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AMATEUR TALKING PICTURES
precisely from the output terminals though in a magnified form. To make an amplifier which gives perfectly straight line amplification is a difficult matter, but most of those at present marketed by reputable firms are sufficiently good for home recording.
The next criterion of the amplifier is that it amplifies sufficiently to work a recording head (actually a pick-up). Generally speaking, most radio sets or amplifiers working from the mains and employing mains valves will possess sufficient power to work an electric recorder. Nevertheless the greater the amplification the greater will be the scope. Battery-operated radio sets or amplifiers can also be used providing they are sufficiently powerful.
The usual method of estimating the power of an amplifier is by stating its output in milliwatts. This may be found very simply by the following formulae.
Triode Valves. Output in milliwatts
anode current in milliamps x anode volts
Pentode Valves. Output in milliwatts
anode current in milliamps x anode volts
It is somewhat difficult to set the minimum power required for electric recording, but we might say that 500 milliwatts is a fair standard, and this will require some care in operation if good results are to be obtained. In the experience of the writer a convenient value of output is about 2,000 milliwatts, which corresponds roughly to that of the modern indirectly heated pentode valves.
In making a choice of amplifier for home recording the question of portability must be considered. If we wish to carry equipment from one place to another then obviously the battery amplifier is most suitable. On the other hand, if we intend to record only at home, then undoubtedly the mains operated variety is preferable, since it is cheaper to run and produces greater power. This same question relates also to the choice of a motor-spring or electric for driving the turntable.