GB2187607A

GB2187607A - Apparatus and method for driving loudspeaker systems

Info

Publication number: GB2187607A
Application number: GB08605400A
Authority: GB
Inventors: Malcolm John Hawksford; Paul Geoffrey Leonard Mills
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-03-05
Filing date: 1986-03-05
Publication date: 1987-09-09
Anticipated expiration: 2006-03-05
Also published as: GB2187607B; GB8605400D0

Abstract

A method and apparatus for powering, protecting and controlling the behaviour of a loudspeaker, with particular application to high fidelity sound reproduction systems. A method of achieving correct damping of the loudspeaker drive unit independent of its voice coil temperature and improving its linearity is disclosed whereby a high output impedance amplifier is used to power the loudspeaker, with damping maintained by motional feedback techniques. The high output impedance amplifier is also described. <IMAGE>

Description

SPECIFICATION Apparatus and Methods for Driving Loudspeaker Systems This invention relates to apparatus for and a method of powering, protecting and controlling the behaviour of a loudspeaker and has particular applications to high fidelity sound reproduction systems.

In a conventional amplifier and loudspeaker arrangement, the amplifier provides a low output impedance and may be considered as a voltage source. The amplifier thus provides electrical damping for the loudspeaker drive unit. Because of this, variations in the static and motional inpedance of the drive unit give rise to incorrect damping, particularly due to heating of the voice coil causing an increase in its resistance.

Attempts have been made partially to negate the static impedance of the loudspeaker drive unit by providing the associated amplifier with a negative output resistance (as described in United States Patent Specification No. 4,118,600). While this method does provide an electronic means of defining the system damping constant, errors due to heating of the voice coil will still occur. A paper presented at the 79th Convention of the Audio Engineering Society (October, 1985, New York) by Hsu et al. entitled Electromagnetic Damping of High Power Loudspeakers', concluded that a satisfactory method of compensation for this effect had yet to be found.

In addition, as the driving force on the loudspeaker cone is proportional to current rather than voltage, distortion and reduced fidelity arise not only due to thermal effects, but due to the inductance of the voice coil being dependent on its position in the magnet gap.

Many attempts have been made to improve the performance of a loudspeaker and the control of damping under conventional voltage driven conditions (for example in British Patent Specification No. 1,534,842) by motional feedback techniques, but only a partial correction of the problems noted is possible and only then at low frequencies. Further problems arise due to the damping requirement on the amplifier, as the energy returned by the loudspeaker is time-smeared and non-linear, thus giving rise to interface distortion effects. Such effects have been discussed by Otala and Lammasniemi, for example see 'Intermodulation at the amplifier-loudspeaker interface' Wireless World, November and December 1980.

It is therefore a principal aim of this invention to provide amplifier and loudspeaker configurations and methods of operation thereof which at least reduce the problems and disadvantages discussed above of the known arrangements.

Accordingly, one prime aspect of this invention provides a high output impedance amplifier and loudspeaker combination, the loudspeaker having a drive unit provided with a voice coil and means to sense the motion of the voice coil, and there being means to control the output of the amplifier dependent upon the sensed voice coil motion.

The term "high output impedance", as used herein referring to an amplifier, is intended to mean that the output impedance of the amplifier is relatively large as compared to the characteristic impedance of the load driven thereby, such that the amplifier may be regarded primarily as a current sourceforthe load, rather than as a voltage source.

Since the impedance of a load may rise significantly at resonance, and yet the amplifier still should have a relatively high output impedance as compared to the load, it is preferred for the load inpedance at resonance to be taken into account in determining whether the amplifier has a sufficiently high output impedance. Though no clearly defined line may be drawn, it is envisaged that the output impedance of the amplifier should be at least five times the impedance of the load at resonance, but preferably is at least ten times that impedance.Typically, however, the output impedance of the amplifier may be at least two orders of magnitude higher than the load inpedance, and practical designs of amplifier for driving a loudspeaker may have output impedances in the range of from 1 Okn to 1 OMn, as compared to loudspeakers with an impedance at resonance ranging from say 10Q to 100R.

The arrangement of this invention serves to restore system damping, which otherwise in lost should a high impedance source be used to drive a loudspeaker.

Adequate damping may in this way be obtained, and the loudspeaker will be provided with a precisely defined driving current to improve its linearity and the degree of damping attained can be made totally independent of the voice coil temperature.

