A Home-made Dynamic Cone

Radio Record, Volume II, Issue 33, 1 March 1929, Page 28

A Home-made Dynamic Cone

Constructional Details of Radio’s Most Popular Speaker

By

PENTODE

ROBABLY no other accessory . has aroused such universal interest among all the wireless enthusiasts throughout the world as the moving coil speaker. ; There is no doubt, that, when properly used, it gives the most faithful reproduction of the original, and although it is not a new invention it is the very latest development offered by science to a critical public. No one can deny that the reproduction of gramophone records by any of the recently introduced electrically controlled gramophones is far short of listening to the original. Hach of these instruments uses a moving coil speaker fed by a powerful amplifier, similar; but on a larger scale, to that used in any multi-valve receivers. he amplifier is, of course, partly responsible for the high degree of perfection, but the speaker does its share in reproducing faithfullly what is fed into it by the amplifier. This is more than ean be said of any other type of speaker. The tendency nowadays among the greater part of the wireless enthusiasts is to improve their amplifiers. Power valves are used, reliable audio transformers are substituted, ete., but improvement in actual reproduction can only be advanced to one point, namely, the efficiency of the loud speaker to handle faithfully all that is delivered to it. After this, further improvement to the amplifier is futile. It is the ambition of many to own @ high-class speaker, one which will give true reproduction providing the amplifier approaches perfection. Knowang that the loud speaker is as near perfect as possible, it gives one far more comfort to realise that everything depends on the amplifier. Adjustments

can be made, and their effect noted, knowing that as far as the speaker is concerned it is quite reliable. Types of Dynamic Cones. BEFORD dealing with the constructional détails of a dynamic speaker, it would be just as well to enumerate the various types existing and describe the working principle of each. The moving coil speaker was discovered years ago, before the advent of broadeasting, but it is not until recently that it has been taken seriously, and improvements made. The first commercial speaker was known as the Rice Kellogg, and at the time of its release it created world-wide interest. In Bngland it was manufactured by the B.T.H. Company, who had the patent rights in that country. To many the price seemed exorbitant, but the whole unit also contained a power amplifier as well as the speaker, and the speaker could not be obtained without this amplifier. Previous to this came the Magnovox public address speaker, but this, of course, although operating on the moving coil principle, was a horn type of speaker, and was limited to the reproduction of the frequencies that the design of the horn would allow. The only advantage this speaker possessed was the enormous volume obtainable and was chiefly used for speech. Since the introduction of the R.K. only improvements in design and ase sembly have been evolved. The principle in all cases is identical. If a coil of wire through which an insulating current is passing is suspended in a powerful magnetic field with the lines of magnetic force cutting through the turns of wire at right

angles, the coil will move backward and forward in a piston movement, in time with the frequency of the alternations of current. Now, if a cone or other rigid surface is attached to the coil, the air will be made to move in the vicinity of the apparatus depending upon the movement of the coil and cone. Again, if the alternations of current were varying at the rate of 256 times a second, the coil and cone would move accordingly, and the note, middle ©, would be heard. This is the simple principle upon which the moving coil speaker depends and from which the wave is derived. In some types of speakers this magnetic field is produced by permanent magnets, while others use electro magnets. Since it is the latter type in which we are interested the writer will briefly show how the principles are embodied in practical methods. The whole speaker can be divided into two sections. In one the question of the strong magnetic field is studied, while

the other deals with the moving coil and cone. FIGURE I. shows a coil of wire wound around an iron cone. At one end an iron plate is fixed to an iron cover surrounding the whole. At the opposite end is a similar iron plate, except that a small air gap exists between it and the iron cone. Now, it is in-the air gap that the magnetic field has to be produced, and it is in this gap that the moving coil has to be suspended. Attached to the moving coil either direct or indirect, is the cone and various suspending and centring devices that will be mentioned later. Now, to many, .’.e construction of a speaker of this deseription will seem well-nigh impossible. The more one studies the various factory-built units the more

involved will it seem. Let it be said that most of the manufactured ones are built having a high degree of efficiency, but there is no reason why anyone handy with tools and possessed with a fair amount of patience-this latter virtue is important-should not build a good instrument providing they do not aim to have the finished ay@#le as neat and compact as the vt Rus commercial ones. Here are a few of the most important points upon which the efficiency of 1 speaker depends :-- 1. The magnetic field in which the moving coil, or "voice coil" as it is called in America, is suspended. 2. The method of suspension and centring of the coil and cone. 8. The material of which the cone is made and the shape of the same. 4. The number of turns of wire on the moving coil and their relation to the output of the amplifier. In each of these cases it will be found that manufacturers have taken

great precautions. Take point No. 1. This magnetic field in the type of speaker under consideration, is produced by direct current flowing through the larger coil inside the "Pot," This is called the field coil. The amount of magnetism depends upon two or three factors not in the least being the quality of the iron of which the pot is made. Dynamo steel is best but difficult to obtain and machine. To obtain the same effect with ordinary cast iron, a larger amount to be used, therefore necessitating/ a larger pot. ae The density of the magnetism in the gap also depends upon the width of the gap. Now to suspend a coil in a gap of 5-64 of an inch is an operation a little too delicate for the amateur, so to get the same magnetic density in 1

‘larger gap means using a larger field coil and also a larger field current. These were the two points referred, to when it was suggested that the amateur should not attempt to copy, in size and field coil current economy,

i= * a SSS Hee 8 the speakers manufactured by wellown firms who spend thousands in w@estalling machinery to do operations most impossible by the human hand. Now that these points have been cleared up let us get to work on a moyv-

ing coil speaker, considered by the writer to be the simplest as far as construction goes. Obtaining the Pot Magnet. H® first essential is to obtain the pot magnet. Starting with a length of iron water pipe with an internal diameter of at least 44 inches put into the lathe and clean off each end to a length of 4% inches. To anyone who has the necessary lathe, ete.. the diagrams are of sufficient

ape value to work on, Two end plates will have to be cut from 4 inch steel or iron and have to be shaped as shown so that the length of pipe will fit into a 3-16 inch rabbit in the end pieces. Into the middle of one end a 2 1-16 inch hole.

