FI122649B - Protection circuit for a powered speaker - Google Patents

Description

PROTECTOR FOR POWER CONTROLLED SPEAKER

The invention relates to electro-dynamic mobile coil loudspeakers designed to be used with power amplifiers. It is intended to provide a protection circuit that can dynamically limit the power received by the speaker-5 element so that, in an override situation, the element will not overheat while still operating within the set power limit. The invention is suitable for protecting loudspeakers that are used substantially current-controlled, wherein the signal supplying the loudspeaker is essentially a current signal and not a voltage signal as usual.

10 BACKGROUND: QUALITY ADVANCED POWER CONTROL

Today, all commercially available audio amplifier and loudspeaker devices operate on a voltage control principle without significant exceptions. This means that the amplifier acts as a voltage source and thus has a low output impedance. However, both the technical aspects and the experience of Moon-15 clearly show that voltage control is the wrong choice if sound quality is to be considered important. The reason is that the electric motor forces (SMV) produced by the speaker element interfere with the voltage / current conversion left to the speaker by the voltage control principle.

The driving force (F) of the membrane moving is directly proportional to the 20 currents passing through the voice coil (i) according to F = Bli, where B1 is called the force coefficient (B = magnetic flux density; / = wire length in the magnetic field). However, in a voltage-controlled element, this current becomes corrupt in a variety of ways.

The electrical substitution of the moving coil element can be illustrated as shown in Figure 1. Rc ku-25 represents the resistance of the voice coil, the voltage source em represents the elemental SMV (counter-SMV) of the element, calculated as: em = Blv (v = the speed of the voice coil), and the voltage source e, represents the inductance SMV lossy inductance of the voice coil.

em is generally dominant at the lower end of the element's playing region, and e, respectively, is dominant at the upper end of the playing region. In a typical cone or caliper element, the sum of the absolute values of em and e, over the entire operating frequency range is at least of the order of magnitude of the voltage drop at resistance Rc.

Both em and e1 are susceptible to many serious interferences and therefore these voltages do not follow the reproducible signal in a linear fashion but contain a very wide range of distortion components. By controlling the proxy switch of Fig. 1 with a conventional low-output impedance amplifier functionally corresponding to a voltage source, sources em and e, and all 2 indistinct distortion components contained therein, modulate the voltage across the resistance If, and thus the circuit current with very damaging world quality.

Instead, by controlling the alternate circuit of Fig. 1 with a high output impedance amplifier 5 operatively corresponding to the power source, the sources em and e, and the indeterminate distortion components contained therein, cannot modulate the current in the circuit, now determined directly by the controlling power source following the interference-free signal.

The following distractors mix the power of the voice coil with voltage control, but of course not with 10 power controls: 1) The voice coil for microphones reflecting from inside the housing and passing through the membrane 2) .

4) Fluctuation of the impedance direction angle due to the / / vaiht variation which causes phase current to be modulated at the center frequencies.

5) Spindle inductance dependent on deviation position, which causes both amplitude and phase modulation of current 20 6) Spindle inductance depends on signal strength level causing unharmonic distortion 7) Resistance variations due to temperature variations and manufacturing tolerances 8)

Detailed information on these serious distraction mechanisms of voltage control and current control in general is available in: Esa Meriläinen, Current-Driving of Loudspeakers, CreateSpace 2010, USA (ISBN: 1450544002).

The loudspeaker element operates substantially in the current control state when the impedance seen by the element is high relative to the element's own impedance at the frequencies that the element is intended to reproduce. In other words, if an impedance meter were to be substituted for the element, this should show the greatest possible impedance absolute value. To achieve this, the output impedance of the amplifier must, of course, be high, but in addition, any divider filter must also be designed to be high impedance, whereby first order filters are concerned. (Note that similar restrictions would also apply to conventional voltage control circuits, provided that they respectively adhere to the requirement of low impedance seen by the element. , however, usually closer to voltage control.)

The current control principle is thus suitable for use with both active and passive loudspeakers, as is the case with the voltage control principle. It is therefore deeply regrettable that the interference mechanisms listed above are allowed to influence and degrade the sound quality in all current audio output systems where voltage control is considered standard practice regardless of physics.

