GB2380309A

GB2380309A - Magnetic device for reduction of EMI in audio circuitry

Info

Publication number: GB2380309A
Application number: GB0120278A
Authority: GB
Inventors: Richard Wolfe
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-08-20
Filing date: 2001-08-20
Publication date: 2003-04-02
Anticipated expiration: 2021-08-20
Also published as: GB0120278D0; GB2380309B

Abstract

A magnetic device for reduction of EMI in a high fidelity audio apparatus (9) such as a preamplifier or CD player in a stereo system. The device comprises two disc-shaped permanent magnets (6) with their south poles in close proximity and their magnetic axes vertically co-aligned, resulting in a magnetic field gradient above and below a horizontal plane (P). Incident radio waves (8), eg. from a car ignition system or mains wiring, are deflected away from the audio apparatus (9) by the magnetic field gradient. The effect is enhanced by laminar mild steel plates (2) above and below the magnets (6) and by a loop antenna (4). A quartz crystal (7) is provided for absorption of electromagnetic waves (8) in the microwave region and the device has a housing of carbon-loaded plastics material (1) for additional EMI absorption. The perceived sound quality of the system is improved.

Description

Magnetic device The present invention relates to magnetic shielding devices, particularly but not exclusively for use with audio equipment.

It is known that electromagnetic interference (EMI), originating from eg the mains or from digital circuitry in CD players can adversely affect the perceived sound quality of high fidelity audio equipment and it is known to use materials such as PTFE which have very low dielectric losses at RF frequencies as insulation in eg high quality interconnect and speaker cables and in the phono plugs and sockets of eg preamplifiers and CD players. It is likely that such dielectrics filter out RF which would otherwise subtly degrade the sound quality.

It is also known to use non-magnetic (ie non-ferromagnetic) components in order to improve sound quality. For example Holco (RTM) resistors have copper leads which are free of nickel and are highly regarded for their sound quality and certain companies such as DNM employ non-metallic materials in the casings of their preamplifiers. Valve amplifiers sometimes have stainless steel or copper casings which are claimed to contribute to their sound quality.

On the other hand a steel'brick'has been sold for some years as an audio accessory to be placed on top of an amplifier or other stereo equipment with the aim of localising the magnetic field emanating from the mains transformer and thereby reducing the magnetic field in the surrounding audio cicuitry. It is also considered that the weight of the'brick'reduces vibration which is also considered to affect sound quality.

These effects are subtle and difficult to measure but to varying degrees are accepted 0 as real by persons skilled in the art of audio design. Typically the effect of such components is to make the sound richer, more natural and less fatiguing.

More controversially, other devices notably Shakti stones are available for placing on or around audio equipment. The Shakti stones incorporate embedded quartz crystals and magnets which are claimed to improve sound quality (see US 5,814, 761) by absorbing EMI of a wide range of frequencies with permanent magnets, conductinve surface coatings and quartz crystals. However their physical

mode of action is questionable. It is noted that the magnets are not located in close proximity and there does not appear to be any reliance on deflection of incident EMI fields.

In one aspect the present invention provides a magnetic device comprising two

mutually opposed permanent magnets permanently held in close proximity against c their mutual repulsion by connecting means.

It has been found that such a device has a beneficial effect upon the sound quality of audio equipment including loudspeakers, CD players and amplifiers when placed in the vicinity of the equipment, eg on top of or to one side of the equipment.

Without wishing to be bound by theory, it is envisaged that the magnetic field gradient developed at the interface of the permanent magnets deflects incident EMI, particularly radio or microwave radiation eg from vehicle ignition systems, mobile phone base stations or from nearby mains wiring which would otherwise be picked up by the equipment and interfere with the circuitry either of that equipment or of other audio equipment in the system. For example a loudspeaker, whilst having little or no electronic circuitry of its own, will be connected to a power amplifier and thence to other items of audio equipment and, it is postulated, will in the absence of protection pick up radio or microwave radiation and transmit it via the speaker cables to the power amplifier and (via interconnects) other items of audio equipment in a stereo system.

Preferably the permanent magnets are spaced apart by Imm or less, more preferably by 0. 3mm or less, still more preferably by 0. lmm or less and ideally are in contact ie with zero spacing between them.

Preferably the permanent magnets are bar magnets with their magnetic axes substantially coaxial. In preferred embodiments each magnet has a transverse dimension greater than its longitudinal dimension.

Preferably the magnets have a strength (maximum energy product) of at least 215 c kJ/m3 (27 million Gauss Oersted).

The device of the invention can be used to shield a high-fidelity audio apparatus

from incident EMI.

In another aspect the invention provides a method of screening a high-fidelity audio apparatus from incident EMI wherein a generally laminar magnetic field is established adjacent the apparatus.

Preferred features are defined in the dependent claims.

A preferred embodiment of the invention is described below by way of example only

with reference to Figures 1 and 2 of the accompanying drawing, wherein : tn Figure 1 is a somewhat diagrammatic sketch perspective view of a magnetic device in accordance with the invention, and Figure 2 is a vertical cross-section taken on II-II of Figure 1.

