operate the amplifier at high frequencies, a particularly bad problem with some quasi- complementary designs. In a class A output stage, however, there are no …

In spite of their high cost and low efficiency, class A power amplifiers have recently been receiving more attention from audiophiles who demand uncompromising accuracy. Both the price and quality of these amplifiers result from the operation of their output stages in class A mode, where the amplifying devices are constantly operated in their linear region, above cutoff and below saturation. Whether made from tubes or semiconductors, circuits operating in class A mode yield the smoothest transfer functions and widest bandwidths, hence their near universal application in preamplifiers and other low power circuitry.

Most audio power amplifiers use class A circuitry except in the drive and output stages, where they use class B or AB operating modes to achieve high efficiency. In class B and AB modes, the output stage operates in a push-pull configuration, where one set of output devices delivers positive voltage and current and another set delivers negative voltage and current. When one set is working, the other set is turned off. This scheme operates efficiently, but has two serious flaws, the extremely nonlinear characteristic of the transistors at the collector cutoff region and the turn-on/turn-off times of the devices. Designers of transistor amplifiers have tended to use large amounts of negative feedback to correct for the nonlinearities, but this works well only at low frequencies. At high frequencies, the feedback loop is unable to make adequate corrections, and the distortion that occurs at the output is aggravated by overloaded front-end circuitry.

The usual total harmonic and intermodulation distortion figures do not reveal the abrupt output stage distortions accurately because of the averaging factor involved in such measurements. A spike of crossover distortion may reach 2 per cent, but if it occurs only over 5 per cent of the waveform, it averages out to a respectable 0.1 per cent distortion figure. Considering this error factor, it is easy to see why two amplifiers with the same specifications can sound so different. To properly evaluate the distortion, peak distortion and armonic distribution must be considered…

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