New memberUsername: Flyfishin4trout
Park City, UT USA
Post Number: 10
What youre speaking of is Class D amp.
They call em "digital" because they resemble
Class D amps are more efficient(about 20%) than a/b amps,
and can run more current without heating up.
Class D amps are only good for subwoofers.
Gold MemberUsername: Glasswolf
NorthWest, Michigan USA
Post Number: 5887
Class A amplifiers are the most sonically accurate. On the other hand, they have some drawbacks that make them not be the most common choice. Class A amplifiers use only one output transistor that is turned "on" all the time, giving out tremendous amounts of heat. Class A amplifiers are very inefficient (~25%). More heat means more heatsink area, so even though most class A amps have built-in cooling fans, they are big. Pure class A amplifiers are usually expensive.
Class B amplifiers use two output transistors. One for the positive part of the cycle and one for the negative part of the cycle. Both signals are then "combined". The problem with this design is that at the point when one transistor stops amplifying and the other one kicks in (zero volt line), there is always a small distortion on the signal, called "crossover distortion." Good amplifier designs make this crossover distortion very minimal. Since each transistor is "on" only half of the time, then the amplifier does not get as hot as a class A, yielding to a smaller size and better efficiency (~50%).
Class A/B amplifiers are a combination of the two types described above. At lower volumes, the amplifier works in class A mode. At higher volumes, the amplifier switches to class B operation. These are most common for multi-channel full-range mobile audio amplifiers.
The class D amplifier (known as digital amplifier) is the last of the solid-state types. These amplifiers are not really digital (there is no such thing), but operate similarly in manner to a digital-to-analog converter (DAC). The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifier produces almost no heat and is very small in size. Efficiency is much higher in class D amplifiers (~80%).
The sound quality of a Class-D amplifier is much lower than that of other solid-state amplifiers, which is why Class-D amplifiers are only used for subwoofers in car audio. This is because the switching speed of the transistors, and lower sound quality are masked by the lower frequencies being produced by the subs, since distortion is harder to discern at low frequency.
As technology improves, Class'T (tri-path) amplifiers are becoming mroe common. These are full-range class-D amplifiers, with performance acceptable for operation at greater than low frequency range. While class T amplifiers still can't compete with a class-AB amp for sound quality and accuracy, they are an affordable solution for high power, full range applications where heat, current draw, or footprint size are a concern.
Lastly we have tube amplifiers, which aren't often used in car audio. Tube amplifiers have about 50 to 60% efficiency.
Tube amplifiers are said to sound more musical. The reason is that tube amplifiers produce even ordered harmonics. Musical instruments give off harmonics in even orders. Transistor amplifiers tend to give off harmonics that are odd ordered. These odd-ordered harmonics are not as pleasing to the ear as second order harmonics are. Modern solid state amplifiers have very low distortions but their distortions are less tolerated by the ear than even ordered harmonics. This means that when you hear someone say a Tube amp is "warm" sounding, they are actually talking about the second order distortion produced by that tube amplifier, which they find pleasing to the ear. A good example of this is in guitar amplifiers, which often pride themselves on their second order harmonics.
One should note that while most solid state amplifiers have very low distortions (Total Harmonic Distortion) for the left and right channel, other channels are often much higher as these specifications are rarely noted. Subwoofer amplifiers are particularly bad at creating odd ordered harmonics.
I believe that the best tube and solid state amplifiers sound amazingly alike. Bad tube amplifiers sound tubby and slow. Bad transistor amplifiers sound harsh, bright and strident.
Just like you can't judge a good book by its cover, you can learn very little about an amplifier without digging in and seeing what is inside. Generally speaking, the most important component of any amplifier is its power supply. Is it sufficient? Is it accurate? Is it fast? Unfortunately, almost no amplifier company talks about their power supplies or what transformers they use (An example of a good company would be Eclipse, who uses dual toroidal transformers in their amplifier power supplies.)
I think most manufacturers would prefer you not ask.
Bronze MemberUsername: Flyfishin4trout
Park City, UT USA
Post Number: 13