Assuming that the audible character of valve amplifying equipment results from a combination of its frequency response and linearity, it should be possible to associate this character with results of straightforward measurements. The linearity and frequency response of an amplifier may be visually summarized in a set of surface plots with frequency and level as the independent variables, and various harmonics as the dependent (z-axis) variables. A bus-controlled measurement system has been developed to carry out measurements and give such gain-derivative surfaces. Sets of surface plots, for example solid-state and valve amplifiers, are presented. There is some hope that a solid-state amplifier design may be realized whose sound is characteristic of that of valve designs.
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