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As is the case with loudspeakers, there are countless models available, using different technologies and with different strengths and compromises. And just like loudspeakers, the models that really excel are few and far between. This discussion is limited to two channel hi-fi, and by necessity has to be somewhat of a generalisation. To every rule however, there is at least one exception.
POWER AMPLIFIER SPECIFICS
Power amplifiers fall into two main groups, valve (tube) and transistor (solid state). Within these groups are a good many subdivisions. While much debate occurs on the relative merits of each type and variant, the design competence of each individual amplifier is the major issue. Some amplifiers use valves for the input stages and transistors for the output or visa-versa, in an attempt to get the best of both worlds, and each must be judged on its own merits.
These are generally defined as Class A and Class B, with Class B being the most common. With Class B, the output transistors are run with a small standing current to overcome the worst of the distortion as they are turned on and off by the signal. Any further reduction in distortion relies on the overall negative feedback, or possibly local feedback or error cancelling circuitry. A poorly designed Class B amplifier can generate large amounts of odd order distortion, especially 3rd order- the so called transistor sound? The output devices may be either bipolar transistors or MOSFETS, with debate on which sound best. The reality should probably be that it’s how they are used in the overall circuit context.
In a Class A amplifier, the output devices are run at a high standing current all the time, to reduce distortion. Such amplifiers are large, heavy and expensive and generate large amounts of heat. Evidence suggests that such measures are unnecessary given a very well designed and executed Class B design.
Unlike transistors, valves are high voltage/ low current devices that mean a transformer is needed to step the voltage down and increase the current for driving a low impedance loudspeaker. The transformer is critical to the performance and sound quality of the amplifier, adding much to the weight and cost of such an amplifier. The primary inductance of the transformer determines the low frequency performance, while stray capacitance governs how well it behaves at high frequencies.
Valves are lower gain/ lower distortion devices than transistors, so lower levels of overall negative feedback are generally needed, just as well, because it would be limited at high frequencies by the phase shift of the transformer, causing instability.
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