Most preferably, the sensing means comprises an auxiliary coil wound adjacent the loudspeaker voice coil so as to produce an output dependent upon the motion of the voice coil. The output from the sensing means may then be processed as required, for example by means of active filters, to obtain a feedback signal applied to the amplifier at a suitable point.

Protection for the voice coil may be added to the combination described above, to prevent the voice coil over-heating and failing, especially for highpower applications. This invention therefore further provides a high output power impedance power amplifier and loudspeaker combination in which the loudspeaker includes a drive unit driven by the amplifier and having a voice coil, wherein means are provided to sense the temperature of the loudspeaker drive unit voice coil which means comprise a network simulating the loudspeaker impedance at a given temperature, a second amplifier driving current through the simulating network and the input to which is connected to the same input signal source as is the power amplifier, comparator means to compare the voltages applied respectively to the loudspeaker voice coil and the simulating network, and division means to produce a signal indicative of the temperature of the voice coil, which division means operates on the input signal to the amplifiers and the output of the comparator means.

The invention also extends to a high output impedance amplifier per se, especially suitable for use in the arrangements described above. In this connection, this invention provides a high output impedance power amplifier comprising a driving amplifier stage serving as a current source and an output amplifier stage driven by the driving amplifier stage, which output amplifier stage has a pair of amplifying elements connected in a common-base mode and a pair of series-connected floating power supplies arranged across the pair of amplifying elements, whereby a load may be connected to the common point of the power supplies so as to be driven thereby.

This just-described arrangement is considered preferable to the commonly-known technique of applying current feedback around a voltage amplifier to give a high output impedance; a technique which would not correctly isolate a conected loudspeaker from the amplifier and would give rise to a frequency-dependent output impedance.

Further aspects of this invention extend to a method of driving a loudspeaker from a high output impedance power amplifier, in which the motion of the loudspeaker drive unit is sensed and the input to the power amplifier is modified dependent upon the sensed motion of the drive unit thereby to obtain damping forthe loudspeaker drive unit.

Where the motional feedback signal used to obtain damping is generated by an auxiliary coil arrangement within the loudspeaker drive unit, the invention proposes methods of substantially negating electromagnetic coupling between the driving and sensing coils, using electronic compensation.

Also included within the scope of the invention is a method of protecting the loudspeaker drive unit from over-heating by comparing the output voltage of the high output impedance amplifier to that developed across a reference network which models the behaviour of the loudspeaker at room temperature. In such a method, a current proportional to the loudspeaker drive current may be supplied to the network simulating the loudspeaker impedance at a given temperature, the voltages appearing simultaneously across the loudspeaker voice coil and simulating network are compared and an output signal obtained on processing the comparator output and the input to the amplifier by dividing one by the other, the output signal then being related to the temperature of the voice coil.

The invention will now be further described, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a high impedance amplifier and loudspeaker combination, arranged in accordance with one aspect of this invention; Figure 2 shows an amplifier and loudspeaker combination corresponding to that of Figure 1 but including thermal protection for the loudspeaker voice coil; and Figure 3 shows a high impedance amplifier and loudspeaker combination utilising motional feedback, in accordance with this invention.

Referring now to Figure 1, there is shown a driving amplifier 1, which is configured as a current amplifier and which defines the current in a resistor 2 dependent upon the current 1 supplied to the inverting input of the amplifier. The current in resistor 2 drives a pair of transistor amplifiers connected as a common-base stage 3 utilising two floating power supplies 4. The driving amplifier 1 is referenced to the input of the common-base stage 3, thus making that amplifier insensitive to any distortion in the common base stage. The amplifier 1 may in principle operate in class A, class AB or use digital switching techniques, while the commonbase stage 3 may similarly embody a variety of bias techniques.

The circuit of Figure 1 serves to provide a current drive for the loudspeaker 6; that is to say the input current lin defines the current delivered to the loudspeaker. This is achieved by virtue of the common-base amplifier 3 having a very high output impedance, typically of the order of 1 MR. This serves to desensitise the system amplifier from the effects of any non-linearity in the nature of the load being drived, but has the disadvantage that the damping which otherwise would exist for the loudspeaker is lost.

Both the floating power supplies 4 and those associated with the amplifier 1 may be of a conventional design, or more preferably may use digital switching techiques. In the latter case, the switching frequency of these power supplies may be locked to an external source, such as the sampling frequency (or a multiple or sub-multiple thereof) of a compact disc player, or other source of digitally-encoded signals. Moreover, the output voltage of the floating supplies 4 may be made signal dependent to increase efficiency.

