Since Class D (“switching”) amplifiers can turn fully on or off, they can dramatically increase the mid and bass output power while minimizing the loss of power, making for a highly efficient amplifier. And since it eliminates the need for huge power transformers, banks of output transistors, and huge extruded aluminum heat-sinks, Class D amplifiers are remarkably compact and lightweight for the amount of power they produce.

The Direct Energy HD amplifier - A class D amplifier that is now responsible for the high efficiency sound amplification in the new SC series of home AV Receivers.

The Following References may give you a Fair idea of Class D Amplifiers, their Circuitry etc.

1.  Class-D amplifier

From Wikipedia, the free encyclopedia

        A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, instead of as linear gain devices as in other amplifiers. The signal to be amplified is a train of constant amplitude pulses, so the active devices switch rapidly back and forth between a fully conductive and nonconductive state. The analog signal to be amplified is converted to a series of pulses by pulse width modulation, pulse density modulation or other method before being applied to the amplifier. After amplification, the output pulse train can be converted back to an analog signal by passing through a passive low pass filter consisting of inductors and capacitors. The major advantage of a class-D amplifier is that it can be more efficient than analog amplifiers, with less power dissipated as heat in the active devices.

Output stages such as those used in pulse generators are examples of class D amplifiers. However, the term mostly applies to power amplifiers intended to reproduce signals with a bandwidth well below the switching frequency.

Link:

   http://en.wikipedia.org/wiki/Class-D_amplifier

2.   APPLICATION NOTE 3977 

     Class D Amplifiers: Fundamentals of Operation and Recent Developments

           A Class D amplifier's high efficiency makes it ideal for portable and compact high-power applications. Traditional Class D amplifiers require an external low pass filter to extract the audio signal from the pulse-width-modulated (PWM) output waveform. Many modern Class D amplifiers, however, utilize advanced modulation techniques that, in various applications, both eliminate the need for external filtering and reduce electromagnetic interference (EMI). Eliminating external filters not only reduces board-space requirements, but can also significantly reduce the cost of many portable/compact systems.

Link:

    http://www.maximintegrated.com/en/app-notes/index.mvp/id/3977

3.   Youtube Video on Class D Amplifier-Tutorial

     An explanation of how class D amplifiers work, and how to build your own 2x15W stereo amplifier.

Link:

      www.youtube.com/watch?v=O1UagNkcxi4

4.    Class D Direct Energy HD Amplifiers

Link:

    http://www.pioneerelectronics.com/PUSA/Home/AV-Receivers/Elite+Receivers/Class+D+Amps+and+Direct+Energy+HD+Amplifiers

5.  Class D Amplifiers – Not 'Just Audio'

   by Steve Somers

Today, audio is about more than dynamic range or Total Harmonic Distortion (THD). It's also about power... more of it for less. Class D audio systems have come into their own as a practical choice whether you need more audio power from a small system or more audio power from a small battery.

Link:

   http://www.extron.com/company/article.aspx?id=ts122001

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