JP2016178560A - Power amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology of capable of detecting disconnections or the like of speakers immediately at timing that disconnections or the like in all the speakers occur without necessity of attaching signal generation means for inspection used to detect such disconnections.SOLUTION: A power amplifier 1 includes a class D amplifier part 100 that performs self-oscillation at a predetermined self-oscillation frequency through a feedback loop including a speaker SP, generates a pulse duration modulation signal subjected to pulse duration modulation according to an audio signal AIN and supplies the pulse duration modulation signal to the speaker SP. A disconnection detection circuit 300 determines the presence/absence of a disconnection in the speaker SP according to whether or not the self-oscillation frequency of the class D amplifier part 100 changes from a predetermined reference frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power amplifier that drives a load such as a speaker.

  Various game machines such as pachinko and pachislot machines are equipped with a plurality of speakers, and music is emitted from the speakers as the game progresses. In addition, some game machines may be equipped with a speaker for sounding an alarm when an illegal action occurs as a countermeasure against an illegal action (so-called goto action) that obtains a ball in an illegal manner. . In the former case, music playback stops when the speaker is disconnected. In addition, when the wiring connecting the speaker and the amplifier that supplies power to the speaker deteriorates and a state close to disconnection occurs, the music playback volume decreases or becomes unstable. In the latter case, if the line to the speaker is cut, even if an illegal act is performed, an alarm sound cannot be emitted from the speaker, and the illegal act cannot be detected and prevented. Speakers are not only installed in the above various gaming machines, but are also installed in game facilities that handle gaming machines, and in various facilities such as offices and public facilities. In such a facility, a plurality of speakers may be installed on a ceiling or the like, and may be used as a speaker for issuing an alarm or a speaker for guidance. Further, a speaker for sound expansion may be installed at a high place in the facility (for example, on the stage of the hall of the game hall). In any speaker, in order to detect disconnection of the line to each speaker (hereinafter referred to as speaker disconnection), it is necessary for a human to actually confirm the sound emitted from each speaker. However, it is difficult to check the disconnection of each of the plurality of speakers one by one. In addition, when the speaker is installed at a high place, the above operation becomes more difficult. Therefore, a means for detecting disconnection of the speaker is required.

  Patent Document 1 discloses an acoustic device including a disconnection detection unit that detects disconnection of a piezoelectric speaker (capacitive speaker). This acoustic apparatus has a disconnection detecting means for detecting the number of disconnections of a plurality of speakers connected in parallel. When detecting the number of disconnections, the disconnection detecting means applies a peak voltage of a PWM waveform (High voltage of the PWM waveform) to a class D amplifier that drives a plurality of speakers for a certain period, and a reference voltage corresponding to the peak voltage Detection voltage generating means for outputting Vi from a class D amplifier to a plurality of speakers for a certain period is provided. The disconnection detection means acquires a rising waveform of the output voltage Vo of the speaker while the reference voltage Vi is output from the detection voltage generation means, and obtains the waveform of the ideal output voltage Vo acquired in advance when the disconnection is not disconnected. The number of disconnections of the speaker is detected by comparison.

JP 2013-192195 A

  By the way, the technique disclosed in Patent Document 1 described above applies the reference voltage Vi to the speaker when detecting the number of disconnections of the speaker, and the disconnection of the speaker or a state close thereto (hereinafter referred to as disconnection or the like). When it occurred, the disconnection or the like could not be detected immediately. In the example of the above-described game hall, it is necessary to detect the disconnection at the timing when the disconnection of the speaker occurs in order to prevent fraud. However, the technique disclosed in Patent Document 1 does not meet such a requirement. In addition, in order to generate the reference voltage Vi, it is necessary to provide an inspection signal generation means (detection voltage generation means). In this way, when a dedicated wiring or electric circuit is separately provided for disconnection detection, the circuit scale of the entire device increases, and in places where installation space is limited (such as the ceiling of a game arcade), the device concerned It will be difficult to install. The rising waveform of the reference voltage Vo is obtained based on the change in the accumulated voltage of the piezoelectric speaker, and the above technique cannot be applied to an electrodynamic (dynamic) speaker. For this reason, the said apparatus cannot be utilized in the facility which has installed the electrodynamic type speaker.

  The present invention has been made in view of the circumstances described above, and immediately detects the disconnection or the like at the timing when any speaker disconnection or the like occurs without providing an inspection signal generation means for detecting disconnection. The purpose is to provide technology that enables this.

  The present invention provides a power amplifier having a class D amplifier that performs self-excited oscillation, by comparing the frequency of the self-excited oscillation of the class D amplifier with a predetermined reference frequency, and Provided is a power amplifier comprising disconnection detecting means for determining the presence or absence of disconnection with a load of a class D amplifier.

  According to this invention, when a disconnection occurs between the class D amplifier and the load due to the self-excited oscillation of the class D amplifier, the self-excited oscillation frequency changes from the predetermined reference frequency. Therefore, by detecting the change from the reference frequency, it is possible to determine whether or not there is a disconnection between the class D amplifier and the load without separately providing a test signal generating means. Moreover, since the presence or absence of the said disconnection is determined based on the change of the self-excited oscillation frequency of a class D amplification part, the presence or absence of the said disconnection can be determined with respect to every speaker.