A shorting element 5, such as a relay, is arranged in parallel with the loudspeaker 6, to provide protection forthe loudspeaker drive unit against, for example, switching transients and amplifier faults.

Figure 2 shows how the temperature of the loudspeaker voice coil can be determined when driven in the manner according to Figure 1. The high output impedance power amplifier 7 is driven by a transconductance pre-amplifier 8 from an input voltage Vie. The input voltage Vin is also used to feed a second transconductance stage 9 which drives a network 10 simulating a model forthe loudspeaker drive unit at room temperature. In the network 10, Re is a resistor representing the voice coil resistance at room temperature and Zm an impedance representing the motional and static impedance of the loudspeaker drive unit 6. The difference in voltage across the actual loudspeaker drive unit 6 and the simulating network 10 is measured by a differential amplifier 12, appropriate scaling being applied by attenuator 11 to take into account the respective current fed to the loudspeaker drive unit 6 and simulating network 10. The RMS values of the input voltage Vlnl which is proportional to the current in the loudspeaker drive unit 6, and of the output of the differential amplifier 12 are respectively applied to RMS converters 13 and 14.

The RMS output from converter 13 is then divided by the RMS output from the converter 14 by a divide circuit 15 to give a voltage Vout, and this represents any increase in voice coil resistance due to heating.

Presuming the temperature coefficient of the voice coil material is known, a measure of temperature may then be determined.

The output voltage Vout may be used to drive a comparator to shut down power to the loudspeaker drive unit 6 at a predetermined temperature, or may be used progressively to attenuate the current fed to the loudspeaker drive unit 6, down to a safe level.

The latter technique is of particular application to studio monitors, where high sound levels and high reliability are required.

Figure 3 shows a complete amplifier and loudspeaker system, employing velocity sensing of the loudspeaker 6 drive element movement, in this case achieved by means of an auxiliary coil 16. An electronic compensating filter 18 is used substantially to negate any electromagnetic coupling with the voice coil of the loudspeaker drive unit, based on a knowledge of the current waveform in the loudspeaker and of the frequency dependency of the induced error. In this arrangement, velocity feedback is shown applied to the system via a low pass filter 17, in order to define the system damping.

As the open-circuit output voltage of the sensing coil is used as a control signal by working into a high-impedance amplifier, both drive and sensing coils are uninfluenced by the effects of temperature, thus eliminating the thermal sensitivity of system damping.

Alternative motion sensing methods may be used instead of the coil arrangement shown in Figure 3, and may employ positional, velocity or acceleration detection. In addition, acceleration feedback may be applied along with velocity feedback as described.

The techniques presented herein may be used within the context of an active loudspeaker system, where each loudspeaker drive unit is powered by its own high output impedance amplifier, with crossover filtering between drive units performed at a low signal level. For high frequency drive units, damping may be achieved mechanically within the loudspeaker drive unit. In addition, a number of drive units may be fed in series from a common high output impedance amplifier without errors being introduced by impedance or thermal effects in the said drive units. When operated in this configuration, cross-over filtering for the loudspeaker drive units may be implemented by impedances connected in parallel with and/or in series with a loudspeaker drive unit, instead of at a low signal level. Drive units may also be connected in parallel to a common high output impedance amplifier, whereby the positive temperature coefficient of the voice coil resistance gives a thermal load sharing between drive units.

When the loudspeaker is supported by a metal stand, the stand may be used as a heat sink, to assist dissipation of the heat generated by the electronic circuitry employed in implementing the described arrangements.

Claims

1. A high output impedance amplifier and loudspeaker combination, the loudspeaker having a drive unit provided with a voice coil and means to sense the motion of the voice coil, and there being means to control the output of the amplifier dependent upon the sensed voice coil motion.

2. The combination according to claim 1,wherein the sensing means comprises an auxiliary coil wound adjacent the loudspeaker voice coil so as to produce an output dependent upon motion of the voice coil.

3. The combination according to claim 2, wherein means are provided to substract from the auxiliary coil output a proportion of the input to the amplifier thereby substantially to reduce electromagnetic coupling between the voice and auxiliary coils.

