A New Method of Testing for Distortion in Audio-Frequency Amplifiers

A periodic voltage wave consisting of a series of straight lines is distorted into a series of curves when it passes through an amplifier which gives nonuniform amplification. As such distortion can very readily be detected visually with an oscillograph, it affords a means of testing for uniformity of amplification. Mathematical analysis shows that a "saw-tooth" voltage wave is distorted into a series of exponential curves in passing through a resistance-capacity-coupled amplifier, the distortion being considerable at low audio frequencies if the coupling capacity or the grid-leak resistance is too small. The analysis proves that a falling off in amplification of less than 1/2 per cent at low audio frequencies can be detected with ease. In other types of amplifiers the distortion is not necessarily exponential, but curvature of the output wave is in general a sign of nonuniform amplification or phase shift. The method is applicable to the testing of any type of coupling circuit. The apparatus required for this method of testing is very readily constructed and is of general laboratory usefulness.