  Further, according to the present invention, in a power amplifier having a class D amplification unit that performs self-excited oscillation, an input signal to the class D amplification unit is compared with an output signal of the class D amplification unit, and the comparison result is output. There is provided a power amplifier comprising: a comparison unit; and a determination unit that compares a comparison result acquired from the comparison unit with a predetermined threshold value and determines a load state of the class D amplification unit.

  According to this invention, when a disconnection occurs between the class D amplifier and the load connected to the class D amplifier, the gain of the class D amplifier changes. Therefore, by comparing the input signal and output signal of the class D amplifier with a predetermined threshold and detecting the change in the gain, it is possible to determine whether or not there is a disconnection between the class D amplifier and the load. .

1 is a circuit diagram showing a configuration of a power amplifier 1 according to a first embodiment of the present invention. It is a circuit diagram which shows the structure of the frequency determination circuit 310 in the embodiment. It is a circuit diagram which shows the structure of the frequency determination circuit 320 in the embodiment. 3 is a time chart illustrating signal waveforms of respective units of a frequency determination circuit 320 in the same embodiment. It is a circuit diagram which shows the structure of the frequency determination circuit 330 in the embodiment. In the embodiment, it is a time chart illustrating the signal waveform of each part of the frequency determination circuit 330 when the speaker SP is not disconnected. In the embodiment, it is a time chart illustrating the signal waveform of each part of the frequency determination circuit 330 when the speaker SP is disconnected. It is a circuit diagram which shows the structure of power amplifier 1A which is 2nd Embodiment of this invention. It is a circuit diagram which shows the structure of the disconnection detection circuit 400 in the embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<First Embodiment>
FIG. 1 is a circuit diagram showing a configuration of a power amplifier 1 according to the first embodiment of the present invention. In FIG. 1, a speaker SP that is a load of the power amplifier 1 is also illustrated in order to facilitate understanding of the configuration of the power amplifier 1.

  As shown in FIG. 1, the power amplifier 1 includes a class D amplifier 100 and a disconnection detection circuit 300. The class D amplifier 100 includes an operational amplifier (or comparator) 110, an output stage 120, a filter 130, and a feedback resistor 140.

  The operational amplifier 110 is a circuit that forms an input unit for the input signal of the class D amplification unit 100. The audio signal AIN is input to the non-inverting input terminal of the operational amplifier 110 through the input terminal 111. The inverting input terminal of the operational amplifier 110 is grounded via a resistor 150.

  The output stage 120 has a transistor 121 interposed between the positive power source + B and the output terminal 123 of the output stage 120 as a switching element, and is interposed between the negative power source −B and the output terminal 123 of the output stage 120. The transistor 122 is provided. In a preferred embodiment, the transistors 121 and 122 are MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). The output stage 120 turns on the transistor 121 and turns off the transistor 122 and connects the positive power source + B to the output terminal 123 according to the output signal of the operational amplifier 110, or connects the transistor 121 to OFF and turns on the transistor 122 and turns on the negative power source -B. Is connected to the output terminal 123. Therefore, the waveform of the output signal of the output stage 120 is a rectangular wave.

  The filter 130 is a low-pass filter that removes high-frequency components of an audio band or higher from the output signal of the output stage 120 and supplies the remaining frequency components of the audio band to the speaker SP. The filter 130 includes an inductor 131 and a capacitor 132. Here, the inductor 131 is interposed between the output terminal 123 of the output stage 120 and one end of the speaker SP. The capacitor 132 has one end connected to a node 133 between the inductor 131 and the speaker SP, and the other end grounded. That is, the capacitor 132 is connected in parallel to the speaker SP that is a load.

  The feedback resistor 140 is interposed between the node 133 between the inductor 131 and the speaker SP and the inverting input terminal of the operational amplifier 110. This feedback resistor 140 constitutes a self-excited oscillation feedback loop that feeds back the output voltage to the speaker SP to the operational amplifier 110 that is the input of the class D amplifier 100 and causes the class D amplifier 100 to self-oscillate. The class D amplifier 100 outputs from the output stage 120 a PWM pulse train signal that has been pulse-width modulated based on the input audio signal AIN while oscillating at a predetermined self-excited oscillation frequency. The filter 130 serves to remove a high frequency component higher than the self-excited oscillation frequency from the PWM pulse train signal and supply it to the speaker SP. The above is the configuration of the class D amplification unit 100.

  In the power amplifier 1 according to the present embodiment, a load current feedback circuit 200 is connected to the class D amplification unit 100. The load current feedback circuit 200 is a circuit that negatively feeds back the load current flowing through the speaker SP that is the load of the class D amplifier 100 to the operational amplifier 110 that is the input unit. The load current feedback circuit 200 includes a current detection resistor 210, an amplification unit 220, and a coupling unit 230.

