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D-Class Amplifiers: Small Size, Lower Power, Big Sound

Tech advances coming from consumer audio By Gordon Moore

Some industry experts have said that in the coming years, technical advances will be coming to us not from the professional world but more from the consumer world. In effect, the early adopters (in their higher numbers in consumer products) will help fund the research and development that used to lie in the realm of professional products. The phenomenon holds particularly true in the field of audio. Case in point: D-Class amplifiers.

Every audio system, whether that found on a laptop computer, or in a full blown cinema or anything in between, relies ultimately on amplifiers to deliver the audio to the user. For the typical AV system, the amplifiers have long presented some design problems and issues. The first has been inefficiency. Class A or A/B amplifiers typically delivered required power levels only by using a great deal more power. To generate a sine wave of 25 watts, the amplifier would consume 50 watts. Where does this extra power go? Literally as heat. So, in addition to adding considerable load to the electrical system for a fairly low return, the amplifier also serves as a heat source in the rack, warming up every other component in the system. Add to this the coming dominance of DSP processors in the rack with their attendant higher power consumption and heat generation. So, now you have to take precious rack space up with in rack fans and their increased power load. Perhaps the heat load becomes sufficient now to require a cooling duct from the HVAC system into the equipment room to prevent thermal overload. It can become a cascading problem sometimes overlooked by the system designer.

Now consider also that surround sound systems also require larger bass amplifiers which require much higher power levels than those needed for speech frequency response. The human ear perceives bass levels with much lower sensitivity. For a typical listener to say they hear 40Hz equal in level to 2KHz, you have to design a system that delivers at least 40dB more power for that 40Hz signal. In other words, if you only need 1 watt for the required level at 2KHz, to deliver "equal volume" at 40Hz, the amplifier will have to put out 200 watts. In short, to deliver that heavy bass beat in an AV installation, you will have to have a relatively powerful amplifier dedicated to that subwoofer. For example, in my home theater system, my front and rear amplifier channels probably idle along at less than 1 watt in each channel for about 75dB level. But I have a 250W subwoofer.

Here's where the consumer world has driven audio amplifier development. Modern auto sound systems can now deliver incredible levels of power without overloading the car frame with massive and oversized amplifiers. Small portable DVD players offer great headphone audio while running a long time on batteries. How do they do it? The D-class amplifier.

The D designator is sometimes incorrectly referred to as standing for Digital. This is incorrect. Basically the D designator is just that, a designator for fourth in the chain. The design does have a "digital" component to it however but they are not considered digital devices and typically have analog inputs.

The D class amplifier uses Pulse Wide Modulation (PWM sometimes also called pulse duration modulation). The analog signal is modulated (ie combined) with a very high frequency signal or carrier wave in a sawtooth wave form. Philips Semiconductors, for example, uses a 360KHz waveform for their line of D-class components, well above the human hearing response.

The resultant signal is a digital form that maintains the same frequency of the sawtooth waveform with elements of the original analog signal. This digital signal is amplified (in the digital realm) then remixed at the output stage (at the higher amplitude) for restoration of the original signal at higher levels. The design MUST include a low pass filter at both ends of the output stage to filter out the high frequency sawtooth waveform. If that wasn't filtered out, we would not hear it but the poor speaker would either be working its tail off to generate something we can't hear OR (more likely), it would be generating a lot of heat and failing prematurely.

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Related Keywords:Class A, D-class amplifiers, audio, power load, subwoofer, surround sound, home theater, technical advances, early adopters, research and development

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