The introduction of current control may have been constrained by the prejudice that the basic resonance of the bass element 10 could not easily be suppressed by the so-called voltage control. with no electrical suppression. However, the necessary resonance damping is achieved by using a low mechanical δ value element, filling the housing with an effective damping agent, and using a suitable active or passive damping coupling for fine tuning the bass response.

15

According to a widespread but purely wishful thinking, the amplifier output-luminosity should be as low as possible, so that the motion of the diaphragm would be "controlled" and the SMVs produced by the element would somehow disappear. The physical fact is that the motion of the diaphragm (more specifically, the acceleration) can only be controlled by current, and the electric motor forces themselves can never be suppressed without suppressing the entire speaker, simply because the em = Blv law can never cease to hold its position, whatever impedances or circuits . Instead, harmful SMV-derived currents are what can and must be suppressed And this is only done with careful current control design.

25 CURRENT SECURITY TECHNIQUES

The current means of protecting the loudspeaker elements are mainly a series of current limiting devices or circuit breakers that are connected to the element. The cut-off function is accomplished by a fuse or relay which disconnects the element when the average signal level exceeds a certain limit. Various temperature-dependent resistors, such as PTC resistors, incandescent lamps, and specially designed thermosensitive components ("posistor"), have also been used to limit current.

On the other hand, it is not possible to use the protection elements connected only to the element in voltage-controlled 35 loudspeakers, since the current supplied by the element does not reduce the current received by the element, but only places an additional load on the amplifier.

4

The use of fuses and relays is also possible in the power control speaker, but due to the slowness of the fuses and the inaccuracy of the relays, setting the activation threshold is difficult. Also, a sudden loss of signal is often undesirable.

Patent application GB2115628A discloses a protective circuit for a conventional voltage control loudspeaker with a rectifier bridge and a voltage sensitive switch. However, the rectilinear signal is not filtered in any way, so that the switch is triggered from the first cross-border wave brush.

10 GENERAL PRINCIPLE OF THE INVENTION

The current control also provides the possibility of using transistors as a limiting element, since the current passed through the element is absent from the element itself. The present invention describes a way of protecting a power-controlled speaker element from over-power by using the protective switching principle of Figure 2.

15

The voltage across the protected object 1 is rectified by a full-wave rectifier bridge 2a-d whose voltage is low-pass filtered by an equalization filter 3 whose time constant approximates the warm-up time constant of the voice coil of the protected object 1. A voltage proportional to the rise in temperature of the protected voice coil thus produced by the equalization filter 3 is applied via a control circuit 4 20 to control a power transistor 5 which is switched on when the voltage produced by the equalization filter 3 exceeds a set limit. With the power transistor 5 conducting, the current and power supplied by the protected object 1 remain within the permissible range, even if the current supplied 6 is much greater than the current tolerated by the protected voice coil.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 illustrates a simple embodiment of the invention in which a power MOSFET transistor is used as a limiting member. Figure 3 illustrates a circuit similar to that of Figure 2, but a simple equalization is introduced. Fig. 4 illustrates a circuit similar to that of Fig. 30, except that a bipolar control transistor is used to refine operation. Figure 5 illustrates a circuit similar to that of Figure 4, but so-called control transistors and power transistors are used. Darlington pairs.

In Figure 2, the resistor 7 and the capacitor 8 form an equalization filter 3 which produces an approximate short time average of the rectified voltage. A resistor 9, which is much smaller in value than resistor 7, is required for the capacitor 8 to charge toward the average of said rectified voltage instead of a peak value. The averaging function is better because the warm-up of the voice coil ♦ 5 is determined by the effective value and the ratio of the peak values to the effective value varies with practical waveforms more than the ratio of the power value to the mean. (The effective value is always higher than the average.) 5 The resistance 10 is greater than the resistance 7, so that the capacitor 8 is not overloaded. The protection response rate is then determined by the time constant RC, where R is the resistance of resistor 7 and C is the capacitance of capacitor 8.

The voltage drop across the diode circuit 11 is set to correspond to the difference between the charging voltage required for activating the shield 10 (capacitor 8) and the threshold voltage of the MOSFET 5.

By using the diodes 11, the protection switching is made sharper and at the same time the dependence on the dissipation of the threshold voltage of the MOSFET 5 is reduced. However, for the lowest wattages, the diodes 11 cannot be used because the required voltage drop would be non-existent.