  The current detection resistor 210 is inserted between the terminal on the speaker SP opposite to the node 133 and the ground line. The amplification unit 220 includes an operational amplifier 221 and resistors 222 and 223. The inverting input terminal of the operational amplifier 221 is grounded via a resistor 222 and connected to the output terminal of the operational amplifier 221 via a resistor 223. The voltage across the current detection resistor 210 is applied to the non-inverting input terminal of the operational amplifier 221. Therefore, when the resistance value of the resistor 222 is Ra and the resistance value of the resistor 223 is Rb, the amplifier 220 amplifies the voltage across the current detection resistor 210 with a gain of (Ra + Rb) / Ra and outputs the amplified voltage. In the present embodiment, since the voltage across the current detection resistor 210 is amplified to a sufficiently large voltage by the amplifier 220 in this way, the resistance value of the current detection resistor 210 can be reduced. The coupling unit 230 includes a resistor 231 and a capacitor 232 that are inserted in series between the output terminal of the amplification unit 220 and the inverting input terminal of the operational amplifier 110. The coupling unit 230 serves to adjust the frequency characteristics of the feedback amount of the negative feedback performed through the load current feedback circuit 200. More specifically, the coupling unit 230 restricts the phase rotation in the high frequency band to prevent the self-oscillation frequency from being lowered.

Here, the self-excited oscillation frequency of the class D amplifier 100 is determined by the frequency characteristic of the feedback amount of the negative feedback performed through the load current feedback circuit 200 and the two feedback circuits of the self-excited oscillation feedback loop. . For this reason, when the negative feedback performed via the load current feedback circuit 200 is cut off due to the disconnection of the speaker SP, the self-excited oscillation frequency of the class D amplification unit 100 changes. In this embodiment, the self-excited oscillation frequency of the class D amplifier 100 is set to, for example, 384 kHz when the speaker SP is not disconnected, and negative feedback via the load current feedback circuit 200 is caused by the disconnection of the speaker SP. In the case of interruption, the phase of the load current feedback circuit 200 is set so as to be, for example, 645 kHz (a frequency that is 1.5 times the self-excited oscillation frequency when the speaker SP is not disconnected). Therefore, in the disconnection detection circuit 300 according to the present embodiment, whether or not the self-excited oscillation frequency of the class D amplifier 100 has changed from a predetermined reference frequency (that is, the self-excited oscillation frequency when the speaker SP is not disconnected). By detecting this, it is detected whether or not a disconnection has occurred in the speaker SP. Therefore, the disconnection detection circuit 300 includes a frequency determination circuit that detects whether the self-excited oscillation frequency of the class D amplification unit 100 has changed from the reference frequency. A specific configuration example of the frequency determination circuit will be described later. In the present embodiment, the disconnection of the speaker SP (disconnection between the class D amplification unit 100 and the speaker SP (load)) refers to the disconnection of the line (cable) from the class D amplification unit 100 to the speaker SP and the speaker. It means both the disconnection inside the SP (for example, voice coil).
The above is the configuration of the power amplifier 1 according to the present embodiment.

  Next, the operation of this embodiment will be described. In the class D amplification unit 100, the output signal of the output stage 120 is fed back through the filter 130 and the feedback resistor 140 to the operational amplifier 110 serving as the input unit with phase rotation. As a result, the class D amplifier 100 oscillates by itself. The self-excited oscillation frequency of the class D amplifier 100 is as described above, and is set to a frequency sufficiently higher than the frequency band of the input audio signal AIN.

  Here, the output signal of the output stage 120 is a rectangular wave, but since the filter 130 performs first-order integration of the rectangular wave by the capacitor 132, the signal waveform of the node 133 is a triangular wave. In the operational amplifier 110, the triangular wave of the node 133 fed back through the feedback resistor 140 is compared with the input audio signal AIN. As a result, a PWM pulse train signal pulse-width modulated by the input audio signal AIN is output from the operational amplifier 110, and this PWM pulse train signal is output to the filter 130 via the output stage 120. The PWM pulse train signal is removed from the high frequency component through the filter 130 and supplied to the speaker SP.

  Specifically, when the voltage value of the input audio signal AIN is 0V, a PWM pulse train signal with a duty ratio of 50% is output from the output stage 120, and the voltage applied to the speaker SP is 0V. When the voltage value of the input audio signal AIN changes from 0V in the positive direction, the duty ratio of the PWM pulse train signal output from the output stage 120 changes from 50% to a maximum of 100%, and the voltage applied to the speaker SP is 0V. To max + B. On the other hand, when the voltage value of the input audio signal AIN changes in the negative direction from 0V, the duty ratio of the PWM pulse train signal output from the output stage 120 changes from 50% to the minimum 0%, and the voltage applied to the speaker SP is 0V. To minimum -B. In this way, a signal having a waveform approximate to the input audio signal AIN is given to the speaker SP.

  While the amplification operation by the class D amplification unit 100 described above is performed, the load current feedback circuit 200 negatively feeds back the load current flowing through the speaker SP to the operational amplifier 110 that is the input unit of the class D amplification unit 100. As a result, the following effects can be obtained.

