However, in the real world, instantaneous on/off switching is a physical impossibility. The benefits of deploying transistors as two-state devices (‘on’ or ‘off’) are 1) higher energy efficiency and 2) lower heat dissipation which have led designers to create smaller, lighter and less costly amplification. This is often referred to as Pulse Width Modulation (or PWM). The wider the pulse, the louder the ‘on’ signal. And no, that D does not stand for ‘digital’, even though many Class D amplifiers use a digital audio signal’s pulses to turn the output transistors on and off at a very rapid rate. It typically isn’t as low as Class A amplifiers and is often treated with feedback.Ī Class D amplifier works in a very different way to Class A and Class A/B. It’s why many amplifiers available online and on the high street are Class A/B designs. The Class A bias theoretically eliminates any crossover distortion but with a significantly lower heat penalty than pure Class A operation. always on – and the Class B portion of the amplifier delivers more power whenever the music calls for it. The compromise is to inject some Class A into our Class B design where the first few Watts operate in Class A – i.e. It’s why we rarely, if ever, encounter Class B amplifiers. This is called crossover distortion and is (unpleasantly) audible. The downside of Class B is significant: any signal hand-off taking place early or late distorts the waveform. The ‘upper half’ transistors in a push-pull topology turn off as soon as they have handed the signal off to the ‘lower half’ transistors and then turn on again when the ‘lower half’ transistors hand the signal back, at which point they turn off. One way around Class A’s heat problem is to momentarily turn off the transistors when they are not in use. It’s why many high-power Class A amplifier owners complain of warmer listening rooms and higher energy bills. But more power calls for more transistors and more transistors mean more heat. Hello HEAT! In order to lower a Class A amplifier’s operating temperature, we must play music through it. A Class A amplifier will draw the same amount of power irrespective of whether or not it’s playing music. If it’s a push-pull design, one transistor will handle the upper half of the signal and a second will take care of the lower half.īut there is a significant drawback to always being on. If it’s a single-ended amplifier, a single transistor will handle both the upper half and the lower half of music’s ‘sine’ wave. That’s the state of the output transistors in a Class A amplifier.
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