Referring to Figure 1, the device comprises a housing in the form of a thin-walled cuboidal box 1 of black ABS plastics material. The box material is loaded with carbon black (as a filler/colourant) which is believed to have some EMI-absorbing effect by virtue of its very slight electrical conductivity. At least some of the enhancement in sound quality of a high fidelity audio apparatus in the vicinity of the device is believed to be attributable to the box. In a variant the box could be tapered in at least two dimensions.

Within the box is mounted a laminar magnetic field-concentrating assembly comprising two mild steel plates 2 of dimensions 25mm x 25mm and thickness lmm, between which is sandwiched a spacer 3 of MDF (Medium-Density Fibreboard).

On this assembly is mounted a magnet assembly comprising a vertical copper tube 5 of internal diameter 20 mm which encases two powerful disc-shaped neodymiumiron-boron bar magnets 6 of diameter 20 mm and thickness 5 mm which are held in contact or with only a minimal spacing between them by inwardly-turned upper and lower edges of the copper tube. The permanent magnets 6 have their magnetic axes vertically co-aligned with their South poles adjacent, as shown, resulting in a substantial magnetic field gradient concentrated at and transverse to their interface at

a horizontal plane P.

On top of the magnet assembly is mounted a further laminar magnetic fieldconcentrating assembly comprising a further MDF spacer 3 carrying a further square mild steel plate 2. These have the same dimensions as the previously-mentioned spacer 3 and plates 2. It is believed that the laminar magnetic field-concentrating assemblies tend to concentrate and flatten the magnetic field at the North poles of the magnet assembly, not only indirectly enhancing the field gradient across plane P but also enhancing the much lower magnetic field gradients in these North pole regions. These latter magnetic field gradients may have some EMI deflecting effect which enhances that of the magnetic field gradient centred on plane P.

A circular wire loop antenna 4 of diameter 50 mm is mounted over the magnet assembly with its axis substantially vertical (but shown tilted out of the plane of the drawing for ease of illustration). As shown, the antenna comprises a downwardlyextending length of wire which has its free end attached to the exterior of copper tube 5. A quartz crystal 7 is located on top of the uppermost steel plate 2 beneath loop antenna 4 and is believed to have some EMI absorption capability in the microwave region of the spectrum, which is considered to have a beneficial effect on sound quality.

Referring now to Figure 2, the device is shown mounted on top of a high-fidelity audio apparatus 9 which is susceptible to EMI. The apparatus 9 could for example be a loudspeaker (in which case the device will normally be adjacent the tweeter coil of the loudspeaker) or a preamplifier or a CD player for example. An incident beam 8 of interfering radiation from eg a car ignition system or mains wiring is assumed to be directed at the apparatus 9. As shown, the magnetic fields F of the magnets are intensified above and below the plane P. It is known that the velocity of electromagnetic radiation is reduced by a magnetic field extending parallel to the direction of propagation. This results in an increase in wavelength in the region of high magnetic field intensity and hence an upward deflection of the beam 8 away from apparatus 9 as shown.

In practice it has been found that the device enhances the perceived sound quality of audio apparatus in its vicinity-specifically the spaciousness and dynamics are enhanced. These improvements are typical of those obtained by other methods of

reducing EMI in audio equipment.

Claims

Claims 1. A magnetic device comprising two mutually opposed permanent magnets permanently held in close proximity against their mutual repulsion by connecting means.
2. A magnetic device according to claim 1 wherein the permanent magnets are spaced apart by 1 mm or less.
3. A magnetic device according to claim 2 wherein the permanent magnets are zl spaced apart by 0.3 mm or less.
4. A magnetic device according to any preceding claim wherein the permanent magnets are bar magnets with their magnetic axes substantially coaxial and each magnet has a transverse dimension greater than its longitudinal dimension.
5. A magnetic device according to any preceding claim wherein the magnets

have a strength of at least 215 kJ/m3 (27 million Gauss Oersted).

0
6. A magnetic device according to any preceding claim wherein the south poles of the magnets are mutually adjacent.
7. A magnetic device according to any preceding claim having at least one laminar magnetic field-shaping member disposed adjacent one of the non-adjacent poles of the permanent magnet assembly.
8. A magnetic device according to any preceding claim, further comprising an antenna having its axis transverse to the plane of the magnetic field at the interface of the permanent magnets.
9. A magnetic device according to claim 8 wherein the antenna has a resonant

frequency in the range 0. 2GHz to 2Ghz.

0
10. A magnetic device according to claim 8 or claim 9 wherein the antenna is in the form of a rod having a length of 20 to 200 mm.

<Desc/Clms Page number 7>
11. A magnetic device according to claim 8 or claim 9 wherein the antenna is in the form of a loop having an area of from 100 mm2 to 10,000 mm2.
12. A magnetic device according to any preceding claim wherein the magnets are housed within a box of electrically insulating material.
13. A magnetic device according to claim 12 wherein the box has a volume of from 500 mm3 to 250,000 mm3.
14. A magnetic device substantially as described hereinabove with reference to Figures 1 to 2 of the accompanying drawing.
15. Use of a magnetic device as claimed in any preceding claim to shield a high-fidelity audio apparatus from incident EMI.
16. A method of screening a high-fidelity audio apparatus from incident EMI wherein a generally laminar magnetic field is established adjacent the apparatus.
17. A method of screening a high-fidelity audio apparatus from incident EMI, substantially as described hereinabove with reference to Figures 1 and 2 of the accompanying drawing.