The Audio DesignLine website has posted a series of Class D Audio Amplifier articles. These were written by Eric Gaalaas of Analog Devices and are based on his original article which was published in EE Times magazine. Some of the data used to show the performance of Class D amplifiers in these articles is based on technology from Analog Devices, like the AD1994 Class D amplifier. The 25W AD1994 doesn't necessarily meet the performance specifications of a high end home theater system, like the Class D amplifier modules from Hypex, ICEpower or ColdAmp, because it is designed primarily for applications like automotive audio, or integration with flat panel televisions. However, this series of articles does provide a fairly good overview of the technology.
The first article, Class D Audio Amplifiers: What, Why, and How, describes the different topologies of Class A, B and A/B linear amplifiers and their power dissipation characteristics. The main point here is the Class A amps dissipate a lot of power in the form of heat (in fact, more power is dissipated than is sent to the loudspeakers), but the sound quality is very good; Class B amps don't dissipate as much power, but the sound quality isn't very good; and, Class A/B, which is a hybrid of the two, dissipate less power than Class A and a little more than Class B, but also sounds much better than Class B.
The second installment, Class D Audio Amplifiers: What, Why, and How - Part 2, describes the basic architecture of a Class D amplifier and explains how they are much more efficient with regards to power dissipation. They mention how the output efficiency effects system design and that products with less efficient amplifier topologies sometimes use large, heavy heat sinks or fans to avoid over heating the components.
In the third article, Class D Audio Amplifiers: What, Why, and How - Part 3, the author talks about Class D amplifier terminology and compares single ended to differential designs.
The fourth article, Class D Audio Amplifiers: What, Why, and How - Part 4, describes some of the challenges with Class D technology and discusses techniques for improving sound quality by using feedback circuits.
In the fifth installment, Class D Audio Amplifiers: What, Why, and How - Part 5, he discusses the modulation techniques used to convert the analog signal into pulses. The most common method used is pulse-width modulation (PWM), which is why you may find Class D amplifiers sometimes referred to as PWM amplifiers. The PWM modulators may have problems with distortion in some implementations and can generate EMI (electromagnetic interference). The author also explains that with PWM, pulse widths become very small near full modulation, and because of the limited drive capability of the output-stage gate-driver, which cannot switch fast enough to produce the very short pulses, it is almost impossible to get full modulation. This means you cannot achieve full theoretical power. Alternatives to PWM include pulse-density modulation or PDM, which has an advantage when it comes to EMI, but has limited power efficiency. Another alternative, uses a self oscillating technique instead of the fixed frequency modulator used with PWM. This is the method used with the Hypex amplifers that we included in our prototype system. The self oscillating amplifier avoids some of the problems associated with PWM.
The sixth article, Class D Audio Amplifiers: What, Why, and How - Part 6, mentions that the high-frequency components of a Class D amplifier can generate a lot of EMI, which can interfere with the operation of other nearby equipment. The author then discusses several techniques used to minimize EMI.
The seventh and final article in the series, Class D Audio Amplifiers: What, Why, and How - Part 7, talks about the cost savings associated with audio systems that use Class D amplifiers. These costs savings are due to the higher efficiency of the Class D amplifers. Since they don't generate nearly as much heat, the systems don't need the large expensive heat sinks and chassis. However, the advanced circuitry for feedback loops and filtering adds to the expense. So in reality, using Class D amplifiers doesn't necessarily mean they will be less expensive than a system that uses Class A/B amplifiers.