15

It is preferable to select at least about 10 amperes for the transistor 5, even if the expected currents are smaller, since the higher the transconductance of the transistor 5, the more closely the protection will operate. The threshold voltage should be low and its dispersion reasonable. However, MOS-FETs are quite cheap, so there is a bit of a choice of threshold voltages.

The cooling demand of the MOSFET 5 can be estimated by the maximum current of the amplifier and the maximum voltage of the element 1 to be protected. However, once the shield is activated, most of the loss power is consumed by the amplifier, which must otherwise take into account the possibility of a short circuit.

25

When the impedance of the object to be protected 1 is strongly frequency dependent, the voltage across this impedance does not correspond very well to the voltage lost in the resistance of the voice coil, which determines the heating of the voice coil. In this case, the voltage to be monitored can be filtered before rectifying and averaging. An example of such a connection is shown in Figure 3.

30

The resistors 12, 13, 14 and the capacitor 15 form a symmetrical echo filter 16 which makes it possible to attenuate high frequencies from 0 to 6 dB, for example to compensate for voice coil inductance. During the second half-wave, the equalization circuit 17 receives its voltage from the coil filter 16 through diodes 18 and 2c and during the second half-wave through diodes 19 and 2d. Diodes 18 and 19 may be conventional small signal diodes such as gate-conducting diodes 11.

The compensation circuit 17 must not overload the filter 16, and the filter 16, in turn, must not create too low impedance alongside the element 1 to be protected. Thus, with resistances 12 and 14 being, for example, a few hundred ohms, the resistance 9 should be at least a few kilo-ohms and the resistance 7, respectively, still higher.

It is possible to further sharpen the protection function and reduce the threshold voltage dependence by using a control transistor in front of the power transistor 5.

The PNP transistor can be used to control the N-type MOSFET as shown in Figure 4. The reference potential of the equalizer filter 3 is now positive pole 20, and the protection is activated when the base emitter voltage, i.e. resistor 22, of PNP tran-10 reaches a certain limit ( 0.6 V), whereupon the resistor 23 begins to receive current and opens the MOSFET 5.

The base-emitter voltage dispersion of the bipolar transistors is small, so the accuracy of operation is good even at low power. The current gain factor is not of great importance here since the base 15 is controlled mainly by voltage.

However, the base current 24 required for activation should be at least one order less than the current of the resistor 22, which in turn should be much less than the current charging the capacitor 8, which again should be small relative to the current of the resistor 9. Thus, in practice, the carrier 24 may have only a few microampers 20, so to avoid interference it is not necessary to place the high current conductors in the immediate vicinity of the control transistor 21, even though it is not a precision circuit.

The waveform of the limited voltage is quite different with the connections of Figures 4 and 2, since the collector current of the controller-25 transistor 21 and the control voltage of the MOSFET 5, which is proportional to it, depend slightly on the instantaneous value of the signal. The coupling in FIG. 4 works by smoothly cutting signal peaks above a certain limit, while the coupling in FIG. 2 tends to cut the waves from the base, leaving the peaks almost intact.

The use of a controller transistor also makes it possible to use a Daripington bipolar pair as a power transistor, as shown in Figure 5. In order to provide sufficient base current for this pair, a Darlington transistor can also be used as a controller transistor. However, the carrier voltage required for activation is then double that of Figure 4.

The echo filter 16 shown in Figure 3 can also be added to the connections of Figures 4 and 5, provided that the load capacities of the various degrees are taken into account.

Claims (4)

A loudspeaker circuit suitable for a current-controlled loudspeaker which monitors and limits the signal voltage present in the loudspeaker system, characterized in that said circuit comprises: a rectifier bridge (2a-d) with alternating voltage terminals connected to said signal voltage and . a power transistor (5) which, when open, conducts current between said plus terminal and said negative terminal. 15. at least one RC time constant circuit (3) for adjusting the response rate of said protection circuit so that the intensity of said signal voltage can transiently exceed the static activation limit of said protection circuit. Protective circuit according to claim 1, characterized in that said signal voltage 20 is filtered before rectifying and directing to said circuits controlling the power transistor (5). A loudspeaker, characterized in that it comprises a protective circuit according to claim 1. A loudspeaker according to claim 3, characterized in that said signal voltage is filtered before rectifying and directing to said circuits controlling the power transistor (5).