JP2016171507A - Acoustic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a counter-electromotive force and an overcurrent generated when switching a speaker or an amplifier and to prevent the speaker from being broken when a DC voltage occurs in the output of the amplifier.SOLUTION: The acoustic device monitors an output voltage of an amplifier and switches a speaker or the amplifier when the output voltage of the amplifier is equal to or lower than a threshold value, increases a transition time of a switch for switching the speaker or the amplifier, and monitors the output voltage of an amplifier and shuts down the switch for switching the speaker or the amplifier when a DC voltage occurs in an output of the amplifier.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an audio device having a speaker switching function or an amplifier switching function.

  2. Description of the Related Art A speaker switching device and a plurality of amplifiers for switching a plurality of speakers to reproduce a sound and a amplifier switching device for reproducing a sound by switching a plurality of amplifiers are practically used. Also, an amplifier / speaker switching device that combines a speaker switching device and an amplifier switching device has been put into practical use.

When a speaker or an amplifier is switched during sound reproduction by an audio device incorporating the speaker switching device or the amplifier switching device, there are the following problems.

Since the voice coil of the speaker has an inductance component, a back electromotive force is generated from the voice coil of the speaker when the speaker is disconnected from the amplifier during sound reproduction. If the inductance of the speaker is L, the current flowing through the speaker's voice coil just before disconnecting the speaker from the amplifier is i, and the time required to disconnect the speaker from the amplifier is t, then the voltage V generated between the terminals of the speaker is V = L x (di / dt). This voltage may be applied to the components inside the speaker and the switching device, causing the speaker to be destroyed.
In an amplifier having an output transformer such as a vacuum tube amplifier, when the amplifier and the speaker are separated, a back electromotive force is generated at both ends of the output transformer on the same principle as the speaker, and the voltage is applied to the components of the amplifier to destroy the amplifier. It can be a cause.

When a speaker and an amplifier are connected during audio playback, a loud sound may be suddenly emitted from the speaker, which may make it uncomfortable.
When an amplifier and a speaker are connected during audio playback, the amplifier charges the capacitance of the speaker and the connection cable, and an overcurrent flows through the amplifier, speaker switching device or amplifier switching device, and the amplifier, speaker switching device or amplifier switching. It may cause damage to the equipment.
When the maximum value of current flowing out from the amplifier is Imax, the output voltage of the amplifier is V, the sum of the output impedance of the amplifier, the resistance of the changeover switch and the resistance of the connecting cable is R, the current of Imax = V / R is instantaneously output from the amplifier Decreases with flow time.

When the amplifier fails and a DC voltage is generated at the output of the amplifier, there is a problem that the DC voltage is applied to the speaker via the speaker switching device or the amplifier switching device and the speaker is destroyed. When an amplifier switching device or a speaker switching device is inserted between the amplifier and the speaker, it is desirable that the switching device can be used to protect the speaker when a DC voltage is generated at the output of the amplifier.

To reduce the back electromotive force generated from the speaker or amplifier output transformer immediately after disconnecting the speaker and amplifier, decrease the current i flowing in the speaker or output transformer immediately before disconnecting or increase the time t required for disconnection. It ’s fine. Since the current i flowing through the speaker or the output transformer is proportional to the output voltage of the amplifier, the output voltage of the amplifier just before disconnection may be reduced.

In order to prevent a loud sound from being suddenly emitted from the speaker when the speaker and the amplifier are connected, the speaker and the amplifier may be connected when the output voltage of the amplifier is low. In order to reduce the current charged from the amplifier to the electrostatic capacity of the cable or the like, it may be connected when the output voltage of the amplifier is low or R at the time of connection may be increased.

In Non-Patent Document 1 and Non-Patent Document 3, the speaker is switched after the output voltage of the amplifier is set to zero, and the problem is solved by returning the output level of the amplifier to the original state after the switching is completed. Although this method can reduce the back electromotive force and overcurrent to zero, there is a problem in that the cost increases because a dedicated controller or amplifier that operates in conjunction with the speaker switching device is required.

Further, in the speaker switching device and the amplifier switching device using the mechanical switch shown in Non-Patent Document 2, it is difficult to suppress the back electromotive force and the excessive current that are generated when the switch is switched during sound reproduction.

