A Quality Loud-Speaker

Radio Record, Volume II, Issue 42, 3 May 1929, Page 29

A Quality Loud-Speaker

A Novel Cone-Speaker Easily Constructed

By

MEGOHM

HIS loudspeaker that has been specially designed to suit the "Blue Spot" 66A unit, which has a four-pole balanced ar‘mature, which makes it particularly. suited for handling large volume with consequent emphasis. of the bass. The speaker will be found to have a pleasing mellow .tone,-not always a feature of homeafstiatas speakers, and owing to its design, static and unwanted gh frequencies are well subdued, whilst, provided the amplifier is reasonably efficient, the quality of music and speech will leave little to be desired. Figure 1 shows the completed speaker, which may be finished in any parway, the constructor fancies,

but dark;.stain produces a very satisfactory finish. The front grille is not an essential portion of the speaker,- but givcs a pleasing and ornamental finish. The design of the front may be altered: to suit the taste of the constructor, or it may. be dispensed with altogether. An essential feature of the woodwork is. that it should be a good and

solid job. The main portion is constructed of three pieces of inch rimu as shown in Figure 2. The 1-in. timber will be actually 7-8 thick. The unit is uttached to the back by a strip of wood through which a wood screw passes. This method of attachment makes it an easy matter to alter the position of the unit slightly when placing tHe cone, The unit is first to be secured with its driving spindle exactly over the centre of the back of the case. + Sides of 3-8 or din. timber are now to be fitted and screwed on, the joints to be filled with glue or seccotine. A light frame to hold the cone ts now made from two strips of rimu 1 by jin. and two strips 5-8 by tin. The length of the latter will be about 9in., and the former about 12hin. The method of joining the corners is clearly shown in figure 3, a }in. hole being drilled through the end of the tin. strips to take a No. 5 Sin. screw. This frame should be close-fitting: to the inside of the case, so that it will "stay put" whilst adjusting the cone. The cord to the unit is led out at the most convenient place. ° The front may be of din. rimu, and the bars a full din. wide and #in. thick. These should be drilled at the ends and secured by thin screws in order to avoid splitting the front. The total height of the front is 17 inches, and width 1034 inches, which allows it to project about jin. each: side. The method of marking out the oval opening is shown in figure 9. A vertical centre line is drawn, and at right angles to this at a height of 8% inches another line is drawn, the intersection being the centre of the oval Ata distance each side of centre on the vertical line, 3fin., a pin or small brad is inserted (B and ©), and at A, 33in from the’ centre, a pin is also inserted. Now tie a piece of thread round the outside of the three pins, forming ‘a triangle. Next withdraw the pin A and in its place hold the point of a pencil, which can be moved around as if to.form a circle, keeping the thread taut, and an oval of the required size, 103in. by Ttin., will be drawn. This is then cut out with a keyhole saw and the front edge well rounded off with a spokeshave and glags-paper. Two ‘strips lin. by #in. are screwed to the back as shown in figure 8. When attaching the front to the case for the’ first time, a few scraps of thick cardboard should be placed on the edges of the case so that a space of 1-16in. or so will be left between case and front to prevent any chance of rattle. When finished,the under side of the speaker is 1 5-16in. above the table, and the back support is made the requisite height. Making the Cone, ATERIAL for the cone is made by pasting together two thicknesses of soft "cover-papet" obtained from a printer. "The most suitable kinds are usually in good grey and brown shades. Paper with a hard, glossy finish is un-

suitable. Two pieces 15in. square pasted together will be sufficient. Use cornflour paste, and dry for a few hours under flat pressure, Fig. 6 shows how «Oo mark out the cone. First describe a circle with a radius of 7Tiin. In lieu of compasses, use a strip of cardboard with a pin at one end, and a hole for the pencil point, just 7iin. from the pin. Now mark a point on the circle, and from this measure off a straight line 10 5-16 inches long, and from each end of this in the same manner mark off 7 7-8in. and another 10 5-16in. Now join these points to the centre, and at X draw a Une jin. outside the one running to the centre. This jin. is for joining up. Fold the cone along the four dotted lines, keeping the pencil marks outside, and join up with'seccotine, keeping the fin, piece on the outside, where it will not show. We now have a cone of pyramid shape. A floating edge has now to be attached, and this may be of thin chamois leather preferably, but thin fabric such as serge or calico. may be used if cut "on the bias." One strip is required tor each of the four sides, width a little over one inch. . Now apply seccotine to.a width of qin. to the outside edges of the cone, and attach the leather or fabric strips, overlapping fin. At the corners mitre the strips, but. do not overlap them. When the strips are firmly attached to the cone the frame is taken, and to the side on which the wood strips are all level the floating strips are carefully pinned down in position. When corréctly centred, remove the pins one by one, and apply seccotine to hold the leather or fabric,. which should not be

pulled too tight, though slackness must be removed. Adjusting the Cone. WHEN all is set, the speaker is plac ed on its back, and the cone, with hole at the apex, is gently lowered, keeping even with sides of the case. If it does not drop exactly over the end of the spindle, feel for its position to show how far the unit is to be moved, and finally get the spindle to its exact position. . The back nut should be about half an inch from the end of the spindle, and should be secured to it with seccotine at the back. Figure 7 shows the order in which washers are placed upon the spindle. The card washers may be thick paper, and the metal washers should not exceed 8-8in, diameter. Screw up tightly, as any slackness means loss of volume. — Finally, secure the front nut with seccotine to prevent it working loose and rattling. Now, with the speaker connected to the receiver giving good volume, adjust the cone by moving backwards or forwards until the best position is found. The tendency should be to keep the cone forward as, much as possible. When the position has been settled, the cone may be held in place by a couple of small brads through the -tin. strips of the frame. The 66a unit retails at 30s, and may be procured through any radio dealer, but in case of difficulty may be. obtained. from the Rodger Importing Co., 159 Manchester Street, Christchurch, for 31s. post free. A Baffle-board Cone. A cone with less work in construction can be made for a balanced-arma-ture unit by pasting together three thicknesses of cover paper as already mentioned. Out out a _ circle nine inches diameter, cutting out a wedgeshaped section of 50 degrees or 1-7th the circumference. Join with seccotine. The baffle-board is three-ply 2ft. by $ft., or larger, with a round hole in the centre half an inch larger in dia-

meter than the cone. -A quarter-inch is ‘turned flat all round the edge of the X cone, and ‘to this is attached leather

cut to the correct curve, in strips iin. wide. The outside edge of leather is then glued to the. édge of thé hole in baffle. A 4 by 1 strip is screwed to top and bottom edge of back of baffle, and to this is secured a similar strip to hold the unit at the correct distance behind the cone. The unit is clamped to the strip with a cross-piece of wood and a screw. Wither side of the cone may face the front.