High Fidelity Amplifiers operate in what is known as Class A, Class AB, Class D, or Class H. In Class A, the bias (the amount of voltage applied to the grid in tubes – see discussion of tubes above – and to the corresponding controlling connector in transistors) is such that there is current flowing through the system whether there is an input signal (music) or not. This makes the tubes or transistors get hot, as the power not being used for music is dissipated as heat. In fact, a Class A amplifier at idle (no music signal) dissipates about 4 times the rated audio power as heat. So, if the amplifier is rated at 10 watts per channel of audio power, it dissipates 40 watts of heat per channel at idle. When a signal is applied, the power that was being dissipated as heat becomes audio power to the speakers. So, it runs hot when no music is playing, and cooler when you crank it up! The value of Class A amplification is that, because current is always flowing in the system, the musical signal is “instantly” diverted to the output, rather than current flow having to be turned on and off, so the response to the input signal is extremely fast, and a very clean sound results. Class AB amplifiers operate in Class A for a small portion of their potential power output, then transfer to Class B (where additional current has to be drawn) as higher power demands are made.

Class A amplification is not very efficient, and the quality of components has to be high, especially if it produces high power. If the amplifier operates in Class A all the way to the limits of its power output, and, if all stages of the amplifier are operating in Class A, then the amplifier is said to be Pure Class A. This is something to watch for in shopping for an amplifier, since some will advertise Class A operation, but do not operate in Class A throughout their output range, nor operate in Class A in all stages of their circuitry. Class AB is much more efficient, allowing Class A operation when soft passages in the music are being played, and then switching to the more efficient Class B operation when loud passages come along. The single ended triode tube amplifier mentioned above is usually a Pure Class A amplifier.

There are numerous solid state as well as tube Pure Class A amplifiers, and they are all quite expensive. The majority of separate component high fidelity amplifiers (power amplifier separate from the preamplifier) are Class AB. Many of the amplifiers (specifically, the integrated amplifiers, i.e., preamplifier and power amplifiers in one chassis) sold in large home electronics chain stores are likely to operate almost entirely in Class B. That does not mean that they sound bad, only that you don’t get any Class A, even at low volume. However, a few of the integrated units will be Class AB, and they will advertise that fact, because the sound quality is improved. They will say something to the effect of Class A operation, but actually meaning that only some of the power is in Class A. If you become interested in one of these amplifiers, including surround sound integrated packages, check to see how much of the power is devoted to Class A. Also, feel the top of the amplifier after it has been sitting with the power on for a while. It may be quite warm to the touch, and if you buy such a unit, you will need to be careful as to where you place it in your home. Better than average ventilation is important in this case.

Class D amplifiers are switching amplifiers. The + and – output transistors are switched on and off at varying frequencies, with the higher frequencies producing a larger voltage on the waveform (more volume). The + and – transistors are never on at the same time, but when they are switched on and off, they are completely on and off, rather than somewhere in between. Class D is extremely efficient, so almost no heat is produced. However, distortion at the crossover point (+ to -) can be introduced because of “dead time” which is caused by the fact that the + and – transistors cannot ever be on at the same time. Class D amplifiers are often used at concerts for musical groups because they put out lots of power but don’t weigh a lot. However, these models don’t make good amplifiers for home use because they have noisy fans, and they usually have 1/4″ phone jacks for inputs rather than RCA jacks, so adapters are needed. The switching frequency makes a lot of of difference to the high fidelity, and the best ones have a high switching frequency. These are the ones to consider in home audio systems. Class D amplifiers are also great for subwoofers, and there are many available with 500 watts of power and more.

Class H amplifiers have several rail voltages and depending on the intensity of the signal, the low rail voltage wil be used (low volume) or the high rail voltage (high volume). Because the output transistors always “see” the rail voltage, and have to dissipate the difference between the rail voltage and the voltage across the speakers multiplied by the current (which equals watts of heat), Class H is efficient and not much heat from the amplifier is generated. Its drawback is that there is finite time to switch between one rail voltage and the others, so in the mid volume level, there can be distortion when the rails are moving back and forth.