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RECORDING NOTES
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control should be adjusted so that blasting does not take place, then whatever movement is made to the microphone volume control distortion will not be introduced.
Loud Speaker as Microphone. We have mentioned that the moving coil microphone at present used in America for talking picture work and in this country for the making of gramophone records by the Columbia Graphophone Co., is the converse of the ordinary moving coil loud speaker.
Working from this assumption the writer carried out a series of tests to determine whether or no it was possible to employ an ordinary moving coil loud speaker as a microphone. The results of the tests were satisfactory, and the
Microphone
Vol. Control
/
'Mic. Transformer
Fig. 20. Volume Control or Microphone
arrangement employed is shown in Fig. 27. To the left we have an ordinary moving coil loud speaker of the permanent magnet class and with a cone diameter of 9 in. The leads from the speech coil are connected to a transformer — having a step-up ratio of the order of 30 to 1 or even higher. As a matter of fact the ordinary output transformer used with the speaker may be employed without making a change to its connections. Normally it is used as a step-down coming from the amplifier to the speech coil. When, however, the fluctuations are produced in the loud speaker and the transformer is connected to the input terminals of the amplifier naturally the transformer acts as a step-up.
It was obvious at the beginning that this arrangement would work, but the doubt was that a two valve amplifier of the ordinary mains type would be sufficient to bring the level suitable for home recording. This was. however, found perfectly satisfactory, and furthermore the recording obtained by using a loud speaker as a microphone in this
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