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DISTORTION MEASUREMENT IN AUDIO AMPLIFIERS

DISTORTION MEASUREMENT IN AUDIO AMPLIFIERS,Michael Renardson

DISTORTION MEASUREMENT IN AUDIO AMPLIFIERS  
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SUMMARY The requirements for the measurement of non-linear distortion in audio power amplifiers are examined. The limitations of conventional measurement methods are discussed and several improved test methods described in which complex test signals could be used. A detailed examination is made of one method in which the input and output signals of an amplifier are combined in such a way that the undistorted component of the output signal is cancelled by the input signal and the distortion component isolated. The existing literature concerning this method is surveyed. The sources of error when using this technique are examined. These include phase and gain errors at high and low frequencies, earth connection arrangements and the effects of complex loads. Methods of reducing the errors are explained and a practical measuring instrument circuit designed. The instrument has a differential input so that inverting, non-inverting or differential amplifiers can be tested and uses a simple adjustable second order high frequency phase and gain compensation network. The distortion and noise of the instrument are analysed. The practical performance of the instrument is evaluated and its distortion contribution shown to be of an extremely low value. The rejection of test signal distortion is calculated for a particular amplifier test and shown to be more than adequate even when measuring extremely low harmonic distortion. The effectiveness of the load effect compensation arrangement derived is demonstrated. Finally some of the uses of the instrument are illustrated in tests on a typical class-B power amplifier to detect crossover distortion, transient intermodulation distortion and phase modulation.
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