GREAT WIRELESS ADVANCE

Hokitika Guardian, 29 November 1930, Page 6

GREAT WIRELESS ADVANCE

A NEW RECEIVER. DISTORTION ALL BUT OVERCOME. LONDON, November 8. The new broadcasting receiving apparatus has been brought, into use at the Science Museum, South Kensington, and will give daily expositions of the very best that is possible,, in the reception of wireless speech and music. The new receiver, replaces equipment; that lias been in use for some years, and which has, for some time, been out of date and behind technical progress. , “ Designed solely for local station reception, this imposing receiver makes use of no fewer than n ne valves, five being of tiie “indirectly • heated” mains type, and the remainder heavy filament power valves heated from the. mains. Despite the number of valves employed, the receiver has only five stages, one being a single stage of high frequency amplification, the 6econd a detector, and the remaining three low frequency amplifiers arid an output stage.

Such a receiver is beyond the needs of any household, and most certainly it is beyond the contructional powers of any but the highly skilled with considerable workshop facilties. Its use is to give a standard of reference for those who are interested in obtaining tilie very besty possible reproduction, and to show.what- can be done when money has not to be considered. ~lt will also serve to keep a watchful eye on the quality of the B.R.C. transmission from Brookman’s Park, for if anything is amiss it will most certainly not be in the receiver.

SIMPLIFIED TUNING. For those with some technical knowledge a brief description may be of interest, though complete details would be out of place, here. the single screen-grid H.f, stage is followed by a . stage of push-pull powergrid rectification. Here it may be noted that the high frequency stage is duplicated so that the. tuning being pre-set, either the programme of the National or the Regional, station may be used by the simple turning of a switch. This makes the actual number of valves in use up to ten, but only nine are working at any ono time. The rectifier is followed by an elaborate filter circuit cqmposcd of resistances and condensers, the purpose of which is completely to cut off any high frequency component of the rectified wave The low frcr quency amplifiers are all in push-pull and consist of low-gain .resistance capacity coupling The nature of the output valves may be gathered from the fact that they are fed from a 15Q0 volts supply, The early, stages use something under 500 volts The loud-speaker consists of a mov.ng coil unit combined with a large and specially designed horn This is held to be the very best possible arrangement in the light of present knowledge when a very large output has to be handled and when a large space has to be filled with sound. The new receiver is “ail mains”, driven, even to grid bias, and is thus in the same class with those more simple pieces of apparatus which are rap’dly replacing battery-operated sets in all homes with electric power available. So good are many of these comparatively simple sets that it is possible that their owners will not be able to recognise that the Science Museum apparatus gives quality that is sufficientlly superior to warrant the great cost, both in tial and upkeep of such a complicated electrical device. The fact remains, however, that the measured output of this new receiver is practically distortionless and gives an almost straight-out response over all frequencies between 50 and 10,000 cycles. EXPECTATION FULFILLED /. . . Many.- years ago the opinion was given that ..such an even response was, possible, and, should be aimed at by all manufacturers. It may now be said that the great- difficnlt’es were not then ful.ly recognised; but the thing has been done', on a laboratory scale, and there is no. reason at all why the knowledge that has been gained should not, be used by .manufactures, on a more modest scale, to improve the already excellent apparatus that can now be bad ifor between £3O and £SO. Manufacturers have to work to defined limits, and the majority of loudspeakers are still behind the development of receivers. For all that, the new Science Museum receiver is not now so far in advance of the majority of household sets as was its predecessor four years ago.. The magnitude of the task which the designers of this receiver set themselves is shown by the time which lias elapsed since it was first mooted. The work has been , well worth doing, 3ven iff only because push-pull detection and power-grid defection have been made known to research workers ind through them to the trade.