  The impedance of the speaker SP changes depending on the driving frequency of the speaker SP. Here, when the impedance of the speaker SP increases due to a change in the driving frequency of the speaker SP and the load current flowing through the speaker SP decreases, a feedback signal to the input unit of the class D amplification unit 100 via the load current feedback circuit 200 is obtained. It decreases, and the output signal of the class D amplifier 100 increases. As a result, the effective voltage applied from the class D amplifier 100 to the speaker SP increases, and the load current flowing through the speaker SP increases. On the other hand, when the impedance of the speaker SP decreases due to a change in the driving frequency of the speaker SP and the load current flowing through the speaker SP increases, the feedback signal to the input unit of the class D amplification unit 100 via the load current feedback circuit 200 increases. As a result, the output signal of the class D amplifier 100 decreases. As a result, the effective voltage applied from the class D amplifier 100 to the speaker SP decreases, and the load current flowing through the speaker SP decreases. As a result of such negative feedback control, the load current flowing through the speaker SP becomes constant regardless of the driving frequency of the speaker SP.

  As described above, in the present embodiment, the load current flowing through the speaker SP is negatively fed back to the input unit of the class D amplification unit 100, thereby effectively increasing the output impedance of the class D amplification unit 100, and the class D amplification unit 100. The load current flowing from the speaker to the speaker SP can be kept constant. Therefore, in a frequency band near the frequency f0 (for example, 80 to 100 Hz) at which the impedance of the speaker SP increases, a sufficient load current can be supplied to the speaker SP to achieve sound emission at a large volume. Further, in the present embodiment, control is performed to keep the load current flowing through the speaker SP constant regardless of the driving frequency of the speaker SP. Therefore, in the frequency band other than the frequency band near the frequency f0, the speaker SP is excessive. A large current can be prevented from flowing, and the speaker SP can be prevented from being damaged.

The above is the normal operation of the power amplifier 1 according to the present embodiment.
In the power amplifier 1, when the disconnection of the speaker SP occurs, the self-excited oscillation frequency of the class D amplification unit 100 changes from the reference frequency 384 kHz to 645 kHz, which is 1.5 times the reference frequency. The disconnection detection circuit 300 detects a change from the reference frequency of the self-excited oscillation frequency based on the output signal of the operational amplifier 110, and outputs an alarm signal indicating that the disconnection of the speaker SP has occurred.

The above is the basic operation when the disconnection of the power amplifier 1 according to the present embodiment is detected.
Next, details of operations performed by the frequency determination circuits 310, 320, and 330, which are examples of the frequency determination circuit used in the disconnection detection circuit 300, when disconnection is detected will be described in order.

<Frequency determination circuit 310>
FIG. 2 is a circuit diagram showing a configuration of the frequency determination circuit 310. The frequency determination circuit 310 includes a phase comparator 311, a charge pump 312, a comparator 313, and a malfunction prevention capacitor 314. The PWM pulse train signal output from the operational amplifier 110 is input to one end of the phase comparator 311. A reference clock signal is input to the other end of the phase comparator 311 from a reference clock signal generator (not shown). The frequency of the reference clock signal is set to the same self-oscillation frequency as that when the speaker SP is not disconnected, and is set to 384 kHz as a predetermined reference frequency. The phase comparator 311 detects a phase difference between the PWM pulse train signal and the reference clock signal, and outputs a phase difference signal having a pulse width corresponding to the phase difference to the charge pump 312.

  The charge pump 312 integrates the phase difference signal and outputs an analog signal indicating the integrated value to the comparator 313.

  A threshold signal having a predetermined voltage value is input to the comparator 313 from a threshold signal generator (not shown). The comparator 313 compares the output signal of the charge pump 312 with the threshold signal. If the output signal of the charge pump 312 is smaller than the threshold signal, the SP disconnection determination flag is set to the low level, and the output signal of the charge pump 312 is the threshold signal. If it is greater than the value, the SP disconnection determination flag is set to the high level.

  The malfunction prevention capacitor 314 is a capacitor provided to prevent malfunction of the comparator 313. One end of the malfunction prevention capacitor 314 is connected to the output terminal of the comparator 313, and the other end is grounded.

  In the above configuration, when the speaker SP is not disconnected, a PWM pulse train signal having a frequency of 384 kHz is output from the operational amplifier 110 of the class D amplification unit 100 to the phase comparator 311 of the frequency determination circuit 310. Here, when the speaker SP is not disconnected, the frequency of the PWM pulse train signal output to the phase comparator 311 is equal to the frequency of the reference clock signal, and the phase difference between the two is small. For this reason, the pulse width of the phase difference signal output from the phase comparator 311 is narrowed. As a result, the level of the output signal of the charge pump 312 becomes smaller than the voltage value of the threshold signal given to the comparator 313. For this reason, the comparator 313 notifies the speaker SP that no disconnection has occurred by setting the SP disconnection determination flag to the Low level.

  On the other hand, when the speaker SP is disconnected, the self-excited oscillation frequency of the class D amplification unit 100 changes from 384 kHz to 645 kHz. For this reason, the frequency of the PWM pulse train signal output from the operational amplifier 110 changes from 384 kHz to 645 kHz, and the PWM pulse train signal having a frequency of 645 kHz is input to the phase comparator 311. In this case, the phase difference between the PWM pulse train signal input to the phase comparator 311 and the reference clock signal changes over time, and a phase difference signal having a large pulse width is output to the charge pump 312. As a result, the level of the output signal of the charge pump 312 becomes larger than the voltage value of the threshold signal given to the comparator 313, and the comparator 313 sets the SP disconnection determination flag to the high level to notify that the speaker SP is disconnected. To do. The details of the operation of the frequency determination circuit 310 have been described above.

