Protecting the Loudspeaker

Radio Record, Volume I, Issue 29, 3 February 1928, Page 12

Protecting the Loudspeaker

OUTPUT FILTER CHOKES Se

Lov cost of production probably accounts for the large number of receivers in which no provision is made to keep the D.C. component out of the loudspeaker. Of sets manufactured in Britain during the past year, 83 per cent. provided for direct coupling to the speaker, 9 per cent. had a built-in choke filter, and 8 per cent. were fitted with an output trausformer. Where the ontpnt of .a set is not Jarge, H.T. voliage low, and only a few milliamps being passed by the last valve, there is no great need for a filter, but the speaker must be connected with the correct polarity. But when the output of the last valve is from 10 to 20 milliamps and over 90 volts somé means of protecting the speaker windjugs is very necessary. The output of the last valve of a reeeiyer is composed of direct current from the high-tension battery, and a superimposed alternating current which actuates the loudspeaker. In order to get sufficient turns of eire on the speaker inagnets so that they may be influenced by the small alternating current, the wire used must be very fine-too fine to properly carry even a sinall direct current without offering heavy resistance. And as the direct current component does no usefnl work in the speaker portion of the circuit, it may be shunted or by-passed back to the battery without traversing the speaker windings. This is accomplished by means of a low-frequency iron-cored choke coil of many turns of fine wire. This coil allows direct current to pass fairly easily provided that its resistance is not made tao high by having the wire of too great a length or too fine a gange. This coil, consisting as it does of a large number of turns of wire over an iton core, has the property of "choking" back or stopping the progress of alternating current, which must therefore pass through the speaker windings and by influencing the Strength of the magnets, actuate the diaphragm. A fixed condenser of larze capacity. is also included in the Cireuit

If an output filter is necessary, .then it is because the output of the receiver | is comparatively large, so that. the direct-current component will be large, and must not be. unduly retarded in its path, otherwise the plate voltage on the last valve is -unnecessarily reduced, A choke coil such as is under discussion is measured in two different ways, firstly there. is the resistance in ohms offered to the passage of direct current, and consistent with the number of turns necessaty, this factor must be kept down by not employing too fine a gauge of wire. Secondly, there is the value of the coil as an inductance, which is expressed in ‘‘Henries." Inductance is the value of the tendency to produce ‘back voltage" due to the action between the turns of wire and the magnetic flux surrounding them. ‘This intersection of turns and lines of force produces a voltage in the coil that tends to resist the change in current, and this ‘"‘back voltage" is always opposed to the applied E.M.F., and by opposing any rapid change in direction, stops the progress of alternating current. Another factor demanding, within reason, the limitation of turns is that of "saturation.’? For every case there is a number of turns that will produce magnetic saturation of the core, and once this saturation point is reached, it cannot be exceeded, but it is important to note that when this point is being approached, the impedence or "choking" qualities of the coil are greatly impaired. In purchasing a choke coil, it is as well to know, if possible, the amount of direct current it will carry without saturation. As the number of turns to be employed is necessarily large, saturation by a small current is avoided by making one or more ‘gaps’? in the core, and although such gaps may be less than one sixtyfourth of an inch, with them much more current may be carried without saturation of the core. CHOKE FILTER CIRCUITS. Three regular methods of coupling are in general use No. 1 being the

most popular, as shown in the diagram, in which a low-voltage conden--ser of two microfarads may be used, as there is little danger of it breaking down, as the only voltage impressed across it is the drop in the choke. The second method is a variation of the first, effected by placing an additional 2 mfd. condenser in series at A. This method cuts the speaker from any direct connection witn the set, and is good where long leads are in use to’ other rooms or the garden. The third method, shown in figure 2 is a good circuit, and is particularly recommended where a B eliminator is_ in use, as the grounding by connection to negative A assists in eliminating lum. In this circuit the loudspeaker has no high voltage upon it,

re re but as the full B potential is only prevented from shorting through the spcaker by the fixed condenser, this latter taust be of a guaranteed high voltage type. The value of coupling condensers may be from 2 to 6 microfarads, but the former is usualiy quite sufficient capacity. It is not wise to go below this value as the larger the capacity, the more will the low audio frequencies be heard. VALUES OF CHOKE COILS Although choke coils of as high an inductance as 100 henries haye been used in filter circuits, such a high. value is not recommended, as the direct current resistance will probably be much too high. On the other hand a value as low as 15 henries may be very successfully used, and 20 henries is quite a tegular standard value. This type of choke coil is essentially one of low inductance, and the employment of those intended for impedance coupling in amplifiers will not necessarily be successful. Values up to 30 and 50 henries will in many cases be quite suitable for the purpose, provided the winding is not unduly fine. MAKING A CHOKE COIL. A good filter choke is easily made of stalloy strips one inch wide, costing 8s. 6d. per dozen in 8ft. lengths (Messrs. Johns, Ltd., Chancery Street, Auckland). About 16 lengths will be required. The core is built up one inch in thickness. Nothing is gained by cutting down the cross-section of the core, as much more wire must be used to obtain the same inductance value. The stalloy is cut wita ordinary snips, but as this process is inclined to curve the ends where cut, this curve should be tapped out on an iron surface with a hammer, Tour sizes of strip must be cut, 83, 24, 1 11-16, and 11-16 long respectively. Sufficient of each must be cut to form a pile one inch high when compressed. For clamps four pieces of wood are required 4in. long, lin. wide, } thick, drilled at each end, centres 3}in. apart, The spool-ends are cut from millboard, Qhin. square, with a centre hole just over one inch square in order to glue over the ends of thin card used for centre of spool, and bent round former. A handy winding former may be built up as shown, or made of a sinall block drilled through the centre or as convergent for mounting on a winding spindle. The spool is wound with three-quar-ters of a pound of 36’s s.w.g. enamelled i eh ml es te te ec te a en a wt

wk Sad Ale ba "0 Le ladle tert Aah treme ort sah et rd ncn wire, winding irregularly in’ groups, putting a tissue-paper in- occasionally. When winding is complete, with the two ends of the wire through the spool ends, the 3}in. strips are packed in to fill the centre of the spool. Then the other three sizes of strips are assembled as shown, brick-wise, so that joints do not coincide in adjacent layers. , The ends AA are butted against BB with a piece of visiting card or slightly thicker .card, ‘separating the point of contact: The pair of clamps is then placed upon each end and the whole held: together with four brass bolts 2in. long by 3-16 thick. A small piece of ebonite is then screwed to the top clamps, projecting each side to take terminals for the two leads. This is a good choke for average conditions, voltages from 90 upwards. The theoretically correct gap for.a stalling iron choke is 0.005 (5-1000ths) of an inch for every inch of core length, but the best amount can only be properly determined by trial upon the work for which the choke is intended, but in. the case of a filter is not usually very critical. ‘The inductance of the choke coil is proportional to the cross-section of the core, to the square of the number of turns of wire, and inversely to the length of the air gap. The alternative to choke-condenser coupling is the use of an output transformer of 1-1 ratio specially made for the purpose, passing only alternating current to the speaker, but it is important for good results that the impedance of the transformer secondary winding should be approximately equal to that of the loudspeaker. (End of Construction Section.)