What does "Class D" mean
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A class "D" amp is what's called a "digital" amp, although that isn't really all that accurate.
A class "A" amp is the simplest (and best sounding) amp you can use, but it's very inefficient. With a class "A" amp, the amplifier produces the whole sine wave. Because there's always current passing through a FET even when it's not amplifying anything, a class A amp tends to suck a lot of power and get hot even when you're not using it. It's inefficient because there's always a voltage potential across the FETs, and there's always a current flow. When you have voltage and current present, you have power consumption. In this case, it's wasted as heat.
A class "B" amp basically uses 2 FETs instead of 1 like a Class A uses. With a class B amp, each FET amplifies half of the sine wave, one for the positive half and one for the negative half. This makes the amp more efficient because you can shut the FET off while you're not using it. The problem with this amp is that because a FET has a .7 volt turn-on window, you get a distorted sine wave every time the amp switches FETs. A simple capactior circuit can smooth it out, but it never gets 100% perfect. With this amp, because the rail voltage is 1/2 as big on each FET compared to a class A amp (because each FET only does 1/2 the sine wave, the voltage can be halved for a given output), you have less wasted power.
Most car audio amps are class AB. This type of amp is basically a class B amp, but they bias the FETs so that there's a little current flow at all times. This eliminates most of the crossover distortion, and it behaves like a class A amp at low power levels.
Anyway... on to class D. It's called class D (or digital) because the transistors are either on or off... there's no inbetween; sorta like a digital signal vs. an analog. The way this works is that the amp switches the FETs on and off at a high rate of speed. This results in a square-wave type of output, but when it's filtered it represents a regular sine wave. To produce more power, the amp just stays switched on longer. This means that when the transistors are conducting (switched on) there is basically no voltage across the transistor and when there is full voltage across the transistor (switched off), there is no current flowing through the transistor. This is basically how a switching power supply works too. Anyway, what you wind up with is a choppy representation of the sine wave that gets turned on and off a bunch of times, but once you filter it, you more or less get the intended sine wave. These amps are extremely efficient, but due to their switching nature, they can only be used for low frequency reproduction. They don't switch fast enough to be able to cleanly reproduce high frequency signals. But, since bass is what takes all the power, it's not really necessary.
Hopefully I didn't confuse the hell out of you. If you don't understand how a switching power supply works, you're probably not going to grasp the Class D idea. But the long and the short of it is that the class D amp is a very, very efficient amp that doesn't have good enough fidelity to operate anything except subwoofers.
A class "A" amp is the simplest (and best sounding) amp you can use, but it's very inefficient. With a class "A" amp, the amplifier produces the whole sine wave. Because there's always current passing through a FET even when it's not amplifying anything, a class A amp tends to suck a lot of power and get hot even when you're not using it. It's inefficient because there's always a voltage potential across the FETs, and there's always a current flow. When you have voltage and current present, you have power consumption. In this case, it's wasted as heat.
A class "B" amp basically uses 2 FETs instead of 1 like a Class A uses. With a class B amp, each FET amplifies half of the sine wave, one for the positive half and one for the negative half. This makes the amp more efficient because you can shut the FET off while you're not using it. The problem with this amp is that because a FET has a .7 volt turn-on window, you get a distorted sine wave every time the amp switches FETs. A simple capactior circuit can smooth it out, but it never gets 100% perfect. With this amp, because the rail voltage is 1/2 as big on each FET compared to a class A amp (because each FET only does 1/2 the sine wave, the voltage can be halved for a given output), you have less wasted power.
Most car audio amps are class AB. This type of amp is basically a class B amp, but they bias the FETs so that there's a little current flow at all times. This eliminates most of the crossover distortion, and it behaves like a class A amp at low power levels.
Anyway... on to class D. It's called class D (or digital) because the transistors are either on or off... there's no inbetween; sorta like a digital signal vs. an analog. The way this works is that the amp switches the FETs on and off at a high rate of speed. This results in a square-wave type of output, but when it's filtered it represents a regular sine wave. To produce more power, the amp just stays switched on longer. This means that when the transistors are conducting (switched on) there is basically no voltage across the transistor and when there is full voltage across the transistor (switched off), there is no current flowing through the transistor. This is basically how a switching power supply works too. Anyway, what you wind up with is a choppy representation of the sine wave that gets turned on and off a bunch of times, but once you filter it, you more or less get the intended sine wave. These amps are extremely efficient, but due to their switching nature, they can only be used for low frequency reproduction. They don't switch fast enough to be able to cleanly reproduce high frequency signals. But, since bass is what takes all the power, it's not really necessary.
Hopefully I didn't confuse the hell out of you. If you don't understand how a switching power supply works, you're probably not going to grasp the Class D idea. But the long and the short of it is that the class D amp is a very, very efficient amp that doesn't have good enough fidelity to operate anything except subwoofers.
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