Marantz pm-4

Class A Amplifier

The most commonly used type of power amplifier configuration is the Class A Amplifier. The Class A amplifier is the most common and simplest form of power amplifier that uses the switching transistor in the standard common emitter circuit configuration as seen previously. The transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform producing minimum distortion and maximum amplitude to the output.

This means then that the Class A Amplifier configuration is the ideal operating mode, because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected together to share the high load current.

Thank you Sir .

If you can visualise a simple one note signal as a line that waves upwards and downwards, the waves being equally spaced (as this is one fixed note). There is a middle point in the form of the wave. Imagine this middle point as a line passing along and through the waves, with the top half of the wave as a hump above this line and the bottom half as a trough below the central line. The central line is called the 0db point as there is no wave energy there. Anything above the middle line has a + value in db (energy), anything below, a - value. These values will govern the amplitude (strength) of the wave (height & depth).

Like this complete wave:

http://i68.tinypic.com/1sxd2x.png

A Class B amplifier will use two transistors to create the wave. One transistor 'pushing' upwards to form the hump above the centre line and as the 'push' reduces, the wave declines back down to the centre line, then the signal switches to the other transistor which 'pushes' downwards to create the trough part.

Class A/B amps are very similar, but have a little 'energy overlap' above and below the 0db middle line in the power fed to the two transistors .

A Class A amplifier is a little trickier to describe, but picture another flat line along the the top of the wave hump. Bear in mind that anything above the line is a 'push' in that upwards direct, so your new line is 'pushing' upwards, so to speak. But, it also forms the new centre line for your Class A signal. The wave oscillating up and down above and below the new line. This wave is created around the new line by varying the amount of 'push' so that the signal wave is formed by the rising and falling energy. As all the signal energy is in one direction (above the 0db centre line) one transistor can deliver this continually as it is just being fed varying energy in one direction and there is no need for another transistor to switch to. The position of the new (second) line around which the Class A signal operates is governed by a potential called the DC offset, which basically pushes the waveform away from the 0db central point.

I've simplified a bit and I'm no electronics wiz, but I think I've got the basics here.

Well now this simpleton understands , kind of !!! Why is it better ? And what is best , Valve or class A ?

If you take a car analogy Class A is like running the engine at maximum revs all the time, no matter how slowly you are moving.

Maximum energy is available all the time (maximum output for an amplifier) but just enough for the required speed is used (volume setting on an amplifier).

In both cases efficiency is very low because energy is being consumed all the time, being dumped as heat when not 'used'.

:)

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Originally Posted by andyrlb

Well now this simpleton understands , kind of !!! Why is it better ? And what is best , Valve or class A ?

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No distinction here, both valves and solid state can be Class A. Or not.

Class A is often held to be better for it's purity of sound - the reason for this is that there is no switching distortion caused by the output devices handing the signal over to each other between the positive and negative parts of the signal as shown in the drawing earlier. In Class A the whole signal is handled by a single device (valve or transistor) so this switching distortion is eliminated.

Aha now I've got it , I always use car analogies myself , plus that'll explain the heat and slight smell !!

Cheers Ali

:thumbsup:

Quote:

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Originally Posted by andyrlb

Well now this simpleton understands , kind of !!! Why is it better ? And what is best , Valve or class A ?

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Class A avoids 'crossover distortion' or 'notch distortion'. In B Class and A/B Class amps this can occur at point where the pair of transistors that share the handling of the wave, switch from one to the other.

There's an article here: http://www.aikenamps.com/index.php/w...ver-distortion

In Class A there can be no chance of this as there is no switching between two devices.