Input Transformers

Author: N.H. Crowhurst

An input transformer is used to improve the signal-to-noise ratio in certain circumstances. If the noise resistance of a tube is 2000 ohms and the impedance or resistance of a microphone is only 500 ohms, the predominant noise when the microphone is connected in directly will be due to the tube. In fact, the tube will produce twice the noise voltage, for a given bandwidth, that the microphone does.

Input transformer improves signal-to-noise-ratio

With the microphone producing 0.4 microvolt of noise, and the tube 0.8 microvolt of noise, the two add together by the process of "the square root of the squares"; 0.4 squared is 0.16 and 0.8 squared is 0.64. Adding these figures gives 0.8, the square root of which is 0.895 microvolt with an audio voltage of, say, 1 millivolt.

By stepping up the audio and microphone noise with a 10:1 input transformer, the microphone gives 4 microvolts of noise on the transformer secondary. The tube still gives 0.8 microvolt of noise. Now the total noise voltage is V 42 + .82 = V 16 + .64 = 4.08 microvolts, which is very little more than the microphone alone. The audio voltage, however, has been stepped up to 10 millivolts in place of the original 1 millivolt. Thus signal-to-noise ratio has been improved by better than 2:1. Beyond this point, however, using a transformer with greater step-up will not improve matters, because it will step up the audio and the noise in the same ratio. In addition, making a transformer with a big step-up gives problems in getting uniform response over a wide range of frequencies.