The Microphone Matching Transformer

Author: N.H. Crowhurst

A moving-coil or ribbon microphone directly converts acoustic power into electrical power; however, there is very little power to use, and the microphone must make the best use of what there is. (Electrical power is measured in watts, found by multiplying current in amperes by potential difference in volts; for microphones, the output is in fractions of a microwatt,)

The basic microphone matching transformer.

In the moving-coil microphone, there may be as many as 100 turns in the coil, each of which generates 5 microvolts for a particular sound intensity. The whole coil, therefore, gives a total of (100 X 5) or 500 microvolts. A ribbon does not generate much more than about 1 microvolt (open circuit) because there is not even one whole turn, and the magnetic field cannot be so intense, because of the wider spacing between the poles. However, the resistance of the ribbon is very small, about .05 ohm. Applying Ohm's law, we find that the short circuit current in the ribbon that would flow due to 1 microvolt is about 1/.05 or 20 microamperes. The voltage output of the microphone can be raised by a microphone transformer, which has a small number of turns in one winding, and a larger number of turns on another winding, both wrapped round the same magnetic core.

In a transformer, every one of the turns will have the same voltage "generated" in it; if the single-turn winding generates 1 microvolt, all the other turns will also give 1 microvolt, and a 500-turn winding will give 500 microvolts. The total power will, of course, be the same, and with 20 microamperes available from the microphone into the 1-turn, the 500-turn winding will only give 20/500 or .04 microampere. What the transformer does, in effect, is to replace the actual single ribbon with the equivalent of 500 very much thinner and lighter ribbons, all connected in series.