RADIO PROGRESS

Evening Post, Volume CV, Issue 127, 31 May 1928, Page 19

RADIO PROGRESS

BRITISH AND AMERICAN WHAT DESIGNERS AXE DOING The main .cliuructui'istic vi' the !English sot designer is that ho places quality of musical reproduction in the first place. In America, current literature indicates that, though "quality" occupies a foremost place in the thcoretial Hs't of desired features in a receiver, the ruling passion is still to have the newest thing.' -Wo find,-then; that the; American radio journals are full of descriptions of complicated receivers to be operated direct from the mains, with the new "shielded grid" valves rapidly climbing into popularity. The shielded'-grid- valve, an instrument which theoretically has an enormous amplification factor which is not approached in practice, haa introduced a number of new factors in receiver design, and though the journals on both sides of the Atlantic give plenty of directions for using them, it is probable that much remains to be dou'e before they become as generally satisfactory as the familiar type of valve. America appears to be undergoing a radical change-over- from battery-oper-ated to mains-operated receivers. Both for convenience and ultimate economy, electricity from the mains is better than from batteries, but the fact that a receiver requires a perfectly steady direct current and most • electric supplies are alternating makes a complicated and costly piece of apparatus necessary. The -construction of this, tfiough not radio engineering^ is very interesting and not difficult, though the builder has to be pretty careful if he is to avoid costly and even, dangerous mistakes. Mjainsoperated receivers are sure of a good market in America, because electricity is iii general use; far more so than in England, where' a domestic supply of current is by no means as commoft as in: New Zealand. '.". English amateurs are, however, much interested in using the mains. A large proportion of the. domestic electricity supply is, however, direct current, and one.finds, therefore, that for nearly all the mains-operated receivers described, directions are given for "eliminators" for both/types of current.' The complete eliminator for the high-tension supply, in' the case of. alternating current, consists' of a transformer and a rectifier, which together provide a direct current of the required voltage but pulsating and in that form useless; and a "smoothing circuit/" which-flattens out the pulsations into a practically smooth current. When direct current is available, no transformer-and-rectifier unit is needed, but a smoothing unit is, though it need not be of as large capacity. The direct current sot is thus described with the smoothing apparatus, and the alternating current set with both units. Only, a very limited demand exists for direct-current .sets in America, and a curious absurdity exists in connection with the commercial mains-operated receivers, all of which.are on alternating current. The unfortunate buyer whoso house is supplied with direct current is given the opportunity to buy a motorgenerator, which, driven by the direct current, snpplies alternating, which the set promptly, converts back to direct. The popularity, of the idea of drawing" oil-, the mains has driven the radiofro.quency: side of ..■ wireless technique rather'into the background in America, and' the audio-frequehey amplifier has long been fairly stabilised. In'the vast majority;;of American sets two transformer coupled stages are used, and the most noticeable changes in this department are really the outcome of using the mains; more powerful valves are available for the last stage, and are rightly becoming popular. Amplifier design, however; is rather neglected, and there is little movement awayfrom the general purpose valves which have been standard, in. America for years past, except in the power.stages. What changes there are relate to the shield-ed-grid types already mentioned, and. to a new class in which tho filament is heated by alternating current and in turn heats a cathode which, supplies the electronic stream. ' \ English design is active over a much wider field. In .spite of the limited availability of domestic electric supply, operation from tho mains offers an .entertaining outlet for amateur activity, but in contrast with America, has not gripped the manufacturers yet, though there is a liberal supply of tho parts- required for building eliminators. ". . .'■■ - .. ; . - There.is. : not very much scope now for the development of the radio frequency circuits of receivers; they have been pretty well standardised into . a few well marked varieties. Some English designers have concentrated particularly upon a circuit which is essentially similar to tho Browning-Drake; that is, it provides ono stage of neutralised radio, frequency amplification followed liy a detector with regeneration, though the desiraljlejiesss of omitting regeneration .is .emphasised. As the result oil careful design this circuit lias been developed to very high efficiency. But the enthusiasts have for a long time devoted far more attention to the. audio-frequency amplifiers, and resist-ance-capacity coupling is very popular. There are in England far nioro valves of different characteristics than aroreadily available in America, and all •these types are' thoroughly tested .for their most effective use.. At every step one finds, that tho designer is aiming.. at.-quality of reproduction all the time; and tho further the work goes, the more emphasis is laid upon having a very powerful amplifier ■<. so that ■ it' will never be overloaded, and upon, ensuring that no avoidable causes of distortion shall exist. lii"tho matter, of "loud-speakers,"' everyone who can afford tho luxury"'appears, in England, to be making, or talking, coil-drivcil cones; "?lipso require very-powerful amplifiers to drive them, but the results arc admirable. They enablo the reproduction to approach, if. not actually' to equal, the volume of sound in the original performance. So far America has hardly touched the coil-driven reproducer. Hand in hand with the rapid development of these very accurate amplifiers and powerful, faithful loud-speakers runs the marriage of tho wireless sot and the gramophone; and this, as might be expected from what has been said of English amplifier progress, has advanced rapidly at Home. Quito a number of "pick-ups" are on tho market, some good'and others not so good. A really good pick-up, a really good amplifier, and a really good loud-speak-er make a combination which will beat an ordinary gramophone; but any serious fault in either makes the whole outfit a failure. For those. who contemplate using a "pick-up" and thus electrifying, their gramophones it may be as practically a certainty that if a horn-type loud-speaker of any of the-standard type is used, the re-' suits will not be worth tho expense.