<Frequency determination circuit 320>
FIG. 3 is a circuit diagram showing a configuration of the frequency determination circuit 320. The frequency determination circuit 320 includes a shift register 321 including D-flip flops 322_1 to 322_3.

  The data input terminal D of the D flip-flop 322_1 is connected to the high potential power supply line. The PWM pulse train signal output from the operational amplifier 110 is input to each clock terminal C of the D flip-flops 322_1 to 322_3, and the frequency is 96 kHz (the speaker SP is disconnected) to the reset terminal R of the D flip-flops 322_1 to 322_3. If not, a reset pulse having a self-excited oscillation frequency (a frequency that is 1/4 of the PWM pulse train signal) of the class D amplifier 100 is given. An SP disconnection determination flag indicating whether or not the speaker SP is disconnected is output from the output terminal Q of the D-flip flop 322_3 which is the final stage of the shift register.

  4A and 4B are time charts illustrating the signal waveforms of the respective parts of the frequency determination circuit 320. FIG. More specifically, FIG. 4A shows a signal waveform of each part when the speaker SP is not disconnected, and FIG. 4B shows a signal waveform of each part when the speaker SP is disconnected. Show.

  When the speaker SP is not disconnected, as shown in FIG. 4A, the D-type amplification when the speaker SP is not disconnected from the operational amplifier 110 to each clock terminal C of the D flip-flops 322_1 to 322_3. A PWM pulse train signal having a frequency of 384 kHz, which is a self-excited oscillation frequency of the unit 100, is input. Here, while the signal level of the reset pulse is at the high level, the D-flip flops 322_1 to 322_3 are reset. For this reason, while the signal level of the reset pulse is high, the signal level of the SP disconnection determination flag is low regardless of the rise of the PWM pulse train signal. While the reset pulse signal level is low, the PWM pulse rises twice. Therefore, after the signal level of the reset pulse is switched from the high level to the low level, the high-level signal applied to the data input terminal D of the D-flip flop 322_1 is caused by the rising of the PWM pulse twice. Shift to the data input terminal D of 322_3. However, since the third PWM pulse rise does not occur while the signal level of the reset pulse is at the low level, the signal level of the SP disconnection determination flag is maintained at the low level. Thus, in a situation where the speaker SP is not disconnected and a PWM pulse train signal having the same frequency of 384 kHz as the reference frequency is generated, the SP disconnection determination flag maintains the low level.

  On the other hand, when the speaker SP is disconnected, as shown in FIG. 4B, the frequency determination circuit 320 has a PWM pulse train signal whose frequency, which is the self-excited oscillation frequency of the class D amplifier 100 when the speaker SP is disconnected, is 645 kHz. Is entered. In this case, as illustrated in FIG. 4B, the PWM pulse rises four times while the signal level of the reset pulse is low. Therefore, when the PWM pulse has risen three times after the signal level of the reset pulse is switched from the high level to the low level, the signal at the high level applied to the data input terminal D of the D-flip flop 322_1 becomes D. -Output from the output terminal Q of the flip-flop 322_3, and the SP disconnection determination flag changes from Low level to High level. The SP disconnection determination flag maintains the high level while the reset pulse is at the low level. When the signal level of the reset pulse is switched from the low level to the high level, the SP disconnection determination flag changes from the high level to the low level. Thus, in the situation where the speaker SP is disconnected and a 645 kHz PWM pulse train signal changed from the reference frequency is generated, the SP disconnection determination flag changes in a pulse shape. Therefore, the presence or absence of disconnection of the speaker SP can be determined based on the behavior of the SP disconnection determination flag.

<Frequency determination circuit 330>
FIG. 5 is a circuit diagram showing a configuration of the frequency determination circuit 330. The frequency determination circuit 330 includes a counter 331, inverting circuits 334_1 to 334_3, an exclusive OR circuit 335, and an AND circuit 336. The counter 331 includes D-flip flops 332_1 and 332_2 and an exclusive OR circuit 333.

  The PWM pulse train signal output from the operational amplifier 110 is inverted and applied to the clock terminals C of the D flip-flops 332_1 and 332_2 by the inverter circuit 334_2. The reset terminals R of the D-flip flops 332_1 and 332_2 have a frequency of 96 kHz (a frequency that is 1/4 times the self-excited oscillation frequency (PWM pulse train signal) of the class D amplifier 100 when the speaker SP is not disconnected). ) Reset pulse is inverted by the inversion circuit 334_1. The data input terminal D of the D flip-flop 332_1 is given a negative logic output signal (a signal obtained by inverting the output signal of the output terminal Q) of the D flip-flop 332_1. The exclusive OR circuit 333 includes an exclusive logic of a positive logic output signal Q1 output from the output terminal Q of the D-flip flop 332_1 and a positive logic output signal Q2 output from the output terminal Q of the D-flip flop 332_2. The sum is calculated, and a signal as the calculation result is output to the data input terminal D of the D-flip flop 332_2. The above is the configuration of the counter 331.

