The TF 2337A Automatic Distortion Meter enables rapid and accurate measurement of both level and distortion,factors of audio frequency signals. The need to tune tor fundamental frequency rejection is avoided, thus operation simply consists of the selection by push button switches of the input voltage range, distortion range and fundamental frequency. The instrument measures distortion in the conventional way, that is, by filtering out the fundamental component and comparing the residual with that of the total signal. However, because of the novel design of the ratio circuit the need for reference level adjustment prior to distortion measurement is obviated. Thus fluctuations of the input level (up to -10 dB) will not affect the accuracy of the distortion measurement. The TF 2337A is therefore suitable for repetitive distortion measurements and because of its simplicity of operation is ideal for factory testing of mass production units such as a. f. amplifiers etc. Furthermore as the instrument employs a multi-stage Twin-'Γ active filter having a fairly wide rejection band (66 dB at-+5% of fo), the affect of wow and flutter on the distortion measurement is negligible. This attribute is particularly useful for t. h. d. measurements of output signals on equipment such as tape recorders and record players. Owing to the use of field effect transistors in the input stage, internal noise is low and therefore with input signals as low. as 10 mV, distortion can be measured down to 0. 01%.