GETTING GOOD QUALITY

Evening Post, Volume CXII, Issue 55, 2 September 1926, Page 14

GETTING GOOD QUALITY

1 WHERE TO LOOK FOB FAULTS. "We read now and again—especially in advertisements —that such and such a receiver, by virtue of its circuit, or some particular brand of coil of tuning condenser or what-not, or some special brand of valve, will ensure beautiful tone. These references, so far as they refer to the radio-frequency p*art of a receiver, can be discounted as sheer rubbish. The tuning coils and condensers, unless they are in such bad condition that there are defective contacts such as may cause noises, cannot havo the slightest effect upon the musical quality of the reproduction. The only effect that can be produced by the radiofrequency portion of the sot is distortion due to oscillation, or too much reaction, and good parts are in this respect just as bad as poor ones. Where, then, is quality controlled? Solely in those portions of the apparatus that follow the detector, and only in some of these. If a loud-speaker is used, it is commonly biamed for a good deal of unpleasantness, and usually for more than it deserves; for contrary to a general opinion, the average loudspeaker is fairly good, and really bad ones are nowadays rather uncommon. The principal culprits are the audio-fre-quency transformers and the valves. Until the last year or two, transformers with the characteristics necessary for repeating faithfully the shape of the, current variations put into them were hard to obtain; they displayed "peaks" at certain frequencies, usually fairly high up the musical scale, and were feeble operators in regard to the lower notes upon which the richness of most music is based. Hence aroso the practice of using by-pass condensers, through which some of the higher frequencies could escape. Modern trans formers are designed to provide as even a characteristic as possible, and the "tinniness" which used to be so common is hardly noticeable when they arc used in a set otherwise correctly designed, without any of the by-passing business being necessary. At least as bad as a bad transformer, and perhaps worse, is a valve improperly used in amplifying stage. Insufficient voltage in the plate circuit, improperly adjusted grid-potential, and too low filament voltage all cause distortion, and in every case tho principal effect is to introduce higher frequencies than exist in tho notes being transmitted and to make the sound tinny. The same result is caused by using in the last stage a valve incapable of dealing with the load upon it. For the first stage of an audio-frequency amplifier any good general purpose valve will

serve, but in the second stage a valve having a fair capacity is needed. Its characteristics should be in the neighbourhood of those of the well-known 201 a type—of which there are now many makes, both British and American —or of a small power valve. By using such valves and sticking closely to the recommendation of the manufacturers in regard to filament, plate, and grid-bias voltages, one can be reasonably, easy in mind as to the freedom of the valve from responsibility for unpleasant noise instead of music. Neither the valves nor the extra batteries needed involve much additional expense. Going back to the transformers, it may be found that good ones cost a few shillings more than poor ones, but no money spent on the set can be better invested; in fact, it would be a good thing if the transformers put into many a set built a few years ago were scrapped and replaced by the best available. Audio-frequency transformers and battery-voltages contain between them nearly the whole secret' of good reproduction. There is so much distortion in many standard sets due to these elements being unsuitable, that resistance coupling and choke coupling are having an almost triumphant run at present among the sefc-building enthusiasts; but it is a very open question whether a well-designed transformer outfit suffers at all by comparison with these systems. To go back to the beginning; if your set does not produce a nice round tone, look to the- audio-frequency department, not to the coils and condensers. These may be responsible for the set not_ being selective, but they cannot spoil good music. Unless you are satisfied by comparison with others or by having the testimony of candid friends, do not take it for granted that your receiver is giving a good tone. Now and again inquire into the matter critically; have a gramophone expert in now and then he ■is bound to pick holes in the rival music-box. And if he says tho tone is rotten, listen to him—he may be right It is impossible to trust one's own ear in these matters .when it has been accustomed to listening to the set, for the ear is treacherously tolerant.