Class D Audio Amplifiers – another form of class C without the Morse code.
Simply put a class D device is a fast acting switching circuit – either on or off. Think of class C RF equipment – Morse code transmissions were all more or less class C amplified – when there was no RF applied to the power amplifier the output devices were biased off. Morse code transmissions were only on or off – a dot one third the length of a dash. Simple.
Along came first generation audio amplifiers – what was recorded had to be faithfully reproduced, analogously. Class A amplifiers do this remarkably but at a cost. Class A amplifiers are at maximum 50% efficient – the incoming signal goes through at worst various stages of phase inversion but the end result in a properly designed class A circuit is – sine wave in – sine wave out. A photocopy. Class B amplifiers draw current as soon as there is a signal – not very good for distortion so through a simple modification the designer sets a small idle or quiescent current to the output devices – changing the class to AB. AB amplifiers are noted for their maximum 78% efficiency – they idle at a few milli-amperes (bias current of output devices) and as a signal is applied the amplifier draws more and more current depending on input amplitude and load resistance until the onset of distortion. Yes, there are lots of other criteria involved here – but the biggest drawback is the switchover between output devices in push pull configuration causing crossover distortion – most noticeable at lower output levels. Poorly designed amplifiers often sound very harsh at low outputs. Fuzzy. And no, they aren’t all like this – most amplifiers nowadays running in AB class are designed using perfectly matched output devices and idle current is optimized for low input/output signals. Class AB amplifiers are also categorized AB1, AB2 etc. Single Sideband transmission uses class AB power amplification – this is again all to do with the distortion produced and possibly in very poorly designed circuits – loss of intelligence. SSB transmitters are very expensive because they don’t normally get designed by monkeys. Likewise the first class D amplifiers and modern design.
Class D amplifiers and EMI
Class D amplifiers, if not designed and built properly will sound ghastly and more importantly will have the authorities tapping on your door because of the radio interference (EMI). But because they don’t draw current unless it’s really needed as defined by the audio signal are great for the Greenpeace Movement. How exactly do they work?