I'm Ben from Cambridge Audio.
Today, we're going to talk about Class AB amplification.
An amplifier's job is to take a weak audio signal from a music player and boost it to generate a signal that’s powerful enough to drive a speaker.
There are usually two parts to an amplifier: the part that controls volume and input selection the pre-amplifier
and the part that drives your speakers the power amplifier.
In an ‘integrated’ amplifier, the preamplifier and power amplifier sections are combined in one box or ‘chassis’.
A power amplifier is essentially a ‘push/pull’ device. It creates the backwards/forwards signal, or waveform, that’s needed to drive the speakers.
In Class B and Class AB amplifiers, this backwards/forwards waveform is created by a pair of output transistors
one in the positive domain and one in the negative.
These positive and negative transistors take turns to switch on and off
meaning that the current flows from one to the other, from positive to negative, and back again as the transistors continue to switch on and off.
The problem with a Class B amplifier is that each time the current is switched from positive to negative, it is reduced to ‘zero’ before crossing over.
This is known as the crossover point.
At this point, both of the output transistors are in their off positions, before one of them switches back on to ‘receive’ the current from the other side.
At this split-second moment in time, when both transistors are off, and there is no current travelling through either of the transistors
audio distortion is created.
This is known as crossover distortion.
Class B amplifiers exhibit large amounts of crossover distortion, which has a negative effect on the overall sound quality of the amplifier.
To solve this problem, a Class AB amplifier does things slightly differently.
Each transistor starts to switch on before the last one has fully switched off
This creates an overlap of electrical current, known as quiescent current.
Unlike Class B, this means that electrical current is always travelling through at least one of the transistors
keeping crossover distortion to a minimum because the current is never reduced to zero.
Thanks to this much lower distortion, Class AB amplifiers sound much better than Class B amplifiers.
A simple analogy is to imagine playing with a ball.
My hands are the positive and negative output transistors and the ball is the current.
With Class B, I throw the ball, or current, from one hand to the other.
As I throw and let go of the ball, it travels through the air and is not being touched by either hand, or transistor.
With Class AB, instead of throwing the ball, I pass it.
Because the ball is always being touched by one hand and is never fully let go of, it is never out of control.
Some amplifiers use more than one pair of output transistors, running in parallel to create more power
but the principle of current passing from one to the other is exactly the same.
The CX Series amplifiers use a single pair of output transistors.
The 851A uses two pairs of output transistors, running in parallel for more power.
The 851W uses four pairs of output transistors, running in parallel for a huge power output!
Because of its lower distortion and superior sound, Cambridge Audio amplifiers all use Class AB amplification.
The only exception to this is the 8 Series amplifiers.
Here we use our proprietary Class XD amplification which is a further improvement on Class AB.