Because of the way the human ear responds to sound, the greatest demand. for power occurs in the sub-bass and bass frequencies. In order to supply these frequencies at a sufficient power level to the appropriate speaker, a crossover network needs to reject all higher frequencies without depressing the amplitude of the bass frequencies in any significant way.

Typically, in the past, iron-core inductors have been employed to achieve the required inductance needed in a crossover network, using coils of reasonable size and low DC resistance, to assure an efficient transfer of amplifier power to the driver loudspeaker load while at the same time being able to dissipate the heat resulting from high current. These inductors, however, are typically massive in size. An. example of such an inductor is one formed from two U-shaped pieces and joined together as a rectangular piece of metal and glued together at the ends of their respective arms. Two

opposite arms of each piece are relieved in width so as to permit a coil to be wound about it. The two arms that are glued together form an iron core for the coil.

However, the iron core is discontinuous in the center of the coil where the two pieces are glued together and, more importantly, in the cradle beneath the coil. The discontinuity, especially in the cradle portion, creates losses in the magneticfield generated by the coil and otherwise generally contained within the core. This effects the inductor's figure of merit or"Q"value.

The Q of an inductor describes its loss characteristics at a specified frequency. It is expressed as Q=XL/RAC where XL is the inductive reactance.

RAC is a resistance factor which includes all loss factors including the DC resistance of the inductor. There are other loss factors, however, besides the DC resistance.

There is an induced series resistance RS which represents a loss or barrier to current at the specified frequency and is present as long as that frequency exists at the inductor. This resistance includes copper eddy current loss, iron eddy current loss and hysteresis loss of the magnetic material, all of which must be compensated for by the amplifier into its load. This resistance is often considered as a parallel resistance Rp across the inductor and is related by the dissipation factor D where D=1/Q.

Since Q varies proportionately with XL which, in turn, is a function of frequency, it follows that R, increases with frequency. This means that the bass audio output becomes depressed in the presence of a high-frequency input because of an increase in Rg. In other words, sound that includes a significant high-frequency component leads to a rise in the AC resistance of the inductor thereby decreasing its efficiency for as long as the high- frequency signal lasts. This consideration is therefore a significant problem in broadband performance over several octaves of audio.

In the past, manufacturers have principally turned to iron lamination cores to provide high power handling inductors. These cores are thought to provide low resistance with a minimum number of coil turns.

These inductors are efficient at utility power frequencies (50-60 HZ) but in this usage 400 Hz is considered a"high frequency."Audio frequencies, however, typically encompass the range of 20 to 20,000 HZ and higher. Furthermore, the high frequency content typically supplied to loudspeaker systems by digital. audio components contain harmonics which were, at one time, thought to be beyond the range of human hearing.

Frequencies and amplifier response above 20,000 HZ, however, do effect the quality of the sound sensed by the ear.

Massive iron core inductors such as those described above, if made large enough, can provide high current handling capability for inductor loudspeaker systems. There are numerous problems with such inductors however. One problem is that such inductors generate EMI which can influence the performance of adjacent audio components such as loudspeaker coils. A second problem with such inductors is that they are both massive and expensive. Lastly, the only way to adjust the inductance of these devices is to peel the turns of the coil which is a labor intensive task and expensive for mass production techniques. There is, at present, no effective way of adjusting the impedance of an inductor other than by choice of the number of turns of wire wound about its core. The only other alternative is to produce inductors of different sizes which is also costly and inefficient.

BRIEF SUMMARY OF THE INVENTION A highly efficient inexpensive inductor for high-power broadband frequency applications is provided by a coil having an input and an output end and wound

about a solid continuous core of magnetically permeable material. The core has first and second ends which are magnetically coupled respectively to a substantially U-shaped cradle made of the same magnetically permeable material. The core has a predetermined longitudinal length and the cradle extends longitudinally for a length that substantially does not exceed the length of the core. The core has a cross-sectional area which is related to the cross-sectional area of the cradle such that the ratio of the core cross-sectional area to the cradle cross-sectional area lies in a range from about 1/1.25 to 1/1.5.

The preferred shape of the core is a circular cylindrical shape and the preferred shape of the cross- section of the cradle is rectangular. In order to maximize the surface contact between the core and the cradle when the two are resting together, the opposite ends of the core are beveled at an angle of about 25°.

The end arms of the cradle have a corresponding bevel angle that fits in a butting relationship against the beveled surface of the core.

Both the core and the cradle are made from a powdered iron material characterized by low loss and high magnetic permeability.

In another aspect of the invention, the inductance of the inductor may be adjusted by selectively adding shims or spacers made of non-magnetic material such as paper or plastic interposed between the beveled surfaces of the core and the beveled surfaces of the cradle.

The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS FIG. 1 is a side view of a preferred form of an inductor embodying the present invention.

