File:Portable 5W public address system 1929.jpg

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English: A portable 5 watt vacuum tube public address system the Unipac made by Silver-Marshall Co. from 1929. From R to L it consists of a phonograph needle pickup for playing music from a mechanical phonograph, a Western Electric double button carbon microphone, the amplifier unit, and two horn loudspeakers. This type of long straight horn speakers were called "morning glory" horns. The source says the system is adequate for theaters, auditoriums, churches, or outdoor grandstands with crowds of 2000 to 6000 people (modern audio systems would use far more power for such venues in order to avoid clipping on the signal peaks to ensure high fidelity sound, but these early systems had very low fidelity). It could drive up to 6 speakers. The 5 tube amplifier uses 3 amplifying tubes UX226, UY227, and UX250 output tube, and 2 UX281 power rectifier tubes. It weighed 70 pounds.
Source Retrieved July 10, 2014 from J. E. Smith, "Publick Address Systems" in Radio News magazine, Experimenter Publishing Co., New York, Vol. 10, No. 11, May 1929, p. 991, fig. D archived on American Radio History website
Author J. E. Smith
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This 1929 issue of Radio News magazine would have the copyright renewed in 1957. Online page scans of the Catalog of Copyright Entries, published by the US Copyright Office can be found here. [1] Search of the Renewals for Periodicals for 1956, 1957 and 1958 show no renewal entries for Radio News. Therefore the copyright was not renewed and it is in the public domain.


Public domain
This work is in the public domain because it was published in the United States between 1923 and 1963 and although there may or may not have been a copyright notice, the copyright was not renewed. Unless its author has been dead for the required period, it is copyrighted in the countries or areas that do not apply the rule of the shorter term for US works, such as Canada (50 pma), Mainland China (50 pma, not Hong Kong or Macao), Germany (70 pma), Mexico (100 pma), Switzerland (70 pma), and other countries with individual treaties. See Commons:Hirtle chart for further explanation.

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