In the speaker switching device and the amplifier switching device shown in Non-Patent Document 1, Non-Patent Document 2 and Non-Patent Document 3, the speaker cannot be protected when a DC voltage is generated at the output of the amplifier.

In a speaker switching device or an amplifier switching device using a mechanical switch, since the user of the device operates the mechanical switch, it is difficult to disconnect and connect the speaker and the amplifier at the moment when the output voltage of the amplifier is low. Moreover, since the transition time of opening and closing of the mechanical switch is almost zero, a large back electromotive force may be generated in the output transformer of the speaker or the amplifier. In addition, since the resistance value of the switch immediately after the mechanical switch is closed is low, an excessive current may flow from the amplifier. When direct current is generated at the output of the amplifier, it is difficult to immediately disconnect the speaker from the amplifier.

In a speaker switching device using a relay or a semiconductor switch, the output voltage of the amplifier is monitored, and if the switch is disconnected when the output voltage is equal to or lower than the threshold value, generation of back electromotive force can be suppressed. If the output voltage of the amplifier is monitored and the switch is closed when it is below the threshold, no loud sound is suddenly emitted from the speaker so that no overcurrent flows from the amplifier. In addition, when a DC voltage is generated at the output of the amplifier, if all the relays or semiconductor switches are cut off, the speaker can be protected from destruction.

However, in the speaker switching device and the amplifier switching device, it is difficult to monitor the output voltage of the amplifier without deteriorating the quality of the reproduced sound. FIG. 10 is a configuration diagram of an audio device including a speaker switching device, an amplifier, and a speaker. For simplification, only one speaker switch is shown. In order to monitor the voltage between the output 52 and the output 53 of the amplifier circuit 13 with the switch control means 251 of the speaker switching device 22, the reference potentials of the amplifier 13 and the speaker switching device 22 need to be connected. In FIG. 10, the output 53 connects the amplifier 13 and the ground that is the reference potential of the speaker switching device 22. The output of the amplifier 13 is supplied to the speaker 16 a via the output 52 and is returned to the amplifier via the output 53. The amplifier 13 and the speaker switching device 22 are operated by a commercial power source 51. The power supply circuit 132 and the power supply circuit 252 generate a power supply for operating the amplifier 13 and the speaker switching device 22 from the commercial power supply 51. The power supply circuit 132 and the power supply circuit 252 have a capacitance between the ground and the commercial power supply 51. Therefore, a part of the current that drives the speaker 16 a passes through the path 62. A current for operating the amplifier 13 and the speaker switching device 22 flows through the path 62. Therefore, a voltage is generated in the path 62 due to the impedance of the path 62 and the current. This voltage is superimposed as a noise on the voltage for driving the speaker 16a and deteriorates the quality of sound reproduced from the speaker 16a. In order to prevent such quality deterioration of the sound, the ground of the amplifier 13 and the speaker switching device 22 must be separated. However, if the ground of the amplifier 13 and the speaker switching device 22 is separated, the output voltage of the amplifier cannot be monitored by the switch control means 251.

In the speaker switching device and the amplifier switching device using the relay, the back electromotive force generated in the output transformer of the speaker and the amplifier cannot be made sufficiently low because the switch switching time is short. In addition, it is difficult to sufficiently reduce the current flowing through the amplifier, the speaker switching device, or the amplifier switching device at the moment of connecting the speaker and the amplifier.

JAI Corporation, “Multichanger MC-333 Product Information”, [online], [March 4, 2015 search, Internet (URL: http://www.orb.co.jp/audio/mc333.html ) Audio Design Co., Ltd., “Selector HAS Series Product Information”, [online], [Search March 4, 1995], Internet (URL: http://www.audiodesign.co.jp/HAS.htm)

Sofisonant Audio “Speaker Switcher Product Information”, [online], [Search February 26, 2015], Internet (URL: http://sophisonant.com/products/category1/)

When a speaker and an amplifier are separated while a sound is being reproduced by an acoustic device incorporating the speaker switching device or the amplifier switching device, there is a problem that a back electromotive force is generated in the output transformer of the speaker or the amplifier.

