WO1980002477A1

WO1980002477A1 - Protection device or circuit for loudspeakers

Info

Publication number: WO1980002477A1
Application number: PCT/AU1980/000010
Authority: WO
Inventors: P Scherek
Original assignee: Fortieth Benelux Nominees Pty; P Scherek
Priority date: 1979-05-08
Filing date: 1980-05-08
Publication date: 1980-11-13
Also published as: EP0028613A1

Abstract

A protection device for loudspeakers includes circuitry (16) for continuously measuring the average power of an audio signal delivered to a loudspeaker driver and detecting peak levels of said signal in both positive and negative directions. The circuitry (16) is adapted to provide a particular output to switching circuitry (T1) to activate a relay (13) when the average power exceeds a predetermined maximum for a predetermined time which would cause excessive heating in said driver or when said peak levels in either direction exceed predetermined limits set to correspond to the maximum positive or negative excursion limits of said driver. Activation of the relay causes switching of the audio signal path to the driver into an open circuit condition or switches a resistive load into the path to substantially reduce the power to the driver until overload ceases. Separate said circuitry is provided for each driver in a multi-driver speaker arrangement thus providing protection against distortion components in said audio signal caused by clipping. Optional circuitry (B4, R2, C2, 28, T2) for DC protection is also described as is an arrangement wherein the predetermined levels are user adjustable for incorporation into an audio amplifier.

Description

TITLE: PROTECTION DEVICE OR CIRCUIT FOR LOUDSPEAKERS

This invention relates to a protection device or circuit for a loudspeaker to prevent damage to the speaker, in use, due to excessive electrical signal being fed to the loudspeaker. A loudspeaker is an electro-mechanical transducer, which in operation converts electrical energy into acoustical energy. When applied to high quality musical reproduction, its task is prodigious. At one end of the scale it is required to faithfully reproduce frequencies of up to 20 KHz, while at the other (low frequency) end of the scale it is required to linearly move large volumes of air. Such a loudspeaker is a complex and delicate piece of equipment, and hence is subject to damage by overload. There are two basic mechanisms by which loud- speaker drivers can be damaged by overload:-

1) A loudspeaker driver consists of a driven piston which in moving backwards and forwards in sympathy with the electrical signal causes compression and rarefaction of the air. There are mechanical limitations to the backwards and forwards excur¬ sion of the piston which, if exceeded, can cause physical damage. This is the first way in which loudspeaker drivers can be damaged by overload.

2) The incoming electrical energy is converted into two other forms of energy, sound energy and heat energy. Most drivers convert only 5 to 10%

O PI of the electrical energy into sound energy, the rest being converted to heat which must be dissipated by the driver. This then is the seco limitation to the power a driver can handle - th amount of heat energy that it can dissipate with reaching excessive temperatures. A typical loudspeaker system will have several drivers, each assigned to handle a particular band of fre¬ quencies, normally low, middle and high frequencies. In typical music, the energy distribution versus frequency is such that the mid-frequency driver will be called upon to handle about 20% of the overall signal while the high frequency driver may be called upon to handle only 10% of the overall power. As an economy measure, the majority of loudspeaker system manufacturers take advantage of this fa and fit middle and high frequency drivers rated at 20 and 10% respectively of the overall power rating. In practice this is normally quite satisfactory. However, if an ampli is over-driven, the signal is "clipped" at the peaks, and distortion components thus generated completely negate the normal energy distribution versus frequency ratio. Thus e a relatively low-powered amplifier can destroy middle and high frequency drivers if it is over-driven to the point o gross distortion. Thus it is desirable that the protectio system has some form of protection against clipping.

Furthermore, modern amplifiers have a direct coupling between the output circuitry and the loudspeaker. If a component fails, large values of direct current can b produced. A loudspeaker can handle much less direct curre than alternating current for two._*reasons:-

1) At OHz the efficiency is 0%.

2) At OHz there is no movement of the piston, and hence no air cooling.