  The exclusive OR circuit 335 calculates the exclusive OR of the output signal Q1 of the D-flip flop 332_1 and the output signal Q2 of the D-flip flop 332_2, and outputs an output signal a that is the operation result. The inverting circuit 334_3 inverts the output signal a and outputs the inverted signal to the logical product circuit 336 as the output signal b. A determination flag is input to the logical product circuit 336 from a determination flag generation unit (not shown). Here, the determination flag is a pulse signal having a frequency of 192 kHz synchronized with the PWM pulse train signal (a frequency half the self-excited oscillation frequency (PWM pulse train signal) of the class D amplification unit 100 when the speaker SP is not disconnected). It is. The AND circuit 336 outputs the logical product of the output signal b and the determination flag as an SP disconnection determination flag.

  FIG. 6 is a time chart illustrating the signal waveform of each part of the frequency determination circuit 330 when the speaker SP is not disconnected. FIG. 7 is a time chart illustrating the signal waveform of each part of the frequency determination circuit 330 when the speaker SP is disconnected.

  When the speaker SP is not disconnected, a PWM pulse train signal having a frequency of 384 kHz is input from the operational amplifier 110 to each clock terminal C of the D-flip flops 332_1 and 332_2, as shown in FIG. Here, while the signal level of the reset pulse is at the high level, the D-flip flops 332_1 and 332_2 are reset. For this reason, while the reset pulse is at the high level, the signal levels of the output signals Q1 and Q2 output from the D-flip flops 332_1 and 332_2 are at the low level regardless of the rise of the PWM pulse train signal. As a result, the signal level of the output signal a, which is the exclusive OR of the output signals Q1 and Q2, becomes Low level, and the signal level of the output signal b obtained by inverting the output signal a becomes High level. Here, the determination flag rises once while the signal level of the reset pulse is high. Therefore, as shown in FIG. 6, if the determination flag rises once while the signal level of the reset pulse is high, the SP disconnection determination flag rises once in synchronization with this.

  While the signal level of the reset pulse is low, the output signals Q1 and Q2 change as (Q1, Q2) = (1, 0) → (0, 1) due to two rises of the PWM pulse. As a result, while the signal level of the reset pulse is low, the signal level of the output signal a, which is the exclusive OR of the output signals Q1 and Q2, becomes high level, and the signal of the output signal b obtained by inverting the output signal a The level becomes the Low level. Therefore, as shown in FIG. 6, the SP disconnection determination flag maintains the Low level while the signal level of the reset pulse is the Low level regardless of the rising edge of the determination flag. In such a situation where the speaker SP is not disconnected and a 384 kHz PWM pulse train signal having the same frequency as the reference frequency is generated, the SP disconnection determination flag rises in synchronization with the determination flag only while the reset pulse is at the high level. .

  On the other hand, when the speaker SP is disconnected, a PWM pulse train signal having a frequency of 645 kHz is input from the operational amplifier 110 to each clock terminal C of the D-flip flops 332_1 and 332_2, as shown in FIG. While the signal level of the reset pulse is at the high level, the SP disconnection determination flag rises in synchronization with the determination flag as described above. On the other hand, while the signal level of the reset pulse is low, the PWM pulse rises three times as shown in FIG. Therefore, after the signal level of the reset pulse is switched from the high level to the low level, the output signals Q1 and Q2 are (Q1, Q2) = (1, 0) → (0, 1) → (1,1) As a result, the output signal a which is an exclusive OR of the output signals Q1 and Q2 changes from a = 1 to 1 → 0, and the output signal b obtained by inverting the output signal a is b = 0 → 0 → 1. Change. Here, after the signal level of the output signal b is switched to the high level, the determination flag rises once. Therefore, as shown in FIG. 7, the SP disconnection determination flag rises once in synchronization with the rise of the determination flag while the signal level of the reset pulse is low. Therefore, when a disconnection occurs in the speaker SP, the SP disconnection determination flag rises every time the determination flag rises. In such a situation where the speaker SP is disconnected and a 645 kHz PWM pulse train signal changed from the reference frequency is generated, the SP disconnection determination flag rises every time the determination flag rises. Therefore, the presence or absence of disconnection of the speaker SP can be determined based on the behavior of the SP disconnection determination flag.

Second Embodiment
FIG. 8 is a circuit diagram showing a configuration of a power amplifier 1A according to the second embodiment of the present invention. In FIG. 8, parts corresponding to those in the first embodiment are denoted by common reference numerals, and description thereof is omitted. The power amplifier 1 </ b> A according to the present embodiment includes a class D amplification unit 100 and a disconnection detection circuit 400.

  In the power amplifier 1A, the configuration and operation of the class D amplification unit 100 are the same as those in the first embodiment, and a detailed description thereof will be omitted. In the power amplifier 1A, when the impedance of the speaker SP changes due to deterioration or disconnection of the speaker SP, the gain of the class D amplification unit 100 changes. Therefore, the disconnection detection circuit 400 in the present embodiment calculates the gain of the class D amplification unit 100, that is, the ratio between the input signal supplied to the class D amplification unit 100 and the output signal of the class D amplification unit, and calculates the ratio. By comparing with a predetermined threshold value, it is detected whether or not the disconnection of the speaker SP has occurred.

