GB2235350A

GB2235350A - Improvements in moving coil loudspeakers

Info

Publication number: GB2235350A
Application number: GB9010646A
Authority: GB
Inventors: David Robin Birt
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-08-21
Filing date: 1990-05-11
Publication date: 1991-02-27
Also published as: GB9010646D0; GB8918975D0

Abstract

The invention discloses means for substantially reducing the magnetic flux modulation which normally occurs as a result of the audio frequency current which flows in the voice coil 4. Flux-sensing coil 9 cooperates with amplifier 6 (via interface 8), and subsidiary flux generating coil 3 to form a negative feedback flux control loop which opposes changes in flux induced by current in the voice coil, in proportion to the loop gain. Feedback may also be direct from the voice coil. without the presence of flux-sensing coil 9. <IMAGE>

Description

IMPROVEMENTS IN MOVING-COIL LOUDSPEAKERS The present invention relates to moving-coil transducers such as moving-coil loudspeakers. The invention aims to resolve performance deficiencies of conventional designs, which are apparent at low frequencies; the reproduction of which demands large diaphragm or cone excursions, and high power levels. The invention addresses the problem of non-linearity in the motor system.

Conventionally, the motor system comprises a permanent magnet, and a magnetic circuit constructed of ferrous material, and including an air gap, in which the moving coil is situated.

Non-linearity of the motor system causes harmonic and intermodulation distortion, and this is often of sufficient magnitude to be clearly audible as an impairment in the reproduced sound quality. The levels of distortion at low frequencies below about 100 Hz , in contemporary conventional designs of loudspeaker, can be some two orders of magnitude greater than that occurring in the rest of the system: from microphone, through digital recording and playback, and including the power amplifier driving the loudspeaker.

The cause of the non-linearity which the present invention substantially overcomes, is often known as flux modulation. Flux modulation arises when the gap flux (including fringe fields either side of the front plate) is not static as it would be in an ideal design, but varies as a function of the signal current in the moving coil. The magnetic circuit in a conventional moving coil loudspeaker can be thought of as including in the flux path two serially configured magnets. The first is the permanent magnet supplying the field. The second is an electromagnet comprising the cylindrical centre pole, and the (moving) coil which surrounds it. Current in the coil in a first direction, aids the flux contributed by the permanent magnet; whereas current in a second, opposite direction opposes the flux of the permanent magnet.Hence the total flux in the gap is modulated by the voice coil current. Since the force exerted by the moving coil on the cone or diaphragm is proportional to the magnitude of the flux which surrounds it, it is apparent that in the case of an applied alternating audio current, one half of the alternating cycle wherein the direction of current flow is such as to increase the gap flux, will yield a greater force than that corresponding to a succeeding half-cycle in which the opposite direction of current flow causes a reduction in gap flux. As is well known, this asymmetry about the x axis of time implies even order harmonic distortion; and there is also â shift in the mean position of the cone. This shift in mean position of the cone is analogous to the familiar d.c shift in the operating point of electronic circuits having even-order nonlinearity.

Flux multiplication effects tend to be more significant when ceramic permanent magnets are used, compared with the formerly-used Alnico magnets. This is because of the difference in characteristics between the two types of magnetic material.

Known art has addressed the problem of flux multiplication by use of a "flux-stabilising ring", which is in effect a short-circuited single turn winding, enclosing the centre pole. This known technique is not effective at very low frequencies, because the condition that the time constant L/R of the ring shall be long compared with the period of the lowest reproduced signal, cannot be achieved in practice.

The object of the present invention is to cancel, or substantially cancel the flux due to current in the moving coil, and thereby to maintain a substantally constant gap flux which is substantially independent of the magnitude or direction of current in the moving coil. One technique disclosed is effective down to zero frequency.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which : Fig. 1 is a plot of incremental values of force factor [B13 versus displacement of the moving coil, which illustrates flux multiplication. This plot is the measured characteristic of a contemporary design of loudspeaker.