There is a problem that when a speaker and an amplifier are connected while a sound is reproduced by an audio device incorporating the speaker switching device or the amplifier switching device, a loud sound is suddenly emitted from the speaker and the user feels uncomfortable. In addition, there is a problem that an excessive current flows from the amplifier when the speaker and the amplifier are connected during reproduction of sound by an acoustic device incorporating the speaker switching device or the amplifier switching device.

When a DC voltage is generated at the output of the amplifier due to the failure of the amplifier, there is a problem that the DC voltage is applied to the speaker via the speaker switching device or the amplifier switching device and the speaker is destroyed.

The present invention is intended to solve the problems of the conventional speaker switching device and amplifier switching device, and prevents back electromotive force from being generated even if the speaker or amplifier is switched during sound reproduction. The purpose is to prevent an excessive current from flowing and to prevent sudden loud sound from being emitted from the speaker.

Another object of the present invention is to prevent the speaker from being destroyed by shutting off the speaker switching device or the amplifier switching device when a DC voltage is generated at the output of the amplifier.

The acoustic device according to the present invention includes a changeover switch, switch control means for opening and closing the changeover switch, and voltage monitoring means for monitoring the voltage of the input of the changeover switch.

The changeover switch of the acoustic device according to the present invention may be a relay or a semiconductor switch.

The changeover switch of the acoustic device according to the present invention is a semiconductor switch, and the transition time of opening and closing of the semiconductor switch is a predetermined time.

The output signal of the voltage monitoring means of the acoustic device according to the present invention is connected to the switch control means.

The switch control means opens and closes the changeover switch when the voltage monitoring means of the acoustic apparatus according to the present invention indicates that the input voltage of the changeover switch is smaller than a threshold value.

The switch control means opens the changeover switch when the voltage monitoring means of the acoustic device according to the present invention continues for a predetermined time or longer indicating that the input voltage of the changeover switch is greater than a threshold value.

The changeover switch of the acoustic device according to the present invention may be a switch for switching a speaker or an amplifier.

According to the speaker switching device and the amplifier switching device of the present invention, no back electromotive force is generated from the output transformer of the speaker or the amplifier even if the speaker or the amplifier is switched during the reproduction of the sound, and no overcurrent flows from the amplifier. There is an advantage that a loud sound is not emitted.
The speaker switching device and the amplifier switching device of the present invention have an advantage that the destruction of the speaker can be prevented when a DC voltage is generated at the output of the amplifier.

The figure which showed an example of the implementation method of a speaker switching apparatus. An example of a switch of a speaker switching device. MOS-FET characteristics. The block diagram of the audio equipment containing a speaker switching apparatus. The block diagram of the audio equipment containing an amplifier switching apparatus. 1 is a block diagram of an audio device including an amplifier / speaker switching device. The block diagram of a speaker switching apparatus. The block diagram of an amplifier switching apparatus. The block diagram of an amplifier and speaker switching device. Problems with speaker switching devices. The flowchart of speaker switching. The flowchart of DC protection.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not limit the contents of the present invention described in the claims. In addition, all of the configurations described below are not necessarily essential configuration requirements of the invention.

Prior to the description of the embodiments of the present invention, the configuration of a conventional speaker switching device, amplifier switching device, and amplifier / speaker switching device will be described, and only the embodiment of the speaker switching device will be described. It also shows that the operation of the amplifier / speaker switching device can be easily estimated.

 FIG. 4 is a block diagram illustrating an example of an audio device including a speaker switching device. The source device 11 is a device that outputs sound, such as a CD player or a tuner. The preamplifier 12 is an amplifier for amplifying the audio signal supplied from the source device 11 and adjusting the signal level. The power amplifier 13 is an amplifier for amplifying the audio signal supplied from the preamplifier 12 to power that can drive the speaker. The speaker switching device 22 is a device for selecting a single speaker from the speakers 16 a, 16 b, and 16 c and connecting it to the output of the power amplifier 13. The speaker 16a, the speaker 16b, and the speaker 16c are devices that convert an electric signal supplied from a power amplifier into an air dense wave. In FIG. 4, the preamplifier 12 and the power amplifier 13 are independent, but they may be integrated.

