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CHAPTER II
ELECTRICAL DATA
As will have been gathered from Chapter I, principles of acoustic and electrical recording are identical. Sound causes a diaphragm to vibrate, which by some means is transmitted to a cutting stylus which marks out a spiral groove on a rotating disc. In the case of acoustic recording the cutter is applied direct to the diaphragm, whereas in electrical recording we have an intermediate amplifier.
Before we can proceed to discuss some of the equipments on the market for carrying out this work there are several points in connection with electrical amplification with which we must deal. The subject of electrical amplification if treated thoroughly is a complex one. and most of it is outside the present scope. Those having knowledge of radio work must excuse the elementary nature of the treatment.
The Valve Amplifier. The valve amplifier is simply a means of proportionally magnifying an electric current. Thus coming from our microphone we have a feeble and constantly varying electric current which is insufficient for our needs. This is put through an amplifier, whence it emerges magnified in amount but still bearing precisely similar variations.
As a matter of fact the current which goes to the input terminals of an amplifier does not actually emerge from the output terminals, but instead we get another 1 at larger current.
In a number of the home recording equipments at present on the market special amplifiers are supplied if required, but a good many amateurs already possessing radio sets or amplifiers will not wish to bear this additional and probably unnecessary expense. What. then, is the criterion for an amplifier suitable for home recording ?
First of all it should have a straight-line characteristic, which means simply that if we impress a scries of fluctuations on the input terminals these will be reproduced