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ELECTRICAL DATA
23
Amplifier Connections. In Fig. 8 is a pictorial lay-out of electrical recording including mechanical features of turntable, etc. In Fig. 11 we have the electrical recording circuit which should be studied with care. Starting at the left of the diagram is the microphone, and running from it the leads go to one side of a transformer. Since the microphone by itself is incapable of generating electric current, a battery is included in circuit. We can imagine the current flowing round the circuit as indicated by the small arrow heads. The battery sets up a steady flow of current round the circuit, including the resistance of the microphone and the primary winding of the transformer. Due to the variations in pressure of the diaphragm of the microphone the electrical resistance of the latter is varied, thus causing the current also to fluctuate.
This fluctuating current passing round the primary of the transformer is reproduced in the secondary, but is stepped up in voltage. The actual ratio of the transformer used depends upon the type of microphone and the first valve of the amplifier to some extent. Frequently a ratio of 13 to 1 is employed, but the recommendation of the company supplying the microphone should be adopted, although a little experimental work in this connection usually repays. From the secondary of the transformer the wires run to the input terminals of the amplifier. With theinternalconnections of the amplifier we are not concerned, and so pass to the output terminals shown to the right, and to the output
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