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24 AMATEUR TALKING PICTURES
transformer primary and secondary. From the latter the current flows round the speech coil of the recorder which corresponds to the speech coil already mentioned in Fig. 9 in Chapter I.
What is the object of the transformers ? Why cannot we run direct from the microphone into the amplifier, and direct from the amplifier into the speech coil of the recorder ?
The object of the microphone transformer is twofold. The battery flowing through the microphone cannot be passed directly through the amplifier. By using a transformer the battery current is kept to one side and transmits only the fluctuations to the amplifier.
The second function of the microphone transformer is to raise the voltage of the fluctuations generated by the microphone to such a level that they will be sufficient to work the first valve of the amplifier.
As is well known the fluctuations or voltage variations applied to a valve must be of such a value that the maximum "swings" the valve to operate at its maximum output consistent with linear amplification. Valves vary in this respect and thus so should transformers. However, the type of amplifier used for home recording is likely to have an input valve of a type commonly associated with "detectors" of radio sets, and for this the "swing" is roughly constant. The use of the output transformer is necessary for similar reasons. From the output terminals of most amplifiers we have a complex current consisting of a direct current (the HT supply), and magnified fluctuations the counterpart of those applied at the input. It is not advisable for the direct current to pass through the fine windings (the wire is sometimes only -001 in. diameter) of the speech coil of the recorder or pick-up. Quite possibly if a powerful amplifier were used and put direct to the speech coil it would be burned out. Again, better recording is achieved when the direct current is kept from the speech coil.
A point to be mentioned here is that the output transformers usually employed with low impedance moving coil loud speakers are not suitable for use with home recording. Output transformers for moving coil speakers are of the step-down variety, often in a ratio of 40 to 1. For home