Fig. 2 is a diagram of a preferred topology of magnet circuit incorporating, according to the present invention, a fixed, compensating, flux-generating coil, which is able to establish a magnetic flux in opposition to the flux produced by the audio frequency current flowing in the voice coil.

Fig. 3 is a diagram showing how the said compensating coil may be electrically connected.

Fig. 4 is a diagram of an alternative connection of the compensating coil in an alternative magnetic circuit topology.

Fig. 5 is an illustrative explanatory diagram of an alternative system for eliminating flux modulation, which uses an additional flux-sensing coil to facilitate a negative-feedback loop which operates to reduce change of flux to virtual-zero.

Fig.l is a plot of incremental force factor [B1] ,in the vertical axis, versus voice coil displacement (horrizontal axis). The left of the displacement axis is the condition of forward excursion of the voice coil. The right of the displacement axis is the condition of rearward excursion of the voice coil. The centre of the displacement axis is the natural rest position of the voice coil. The magnetic circuit topology in the loudspeaker upon which these measurements have been made, is that wherein the voice coil is long compared with the front plate depth, and the centre pole is a rod of uniform diameter, terminated flush with the front of the front plate. The upper [Bl] profile curve 1 is plotted with a current of 0.5 amp flowing in the voice coil in a first direction.The lower [ profile curve 2, is plotted with the same current, 0.5 amp d.c., flowing in a second direction opposite to the first directicn. To obtain the two plots 1, and 2, only the direction of current flow in the coil has been changed. This magnitude of current is equal to the peak value of a sinewave which would dissipate one watt of power in the loudspeaker. Normal operating current levels would be some ten times greater, giving 100 watts power, and a proportionally greater separation between the two curves 1, and 2.

It will be noted that with this stated topology, the two curves converge to the left of the diagram. This is because at maximum forward excursion of the diaphragm on the left of the diagram, most of the coil is outside the gap in the forward direction; and the forward part which overhangs the centre pole cannot influence the field in the gap significantly. Conversely, there is no convergence of the two curves 1 Ind 2 with inward displacement of the voice coil, because then, on the right side of the diagram, the whole of the voice coil surrounds the centre pole of the magnet system.

It will be noted that both curves are asymmetrical about the centre of deflection. This is because of the unequal fringe fields foreand aft of the front plate, with this magnetic topology.

Fig. 2 shows a preferred topology according to known art, in which the centre pole is extended forward, so that the centre pole is surrounded by the voice coil throughout the displacement range. This topology gives [31] curves which are symmetrical, or nearly symmetrical about the centre of deflection, and which do not converge. ( A description of this topology is found, for example in the "Loudspeaker and Headphone Handbook" Edited by John Borwick, and published by Butterworths, pp 75-77). According to the present invention,there is provided a securely fixed compensating coil 3 having an equal, or lower(e.g. 70%)number of turns to the voice coil 4. The compensating coil is connected such that its generated flux is in opposition to the flux generated by the voice coil, so that the net flux produced by the voice coil and compensating coil is zero, or near zero. The compensating coil is preferably wound with thicker wire, and has lower resistance than the voice coil. The [31] profile of a loudspeaker with compensating coil has greatly reduced dependency upon voice coil current, and is of parabola-like shape. Means for correcting for displacementdependent [Bl] has previously been described in GB 2,196,815 A.With this simple series connection scheme there tends to be slight under-compensation for voice-coil induced flux when the voice coil is displaced towards its inward excursion limit; and slight over-compensation when the voice coil is displaced towards its maximum forward excursion limit. There is however a substantial reduction in flux modulation which is effective down to zero frequency (d.c.).

Fig. 3 is a shematic diagram showing a preferred arrangement in which the voice coil 4 is connected in series with the compensating coil 3. The dots indicate the ends of the windings which are in-phase.( as in transformer terminology). The power amplifier 5 which drives the loudspeaker, may be preferably a transconductance amplifier giving current drive, although conventional voltage drive may be used.

