The class D amplifier is said to transcend the conventional hi-fi amplifiers. Do not confuse the letter D to stand for digital because it does not. It simply designates the next class of amplifiers after C. They are not a true digital amplifier and only act digitally in the fact that the output drivers are either fully on or fully off. They were created in response to the development of digital music and consumer demand. The class D amplifier is the ideal type for music that is listened to in the 21st century. Some call the class D amplifier a switching amplifier or a PWM amplifier. There are two categories of the class D amplifier. The first is analog controlled and has an analogue input signal. The second category is the digitally controlled class D amplifier which digitally switches the power on or off. The digitally controlled class D amplifier is controlled by digital circuits but the power stage involves voltage and current. The switches on the class D amplifier are either fully on or off. This means they are more efficient and will not lose power to output devices.
The input signal on a class D amplifier is converted to a sequence of higher voltage output pulses. The output pulses are in proportion to the amplitude of the input signal. Actually, they are at least 10 times higher than the highest frequency of the input signal. A low-pass LC filter is used to remove the incorrect sounds coming from the output pulses and restore the signal shape on the load. This makes the output signal an amplified replica of the input signal. Pulse frequency modulation is also used. There are more advanced forms of modulations that are used now as well, such as in AD1990 analog device class D amplifiers. Signals with a bandwidth lower than switching frequency need a class D amplifier. The power efficiency on these amplifiers is 100%. The output is a square wave on a class D amplifier. The digitally controlled amplifier produces a distortion called quantization error. This error is caused when the input signal is converted to a digital value. This is why it needs the low-pass LC filter which can correct the errors. These amplifiers are smaller and more compact in size than a class AB amplifier.
High quality Class D amplifiers are now being produced and are on the market for consumers. The earliest Class D amplifiers were mainly used in car stereo systems as subwoofer amplifiers. They are much less expensive when compared to a class AB amplifier. The class D subwoofer amplifier costs less than $300 and is 80 to 95% efficient, where as a Class AB amplifier can cost several thousands of dollars. A subwoofer switching feed does not have to be as fast since the bandwidth is limited to 150 Hz. However, the output filter on them lowers the dampening factor on the amplifier.
The class D amplifier is intended for use when you need more audio power from a small system or from a small battery. Audio signals can now be amplified using a small component at nominal voltages. Fixed, high frequency carriers with pulses varying in width based on signal amplitude are using class D amplifiers. Portable applications need high efficiency power usage which you can get with they class of amplifier. In fact, a portable class D battery powered audio device can have a battery life of up to 2.5 times longer. You can find these portable devices in electronic stores and various vendors online. For your best listening enjoyment take advantage of these devices with the class D amplifiers in them.
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