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What are the problems with traditional Class-A and A/B amplifiers?


Traditional linear amplifiers such as Class-A and Class-A/B amplifiers are bulky and inefficient. The inefficiency compromises the reproduction of music signal's full dynamic range. Its resulting higher operating temperature also shortens the useful life of the electrolytic capacitors used in abundance in these amplifiers. To get around that problem, today's better amplifiers employ bulky heatsinks and costly linear power supplies to provide enough headroom to handle the full dynamics. These huge power supplies are unregulated and could add noise and ripples at low volume. Besides being inefficient, linear amplifier depends on transistors or MOSFET devices to generate power. Big (high-power) bipolar transistors or MOSFETs have inherently low bandwidth and do not provide adequate audio performance. Therefore, smaller (up to 20+) MOSFETs with decent audio bandwidth performance are paralleled to provide sufficient power. Each MOSFET has an inherent junction noise - actually worse audio low frequency noise than bipolar transistors - and the aggregated noise corrupts music reproduction. What you hear is haziness and a lack of clarity in music reproduction. MOSFETs are used in parallel because technically, they are easier to drive although they have inherently higher distortion than bipolar transistors, which are much harder to drive when they are paralleled. Class-AB amplifiers - the most popular amplifier circuit - have to overcome the inherent crossover distortion that occurs when the audio signal goes from negative to positive and vice-versa, crossing the zero region where gains of transistors are much reduced. They are actually down to zero when the transistors stop conducting current. Close-loop system designers know that lower gain means higher inaccuracy of the amplification loop.

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