  The disconnection detection circuit 400 is a function provided in a DSP (Digital Signal Processor) that performs acoustic processing on the audio signal AIN, for example. Note that the disconnection detection circuit 400 may be provided in a separately prepared IC or the like instead of additionally provided in the sound processing DSP. As shown in FIG. 8, the audio signal AIN is output to the class D amplifier 100 after being subjected to acoustic processing by the DSP. The disconnection detection circuit 400 acquires an audio signal given to the class D amplification unit 100. Hereinafter, an audio signal supplied from the class D amplification unit 100 to the disconnection detection circuit 400 is referred to as an audio signal V1. Further, the disconnection detection circuit 400 acquires an audio signal output from the class D amplification unit 100 to the speaker SP. As shown in FIG. 8, the output signal of the class D amplification unit 100 is divided by a resistor 510 and a resistor 520 and stepped down to a voltage that can be processed by the DSP, and then output to the disconnection detection circuit 400. Hereinafter, an output signal supplied from the class D amplifier 100 to the disconnection detection circuit 400 is referred to as an audio signal V2.

  FIG. 9 is a block diagram illustrating a functional configuration of the disconnection detection circuit 400. As illustrated in FIG. 9, the disconnection detection circuit 400 includes A / D conversion units 410_1 and 410_2, a calculation unit 411, a storage unit 412, and a determination unit 413. When the A / D conversion unit 410_1 acquires the audio signal V1, the A / D conversion unit 410_1 performs A / D conversion on the signal at a constant sampling rate, and provides a sample S1 of the signal to the calculation unit 411. When the A / D conversion unit 410_2 acquires the audio signal V2, the A / D conversion unit 410_2 performs A / D conversion on the signal at a constant sampling rate, and provides a sample S2 of the signal to the calculation unit 411. When the calculation unit 411 acquires the samples S1 and S2 from the A / D conversion units 410_1 and 410_2, the calculation unit 411 stores them in the storage unit 412 in time series. As a result, the storage unit 412 stores the sample S1 and the sample S2 in time series. When the calculation unit 411 reads the samples S1 and S2 stored in the storage unit 412, the calculation unit 411 calculates an average value (or representative value) from a plurality of sample values in a predetermined time unit for each sample. The calculation unit 411 then compares the average value (or representative value) P1 of the sample V1 at a certain time with the average value (or representative value) P2 of the sample V2 that appears most recently at that time (that is, the class D amplification unit). P2 / P1 ratio (100 gain) is calculated. When calculating the P2 / P1 ratio, the calculation unit 411 gives the calculation result to the determination unit 413. Upon receiving the calculation result from the calculation unit 411, the determination unit 413 determines whether or not the P2 / P1 ratio is out of a predetermined range (that is, a threshold value). When the P2 / P1 ratio is within the predetermined range, the determination unit 413 sets the SP disconnection determination flag to a low level. On the other hand, when the P2 / P1 ratio is outside the predetermined range, the determination unit 413 sets the SP disconnection determination flag to a high level and notifies the speaker SP that a disconnection has occurred.

Next, details of the operation of the disconnection detection circuit 400 will be described.
When the speaker SP is not disconnected, the audio signals V1 and V2 are output to the disconnection detection circuit 400 while maintaining the ratio of the amplitudes constant. The audio signals V1 and V2 are A / D converted by the A / D converters 410_1 and 410_2, respectively, supplied to the arithmetic unit 411, and then stored in the storage unit 412. Here, since the average value (or representative value) P1 of the audio signal V1 and the average value (or representative value) P2 of the audio signal V2 maintain a constant ratio, the P2 / P1 ratio calculated by the calculation unit 411 is the speaker. When SP is not disconnected, a constant value is maintained. Since this P2 / P1 ratio does not exceed the predetermined range in the determination performed by the determination unit 413 when the speaker SP is not disconnected, the determination unit 413 maintains the SP disconnection determination flag at the low level, Notifies that the speaker SP is not disconnected.

  On the other hand, when the impedance of the speaker SP increases due to deterioration or disconnection of the speaker SP, the class D amplifier 100 increases the gain, and thus the load current to the speaker SP increases. For this reason, the average value (or representative value) P2 of the audio signal V2 output to the disconnection detection circuit 400 increases as compared to the average value (or representative value) P1 of the audio signal V1. As a result, the P2 / P1 ratio calculated by the calculation unit 411 increases and exceeds a predetermined range in the determination performed by the determination unit 413. For this reason, the determination unit 413 notifies the speaker SP that a disconnection has occurred by setting the SP disconnection determination flag to a high level.

  In the present embodiment, the disconnection detection circuit 400 detects the change in the gain of the class D amplification unit 100 caused by the fluctuation of the impedance of the speaker SP, thereby detecting whether or not the speaker SP is disconnected. By the way, a change in impedance of a speaker is generally caused by deterioration of the speaker prior to disconnection of the speaker. Therefore, by detecting the change in the gain of the class D amplifier 100 by the disconnection detection circuit 400, not only the disconnection of the speaker but also the deterioration of the speaker can be detected. Thereby, disconnection of a speaker can be prevented in advance by detecting deterioration of the speaker in advance and taking measures such as replacing a speaker that may be disconnected.

