SEEKING PURE REPRODUCTION

Radio Record, Volume I, Issue 14, 21 October 1927, Page 14

SEEKING PURE REPRODUCTION

WHAT ARE THE CAUSES OF BAD TONE?

he radio constructor and experimenter, whatever stage of development and -experiertce he may have arrived at, must never consider that he has reached finality in «sireless matters. A mew receiver wtay have been constructed, brimful of little tips and radical improvements, the result of several years of experience. Or a constructive aspirant may have completed his first set, which seems to its owner and producer to be something really fine and wonderful. In either case it must be remembered that radio matters are always on the move-new methods, new accessories, improvements of every kind are constantly making themselves evident And the enthusiast wilh usually be the first to seize upon and try out new ideas One branch of radio reception that 1s now receiving increasing attention is that of pure reproduction, which is an aim to obtain from a receivitig set a faithful duplication of the items broad‘east froin the local station particularly, aud also from others at a greater distatice. A number of factors are responsible for bad tone-overloading of detector and audio stages being the most common, and frequently the loudspeaker is also an offender. DON’T PUSH THE DETECTOR. It is an easy matter to push the detector filament, even slightly in exccss of its rated voltage, and go the Iumit with the high-tension supply. But what is the resuit? Plenty of ‘‘volume," no doubt, but very little more to am ear that appreciates music. Although there is plenty of volume, it is only the noisy volume of distorted signals on account of the ruthless ‘pushing’ of the detector valve with both filament and plate voltages. If such signals are passed along to be amplified, even though the amplifiers do not add to the distortion, they add considerably to the volume, and as they are receiving 1rore or less "mush" to deal with, they can only put out such mush in magnified form. Most listeners will be quite familiar with the harsh, scratchy, irritating tone of a set that is being wrongly operated and thus overloaded. TF'rom this it is clear that if distortion is produced by ovérloading the detector, it is useless to expect good tonal output from the set, however good the audio amplifiers may be. ‘Therefore the lesson must be learnt and remem-bered-‘‘that it does not pay to get too much from the detector’’-its filament and plate voltages must be kept down if good tone is desired. More reduction that is, in a proportional setise, should be made in the plate voltage than in the filament, as too little on the latter may introduce scratchy tone. Good tone may be obtained from a set with poor components, and unsuttable valves, provided the volume of output is kept down, but it might happen that such a set would only give fair *phone strength without distortion, even with four or five valves. Yet perhaps all that would be required to effect a surprising change would be the replacing of audio transformers by a better type, with valves to suit, and possibly the substitution of valves in other parts

‘of the circuit. Even the substitution of*valves alone may make marked im‘provement in a set. Some valves give harsh tone as detectors, though made for the purpose, whilst others, properly handled, give mellow tone. In the audio department there is a very particular relationship between the primary winding of each transformer and the valve that precedes it. If the transformer primary has a high impedance, the detector or first audio valve preceding it must have a similar, but not exactly the same impedance, If the primary impedance is low, that of the pre-. ceding valve must be low. also. Actual test with several valves of suitable impedance will soon show which actually gives the best results. POWER VALVE IN LAST AUDIO. It is impossible to get both good volume and good tone with two stages of transformer-coupled audio with an ordinary general purpose valve in the last stage. If the previous stages throughout the receiver are working efficiently and doing their share in boosting up the signals, the output current of three or four valves can only be handled without distortion by a power-valve, preferably one of the larger type, and of low impedance. Such a valve will handle large yolume without distortion, but it is not at all essential that even here the high-tension voltage should be pushed to the limit given in the manufacturer’s table of particulars. USES OF AN OUTPUT FILTER. Overloading of the speaker windings is a frequent cause of distortion, which may be adding to that already caused in the set. The fine wire of which there is a good quantity on the magnets of the speaker, is not capable of carrying a comparatively large direct current such as the output of a four or fivevalve set, and the best way to improve matters in this portion of the circuit is to instal what is known as an ‘output filter," by means of which the loudspeaker is coupled to the set in such a way as to receive only impulses of voltage, or only the modulated current instead of the full plate current, increasing thereby the useful load that may be given to the speaker, and at the same time protecting its windings from the possibility of a burn-out. This filter coupling is used for the head-phones as well, and has the added advantage of not demagnetising their magnets of those of the speaker, whichever way, they may be connected. The choke filter consists only of a 2 mfd fixed condenser and a choke coil of from 20 to 50 henries, which may be made by replacing the windings of a smfll audio transformer with 384’s or 86’s enamelled wire wound in one continuous length, irregularly, but with a layer of tissue paper now and then, turning up slightly at the inside ends of bobbin, to prevent turns sinking to the layer below. ‘Too high an inductance (henries) should not be allowed in the choke, as tone would suffer accordingly, and therefore a choke of 100 henries or more should not be adopted except after thoroughly satisfactory trial. The diagram shows this arrangement, alsa including the fixed condenser across the output, without which the tone of any receiver is liable to be | harsh.