Thus it is desirable that the protection system has some form of protection against even low levels of dir current. In the past protection devices such as fuses, circuit breakers, limiters or compressors have been used as crude forms of protection for speakers. The use of fuses or circuit breakers is a token gesture as they only protect against gross overload and cannot protect against transients unless they are designed to blow or trip at a level and consequently at a rate which makes listening impossible. Also, fuses or circuit breakers require manual re-setting. Compressors or limiters have an adverse effect on the audio signal to the speaker throughout the range of operation and consequently are most unacceptable particularly in hi-fidelity audio systems. Consequently the known forms of speaker protection are most unsatisfactory.

Accordingly it is one object of the present invention to provide an improved protection device for a loudspeaker, to protect the speaker at least against thermal overload and preferably against thermal overload and over¬ load due to excessive excursion.

According to a further more specific object it is intended to provide an improved protection device for a loudspeaker, which protects the speaker against damage by thermal overload, over excursion, clipping and damaging levels of direct current.

According to the invention there i-s provided a protection device for a loudspeaker comprising circuit means adapted for inclusion in electrical circuitry to a loudspeaker to interrupt said circuitry when overload conditions occur, characterized in that, said circuit means (16) is adapted to continuously measure the average power of an audio signal delivered to said loudspeaker and detect peak levels of said signal in both positive and negative directions, said circuit means (16) is further adapted to provide an output (19,20) when said average power exceeds a predetermined maximum limit for a predetermined time or when said peak levels in either direction exceed a predetermined limit set to correspond to the maximum positive or negative excursion limit, respectively, of the

O PI ^" loudspeaker driver, and said output (19,20) is adapted to actuate switching circuitry (Tl) to substantially switch- off said signal to said driver whilst said overload condi tions exist. In order that the invention may be more readily understood particular embodiments thereof will now be described with reference to the accompanying drawings wherein:-

Fig. 1 is a circuit block diagram of a device according to a first embodiment which is adapted to be incorporated within a spea cabinet or enclosure, Fig. la is a level detector, averaging and peak detector circuit used in the circuit of Fig. 1,

Fig. 2 is a circuit block diagram of a device according to a second embodiment which i adapted to b,e built into an amplifier fo an audio system and Fig. 2a is an absolute value full wave AC to DC convertor circuit as used in the circuit of Reference should now be made to Fig. 1 which sh a self-powered unit or device adapted to be installed inside a loudspeaker cabinet. The unit is custom-designe to suit the particular model of speaker arrangement and separate circuits are provided to protect each driver (no shown) against excessive average power, excessive positiv excursion and excessive negative excursion. The separate level sensing circuits for mid and high frequenc drivers ensure protection against clipping damage and optional circuitry to protect against direct current is shown. The circuit construction should be evident from t drawing and the following description of its operation.

The overall input signal from an amplifier (not shown) appears at terminals 10 and is rectified by diode The rectified signal is used to charge storage capacitor 11 which is ultimately used to energize relay coil 13. T

O overall input signal is also rectified by diode 14_^ and then regulated and filtered in voltage regulator 16 to provide 5 volts DC (Vcc) to power integrated circuits of the unit. According to this embodiment there are three loudspeaker drivers and there is a level detector, averaging and a peak detector circuit according to Fig. la for each driver. In the circuit 16 the incoming signal on input 17 is rectified by diode 18 and fed to an averag- ing circuit, RlCl. Values for RlCl are chosen to suit the thermal characteristics of the particular driver, that is, the thermal characteristics of the driver may be plotted and then the values of RlCl determined. The output from the averaging circuit passes through zener diode ZD3 (which is chosen for the RMS power handling of the driver) and then on connection 19 to inverting buffer Bl (Fig. 1) . The signal rectified by diode 18 is also fed to the input of Bl via zener diode ZD2, which is chosen to suit the peak positive excursion allowable for- the driver. Because it bypasses the averaging circuit RlCl there will be no delay in this signal reaching Bl. From the input the signal also passes through zener diode ZDl (which is chosen to suit the peak negative excursion allowable for the driver) and, via connection 20 to the input of inverting buffer B2 and subsequently on through to buffer B3. Thus it can be seen that if there is an excess of average power or an excess of peak positive power, the output of Bl will go low, and if there is an excess of peak negative power the output of B3 will go low. These two outputs are inverted by buffer B5 which switches on transistor Tl hence energizing the relay coil 13 and breaking the circuit to the speakers by means of relay moving contact 42 which moves from fixed contact 21 to fixed contact 22. An incandescent lamp 23 is wired across the relay moving contact 20. This gives a visual indication that an overload has occurred, as well as allowing some signal to be heard in the event of a cut-off. A normally open contact on the relay is used in a self-latching circuit.