FIG. 5 is a block diagram illustrating an example of an audio device including an amplifier switching device. The source device 11 is a device that outputs sound, such as a CD player or a tuner. The preamplifier 12 is an amplifier for amplifying the audio signal supplied from the source device 11 and adjusting the signal level. The power amplifier 13a, the power amplifier 13b, and the power amplifier 13c are amplifiers for amplifying the audio signal supplied from the preamplifier 12 to power that can drive the speaker. The amplifier switching device 31 is a device for selecting a single power amplifier from the power amplifier 13a, the power amplifier 13b, and the power amplifier 13c and connecting it to the speaker 16. The speaker 16 is a device that converts an electric signal supplied from a power amplifier into an air dense wave. In FIG. 5, the preamplifier 12, the power amplifier 13a, the power amplifier 13b, and the power amplifier 13c are independent, but any one of the preamplifier 12 and the power amplifier may be integrated.

FIG. 6 is a block diagram illustrating an example of an audio device including an amplifier / speaker switching device. The source device 11 is a device that outputs sound, such as a CD player or a tuner. The preamplifier 12 is an amplifier for amplifying the audio signal supplied from the source device 11 and adjusting the signal level. The power amplifier 13a, the power amplifier 13b, and the power amplifier 13c are amplifiers for amplifying the audio signal supplied from the preamplifier 12 to power that can drive the speaker. The amplifier / speaker switching device 41 selects a single power amplifier from the power amplifier 13a, the power amplifier 13b, and the power amplifier 13c, and selects a single speaker from the speakers 16a, 16b, and 16c, A switch for connecting the output of the selected power amplifier and the selected speaker. The speaker 16a, the speaker 16b, and the speaker 16c are devices that convert an electric signal supplied from the power amplifier into an air dense wave. In FIG. 6, the preamplifier 12, the power amplifier 13a, the power amplifier 13b, and the power amplifier 13c are independent, but any of the preamplifier 12 and the power amplifier may be integrated.

FIG. 7 shows an internal configuration of the speaker switching device 22 shown in FIG. The audio signal input to the input terminal 221 is selected by the switch 222, the switch 223, and the switch 224, and is output to any one of the output terminal 225, the output terminal 226, and the output terminal 227. The switch 222, the switch 223, and the switch 224 are configured by mechanical switches, relays, or semiconductor switches.

FIG. 8 shows an internal configuration of the amplifier switching device 31 shown in FIG. Audio signals input to the input terminal 311, the input terminal 312, and the input terminal 313 are selected by the switch 314, the switch 315, and the switch 316 and output to the output terminal 317. The switch 314, the switch 315, and the switch 316 are configured by a mechanical switch, a relay, or a semiconductor switch.

FIG. 9 shows an internal configuration of the amplifier / speaker switching device 41 shown in FIG. The audio signal input to the input terminal 411, the input terminal 412, and the input terminal 413 is selected by the switch 414, the switch 415, and the switch 416, and further selected by the switch 417, the switch 418, and the switch 419, and the output terminal 41a is output. The signal is output to the terminal 41b and the output terminal 41c. The switch 414, the switch 415, the switch 416, the switch 417, the switch 418, and the switch 419 are configured by mechanical switches, relays, or semiconductor switches.

As shown in FIGS. 7 and 8, the speaker switching device and the amplifier switching device have the same configuration except that the input terminal and the output terminal are horizontally reversed. Further, as shown in FIG. 9, the amplifier / speaker switching device has a configuration in which an amplifier switching device and a speaker switching device are connected in series. As described above, the amplifier switching device and the amplifier / speaker switching device have the same configuration as the speaker switching device, and if the embodiment of the speaker switching device is described, the embodiment of the other switching device can be easily guessed. Only the acoustic device including the above will be described below.