  Further, according to the present embodiment, if it is an acoustic device that performs digital acoustic processing, it is not necessary to separately prepare an IC for providing the disconnection detection circuit 400, and the disconnection detection circuit 400 is added to the DSP for acoustic processing. Can be provided. For this reason, it is possible not only to prevent an increase in circuit scale associated with the installation of the disconnection detection circuit, but also to reduce the cost required for measures against disconnection.

  Further, by changing the constants of the resistors 510 and 520 as appropriate to increase the audio signal V2, it is possible to detect a slight impedance change caused by the deterioration of the speaker SP. If the audio signal V2 is increased, the sound quality of the sound emitted from the speaker SP may be deteriorated. However, in this case, the disconnection detection circuit 400 may be used as a measure against disconnection of a speaker or the like used in a guitar performance or a gaming machine where deterioration in sound quality is not a concern.

<Other embodiments>
While various embodiments of the present invention have been described above, other embodiments are possible for the present invention.

(1) In the first embodiment, the feedback circuit of the class D amplification unit 100 is configured by negative feedback, but may be configured by positive feedback. Even in such a configuration, when the speaker SP is disconnected, the self-excited oscillation frequency of the class D amplification unit 100 remains unchanged. By detecting the change of the self-excited oscillation frequency from the reference frequency, Disconnection of the speaker SP can be detected.

(2) In the first embodiment, a shift register or a counter is used as the frequency determination circuit. However, if a change in the reference frequency of the self-excited oscillation frequency of the class D amplifier 100 can be detected based on the number of rising edges or the number of falling edges of the PWM pulse train signal, the frequency determination circuit can be used using another circuit. May be configured.

(3) In the first embodiment, the voltage value of the threshold signal output to the comparator 313 may be adjusted as appropriate. For example, a slight change in the self-excited oscillation frequency can be detected by setting the threshold value more strictly.

(4) In the first embodiment, the processing capability of the shift register or counter used as the frequency determination circuit may be changed as appropriate. Thereby, various conditions for frequency determination by the disconnection determination circuit 300 can be changed.

(5) In the second embodiment, the class D amplifying unit 100 is configured by a self-excited oscillation circuit, but may be configured by a separately-excited oscillation circuit. In this case, the same effect as that of the self-excited type can be obtained.

(6) In the second embodiment, the example in which the disconnection detection circuit 400 is provided in the DSP mounted in the power amplifier 1 has been described. However, the disconnection detection device in which the disconnection detection circuit 400 is mounted is externally attached to the power amplifier 1. Also good. That is, a first signal acquisition unit that acquires an output signal of a power amplifier having a class D amplification unit that performs self-excited oscillation, a second signal acquisition unit that acquires an input signal to the power amplifier, and the first The output signal acquired by the signal acquisition unit is compared with the input signal acquired by the second signal acquisition unit, the comparison result is output, and the comparison result acquired from the comparison unit is set as a predetermined threshold value. In comparison, a disconnection detection device including a determination unit that determines a load state of the class D amplification unit is externally attached to the power amplifier. In this case, even if the power amplifier 1 is not equipped with a DSP, the disconnection detection device can determine whether or not the speaker SP is disconnected.

DESCRIPTION OF SYMBOLS 1,1A ... Power amplifier, 100 ... Class D amplification part, 111 ... Input end, 110 ... Operational amplifier, 120 ... Output stage, 130 ... Filter, SP ... Speaker, 200 ... Load current feedback circuit, 210 ... Current detection resistor, 220 ... amplifier 230 ... coupler 300,400 ... disconnection detection circuit 310,320,330 ... frequency determination circuit 311 ... phase comparator 312 ... charge pump 313 ... comparator 314 ... capacitor for preventing malfunction, 321 ... Shift register, 331 ... Counter, 322_1-322_3, 332_1-332_2 ... D-flip-flop, 333,335 ... Exclusive OR circuit, 334_1-334_3 ... Inverting circuit, 336 ... AND circuit, 410_1-410_2 ... A / D conversion unit, 411 ... calculation unit, 412 ... storage unit, 413 ... determination unit.

Claims (4)

In a power amplifier having a class D amplifier that performs self-oscillation,
Disconnection detecting means for determining the presence or absence of disconnection between the class D amplifier and the load of the class D amplifier by comparing the self-oscillation frequency of the class D amplifier with a predetermined reference frequency. A power amplifier comprising:   2. The power according to claim 1, wherein the disconnection detection unit performs the determination based on a phase difference between a pulse width modulation signal generated by the class D amplification unit and a signal having the predetermined reference frequency. amplifier.   2. The power amplifier according to claim 1, wherein the disconnection detection unit detects a change in a predetermined period of the number of rising or falling edges of the pulse width modulation signal acquired from the class D amplification unit. In a power amplifier having a class D amplifier that performs self-oscillation,
A comparison unit that compares an input signal to the class D amplification unit and an output signal of the class D amplification unit, and outputs a comparison result;
A power amplifier comprising: a determination unit that compares a comparison result acquired from the comparison unit with a predetermined threshold value and determines a load state of the class D amplification unit.

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