REGARDING THE LOUDSPEAKER. If the speaker is used at a good many, yards distance from the set and is cotnected by twisted flexible leads, tone may suffer on this account, but not —

necessarily, It would be worth while to test with straight wires run separately to see if any difference could be detected, It is a good plan to put a pair of say 22’s enamel wire permanently along under carpets, the wires being separated a few inches, and connect: these to short flex in the room where the speaker is to be used. A long twisted flex connection will sometimes cause a continuous whistle in the speaker. No horn type of loud speaker gives a faithful. reproduction of the lower notes of music, even though they are not stippressed by the audio transform: ers, so it must not be expected that the low, humming bass notes that give so much body-tu musical pieces will be heard to any advantage unless a cone speaker is employed. Horn speakers with throats that are too narrow at the narrowest part are the worst suppressors of the bass parts of music. The notes aze there to some extent if listened for, but are distorted, and have lost their musical tone. On the other hand, a horn speaker with too wide a throat will not respond well to weak signals. ‘The diameter of throat sjould not be judged from the outside, for the metal may be one-cighth inch thick or more. The horn should be taken off and the inside diameter noted, and unless intended for use with a small set, should not be less than three-quarters of an inch. ‘There are horns with a minimum inside diameter of only half an inch, but from the outside appeatance it might easily be supposed to be considerably more. But it should be remembered that the size of speaker should be to some extent proportional to the output of the set it is to work "with. At the present stage of development, the loudspeaker is only a comipromise at the best, and it is more wasteful of energy than any other of apparatus in use. Generally speak-_ ing, cone speakers do not give as much volume on a given output as do the horns, but the cones give strengtlr to. the lower audible: frequencies or bass notes, but tend to weaken any highpitched melody of treble or seprano parts. ‘There are now on the market a few cone speakers with improved mechanism enabling them to overcome the two worst defects to a great extent. A constructor should not purchase too small a speaker at the outset, because although only two valves might at first be in use, additions will soon be made to increase the output of the set, and this increase will be well carried by a speaker that is not too small. Overloading of the loudspeaker is a very prevalent sin, and one which may easily ruin the reputation of a set that is capahle of delivering almost faultless output at a reasonable volume. Extra volume with quality is best obtained by using both a cone and a horn speaker connected in parallel, for then the two windings are each taking a share of the output, and extra volume is obtained without overloading either speaker to the point of distortion, whilst the high and low audible frequencies are both properly cared for. THE IMPORTANCE OF GRID BIAS. Constant mention is being made of the importance of the use,of grid bias in the audio amplifier, and its employment is a big factor in securing quality reproduction. Not only does it improve tone, but reduces the consumption of high tension current in the amplifying valves-an important consideration where a dry B battery is in use. An ordinary valve used in the first stage may oily require a few volts of negative bias such as may be supplied by a emall flashlicht unit of perhaps 44

. volts. No actual current is taken from | a grid bias battery, and it lasts just as long in the set as it might upon the shelf in the shop. When we come to the power valve in the last stage, however, grid bias becomes more of a necessity, and the battery to supply it becomes a more formidable accessory, as atiy voltage up to about 40 may he required on this valve, so that a tapped B battery of 45 volts would be used, and from this the smaller voltage required by the first audio valve would also be taken. Grid bias is worth while on the audio side of any sct, and well worth iis cost. Grid bias can be applied to the radio-frequency stages, but is not so necessary, and in some circuits may even be detrimental in the direction of tending to cause instability. ‘Too high a grid bias voltage will cause distortion, and loss of signal strength, so that experimenters should provide a means of varying it easily. New batteries should be used, and should be tested from time to time, and this latter remark applies to bought sets also, in which the grid bias battery is often stowed away in a corner atid forgotten, but lacks voltage, and spoils tone. Grid bias required by any patticular valve is now always listed in the accompanying circular. FIXED CONDENSERS. Three fixed condensers influencing tone are the grid condenser, one across the primary of the first audio transformer, and another across the output, whether a filter is employed or not. Constructors must experiment with different values in each cf these positions, and in the case of the two latter, fairly large capacity may be found to be an im-

provement. In some sets a smaller | capacity than the usua) .00025 will be found an advantage. In condensers controlling tone, large capacity tends to mellowness and too large to woolliness, especially noticeabe on speech. Another fixed condenser that can have a great influence upon tone is one of small value, say .0003 mfd, ‘This is placed across the plate and grid terminals of the first andio transformer. The correct value of this condenser can enly be found by careful trial, judging the tone, changing the condenser, trying on speech as well as music. The right capacity will be found, that cleans up a good deal of noise, gives bright music, and No woelliness | on speech, The addition of this condenser, where suited, will just add the touch of mellowness so often desired but not found. AUDIO TRANSFORMERS, Much has been said in favour of resistance coupling of andio valves, and where it has displaced poor quality transformers, no doubt a good ‘mprovement has taken place. But possibly in many cases the substitution of high-grade, large-sized transformers would have given still more marked improvement. Small transformers will not vive good tone and volume too, aid the sooner such are replaced by the Lest that can be procured, the better. Distortion can be cansed in the radiofrequency side by extreme selectivity resulting in the cutting ont of more than the correct amount of side-band amplitude, but in practice there is not much to fear from this source in the case of the average home-constructed receiver.