive audio amplifiers exhibit extremely low electromagnetic interference (EMI) levels and are designed to be used in OEM systems with strict electromagnetic compatibility (EMC) requirements (such as CISPR 25-L5) without the need for additional (and expensive) filtering or shielding.

Automotive systems are often subjected to extremely harsh operating environments and require enhanced reliability levels in their performance. TI's full-featured automotive Class-D audio amplifiers include load diagnostics for detecting and diagnosing output disconnects to help reduce testing time during manufacturing, along with load dump protection to withstand high-voltage transients commonly found in automotive system power supplies.