The circuit components necessary for DC protecti will now be described and it should be appreciated that DC protection is an option which may be deleted. In operation, because there is a requirement to take separate outputs from the various drivers, full wave rectification is not practical. It is therefore necessary to use separa circuits to protect against positive and negative direct current. a) Inverting buffer B4 applies a positive charge to capacitor C2 on negative transitions of the inpu signal. The time constant circuit R2C2 is set at 500 S. If there is no negative transition for this period, C2 discharges through R2, feedi a signal through B5, Tl and hence activating the relay coil 13. This action provides protection against positive DC. b) If the output is negative for more than 500 mS, the time constant at the base of transistor T2 allows T2 to conduct. The current path to the relay coil is then from the common or negative rail 24 through diode 25, the relay coil, diode 26 and T2 to the input active connection 27 (which is supposed in this case to have a negative potential) . The circuit represented by reference 28 is. a further 500 mS time constan circuitas the circuit used for positive DC protecti The need for any circuit components shown in Fig. 1 or la and not specifically described will be readil apparent to persons skilled in .the art. For example, resistors 29 and 30 are pull-up resistors and resistor 31 a pull-down resistor. The terminals or connections 32 and 33 are connected to the cross-over (not shown) of the spea arrangement. Connection 34 goes to the positive of the high-range driver (not shown) and 35 goes to the positive the mid-range driver (not shown) . The diodes 36, 37 38 an 39 are associated with the DC protection and may be delete

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WIP if this option is not required together with the resistor 40.

The complexity of the unit according to this embodiment may be reduced by using integrated circuits containing six inverting buffer amplifiers and thus produc¬ tion costs would be quite low. If large quantities were envisaged, a custom designed integrated circuit would further reduce costs.

It will be apparent from the above that the unit of this embodiment covers all forms of protection. It is built inside the cabinet of the loudspeaker -system, and is specially designed for a particular model of speaker. It has separate average and peak detecting circuits for each driver, and in addition the maximum peak excursion for negative and positive going signals can be independently set. Because the level is independently set for the middle and high frequency drivers, protection against over-driven amplifiers is automatic. Circuitry is included to give protection against Direct Current although this can be deleted as an economy measure if desired. (From a

Loudspeaker Manufacturers point of view, D.C. protection in this system is not so important because such damage is uncommon and not normally the subject of a warranty repair anyway) . Reference should now be made to Figs. 2 and 2a which show a protector designed to be built into an amplifier. It has user-accessible controls to set maximum average power and maximum peak power. Clipping protection is internally provided, as is protection against direct current. Assuming the circuitry is self-explanatory opera¬ tion is as follows:-

The output from each channel is divided by R5 and R6 to suit the power of the amplifier, and then fed to an active, absolute value unfiltered.fullwave rectifier circuit 41 which is shown in more detail in Fig. 2a.

Since there are identical left and right channels only the upper or left channel in Fig. 2 will be -described.

_OMPI The unfiltered DC from circuit 41 goes to an averaging circuit, RlCl, chosen to suit the "typical" thermal characteristics of a loudspeaker. From averaging circui the signal is divided by the "Set Average Power" potenti meter VR2 and passed through zener diode ZD2 to the transistor switch Tl which, when turned on, energizes th relay coil 13 breaking the circuit to the speaker. This forms the circuit for protection against excessive avera power. Resistor R4 is chosen so that when VR2, the "S

Average Power" control potentiometer is set for maximum power, this power corresponds to the maximum undistorted power of the amplifier, thus providing protection for th speakers against an over-driven amplifier. The unfiltered DC from the rectifier 41 also passes through diode D4 to the "Set Peak Power" potentio meter VRl, then through zener diode ZDl to the transisto switch Tl. Capacitor C3 filters the rectified signal. Because this rectified signal to VRl bypasses the averaging circuit RlCl the relay is tripped immediately a peak overload is detected, thus protecting the speaker- against excessive excursion.