--= The centre of the other end is recessed 13 inch diameter, into which the specially shaped core piece must be bolted from the outside. This core is 1$ inch diameter and must be machined accurately and cleaned on both faces of the gap so that a uniform space of 5-32 isch is left. All machinery must be done accurately so that when the whole is bolted together it is quite rigid. It is essential that the joints are perfect otherwise a magnetic loss will occur and the efficiency of the speaker impaired. Wuile in the hands of the engineer get him to drill three 4 inch holes equidistant in the front piece as shown and 2 inch from the edge of the gap. This section of the construction is passed over quickly as it will be assumed that the reader has entrusted this part to a professional turner who ean obtain all information from the diagrams. The main point to impress is that the gap when finished will be an even 5-32 inch all round and % inch deep. A suitable cast "pot" is obtainable from the Precision Engineering Co., Wellington, for about £3 10. The Field Magnet Wiring. Now comes the field winding and "Pentode" has thought it advisable to give a_ table of the wire to be used and number of.turns, ete., for different supplies of D.C. . The field winding can be used.as a choke in the smoothing system when working from the AC and this is an economical way of producing the magnetic field. Others will want to use their A battery to energise the magnet and reference to the table will show the number of turns and gauge of wi.e, etc. Choose the D.C. voltage supply available and work according to that specification. Say it is desired to work off the 6 volt A battery. Then 1000 turns of 18 D.C.C. SWC wound in 15 layers will be suitable and will consume 2 ampres while in use. This may seem a considerable drain but, as has been mentioned before this is due to the relatively large gap being used and also to make up for losses due to incorrect core material. A word to those who contemplate using the pot magnet as a choke in’ a smoothing: system. Calculate the total current in milli- amps that will be passing through the coil. Say it is 80 m.a,, then by using the 230. volt winding the voltage drop across the choke will be 280 volts. and this will have to be made up on the rectifying valve end. It is expected that most constructors will use the 6 or 12 volt table. If the reader considers 2 ampres too much of a drain on his 6 volt. battery, he can use the 12 volt winding which takes only $ ampres but the sensitivity of the speaker will be cut down accordingly.

"Before the field coil can be wound a bobbin has to be made so that it will slip over the centre core into the pot itself. A piece of wood slightly in excess of 1% inch diameter can be used round which a strip of 20 gauge brass or copper sheet has been wound, Two end pieces 43 inch diameter in which a 2 inch diameter hole has been cut out of the centre are soldered on at each end forming a spool. These end pieces can be cut from No. 16 gauge metal A section of the winding space of the finished spool must be 3% inch x 1 8-16 inch, The wooden former round which the spool has been made can be kept in place until winding has been completed making use of it to mount the

whole on a winder. Drill two holes in one end of the spool to pass the beginning and end of the wires out. Now glue three or four layers of brown paper over the inside of the spool; ends as well so that everywhere the wire touches will be thoroughly insulated. Give the whole a liberal coating of shellac varnish. It must be realised that whenever the field current is shut off quickly a great voltage potential is built up at either end and special care must be given to insulation. Six volts is thought to be nothing, but it is this built-up voltage when the circuit .is broken that is liable to do damage. Some form of winder will be needed and this will have to be left to the oe ees

discretion of the reader. The essentials are that it must be strong and the turns able to be counted as_ they are wound on. The winding just deseribed by "Megohm" is very suitable. Using gauge wire no gearing is recommended, the wire being wound on in layers by hand. Slip a length of insulated slewing over the beginning and ends of the wire where they pass through the spool ends. If any layer becomes irregular wind one or two turns of brown paper before beginning the next. It is advisable to include a turn of brown paper between each layer, but if this is not done, don’t be sparing with the shellac. When the correct number of turns have been wound on, finish off by winding insulating tape round the outside of the wire. Dismantle from .the winder and knock out the wooden centre. Before sliding into the pot mark in the end of the pot where the two ends of the wire will core and drill two holes to pass the insulated field magnet wires for connections outside. One or two turns of stout brown paper over the iron core will let the spool slide on, a tight fit. The Front Plate. HE front plate needs a little attention before it can be finally screwed into place. Drill and tap an 1-8 inch hole down the centre of a short length of }-inch diameter round

or square brass rod. Now cut off three pieces each being exactly 3-8-inch long. Three 14-inch bolts (1-8-inch Whitworth thread) are passed through the three holes in the end plate, from the inside, and fastened on the outsid» by the three tapped pieces of brass. About }-inch of the threaded end of the bolt will be projecting outside. Tighten up each one firmly and screw the front plate firmly to the remainder of the pot. This completes the first half of the speake. and it can be put aside while the cone and moving coil are being constructed. A single article giving all the details would involve too much space, and the next issue will deal with the remainder, and give useful hints for its use.

Volts S.W.G. Ibs. Layers Turns resistance Ourrent Guage Ohms 6 18 d.e.e. 44 15 1,000 2.9 2.1 amps. 12 20 d.c.ec. 44 20 1,700 8.4 1.4 amps. 120 8¢4enam. 34 67 19,000 1500 67 m.a. 230 38 enam. 3 92 87,000 6500 30 m.a. — SS eS SSE 1 |

Table of Wire for Field Coil.