4. The combination of any of claims 1 to 3, wherein the control means comprise means to substractfrom the input to the amplifier a signal derived from the sensed motion of the loudspeaker voice coil.

5. A high output impedance power amplifier and loudspeaker combination which the loudspeaker includes a drive unit driven by the amplifier and having a voice coil, wherein means are provided to sense the temperature of the loudspeaker drive unit voice coil which means comprise a network simulating the loudspeaker impedance at a given temperature, a second amplifier driving current through the simulating network and the input to which second amplifier is connected to the same input signal source as is the power amplifier, comparator means to compare the voltages applied respectively to the loudspeaker voice coil and the simulating network, and division means to produce a signal indicative of the temperature of the voice coil, which division means operates on the input signal to the amplifiers and on the output of the comparator means.

6. A combination according to claim 5, wherein the input signal and comparator output are both supplied to respective RMS converters, and the outputs of the converters are fed to the division means.

7. A combination according to claim 5 or claim 6, wherein means are provided to restrict the amplifier drive current to the loudspeaker drive unit dependent upon the output of the division means.

8. A high output impedance power amplifier comprising a driving amplifier stage serving as a current source and an output amplifier stage driven by the driving amplifier stage, which output amplifier stage has a pair of amplifying elements connected in a common-base mode and a pair of series-connected floating power supplies arranged across the pair of amplifying element, whereby a load may be connected to the common point of the power supplies so as to be driven thereby.

9. A high output impedance power amplifier according to claim 8, wherein the driving amplifier stage has its input circuitry referenced to the input of the common-base mode output amplifier stage.

10. A high output impedance power amplifier according to claim 8 or claim 9, wherein both power supplies have switched mode regulators the switching frequency of which may be locked to an external frequency source, and the output voltages of which may be made signal dependent.

11. A method of driving a loudspeaker from a high output impedance power amplifier, in which method the motion of the loudspeaker drive unit is sensed and the input to the power amplifier is modified dependent upon the sensed motion of the drive unit to obtain damping for the loudspeaker drive unit.

12. A method according to claim 11, in which the loudspeaker drive unit motion is sensed by means of an auxiliary coil associated with the drive unit voice coil, and electronic compensation is employed substantially to cancel the effect of electromagnetic coupling between the voice coil and the auxiliary coil.

13. A method according to claim 12, in which the compensated output of the auxiliary coil is subtracted from the input to the high output impedance amplifier thereby to obtain damping of the loudspeaker drive unit.

14. A method according to any of claims 11 to 13, in which a plurality of loudspeaker drive units are driven by a common amplifier, the drive units being in any series and/or parallel configuration and the motion of at least one unit being sensed to provide the motional feedback to the amplifier.

15. A method according to claim 14, in which cross-over filtering is employed between the drive units, which filtering has impedance connected across or in series with the drive units.

16. A method according to any of claims 11 to 14 and in which a plurality of loudspeaker drive units are employed, each having its own amplifier and cross-over filtering being employed at the inputs to the amplifiers whereby each amplifier handles only a limited frequency range.

17. A method of sensing the temperature of a loudspeaker voice coil when driving the loudspeaker from a high output impedance amplifier, in which method a current proportional to the loudspeaker drive current is supplied to a network simulating the loudspeaker impedance at a given temperature, the voltages appearing simultanously across the loudspeaker voice coil and simulating network are compared and an output signal obtained by processing the comparator output and the input to the amplifier by division, which output signal is then related to the temperature of the voice coil.

18. A method according to claim 17, in which the input signal and comparator output are separately RMS converted prior to the division thereof.

19. A method of protecting a loudspeaker drive unit, comprising sensing the motion ofthe drive unit voice coil by a method according to claim 11 or claim 12, and controlling the current supplied to the drive unit dependant thereon.

20. A method according to claim 19, in which the current supplied to the drive unit is progressively attenuated as the sensed temperature rises.

21. A method according to claim 19, in which the loudspeaker drive unit is short-circuited when the sensed temperature of the voice coil rises above a preset threshold, or whenever protection against transients is required.

22. A method according to any of claims 11 to 21, and in which a metal stand is employed for a loudspeaker driven by the high output impedance amplifier, and the metal stand is employed as a heat sink for the drive circuits for the loudspeaker drive unit.

23. Each and every novel feature taken singly or in any viable combination of a high output impedance amplifier and a loudspeaker, and driving methods therefor, substantially as hereinbefore described.