Electronic Equalizer

Abstract

A description of an electronic equalizer constructed in the Research Department of All India Radio is given in the paper. It consists of an audio amplifier whose frequency characteristics are widely variable at both high and low frequency ends resulting in a variety of equalization curves. By introducing suitable twin T networks in the negative feed-back loop of one of the stages, the requisite boosts at both high and low frequencies have been obtained with the added facility of continuous variation of the frequency from 4 to 12 kc. at the high end and 35 to 120 cycles at the low end, the boosts obtainable being 20 db. in the former range and 16 db. in the latter. Similarly attenuation of the high and low frequencies can also be achieved, the former by a reactance valve shunted across the audio line and the latter by a variable coupling network. An important feature is that a suitable combination of any two of these various characteristics provides a number of useful curves suitable for such varied purposes as recording, reproduction, line equalization, drama effect, etc. Results obtained with it on long telephone lines are included in the paper.