An embodiment of a speaker switching device will be described with reference to FIG. The audio signal input to the input terminal 221 is selected by the switch 222, the switch 223, and the switch 224, and is output to any one of the output terminal 225, the output terminal 226, and the output terminal 227. The switch 222, the switch 223, and the switch 224 are constituted by semiconductor switches. The MCU 228 is a controller that controls the speaker switching device. The input device 229 is a means for operating the speaker switching device and receives a user operation and transmits it to the MCU 228. Examples of the input device 229 include a switch, a keyboard, a variable resistor, and a remote control receiver. The MCU 228 controls the switch 222, the switch 223, and the switch 224 through the output port 230. The photocoupler 231 detects whether the voltage of the audio signal at the input terminal 221 is equal to or higher than a threshold value and transmits it to the MCU 228. Since the forward voltage VF of the LED of the photocoupler 231 is about 1.2V, when the voltage of the input terminal 221 is ± 1.2V or less, the photocoupler output 232 becomes H level, and the voltage of the input terminal 221 becomes ± 1. In the case of 2V or more, the photocoupler output 232 becomes L level. The resistor 233 allows a sufficient current to flow through the LED of the photocoupler, does not exceed the maximum rating of the LED, and is set to a resistance value sufficiently larger than the impedance of the speaker. The display device 234 is for indicating the operating state of the speaker changeover switch, and an LED, an LCD, an EL display or the like is used. In this embodiment, by using a photocoupler to monitor the output voltage of the amplifier, the output voltage of the amplifier can be monitored without connecting the ground of the amplifier and the ground of the speaker switching device. In this embodiment, the voltage at the input terminal 221 is monitored, but the current flowing through the speaker switching device may be monitored using a transformer, a Hall element, or the like.

FIG. 2 shows the configuration of the switch 222 of FIG. The switch 223 and the switch 224 in FIG. As the changeover switch, a MOS-FET 2221, a MOS-FET 2222, a MOS-FET 2223, and a MOS-FET 2224 are used. Since the MOS-FET has a parasitic diode between the drain and the source, the MOS-FET is connected in series as shown in FIG. 2 for use as a switch. The control means 2225 is a voltage source for supplying a voltage necessary for turning on the MOS-FET between the gate and the source of the MOS-FET 2221 and the MOS-FET 2222. The control means 2226 is a voltage source for supplying a voltage necessary for turning on the MOS-FET between the gate and the source of the MOS-FET 2223 and the MOS-FET 2224. The control means 2225 and the control means 2226 always supply a constant voltage between the gate and the source of the MOS-FET even if the source voltage of the MOS-FET changes. As an example, a floating power supply or a photocoupler can be used as the control means. When the MCU 228 sends a signal for turning on the MOS-FET to the control means 2225 and the control means 2226 through the output port 230, the control means 2225 and the control means 2226 are connected to the MOS-FET 2221, MOS-FET 2222, MOS-FET 2223, and MOS-FET 2224. A voltage is supplied between the gate and the source to turn on these MOS-FETs. When the MCU 228 sends a signal for turning off the MOS-FET to the control means 2225 and the control means 2226 through the output port 230, the control means 2225 and the control means 2226 are the MOS-FET 2221, the MOS-FET 2222, the MOS-FET 2223, and the MOS-FET 2224. These MOS-FETs are turned off by setting the voltage between the gate and the source to zero. In this way, the switch 222 is opened and closed under the control of the MCU 228.

If the output impedance of the control means 2225 and control means 2226 is R and the input capacitance of the MOS-FET is Ci, the voltage between the gate and source of the MOS-FET rises and falls with a time constant determined by R and Ci. That is, if the output impedances of the control means 2225 and control means 2226 are set to appropriate values, the rise and fall times of the voltage between the gate and source of the MOS-FET can be set to predetermined values. According to the relationship between the gate-source voltage and the on-resistance of the MOS-FET shown in FIG. 3, it can be seen that when the voltage between the gate and the source rises or falls over time, the on-resistance also changes over time. When the output impedances of the control means 2225 and control means 2226 are set to appropriate values according to the input capacitance of the MOS-FET, the switch 222 opens after the resistance increases over a certain time from the closed state. Further, the switch 222 is closed after the resistance decreases over a certain time from the opened state. As described above, the switch 222 can arbitrarily set the switching time, so that the back electromotive force V = L x (di / dt) t generated in the output transformer of the speaker or the amplifier when the switch is opened is increased. V can be reduced. Further, since the on-resistance of the MOS-FET immediately after closing the switch is large, the maximum current Imax = V / R R flowing from the amplifier to the capacitance of the speaker and the connecting cable through the switch when the switch is closed can be increased. Therefore, Imax decreases.
As described above, the back electromotive force generated when the switch is opened can be reduced by appropriately setting the switch opening / closing transition time. Also, the value of the current flowing from the amplifier when the switch is closed can be reduced.

The speaker selection operation will be described with reference to FIG.

