Hints on Operating a Loudspeaker

Radio Record, Volume II, Issue 26, 11 January 1929, Page 8

Hints on Operating a Loudspeaker

How Good Reproduction may be Obtained

N the following article Mr. Percy W. Harris, M.1.R.E., editor of the ‘Wireless Constructor," discusses a ‘opic vital to listeners. The article is distinctly practical and should prove beneficial to all interested in quality reproduction-and who is not ?

7O matter how good your set 4, may be, it is the loudspeaker which ultimately governs the reproduction, so that it is well that every listener should: know how to test his loudspeaker on those occasions when it seems to have developed a fault. Now, the complete and thoroughly scientific test of a loudspeaker is a task beyond the scope of any’ but a well-equipped laboratory, but if we pay attention to certain fundamental facts it is quite easy, with the apparatus at the disposal of almost every listener, to find out some very important facts. If, too, a second loudspeaker is available either as a permanent acquisition or borrowed for the evening from a friend, then the test can he made still more complete and satisfying. "Falling-off." ASSUMING that you have purchased a good loudspeaker and have in the past been satisfied with its reproduction, then the falling-off in quality ean arise from three sources. The first is from some source prior to the entry of the signal into the receiver, the second is from the receiver itself (including under this heading valves and batteries), and the third is from the loudspeaker. Only too oft » 2 sudden or slow fall-ing-off in qualily of reproduction is blamed to the loudspeaker whereas it —

arises from some source under heading 1 or 2. If another loudspeaker, preferably of the same make and type is available, the first thing to do is to substitute the other loudspeaker for your own, whereupon if the cause is exterior to the loudspeaker the same faulty reproduction will be heard in both instruments. Loudspeakers may be divided under the two headings of "moving iron" and "moving coil." In the first classification come those loudspeakers in which we have an electro-magnet which exerts a varying attraction upon a diaphragm, armature, or a reed according to the varying strength of the incowing signal. , The diaphragm, armature, or reed communicates the motion so set up in it to a cone of paper, fabric, or some suitable material which in turn moves the surrounding air, or in another type sets in motion an air column inside a horn. L.S. Adjustment. N the second classification the currents from the receiver pass through a coil which is suspended in an intense magnetic field, the varying

current, passing through the coil and not through the electro-magnet, causing a variation in position of this coil in the field. As the coil is itself rigidly attached to some form, of diaphragm, the diaphragm is moyed and impulses are communicated to the surrounding air, As the vast majority of loudspeukers now in use fall under the first heading, we will deal with faults in these before the others. Taking first the horn type, in the majority of these a stalloy diaphragm, circular in form and yarying in diameter from that of the diaphragm used in headphones to several inches across, is rigidly held at its periphery above the pole pieces of the electromagnet in the windings of which the currents from the receiver circulate. The sensitivity of this device will be largely dependent upon the air gap between the pole pieces and the diaphragm, the smaller the gap the greater the sensitivity of the instrument. Obviously, there are limits here, for the diaphragm has to move with varying attractions of the electro-magnet, and if we muke the gap too small then a strong signal ill draw the diaphragm down so much that it will touch the pole pieces. In order that one may suitably adjust the gap a screw or lever is fitted to most loudspeakers of this type, the adjustment being such that either the diaphragm is moved nearer to or farther away from the magnet, or the whole magnet itself is moved, the latter being the more general practice. Sometimes one may adjust a loudspeaker of this type to give maximum sensitivity without the diaphragm coming in contact with the pole pieces, even with the strongest signal, and then on leaving the set for a day or two we nay find that even a weak signal makes the diaphragm touch. This is. due in many cases to variations in temperature of the metal, for metal expands and contracts with changes in the room temperature, and these may arise from an alteration in the weather, or the lighting of 1 fire.

Effect of Plate Current. N many receivers loudspeaker windings are included in the plate circuit of the last valve, and when this is of the super-power variety quite a considerable direct current will flow through the windings in addition to the variations set up by the signals. This steady direct current would exert a continuous pull upon the diaphraggad if the direction of current is such th™¥ it tends to accentuate the field of the permanent magnet. On the other hand, if it is the opposite direction it will reduce the pull of the permanent magnet around the pole pieces of which electro-magnetic windings are wound. There is thus a right and a wrong way of connecting a loudspeaker in sets where the loudspeaker windings are directly in the plate of tne last valve. With the right way one not only obtains the best sensitivity, but the steady current tends to prolong the magnetism of the perme .ent magnet, whereas when connection is made in the opposite direction this steady current. is tending all tre time to demagnetise your loudspeaker magnet and reduce its sensitivity. For this reason many makes of loudspeaker are marked with positive or negative leads or terminals, the former using red and black tips, or else the postive lead has a coloured thread wound into its insulation. Some speakers, in deed, are marked positive and negative by the conventional cross and dash. Not all speakers are so marked, however, for there is a particular form of magnet which is not appreciably influenced so far as its life is concerned, but in all eases it is just as well to keep the windings to the correct polarity. Testing Polarity. [ your loudspeaker leads are not marked for polarity, or what is just as important, if the loudspeaker terminals of your set give no indication of which is the positive and which is the negative, here is the way to’proceed. Connect up your receiver, switch on the loudspeaker and carefully adjy the speaker screw or lever until wr ean hear the rattling which indicates that the diaphragm is touching the magnets. Now carefully and delicately

