DISTORTED RECEPTIONS

Radio Record, Volume I, Issue 40, 20 April 1928, Page 13

DISTORTED RECEPTIONS

(Continued from Page 12.) better results, and it is no small problem to produce good quality and good volume. at the same time, but it can be done. ‘he writer’s standard receiver has just been rebuilt on a different plan, and although the change did not affect the audio amplifier, there is a marked improvement in tone in the ordinary four-valve circuit, _ | And here the writer would like to say that his remarks in this column ‘ at different times with regard to good | radio reproduction are all. intended to. be in a helpful spirit. Radio is developing rapidly, and ideas change quickly; hints given to-day may not be iv entire agreement with those given at a much earlier date. | Some listeners suffering from distorted reception would like to see the trausniitting station bear the blame, but nobody has yet advanced any tangible proof that the station (particularly referring to 2YA) is to blame. The writer hears almost eyery afternoon end evening session, and has a good op portunity of judging quality, and finds it consistently good'so far as transmis- | sion is concerted. | Bad transinissien may be ‘almost. as‘ readily detected at close: quarters as at a distance. ‘Che writer, moreover, is no "servant of the company,’’ and is just as interestef as any other listener in making complaints of any shortcomings. It is probable that in most districts from which complaints of distortion emanate listeners could be found whio get fairly consistent good reception. Kyen one such case in a district world prove that the distortion was locally produced GOOD RECEIVERS OR COM: PONENTS INANCIAL © considerations, unfortunately, have a close’ elation With good reception, and in the radio business the article at double the price will usually give double the satisfaction, just as in most other lines of business. Jf you. are a constructor, it is an easy matter to keep testing improvements in the set, but if you are buying a ready-made set for entertainment it pays to get as good a one as vou can afford, and to take careful note of the tone and volume it will vive. GETTING THE DEEP NOTES. [JEEP minsical notes’ are a severe ‘ test for any receiver and loudspeaker, and a combination that handles all deep notes without distortion, and does not lose or unduly weaken the high notes, is worth possessing.

Poor, undersized audio transformers and unsuitable horn loudspeakers are the cause of more distortion than anything else, for they will give distortion when there is no interference, The only way to get good reproduction from such a combination is to cut down volume to a reasonable level. The better the set the more volume it will give withont losing quality of tone. The fumble crystal is a quality-giver, and oniy when we add valve amplification with unsuitable components does _ its quality begin to suffer so far as the action of the receiver itself is concern. ed. If good quality and volume are desired from amplification of crystal reception, then the amplifier must be of just as good a calibre as if intended for the amplifier of a valve set. But a good valve set, properly handled, is capable of giving just as good reception as any crystal receiver, although the latter, without amplification, reproduces the lowest musical notes put over by the station. . In order to test what notes tlie valve outfit, which includes the loudspeaker, is reproducing, every multi-valve op-erator-experimenter within crystal range of a station is recommended to fix up a crystal receiver and listen-in with the ’phones. In many cases there will be heard by means of the erystal the low strumming notes in the bass that are, perhaps, entirely absent or blurred beyond recognition in the loudspeaker reproduction, Constructors may, therefore, set the crystal reception as a standard to work by, atid gradually improve their receivers until they will, in combination with the loudspeaker, reproduce all the notes heard through the crystal. One easy method that has already ‘been reconimended for improving the first audio stage is by substituting for the transformer a resistance snit such as the Philips or similar ones that are ou the market. ‘These are simply connected up just in the same way as a new transformer would be. But there are other considerations. One point is that the detector valve must now be one of high impedance and amplification to work in conjunction with the resistance coupling. Another joint is that as resistance coupling gives greater prominence to the deep notes, the audio gear following must he capable of handling such notes. ‘This responsibility falls chiefly upon the second ‘transformer,’ which must be a thor- oughly good last stage mogel with high impedance primary winding, and upon the last valve, which should. be of not more than, say, 5000 olims impedance in order to be capable of handling the required volume without listortion. Having now reached the output terminals of the set, we have the loudspeaker to deal with. Quite likely this is a horn type, and if it 1s capable of properly handling low notes it is something out of the ordinary. Most horn speakers tend to make "mush" of all notes below a certain pitch, or frequency, and, to a certain extent, the lower that pitch the beiter the speaker. "But a good cone speaker reproduces iow notes with ease, though it may tend to weaken the very high ‘frequencies to some extent, but as most of the distortion arises on the low notes, it is cutting out a lot of trouble to have a speaker that will handle them. Balanced or double action in a cone speaker makes for quality and even reproduction, as also does one with elastic attacliment round the edges of the diaphragm rather than one with rigid fixing, though this does not apply in every case. "ONCE AGAIN THE OUTPUT FILTER. New, the windings upon the magnets consist of a large number of turns of very fine wire, so fine that it will only carry a yery limited amount of current. But when a power-valve is employed in the last stage, a very considerable amount of current may he passing from the B battery through the valve and speaker windings, and this current may be so great as to cause the

loudspeaker, whether horn or cone, to work inefficiently, owing to the comparatively large amonunt of, direct battery current, having a detrimental ‘action (saturation) upon the magnets. When this battery curreut passes through the valve it has indirectly impressedenpon it by the grid what are called yoltage impulses or fluctuations of an alternating character, and it is these impulses. that really actuate the speaker. It has been found that after the battery current has passed through the valve it may be diverted directly back to the battery, leaying the voltage impulses alone to traverse the speaker windings, still transmitting the same amount of power to the diaphragm, with improved quality, owing to the reduced strain upon the windings. This separation of the alternating and direct currents is accomplished by what is usually referred to as an "output filter," and is a combination af two components, firstly, a choke coil, which has the property of passing direct current but not alternating current, and, secondly, a fixed condenser of large capacity (2 microfarads), which has the property of passing the alternating impulses and not the direct current, These are connected up in a way that has frequently been dealt with in these columns. The use of such a filter will allow of the speaker carrying greater volume owing to the carrying capacity of the windings having been relieved ofthe unnecessary direct current. Such filters may be purchased as a complete small unit ready for attachment to the output terminals and speaker. For an output equal to or over 90 volts at 10 milliamperes, it is a necessity for good reproduction, and also protects the speaker windings from damage, and ‘does away with the necessity for connecting the speaker or ’phonés according to polarity (positive and negative) of the leads. The above are some of the main points in audio improvement for better reception. Such improvements should alwavs be made with the fact in mind that one improyement leads to another -that greater yolume or deeper notes passed on by one component must be retained by improving the components that follow, so that they may efficiently handle the increase. WATCH DETECTOR VOLTAGES. MENTION should also be made here of the amouut of distortion that can be produced by pushing the detector too hard, that is, attempting to get too much amplification from it, Both filament and high-tension yoltages should be kept down rather than pushed to the maximum,.and tome will be helped thereby, and speech clarified. Note how the s’s and t’s coine through your loudspeaker. They are sometimes almost lost in an unsatisfactory audio system. An appeal is made to all operators of. receivers to do all in their power to prevent their sets from causing interference, however slight, for the multiplication of slight effects is serious, and the more aggravated cases of howling are most certainly a direct preventive of a certain number of people becoming listeners. Operators may Searcely realise that by keeping the ether clear they may do a great deal to assist in at- | tracting additional licensees to provide the money for the improved pro--grammes. which are so frequently asked or. }