Sure, most of us know what the main settings do on an amplifier or how the watts translate into power – depending on it being valve or solid state, of course – but how many of you out there know about the different classes of amps, which are available, how it affects the way they’re used or what it all means?
First off, when we’re talking about amplifier classes, we really mean the circuits. They are classified as A, B, AB and C for analog designs and class D and E for switching designs that are based on the proportion of each conduit angle, or input cycle, during which an amplifying device is passing current.
In a class A circuit, the amplifier – whether valve or solid state – passes current regardless of the polarity of the input signal. Essentially, this means that in an audio application, the circuit is ‘biased’ in order to let both the positive and negative cycles of an audio signal pass through. The drawback with biasing is that the amplifier has to pass current at all times, making this design relatively inefficient.
Meanwhile, in a Class B circuit, the amplifier only passes current for one polarity of the signal (which polarity depends on the design of the circuit) which makes this class much more efficient compared to A. In this case – where it is required to pass a symmetrical signal using a Class B circuit – the circuit will need to active devices in order to handle each polarity. This type of construction is known also known as ‘push-pull.’
The Class C circuit format is one that you probably don’t need to worry about since it is rarely used in audio due to the fact that it only conducts on signal peaks. Class D on the other hand is becoming steadily more popular in audio applications and works by creating a stream of high voltage pulses at a very high frequency. These pulses are adjusted in such a way that the average energy they convey follows the wanted audio waveform.
Going back to Class-B, while this design is the most efficient, it suffers from a major problem as far as audio applications are concerned. This problem is known as crossover distortion which stems from both of the active devices in the push-pull pair’s propensity to turn off as the signal nears the zero line. In order to counteract this, you’d need to bias the devices so that they don’t turn off, exactly like in a Class A device more or less, only without the power inefficiency.
This is whereClassABcomes in; a Class B design biased to operate a little like a Class A one when it gets to the crossover region. In other words, it’s a push-pull amp design with each active device handling one polarity of the input signal. It should be noted that these type of design can be set to operate fully as Class A if required, which is how some high-powered amps work – not to mention this technique is also a great way to cancel out even-harmonic distortion noise in tube amp designs when necessary.