FIG. 2 is an end view of the inductor of FIG. 1.

FIG. 3 is a partial cut away side view of the inductor of FIG. 1 with an added shim for adjusting the inductance.

FIG. 4 is a side view of a second embodiment of the invention using a second cradle.

FIG. 5 is an end view of the inductor of FIG. 4.

FIG. 6 is a side view of a third embodiment of the invention for use in large power handling circuits.

FIG. 7 is an end view of the inductor of FIG. 6.

FIG. 8 is an exploded side view of a fourth embodiment of the invention employing an enclosure.

FIG. 9 is a side view of the inductor of FIG. 8 fully assembled.

FIG. 10 is a side cut away view of the embodiment of FIG. 9 modified to allow for adjustability of the inductance by use of discreet core segments separated by non-magnetic members.

FIG. 11 is a graph showing the effect on the Q value of the inductor of FIG. 4 by the insertion of shims between the core and the cradle.

FIG. 12 is a graph illustrating how the shims of FIG. 11 adjust the inductance of the inductor of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An inductor 8 includes a solid continuous core 10 made of magnetically permeable material. The core 10 rests in a substantially U-shaped cradle 12 which is also made of the same or similar magnetically permeable material. A coil 14 having an input end 16 and an output

end 18 is wound about the core 10, or on a bobbin (not shown).

Both the core and cradle are made from a powdered iron material of relatively high permeability.

An appropriate material for this application can be purchased from Micrometals of San Diego, California. A material mix having a permeability (pO) of 75 and a material density of 7 g/cm3 is particularly appropriate as the material to be used for this application. Powdered iron materials falling generally in a range of magnetic permeability of about 40-100 and with material densities ranging from 6.9-7.2 g/cm3 may also be acceptable as well.

It is important that the core and cradle be made from a powdered iron material having relatively high permeability. Further, the core should be a single continuous piece without any physical interruption in the area of the core encircled by the coil 14.

The core 10 has a longitudinal length that extends along an axis labeled"A"in FIG. 1. The cradle also has a longitudinal length that extends along a parallel axis labeled"B"in FIG. 1. The longitudinal length of the core along axis A should match the length of the cradle along axis B for maximum efficiency and to avoid the creation of fringing magnetic fields near the end-points of the core, since fringing flux which is not contained within the core causes problems with nearby components such as loudspeaker coils.

The core 10 is preferably made in the shape of a circular cylinder with beveled ends 22 and 24. The top surface is beveled as well so that the ends of the core 10 may be conveniently held in a machine for coil winding. As seen in FIG. 2, the cradle 12 is rectangular in cross-section. In order to mate the core to the cradle in an abutting relationship, the cradle 12 has upwardly extending arms 26 and 28 which have beveled surfaces 30 and 32.

The core 10 and the cradle 12 lie in abutting relationship with each other to complete the magnetic circuit for flux created by current in the coil 14. It is therefore necessary to maximize the flux linkage from the core to the cradle. Given the geometry of each, i. e., the core 10 is a circular cylinder and the cradle has a rectangular cross-section, proper flux linkage requires that the bevel angle at which the core and cradle are joined maximize the contact area between core and cradle. Mathematically, the angle that creates the maximum area for joining a cylinder to a surface of rectangular cross-section is about 25°. This is shown as the angle in FIG. 3. Thus, bevel angles of between about 24° and 26° will work well for similar geometries, that is those which join circular or nearly circular core cross-sectional elements to rectangular cradles.

In another aspect of the invention (refer to FIG. 4), a second cradle 20 may be added to the inductor configuration of FIG. 1. This cradle is identical to cradle 12 and is oriented 180° opposite cradle 12 in the same plane. The core 10 therefore includes a beveled top surface identical to the beveled surfaces mating with cradle 12 so that it mates in an identical way with the second cradle 20. The second cradle 20 is formed from the same or similar powdered iron material as the core 10 and cradle 12. The permeability of the core and that of the cradles need not be identical. This provides the designer with a greater range of inductances when designing particular units.

In yet another aspect of the invention, a larger inductor is provided which is capable of handling the high currents produced by extremely high-powered audio amplifiers. Amplifiers rated to produce wattages of a thousand watts or more may deliver currents on the order of magnitude of 10 amps to acoustic transducer components including loudspeakers and crossovers. Such high current in an inductor calls for a design which can

dissipate heat. In FIG. 6, a cradle 40 includes a pair of yokes 42 and 44 which receive a core 46. A coil 43 of a predetermined number of turns is wrapped about the core 46 or a bobbin (not shown)., The lengths of both the core 46 and the cradle 40, as measured by dimension"L"in FIG. 6, are equal. In addition, in order to provide sufficient surface so as to maximize heat dissipation, the ratio of the diameter"D"to the length of the core "L"should be approximately in the range of between 1/4 and 1/5. As with the embodiment of FIG. 1, shims or spacers of a non-magnetic material (not shown) and shaped to fit within the yokes 44 and 42 may be interposed between the core 46 and the cradle 40 in order to adjust the inductance value of the inductor.

