Radio Broadcast (May 1929-Apr 1930)

DRIVING PIN VOICE COIL SPRINGS View of the inductor loud speaker motor with the permanent magnets removed. than on the armature bar AL>, thus moving the armature in the direction indicated. On the reverse of the cycle the armature moves in the opposite direction in the same manner. The pole legs are cut to the shape indicated to reduce the leakage flux and to bring the greatest flux density to the most desired point. If the inside spacing between the armature bars is equal to the center to center spacing of the pole faces, the flux in the magnetic circuit P1A1P1 varies 180° out of phase with the flux in the circuit P2A2P2 as the armature is moved to its two extremes. This is shown in Fig. 1, where the flux in the two paths is plotted against the displacement of the armature. The sum of these two curves is a straight line. This would give extreme sensitivity but there would then be no magnetic restoring force. If the armature bars are brought slightly closer together, a distance corresponding to 18 electrical degrees, the resulting curves will be those shown in Fig. 2. The sum of these two curves is the curve M, representing the change in total flux. This shows that the total flux is greatest when the armature is in its "at rest" position and represents the magnetic restoring force, or, if you will, the "magnetic stiffness." This is the design used in practice. Another explanation of the action in fewer words is that the opposite forces on the two armature bars cause the armature to rest at a middle position which we might call its "magnetic center." The flow of voice currents in the coils causes this "magnetic center" to shift and the armature moves along with the "magnetic center." It is now apparent that any d.c. component flowing in the windings would change the position of the armature by moving it to one side or the other, thus reducing its limit of motion in one direction. For this reason there must be no d.c. flowing through the windings, thus making it necessary to use an output transformer or a choke and condenser. However, if the loud speaker is to be used on a push-pull amplMier, a third lead may be taken from the windings at the point where the two coils are connected together and used as the mid-tap of the windings. This corresponds to the mid-tap on the primary of the usual output transformer. The d.c. which flows through the windings in this manner does not upset the " magnetic center." On the contrary it should be in such a direction as to aid the permanent flux through the poles. Doing away with the output transformer in this manner does away with its attendant losses and the gain is readily noticed by the ear. Operating Data It has been found that matching the impedance of the inductor dynamic to that of the amplifier with which it is to be used is of greater importance than it Fig. 3 — Schematic diagram explaining the principle of operation of the inductor loud speaker. was with the moving-coil dynamic. The impedance of the moving-coil dynamic may be varied over quite a wide range before the ear will detect any great change in operation. However, this is not the case with the inductor dynamic. If the loud DRIVING ROD ARMATURE vP0LE PIKtS Cutaway picture showing the placement of the armature between the pole faces of the magnet. speaker has too high an impedance for that of the amplifier with which it is used, the efficiency is lowered at the higher frequencies and increased at the lower frequencies. Since the loud speakers are made in four different models, each having a different impedance, this feature affords the listener the chance to pick a loud speaker which will give the balance of high and low frequencies which is most pleasing to him. Many moving-coil dynamics rely upon a mechanical resonance to give the impression that the loud speaker is reproducing the lower frequencies. The high efficiency of the inductor dynamic at these frequencies makes it unnecessary to depend upon any such "false bass." In fact, the resonance has been placed below sixty cycles. The springs supporting the armature are of very thin stock (0.008") and the entire armature assembly including springs, weighs but 4.5 grams as compared to 8 to 15 grams for the usual moving-coil dynamic. It is at the lower frequencies that the greatest difference is found between the two types of loud speakers. With an input of 15 db at 30 cycles the inductor motor moves a ten-inch cone one-eighth inch. The moving-coil dynamic is so inefficient from a standpoint of field excitation that it requires a heavy field structure of a coil and magnet whereas the inductor dynamic is so much more efficient that with two permanent magnets it will give the same output that may be obtained from a moving-coil dynamic using from ten to fifteen watts in the field. It would have been highly desirable before the advent of the inductor dynamic, if it were possible, to build a moving-coil dynamic loud speaker with permanent magnets. This was tried here and abroad unsuccessfully, as, due to the inefficiency of the field system, between 20 and 35 pounds of permanent magnets were necessary. To supply the power to the field of a moving-coil dynamic it has been common practice to use a rectifier which has introduced an objectionable hum in the loud speaker. The inductor dynamic does not add any additional hum to tha t of the set. View of an inductor loud speaker motor with a paper cone attached. This loud speaker, of course, must be placed behind a baffleboard for best results. • JUNE • 1 929 • • 95