WHAT IS CLASS A ?
A true Class A amplifier consumes constant power, whatever the amplitude of the input signal. Whatever the efficiency of your loudspeakers or the impedance of your speakers. The current constantly consumed and required for this power flows through the amplifier components. In this case, transistors. By default, we set the operating point of these transistors at their comfort zone. At rest, i.e. with no signal, this voltage-current characteristic is found in the middle of the transistor characteristic curve, where the component feels at its best! A signal will cause current excursions around the transistor’s median operating zone. This is where the best use is made of the transistor’s amplifying qualities.
Why do we at Kora talk about real Class A ?
It’s not rare to see a class AB schematic, with a lot of quiescent current, referred to as a class A. In fact, this compromise between class A and class B has a rest point located between the two classes of amplification. Depending on the amplitude of the signal voltage, the device will behave in class A or class B. A true class A amplifier delivers a current. It will certainly be clipped if you ask too much of it. But it will never explode, even in the event of a short-circuit between the loudspeaker outputs.
In our case, we are operating in pure class A at 8 Ω. The device switches to class AB on 4 and 6 Ω, to always provide the right amplification.
Why use Class A ?
It is sometimes said that Class A is ‘the best class’ for power amplifiers. By best, we mean amplification quality. Without taking into account sizing, power consumption or anything else. This amplification quality, and therefore sound quality, can be explained by several characteristics :
– The current required for amplification is immediately available, as it is constantly flowing through the transistors. This means that the power supply is less stressed than usual. Even though most manufacturers, like us, guarantee the quality of power supply sizing.
– It operates in a perfectly linear way. The output signal is the perfect image of the input signal, to within one amplitude factor and one phase difference.
– It generates very little distortion, and has a very good signal-to-noise ratio.