It has been discovered by the inventors that an inductor of the configuration of the invention whose inductance is adjusted or determined by the placement of one or more shims or spacers 31 (refer to FIG. 3) of non- magnetic material may have an effect on the"Q"value of the inductor. In FIG. 12, a chart shows the effect on inductance by the placement of shims in a twin cradle inductor of the type shown in FIG. 4. The graph shows that the inductance drops as more shims are placed at either end of the core between the core and the cradle.

Typically, the shims are 4 mils each and made of a paper material. As shown in FIG. 12, the inductance was changed from a high value of about 30 millihenrys to 24 millihenrys by the placement of combinations of shims at either end of between 4 mils and 16 mils. As the graph of FIG. 11 shows, the Q value of the inductor rose as the inductance fell. This result seems counterintuitive because Q varies directly with the inductive reactance which is, in turn, equal to 2nfL. The inventors speculate that the gaps created by the shims or spacers which lower the inductance cause a corresponding reduction in the effective value of the AC resistance, although the reason for this phenomenon is not well

understood. Thus, an inductor which makes use of one or more shims so that a lower inductance is provided will have a larger Q value in this configuration.

In yet another aspect of the invention (refer to FIGS. 8,9 and 10), a coil 52 of an inductor 50 is wound on a bobbin 54 and enclosed within two half-shell cup core shields 56,58 made of highly permeable magnetic material, preferably, a powdered iron. In this case, the shell functions like the cradle of FIG. 1 to complete the magnetic circuit and confine the magnetic flux to the shell material and core. The base of each cup core shield includes a hole to accommodate the insertion of a permeable core 60. One or more associated spacers 63 positioned within the coil breaks the core into segments 57,59 having an air gaps or gaps interposed so as to achieve the desired inductance. The start and finish leads 62,64 of the coil 52 are fitted through slots 66, 68 so as to exit the cup core shield 56,58 for the external attachment to the crossover network (not shown).

In the alternative, small holes in the base of the cup, one near the center hole and the other just inside the cylindrical cup core wall, permit bringing out the leads through the upper half-shell 58. This allows the cylindrical wall of the cup core shield to be a continuous column providing superior strength for improved ruggedness.

A mechanical means of securing the components herein described is shown using a pair of fender washers 61 and 65 fastened with a machine bolt 67 and nut or threaded disc 69. One or more O-rings 70, when compressed, provide a minor adjustment to the desired inductance value. The intrinsically shielded variable inductor eliminates EMI allowing complete freedom in its location within the crossover network assembly. The "black body"nature of the cup core shield provides moderate transfer of radiant heat for this assembly.

The core can be threadingly adjusted along the bolt 67 to lie partially within or partially without the core 52 and bobbin 54. Since the moveable core offers a wide range of inductance values (4 or 5 to 1) for a single coil with reasonable linearity between movement with inductance change, the usual need to peel turns is no longer a requirement. This permits consideration of a single coil for both 8 ohm speaker systems and 4 ohm speaker systems with freedom to select the desired inductance values for each crossover needed with the associated transducer (s)..-- In addition, a continuously variable inductor with ranges of 2 to 1,3 to 1 or 4 to 1, for example, may be provided with the hollow core. An internal screw attached to the bottom half-shell precisely centered within its hole may provide linear motion of the core throughout the length of the coil. The lower end of the core may include an attached threaded disc to mate with the internal screw, and its upper end may provide a slotted disc for a screwdriver blade with which to turn the core and thereby vary the inductance. A non-metallic screw material such as a composite or nylon screw precludes eddy current losses from being inducted into the magnetic field of the coil.

While the core in FIGS. 1 and 6 has been described as cylindrical and shown as a right circular cylinder, other cylindrical shapes may work as well including rectangular and polygonal cylinders. In addition, while the cradle has been described herein as substantially U-shaped, it should be understood that the term"U-shaped"is'intended to encompass other curved or rectilinear shapes that connect the magnetic circuit at either end of the core that might not be, strictly speaking, in the classical shape of a"U." The highly efficient inductor (s) described herein exhibits performance attributes unattainable or difficult to achieve economically heretofore. It

provides broad band rejection capacity out to 100 KHz, imperceptible or"silent swing"with diverse frequencies or widely varying excitation levels, very High Q over a wide spectrum for minimum loss, non-radiating flux-flow, compact high current carrying capability, imperceptible bass depression in the presence of level high frequencies, high volt-second capacity, i. e., difficult to saturate, and minimal introduction of distortion by virtue of linearized magnetizing current.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.