alter the adjustment so that the diaphragm just clears the magnet and does not touch even with quite a strong signal. Next, without altering anything else, reverse the loudspeaker leads and one of two things will happen. Hither the reproduction will sound just as good (or possibly weaker to some degree) or you will once again hear the rattling noise showing that the diaphragm and magnets are touching each other Making Sure. you do hear this, then the connection you have at that time is the correct one, for it shows that in the way you have just joined up the leads the steady current is tending to accentuate the magnetism, and, therefore, vg sis exerting a greater attraction, which _- 3} what you want it to do. All you need now do is to readjust your diaphragm so that it is cleared once more, and you will be sure that your connection is right for this particular loundspeaker. If, however, as first mentioned, reproduction sounds practically the same, then it is probable ‘iat your first adjustment is the correct one, but to check this readjust the loudspeaker diaphragm until it just touches once more, and theu set it back to just clear again. Now reverse to the original loudspeaker connections and you will get the rattlin, noise referred to, confirming that the first connection was the correct one. In the case of loudspeakers which incorporate some output device such as an output choke’ or an output filter, which prevent pny direct current flowing through the loudspeaker winding. then it does not matte. which way round you connect your loudspeaker, because there is n. direct current to do harm. Furthcrmore, ‘the direct current being absent you will not find any difference in attraction one way or the other. Use Filter Output. T is a distinct advantage for two reason to have some form of output device of good quality. The first and most important is, not that the direct current is kept out of the londspeaker windings, as so many people think, but thst the resistance of the output device (if it is properly chosen) is much lower than that of the loudspeaker, and, therefore, you do not lose voltage in the comparatively high resistance of the luudspeaker windings. This is of great importauce with modern valves and, in fact, the latest superpower valves are practically useless without some form of output device. To illustrate this let us imagine we have a loudspeaker with a resistance of 2000 ohms-quite a common figure-and a super-power valve with a 4000-ohm resistance (not impedance). If now we apply *’ ' volts to the H.T. positive terminals, then the voltage applied to the valve itself will not be 120, but app. »ximately 80! By using an output device of suitable impedance, but of low D.C. resistance, we can see to it that the plate of the valve has nearly the full voltage applied to it by the battery. The correct use of super-power valves however, needs a special article, and, as we are dealing here solely with loudspeakers, we must next consider troubles that may arise with the cone type. In a number of these the type of movement known as the balancedarmature movement is used, no adjust-

ing screw of the type used with the horn loudspeakers being provided. In most cases, however, an examinution of the cone loudspeaker will show that a suitable adjusting screw is provided, and in loudspeakers which operate on a principle similar to that described for the horn speakers, an alteration of this adjusting screw will have the same effect as that described above, and the correct way round can be found by the same method. A few types of loudspeaker have no adjusting screw of the ordinary type, but have a tiny :.crew which locks the rod attached to the centre of the cone. When this is released the small rod attached to the actuating mechanism can move in and out of the central hole without actuating the diaphragm. The sound given when this is occurring is similar to that when the loudspeaker diaphragm is touching the magnet, but is much weaker. On some types of speaker this screw needs quite frequent adjustment, for the material of which the cone is made may be susceptible to changes in humidity and temperature. so that it is just as well with this type before starting the evening’s entertainment to release t2 serew for 2 moment and tighten it up again, so as to relieve any tension thit has been set up by atmospheric changes. A loose screw in such a loudspeaker is, therefore, one of the faults to look for when you are getting bad reproduction. Crackling Noises OUD crackling noises in any type of loudspeaker, with, perhaps, periodical cessation of signals, generally arise from a faulty winding. One must assume, of course, that another loudspeaker has been tried, and it has been discovered that this fault is not in the set or previous to it. The wire used to wind the electro-magnets in loudspeakers is extremely fine, in order that a sufficient number of turns may be wound into the small space available and if, during the winding process, a drop of moisture, such as can arise from perspiration, finds its way into the winding, the acidity may set up corrosion which, in course of time, will eat right through the wire. If you have had your loudspeaker for a few years and it has been very gradually falling off in strength, it is probable that the permanent magnets have lost some of their strength. This may oceur even when the loudspeaker ha: beer correctly connected. and the trouble will be accentu ‘ed. of course, when you have used it incorrectly joined up. Re-magnetising permanent magnets of loudspeakers is carried out by a number of firms for a very small charge and will give new life to many an old instrument. Contrary to a common belief, very few loudspeakers give blasting through loudspeaker overloading. One often hears it said that a certain loudspeaker is very good, but "cannot stand strong signols." signals," Nearly all eases of so-called loudspeaker overloading «re due to output put valve overloading.