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What are Class D Amplifiers? A class-D amplifier is a switching amplifier where the amplifying transistors operate as electronic switches rather than as simple linear voltage-controlled devices as in other Class-D amplifiers. The output from Class-D amplifiers is on the large side and their design is such that they can handle large audio loads. This allows them to be placed in live music venues.
There are three terminals on a Class-D amplifier; ground, input signal and output power. Ground is the center ground reference voltage, whilst the + and - controls are designed to set the input signal level and the power supply voltage. It is recommended that one amp be run on a THOROUGHLY grounded system. If one amp is grounded and the other is connected to the + and - controls by touching the + and - terminals of the separate amps, then each amp will only operate at its +12 volt reference.
Class-D amplifiers have three circuits: An AC controller, power amplifier and a high/low signal chopper. The controller, also called a PLL, does not change the power level but controls the operation of the different amp models through the control circuit. The power amplifier controls the transfer of the AC signal to the speaker and makes sure it is strong enough to drive the speaker. Finally there is the high/low chopper, which enables the amplifier to select a high or low frequency for the input signal.
The operation of a Class-D amplifier can be separated into the following types: Full-bridge, Half-bridge and Non-finite bridge. A full-bridge Class-D amplifier divides the input signal into two separate channels. A Half-bridge Class-D amplifier divides the input signal in half. A non-finite bridge Class-D amplifier does not have any sort of crossover. It separates the output power into two different channels, namely the +12 volt and the ground.
Class-D amplifiers use two different power supplies. The +12 V output signal is connected to the positive side, while the ground is connected to the negative. One major advantage of using two separate power supplies is that it lowers the requirement of an amplifier overload. However, a disadvantage is that it makes the audio signal weaker, as the high frequencies are not produced as bright as they would be with one single source, resulting in less gain.
RF Class-D amplifiers operate at higher frequency. Their advantage over Class-B and AB models is that RF Frequencies can travel longer distances than their lower frequencies. Some manufacturers use RF Frequencies in their Class-D circuits, however, some manufacturers do not use RF technology in their Class-D amplifiers at all. There are advantages and disadvantages for both RF and Class-B models, however.
RF Class-D amplifiers can handle high power output, but their efficiency is quite poor. This is because RF circuits often generate radio frequency energy that is stronger than the electrical signal. A lot of heat is also generated, resulting in less efficient performance. On the other hand, RF Class-Ds are not very efficient when it comes to switching power. RF Class-Ds are usually used in systems that use multiple RF sources. They are therefore unsuitable for the design of home audio video systems.
The final difference between a Class-D and a Class-B amp is the level of distortion. In a DC based amplifier, the total harmonic distortion total will be lower with a Class-D compared to a Class-B amplifier. This is because the RF circuits of a Class-D amplifier produce lower frequency distortion. The distortion results are thus less because there are less components that are working at high power states. The heat generation also comes into account as most Class-Ds have a cooling fan that keeps the internal temperature low.