A Home-Built Loud Speaker Unit for Cone

Radio Record, Volume II, Issue 4, 10 August 1928, Page 30

A Home-Built Loud Speaker Unit for Cone

By

Megohm

' ‘recent: number of "Radio ' News" contains. the following: description of a home-built speaker unit which (ill no’ doubt insmopss «terest many: prospective constructors of cone speakers. This ‘description was awardec a fifty-dollar first prize in a competition. THIS loudspeaker unit may be built almost without cost by the me-

chanically-inclined radio amateur. It will operate a 36-inch cone-type loudspeaker very satisfactorily, and most of the parts required may be found in an experimenter’s junk box. All that is needed ure u Ford spark coil, a magnet from 2a Ford mygneto, a small bobbin from’ a buzzer, a piece of soft iron 10 x 3 x 1-8 inches, a piece of thin flex-

ible iron : Ot x 1- 32 iriches; a brass driving rod, nuts and screws. ‘Ta the construction of the. unit the fir st step is to-break t: magnet in two pieces and discard one half; Next, take th. half which is to be-used in building the speaker unit and determine the north and south poles with a pocket compass. This magnet is 43 x ? x 7-16 inches. The 10-inch length of soft iron must be eut into three pieces with a hack saw. The first section i 4% inches in k th and this is formed as illustrated at B (Fig. 2); the second section is 14 inches long and is formed as shown at C: and the third section is used as a mounting bracket for the unit. The last section may be formed after the construction has been. completed, anid the design which should be followed is shown at G. -- Winding the loudspeaker coil is the problem to consider. Take 2 bobbin, similar to the one illustrated at If (Fig. 2), and wine it ith wire — remored from the secondary winding of a Ford spark coil. As much wire as possible should be placed on.the bokbin. After the directions given in the above paragraphs have been followed out, the yarious parts have been prepared and the unit may be assembled. 'In Fig. 1 the mechanical details of the completed unit are shown. As_ the magnet is hard and eannot be drilled, the yarious parts must be. soldered il place. The 21-inch piece of ilexible iron strip (A) serves as the armature and is fastened to section B as ilnstrated. Also holes must be drilled in and B for the adjustment serew 1D. Now proceed by soldering the various parts to the magnet und -the driving rod (KF) to the armature.

After the speaker unit has been completed it may be connected to a cone in the usual manner; and the two wires from the coil connected to the output binding posts of the set. In order ‘to adjust the speaker, the air gap between the armature and the pole-piece of the coil is changed until best results are obtained. This is accomplished by running the two nuts on the adjustment screw up and down. [tz should be understood that the above unit employing only a single electro-magnet, will require heavy volume to operate it successfully. Al-

though the directions advise soldering io the magnet, most coustructors would find this too difficult, and it would not improve the magnet. A. better method would be to clamp on strips of 1-Sth brass by bolts through holes in the projecting ends. art B could be elamped to magnet in this way. Other parts could be soldered to the brass strips. ,

[HIS article is given as an idea for. any who may care to construct a driving unit for he linén-diaphrigm speaker, but is not particularly recommended for that purpose. It would be quite possible for an ingenious constructor -to combine two small units-with pair of coils opposing and armature between, and thus obtain the double action that is employed in some good types of cone speakers. The balanced armature type of driving unit does not appear to be sold separately from the speaker in New, Zealand at present. Inquiries at Harrington’s Ltd., Wellington, elicited the fact. that about the end of next month they will land a shipment of a unit yery suitable for the purpose retailing . at: 308. . *, A Medium-sized & Unit: A NUMBER of inquiries have come to hand regarding the suitdbility of the "Lessenola" unit, which is stocked by Messrs. M. Silver and Co., Manners Street, This is a yery satisfactory unit for the money where medium volume is to be dealt with by either horn. or cone. speaker, but is not sufficiently powerful to bring out the full capabilities of the linendiaphragm speaker. This unit: retails at 17s. 6d., complete with full-size patterns and instructions for makiug what is practically a three-foot semi-ex-ponential horn of either heavy cardboard or three-ply. For 1s. 6d. extra 4 reed is supplied that will allow of the unit being used to drive : a cone dia» phragm. ©