The user operates the input device 229 in FIG. 1 to select the speaker number N. (Step S10)

The MCU 228 in FIG. 1 waits until the level of the photocoupler output 232 in FIG. 1 is H level, that is, the voltage at the force terminal 221 in FIG. (Step S20)

If the level of the photocoupler output 232 in FIG. 1 is H, the MCU 228 in FIG. 1 opens all the switches through the output port 230 in FIG. (Step S30)

The MCU 228 in FIG. 1 waits until the level of the photocoupler output 232 in FIG. (Step S40)

If the level of the photocoupler output 232 in FIG. 1 is H, the MCU 228 in FIG. 1 closes the switch corresponding to the speaker number N selected through the output port 230. (Step S50)
For example, when the speaker 1 is selected, the switch 222 in FIG. 1 is closed.

With the above operation, the changeover switch can be opened and closed when the voltage at the input terminal 221 is ± 1.2 V or less, so that the back electromotive force V = L generated in the speaker and the output transformer of the amplifier immediately after the speaker and the amplifier are disconnected. It is possible to reduce i of x (di / dt) and reduce the back electromotive force V. Further, since the current Imax = V / R flowing from the amplifier when the changeover switch is closed can be reduced, Imax can be reduced. Furthermore, it can prevent sudden loud sound from being emitted from the speaker.

As described above, when the speaker and the amplifier are separated from each other, the back electromotive force V = L x (di / dt) i can be reduced and t can be increased to reduce the back electromotive force V. When the speaker and the amplifier are connected, Imax = V / R V can be reduced and R can be increased to reduce Imax flowing out of the amplifier. An example of theoretical values of the back electromotive force V generated from the speaker and the current Imax flowing from the amplifier is shown below. Amplifier output resistance = 16 mΩ, speaker switching device on-resistance = 10 mΩ, speaker cable resistance = 100 mΩ, speaker cable and speaker capacitance = 3000 pF, speaker impedance = 8Ω, speaker inductance = 0.5 mH, mechanical switch Transition time = 100 nanoseconds and semiconductor switch transition time = 1 ms.
When the output of the amplifier is a sine wave of 100 W, the maximum value of the back electromotive force generated from the speaker when the switch of the speaker switching device is switched is calculated. The effective value of the voltage at the terminal 221 = (100 × 8) ^1/2 = about 28 Vrms. This peak value is about 80 Vp-p. Therefore, the maximum value of the current flowing through the speaker is 80 ÷ 8 = 10A. Immediately after opening the mechanical switch, a back electromotive force V = Lx (di / dt) = 0.5 × 10 ⁻³ x (10/100 × 10 ⁻⁹ ) = 50,000 V is generated. Actually, the back electromotive force is smaller than 50,000 V due to the resistance and capacitance of the components and wiring, but a large voltage is still generated. Next, according to this embodiment, the back electromotive force when the voltage of the input terminal 221 when the switch is opened is 1.2 V or less is calculated. The current flowing through the speaker immediately before opening the switch is 1.2 ÷ 8 = 0.15A. Therefore, the back electromotive force V = L x (di / dt) = 0.5 × 10 ⁻³ x (0.15 / 100 × 10 ⁻⁹ ) = 750V. It can be reduced to the counter electromotive force ^{V = L x (di / dt} ) = 0.5x10 -3 x (0.15 / 1x10 -3) = 75mV when using a semiconductor switch. Although the calculation of the counter electromotive force generated from the output transformer is not shown here, the counter electromotive force can be reduced by the same principle.

The maximum current Imax flowing from the amplifier at the moment when the switch is closed is obtained. Since R = 16 mΩ + 10 mΩ + 100 mΩ = 126 mΩ, a current of Imax = V / R = 80/126 × 10 ⁻³ = 634 A flows. Thereafter, the current of the speaker increases with time due to the inductance of the voice coil of the speaker, and finally a current corresponding to the output voltage of the amplifier flows. In the above example, a current I = 80/8 = 10 A, which is determined by the output voltage of the amplifier and the impedance 8Ω of the speaker, flows, and then the current I changes according to the change of the audio signal. If the voltage at the moment of closing the switch is made 1.2 V or less according to this embodiment, it can be reduced to Imax = 1.2 / 126 × 10 ⁻³ = 9.5 A immediately after the switch is closed. When the amplifier charges the speaker cable and the speaker capacitance, its time constant is 126 mΩ × 3000 pF = 0.378 ns.
The time for closing the semiconductor switch is 1 millisecond, which is sufficiently longer than the time constant, so that no current flows for the amplifier to charge the speaker cable and the speaker capacitance at the moment of closing the switch. Immediately after the switch is closed, the speaker current increases with time due to the inductance of the voice coil of the speaker, and finally the current I = 1.2 / 8 = 0.15A flows, which is determined by 1.2V and the impedance of the speaker 8Ω. The current I changes according to the change of the audio signal.