If direct current signal in excess of about 3 volts is produced by the amplifier circuit (not shown) , the output from the rectifier 41 reverse biases diode,D2 allowing capacitor C2 to be charged by resistor R2. Whe this charge reaches about 1.2 volts current flows throug diode D5 to the transistor Tl, tripping the relay and th providing protection for the speakers against direct current.

Normally open contact 43 on the relay is used to short-circuit resistor R5 in the input divider circui This gives a hysteresis to the cut-out/re-set point, so that the volume must be turned well down before re-set occurs.

The unit according to this embodiment covers a forms of protection and is built into an amplifier. It

OM ^l has two user-adjusted controls accessible on the amplifier panel (not shown) , one to set the maximum average power and one to set the maximum peak excursion power. Because connections to the mid and high frequency drivers are not accessible, the power limit for these drivers cannot be independently set. However, because the maximum undistorted power of the amplifier is known, the protector is pre-set to trip if this power is exceeded, thus providing protec¬ tion for the middle and high frequency drivers if the amplifier is overdriven. Full protection against Direct Current is provided. This is important to an amplifier manufacturer, as a fault within an amplifier, that destroyed a consumer's loudspeaker, would result in bad feelings towards the amplifier manufacturer. It should be evident from the description herein- above that the present invention provides an improved device, unit or arrangement for protecting loudspeakers. Of course the invention is not limited to the particular embodiments described above as clearly the various embodi- ments could incorporate different combinations of the protective features. In a further embodiment the device is provided in the form of an add-on accessory to be connected between audio amplifier and speaker enclosure and has user adjustable controls to match the performance of the device to the particular speaker drivers in use.

Claims

1. A protection device for a loudspeaker comprisi circuit means adapted for inclusion in electrical circuit to a loudspeaker to interrupt said circuitry when overloa conditions occur, characterized in that, said circuit mea (16) is adapted to continuously measure the average power of an audio signal delivered to said loudspeaker and dete peak levels of said signal in both positive and negative directions, said circuit means (16) is further adapted to provide an output (19, 20) when said average power exceed a predetermined maximum limit for a predetermined time.or when said peak levels in either direction exceed a pre¬ determined limit set to correspond to the maximum positiv or negative excursion limit, respectively, of the loud¬ speaker driver, and said output (19, 20) is adapted to actuate switching circuitry (Tl) to substantially switch- off said signal to said driver whilst said overload condi exist.

2. A device according to claim 1, characterized i that, said predetermined maximum limit and said predeterm time are determined according to the particular temperatu characteristics of said driver, and resistive circuit mea (23) are swi-tched into circuitry (24) to said driver to allow a low level signal to reach said driver when said overload conditions occur.

3. A device according to claim 2, characterized in that, said switching circuitry (Tl) comprises a transi adapted to cause energization of a relay coil (13) to ope relay contact (42) for said switching-off of said signal to said driver and switching said resistive circuit means into circuit.

4. A device according to claim 3, characterized in that, monitoring circuitry (B4, C2, R2, 28, T2) is included for the purpose of monitoring positive and negative direct current components in said signal to said driver and said monitoring circuitry is adapted to cause energization of said relay coil when the average level of said direct current components exceeds a predetermined limit.

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5. A device according to any one of the preceding claims, characterized in that, power for the device is derived from said audio signal by means of a rectifier (14) and voltage regulator circuit (15) .

6. A device according to claim 1, characterized in that, said circuit means is adapted for inclusion in an audio amplifier and includes a first potentiometer (VR2) for adjusting the said average power level at which said switching circuitry (Tl) is activated and a second potentiometer (VRl) for adjusting the said peak levels at which said switching circuitry (Tl) is activated.

7. A device according to claim 6, characterized in that, power for operating said device is derived from a power supply within said amplifier and said switching circuitry (Tl) comprises a transistor adapted to cause energization of a relay coil (13) to operate relay contact (43) for said switching-off of said signal to said driver.

8. A device according to any one of the preceding claims, characterized in that, it is adapted for use with a loudspeaker arrangement having a plurality separate drivers and comprises separate said circuit means (16) for each said driver to ensure protection against damage to the drivers due to distortion components in said audio signal caused by clipping, each said separate circuit means (16) being adapted to actuate a single common said switching circuitry (Tl) .