The Moving-Coil Loud-Speaker

Radio Record, Volume II, Issue 7, 31 August 1928, Page 26

The Moving-Coil Loud-Speaker

Principles and Design

(By

Megohm

OVING-COITL or — coildriven loud speakers, | also called "dynamic," | have not as yet been | much used in New Zea- | jJand. beeause un to the

present ve.y few have been imported. There are signs now of an awakening in this respect, owing no doubt to the good reports of this type of speaker, and at least one firm has a shipment to arrive very shortly. Owing to the construction of the speaker, it lends itself to amateur canstruction so long as the necessary castings can be obtained ready made. A great many different makes of ready machined castings are now obtainable a

in England, and no doubt suitable designs will shortly be imported into New Zealand, Prices for the complete speaker in England run from about eight or ten guineas up. Cheaper makes are advertised, but probably their performance will be only in keeping with the price. Essential parts, the castings and diaphragm, are sold in sets at various prices from £4 up. One make is advertised as parts that may be assembled in five minutes, so that the Windings and cone must be compiete.

General Design. py appearance, a moving-coil speaker at once conveys the impression that it is a cone speaker of some kind, but an examination of the principles involved in its working, the great ditfference between this and ordinary cone or horn speakers is very evident. Moving parts are practically down to the irreducible minimum, as the cone-shaped diaphragm and tiny coil attached to its apex form the sole moving portion. This small coil of fine wire is the "moving coil" and is in circuit with the plate of the last valve of the receiver. This coil is situated in a strong magnetic field generally produced by aun electro-magnet, sometimes by permanent magnets, and is free to move as actuated by the alternating plate current traversing its turns, communicating such movement direct to the cone diaphragm. The more powerful and most effective speakers are actuated by a powerful electro:magnet which must be supplied with smooth continuous current during such time as_ the speaker is in operation, so that this current constitutes e running cost that does not enter into the operation of ordinary loud-speakers.. Then again, in order to keep the moving coil as light as possible, it contains comparatively few turns of wire, and on that account a receiver capable of giving heavy volume is essential for satisfactory operation. These two points, however, Will generally be easily overcome, and where electric mains are available, the current supply for the field magnet becomes a comparatively ceonomical

proposition, and the gain in quality of reproduction is well worth the small additional expenditure. Where the supply of energising current is impracticable, as in many country places, there is still the permanent magnet type, which then costs no more to run than does a speaker of ordinary type, and will give volume in proportion to the output of the receiver in use. The "Pot" Magnet. PHS | field magnet is usualy referred as the "pot," and differs in 1 from the ordinary electromagnet in the way the two poles are brought into close proximity, leaving a ring-shaped or annular space in which the moyving-coil ‘works in a highly magnetic field. Referring to the C’agram, it will be seen that the field magnet winding encircles the centre projection inside the po’ casting, whilst the sides act as a connection to the opposite pole, which is a round dise of iron with circular hole in the centre, and screws secure it to the top edge of the pot, leaving the annular space between itself and the central core. Taking the dimensions from a (lescription of castings for the purpose, the central] core is 2 inches in diameter. over 834 inches long, outside diameter of pot 5 inches, and over 44 inches deep. Int. the space in the pot is dropped the winding, the wire and number of turns suited to the current available.. For operation from 240-volt dire-#-current mains the winding wald consist of 87,000 turns of No. 36 s.g.e, Wire. A resistance may also be necessary in the circuit to prevent overheating oJ the winding, though its resistance .,ould be about 1600 ohms, A wire-wound buffer resistance of 4000 to 6000 ohms is essential across the magnet terminals to prevent damage from voltage surges when switching. For operation from a 6-velt battery. the above-mentioned magnet may be wound with 5lb. ef No. 16 d.e.c., givin GOO turns, amps. current, or 1000 turn of 18's d.e.c. From }$ to $ amp. is th usual consumption when working off : 6-volt battery. but the windings men tioned would consuine rather more. Operation from alternating current mains presents a rather more difficult problem, as the current must be rectified and smoothed. One method of overcoming this difficulty is to employ a full-wave A battery charger capable of delivering 20 volts from each side of the transformer secondary, and a filament winding giving two volts. A Philips rectifving valve No, 867 is used in conjunction with the two suitable resistance lamps (840), and an output of current up to G amps. may be obtained. A magnet winding to suit this supply would have about 2600 turns of 22's d.cae. (4lb.), taking about 6 amp. The Moving Coil. Two types of moving coil are in use, one high and the other low resist-

ance. ‘To suit the castings above res ferred to, the inside diameter of the coil would be just over 2 inches, a loose fit over the central core, just sufficient clearance being given to allow of free movement of the coil without touching the core, The annular space in the magnet is 3-32nds. of an inch in width, so that the thickness of the coil must be rather less than this, with a depth of about 5-8th inch. A low resistance coil is wound with 38’s or 40’s enamelled wire, about 250 turns being put on a former of thin paper. When a low resistance coil is used, a step-down transformer of perhaps 25 to 1 ratio is necessary between the receiver and speaker. A high resistance coil is wound with 47’s enamelled wire, is difficult to wind, . and burns out easily, and for this reason constructors are inclined to favour the low- resistance coil. As many as 1500 turns of fine wire may be wound in the small space available, but a choke-condenser filter must be used to protect the fine winding from plate current. A low resistance coil will give more even response in the range between 50 and 8.000 cycles than will. the coil of high resistance. The Diaphragm. qt -is important that the weight -or mass of the cone and coil combined be kept as low as possible. Wxtra size favours the low frequencies at the | expense of the high. Some speakers have diaphragms only . G inches in diameter, but are liable in | some cases to lack the bass notes, and to make the letter "s" whistle. An average size that is used is 9 or 10 inches in diameter. Drawing-paper known as "detail paper" is suitable for the construction of the cone. If the cone is not of good design it is liable to have objectionable resonance at certain frequencies. . The ecge or the cone is attached to | the outer iron frame by means of a.

circular strip of sheepskin or similar very pliable mat:rial, as a more free method of suspension tends towards — weakening of bigh notes. It will be seen from the fact of the coil having such small space in which to move freely, that its correct adjiistment is highly important if noise? ‘ r caused by friction with the magnet is , to be avoided. In many models three or four threads are attached to the back of the eone, branching out radially, and ailjusted by means of fine thread screws, so that the coil may be kept central in the annular space.

»--- ( Sometimes 'a small cone with ape® facing outwards is secured to the inside of the large cone, its diameter equalling that of the moving coil. This avdition will in some cases make an jixprovement in bass note , reproduction. In order to get good reproduction 4 baffle-board of some kind should be used. This should be of three-ply or heavy millboard three or four feet square, With a round hole in the centre through which the speaker works. Merits of the Moving Coil. "tHE above details are not given as working instructions, but with a view to giving readers a fairly accurate idea of the construction work involved in a moving-coil speaker. Its construction allows of liberal movement of the diaphragm without obstruction such as causes "chattering" in ordinary speakers. Coil-driven speakers have far less resonance than ordinary types, and give more even amplification of all frequencies, as the diaphragm correctly follows the voltages applied to the grid of the last valve, The bass note reproduction gives the effect of overall sound intensity, and the higher frequencies give the quality of the transmission. The coildriven diaphragm is free to be moved through the liberal amplitude necessary to reproduce intensities of volume in proportion to the frequency.