In this embodiment, a photocoupler is used for the voltage monitoring means, and a semiconductor switch is used for the changeover switch. Thereby, suppression of back electromotive force, suppression of overcurrent, prevention of sudden loud sound generation and high sound quality are realized at the same time. If the deterioration of sound quality is allowed, a comparator or an A / D converter can be used as voltage monitoring means. A relay can also be used as a changeover switch if the back electromotive force and overcurrent are not sufficiently suppressed.

The operation when a DC voltage is generated at the output of the amplifier will be described with reference to FIG.
The MCU 228 generates an interrupt at regular intervals by an internal timer.
As an example, a case where a timer interrupt occurs every 5 milliseconds will be described. When the timer interruption occurs, the process of FIG. 12 is performed.
The counter value is set to 0 before the timer interrupt is permitted. (Step S100)

Timer interrupt occurs every 5 milliseconds

Check the value of the photocoupler output 232 of FIG. If the value is L level, that is, if the voltage between the terminals of the input terminal 221 in FIG. 1 is ± 1.2 V or more, the process proceeds to step S120. Otherwise, the process proceeds to step S150. (Step S110)

Increment the counter. (Step S120)

Check if the counter value is 100 or more. If the counter value is less than 100, the interrupt process is terminated.
If the value of the counter is 100 or more, the process proceeds to step S140. (Step S130)

If the counter value is 100 or more, it is determined that a DC voltage is generated, and the switch 222, the switch 223, and the switch 224 in FIG. 1 are opened. Thereafter, the interrupt process is terminated. (Step S140)

The counter value is set to 0 and the interrupt process is terminated. (Step S150)

As described above, the output voltage of the amplifier is sampled every 5 milliseconds, and if it is 100 times, that is, ± 1.2 V or more continuously for 0.5 seconds, it is determined that a DC voltage is generated at the output of the amplifier. Open the switch to protect the speakers. The sampling interval of the photocoupler output and the conditions for determining that a DC voltage is generated are merely examples, and other methods may be used. In this embodiment, the DC voltage is detected using integration by software, but integration may be performed by hardware.

11 Source equipment 12 Preamplifier 13 Power amplifier 22 Speaker switching device 16a Speaker 16b Speaker 16c Speaker 221 Input terminal 222 Switch 223 Switch 224 Switch 225 Output terminal 226 Output terminal 227 Output terminal 228 MCU
229 Input device 230 Output port 231 Photocoupler 232 Photocoupler output 233 Resistance 234 Display device 2221 MOS-FET
2222 MOS-FET
2223 MOS-FET
2224 MOS-FET
2225 control means 2226 control means

Claims (7)

An audio device,
A changeover switch,
Switch control means for opening and closing the changeover switch;
An acoustic device comprising voltage monitoring means for monitoring the voltage of the input of the changeover switch. The acoustic device according to claim 1,
The acoustic device is a relay or a semiconductor switch. The acoustic device according to claim 1,
The changeover switch is a semiconductor switch,
An acoustic device in which a transition time of opening and closing of the semiconductor switch is a predetermined time. The acoustic device according to claim 1,
An acoustic device in which an output signal of the voltage monitoring means is connected to the switch control means. The acoustic device according to claim 1,
When the voltage monitoring means indicates that the input voltage of the changeover switch is smaller than a threshold value,
An acoustic device in which the switch control means opens and closes the changeover switch. The acoustic device according to claim 1,
When the state where the voltage monitoring means indicates that the input voltage of the changeover switch is greater than a threshold value continues for a predetermined time or more,
An acoustic device in which the switch control means opens the changeover switch. The acoustic device according to claim 1, wherein the changeover switch is a switch for switching a speaker or an amplifier.

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