Fig 4 shows an alternative connection which is applicable to other magnetic topologies wherein the influence of current in the voice coil on the gap flux is greatly dependent on the position or displacement of the voice coil.

(One such topology has been reffered to under Fig.l) A separate power amplifier 6 is used to drive the compensating coil, and signal-processing circuits 7, responsive to voice coil displacement, operate to modify the current in the compensating coil such as to cancel the flux from the voice coil irrespectively of its displacement from a natural rest position. In this configuration using a separate power amplifier 6, the compensating coil turns may be different in number to the voice coil turns.

Fig 5 illustrates a further method for controlling flux modulation which automatically responds to postiondependent induction from the voice coil. As in Fig 4 herein above, flux-generating coil 3 cooperates with auxilliary amplifier 6 to influence and control the total flux interacting with the voice coil 4. However, in this further method now described, the input to the auxilliary amplifier 6 is connected via interface circuit 8 to flux-sensing means 9 said flux-sensing means being responsive to the total flux interacting with voice coil 4. Said flux-sensing means may comprise a Hall Effect probe, in which case interface circuit 8 contains d.c. biasing means.Alternatively, the flux-sensing means may comprise a search coil, typically wound round the centre pole, which will generate an EMF equal to N d/dt, where N is the number of turns in the search coil (9), and d/dt is the rate of change of flux. When flux-sensing means 9 comprises a search coil, then interface circuit 8 may contain an integrating device or resitor-capacitor combination. Flux sensing means 9, amplifier 6, and flux generating coil 3 are configured to form a negative feedback/servo system such that the total flux interacting with the voice coil is maintained constant, and/or the change of flux resulting from current in the voice coil 4 is controlled to virtual zero.The performance of the flux-control loop comprising (feedback) sensor 9, amplifier 6 , and flux-generating coil 3 is, in accordance with standard negative feedback theory, dependent on the gain of amplifier 6. Said flux control loop operates independently of the position of the voice coil 4, and so a voice coil displacement signal is not required in this method.

Successful operation may be obtained down to very low frequencies, because the amplifier is able to make up the resistance losses in the flux generating coil. The system simmulates a conventional flux-stabilising ring(shorted turn) having a very long L/R time constant (i.e. near-zero resistance), but need not occupy a large volume.

Such a simmulation of a low resistance flux-stabilising ring could also be achieved by connecting the flux generating coil 3 to a negative resistance, (simmulated electronicelly) so as to cancel the natural positive resistance of the flux generating coil. Such a system is vulnerable to oscillation if, as a result of temperature changes, the total loop resistance becomes negative.

Claims

IMPROVEMENTS IN MOVING COIL LOUDSPEAKERS

CLAIMS 1.

A moving coil loudspeaker motor, wherein modulation of the field-magnet flux interacting with the voice coil, by audio frequency current in the voice coil, is substantially eliminated by means of at least: a) subsidiary flux generating means, b) further flux sensing means, and c) electronic amplifying means; all of which cooperate to provide a negative feedback flux control system.
2.

A moving coil loudspeaker containing subsidiary flux generating means connected to an amplifier as in claim 1; where the input to said amplifier is derived from signal processing means having at least two inputs.
3.

A system as in claim 2, wherein the inputs to the signal processing means are at least an analogue or digital signal representative of voice coil position, and a signal representing the voice coil current.
4 A moving coil loudspeaker containing a subsidiary flux generating coil, as in claims 1 and 2, wherein said subsidiary coil is connected to an electronically simmulated negative resistance, which is not greater than the positive resistance of the subsidiary coil at its minimum operating temperature.
5.

A moving coil loudspeaker containing a subsidiary flux generating coil, connected in series opposition with the voice coil, and interacting with the main magnetic flux path.
6.

Apparatus containing a loudspeaker according to any of the preceeding claims.