Abstract
An analysis of the required sensitivity of electromyograph amplifiers is given in terms of the source impedance. Possible sources of noise in low-frequency amplifiers are discussed and estimates of their magnitudes made on the basis of previous published figures. An account is given of an experimental comparison between valve types, and an investigation of the effect of operating conditions; the noise depends on both the grid bias and the anode current. Spectral analyses are given of the noise from selected valves in the range 2 to 16 000 c/s. Persistent selection and good fortune have brought to light a pair of valves with a noise level lower than any previously reported in this frequency range (as far as the author is aware). Finally it is shown how, under certain conditions, the noise performance can be improved by means of an input transformer.