Class D Amplifiers – Back to School

The EA iAMPs are all “Class D” amplifiers. Class D amplifiers are highly efficient and have a great power to size ratio. EA’s Class D amps are so efficient in fact (85%) that the need no cooling fan (a source of noise and weight).

In the early days of the iAMP ® 800 we put a thermometer on one during a 4 hour bar gig. The amp was pushing a hefty load (2 2x12 cabinets). The iAMP ®800 never went past 117º Fahrenheit. The question that you’re probably asking is, “It Class D is lightweight, efficient AND powerful, why doesn’t everybody make one?” There are many answers. Some companies rely heavily on tubes as a source to generate power. Perhaps the most important reason is that for very high quality sound reproduction (a hallmark of EA), Class D is hard to do right.

Other Class D designs suffered from a “brittle” midrange sound. One that worked well for upright bass players but lacked certain fullness in the other frequencies that electric bass players, keyboard, and acoustic guitar players demand.

EA’s solution to this was to compromise a bit of weight for much better sound. EA uses a toroidal power transformer as opposed to a “switching” transformer as in other Class D designs. The net difference is that the EA iAMP ®800 weights 19 pounds as opposed to 10 pounds but has a full range sound that is second to none.

The bell has rung and it’s time for class, amplifier class that is. Here is a description and comparison between Class A, Class B, Class AB and Class D amplifier designs. Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode while Class D amplifiers operate their outputs in 'switch' mode.

Mode examples:

Linear mode:

Imagine that you are the amplifier's output device(s) and you must support a 10-pound iron weight (the speaker load). The most difficult method (linear mode) would be to hold the weight straight out in front of you. This would very roughly simulate the linear mode architecture. Your muscles would start to ache in a short amount of time. Think of this pain as the power dissipation in output transistors.

Switch mode:

In this example, you can support the weight in one of two positions. In the first position, you can hold the iron weight directly over your head with your elbows locked so that you're not really using very much effort to support the weight. In the second position, you would let the weight hang down by your side. This would also use very little effort from your muscles. If you held it directly over your head half of the time and by your side for the other half of the time, it's position would 'average' out to be the same as if you held it out straight in front of you like in the previous (linear mode) example. This would roughly simulate the switch mode of the Class D amplifier. You can see that with this method (switch mode), there would also be little pain (power dissipation) involved in supporting the weight.

CLASS 'A'

Many class A amplifiers use the same transistor(s) for both halves of the audio waveform. In this configuration, the output transistor(s) always has current flowing through it, even if it has no audio signal (the output transistors never 'turn off'). The current flowing through it is D.C. A pure class 'A' amplifier is very inefficient and generally runs very hot even when there is no audio output. The current flowing through the output transistor(s) (with no audio signal) may be as much as the current that will be driven through the speaker load at FULL audio output power. Many people believe that class 'A' amps sound better than other configurations. This may have been true at one time, but a well-designed amplifier won't have any “sound”.

CLASS 'B'

A true Class B amplifier is NOT generally used for audio, due in part that in a Class B amplifier, there is a small part of the waveform, which will be distorted. The distorted part of the waveform is called 'crossover' or 'notch' distortion. Remember that distortion is any unwanted variation in a signal (compared to the original signal).

CLASS 'AB'

As we said earlier, a Class A amplifier is very inefficient. We also said that a Class B amplifier will cause a signal to be distorted, which is not good in any audio amplifier. A Class AB amplifier is the best compromise. A Class AB amplifier is a Class B amplifier which has a small amount of "bias" current flowing through the output transistors at all times. This eliminates virtually all of the crossover distortion. The bias current is flowing because the output transistors are always conducting current (even without an audio signal). This differs from a pure Class A amplifier in the amount of current flow. A pure Class A amplifier has an enormous amount of current flowing through its output transistors with NO audio signal. A pure Class B amplifier has NO current flowing through its outputs with no input signal. A Class AB amplifier is much more efficient than the Class A but without the distortion of the Class B.

CLASS 'D'

We said that Class A amplifiers were VERY inefficient. Class AB amplifiers are also inefficient but are more efficient than Class A amplifiers. The reason that these amplifier configurations are inefficient is because there is a difference of potential (voltage) across the output transistors and current flowing through the output transistors. When you have voltage across the device and current flow through the device, there will be power dissipation in the form of heat. The power needed to produce this heat is wasted power. When there is (virtually) no voltage drop across a device (such as a large piece of wire or a transistor), there can be a significant amount of CURRENT flow through the device with (virtually) no power dissipation. This means that there is virtually no heat given off (highly efficient). The inverse is also true. If you have a significant amount of VOLTAGE across the device (transistor, wire...) but no current flow through the device, again, there will be no wasted power.