JP2009267615A - Amplifier for audio - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an amplifier for audio, wherein over-current is prevented by detecting a decrease in impedance of a load and its erroneous detection is prevented. <P>SOLUTION: An input audio signal is amplified by amplifiers 11 and 12 and sent out as an audio amplified signal to an output transformer 3. A current detector 13 and a voltage detector 14 detect the current and voltage of the audio amplified signal, and an over-current protection circuit 15a controls the amplification factor of the amplifier 11 based upon differences of the current and voltage to prevent an over-current from being caused by the decrease in impedance of the load. Further, low-pass filters 23i and 23v corresponding to characteristics of the output transformer 3 are provided in the over-current protection circuit 15a to attenuate frequency components of frequency close to the high cutoff frequency of the output transformer 3, thereby preventing the erroneous detection. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an improvement of an audio amplifying apparatus that amplifies an audio signal and supplies it to a transmission line.

  The loudspeaker system is an audio broadcasting system configured by connecting an audio amplifying device and a speaker via a transmission line. When such a loudspeaker system is applied to a large-scale facility such as an airport, a school, or a shopping center, the length of the transmission line may reach several kilometers. For this reason, such a loudspeaker system employs high-impedance transmission with good transmission efficiency.

  High-impedance transmission has good transmission efficiency, but has a drawback that an overcurrent flows when a low-impedance load is connected. Therefore, care must be taken not to short-circuit the transmission line for high-impedance transmission or to connect a low-impedance load by mistake. However, in a loudspeaker system that uses high-impedance transmission, the user may mistakenly connect a general low-impedance speaker. In such a case, overcurrent flows and damages the audio amplifier. There was a problem of doing.

  Therefore, in the previous application (Patent Document 1), the applicant of the present application has proposed an audio amplifying device that prevents the occurrence of overcurrent with a simple circuit configuration. This audio amplifying device detects the current and voltage of an audio amplified signal output from the audio amplifying device, and controls the amplification factor of the audio signal based on the difference therebetween. With such a configuration, it is possible to detect an increase in current and a decrease in voltage due to a decrease in load impedance, and to prevent the occurrence of overcurrent.

  FIG. 6 is a diagram showing a configuration example of the audio amplifying device 2c proposed in Patent Document 1. In FIG. The audio amplifying device 2c includes an input terminal 10, a voltage control amplifier 11, a digital amplifier 12, a current detector 13, a voltage detector 14, an overcurrent protection circuit 15c, and an output terminal 16. The audio signal generated by the sound source device 1 is input via the input terminal 10, amplified by the voltage control amplifier 11 and the digital amplifier 12, and then output from the output terminal 16 as an audio amplification signal.

The current detector 13 detects the current of the audio amplification signal and generates a current detection signal Di. Similarly, the voltage detector 14 detects the voltage of the audio amplification signal and generates a voltage detection signal Dv. The overcurrent protection circuit 15 c generates an amplification factor control signal G based on the current detection signal Di and the voltage detection signal Dv, and controls the amplification factor of the voltage control amplifier 11. That is, the difference is obtained after full-wave rectification of the current detection signal Di and the voltage detection signal Dv, and the load state is monitored based on the difference signal. When the load impedance decreases, the amplification factor of the voltage control amplifier 11 is also decreased to prevent overcurrent from flowing.
Japanese Patent Application No. 2007-134058

  In order to perform high impedance transmission using the audio amplifying device 2c described in Patent Document 1, an output transformer is connected to the output terminal of the audio amplifying device 2c, and the transmission line is connected to the output line via the output transformer. Need to connect. However, when an output transformer is connected to the audio amplifying device 2c, a short circuit does not occur in the transmission line, and the load below the specified level is not connected to the transmission line, but the load impedance is reduced. It was found that there is a possibility of false detection.

  Also, even in the case of a low-impedance transmission amplifying device connected to a transmission line without going through an output transformer, in order to prevent overcurrent that occurs when a short circuit occurs or when a load less than the specified value is connected. Some have a similar overcurrent detection function. An amplifier for low-impedance transmission usually has poor transmission efficiency and is rarely used with a long transmission line connected. However, when used for a long transmission line, the same erroneous detection may occur due to the capacitive load and inductance load of the transmission line.

  The present invention has been made in view of the circumstances as described above, and is an audio amplifying device capable of accurately detecting a drop in load impedance with a simple circuit configuration and preventing occurrence of overcurrent and erroneous detection. The purpose is to provide.

  An audio amplifying device according to a first aspect of the present invention includes an amplifying device for amplifying an audio signal and supplying the signal to a transmission line; a current detecting unit for generating a current detection signal indicating a current waveform supplied to the transmission line; Voltage detection means for generating a voltage detection signal indicating a voltage waveform supplied to the output transformer, and a difference between the current detection signal and the voltage detection signal, based on a signal obtained by attenuating a specific frequency component, And overcurrent protection means for controlling the amplification factor of the signal amplification means.

  With such a configuration, it is possible to prevent the occurrence of overcurrent by detecting a decrease in load impedance based on the difference between the current detection signal and the voltage detection signal. At that time, by using a signal in which a specific frequency component is attenuated as the difference, the phase difference between the voltage and current is detected as the difference and prevents the amplification factor of the signal amplifying means from being lowered by mistake. can do. Therefore, it is possible to prevent the occurrence of overcurrent by detecting a drop in the impedance of the load, and to prevent the erroneous detection.

  In the audio amplifying device according to the second aspect of the present invention, in addition to the above configuration, the overcurrent protection means includes a first low-pass filter that attenuates a frequency component higher than a threshold frequency from the current detection signal, and the voltage detection. Difference calculation for obtaining a difference between a second low-pass filter that attenuates a frequency component higher than the threshold frequency from the signal, the current detection signal that has passed through the first low-pass filter, and a voltage detection signal that has passed through the second low-pass filter Means. Therefore, with a simple circuit configuration, it is possible to prevent the occurrence of overcurrent by detecting a drop in the impedance of the load, and to prevent erroneous detection.

  In the audio amplifying device according to the third aspect of the present invention, in addition to the above configuration, the overcurrent protection unit includes a difference calculation unit for obtaining a difference between the current detection signal and the voltage detection signal, and a threshold value based on an output signal of the difference calculation unit. A low-pass filter that attenuates frequency components higher than the frequency. Therefore, with a simple circuit configuration, it is possible to prevent the occurrence of overcurrent by detecting a drop in the impedance of the load, and to prevent erroneous detection.

  In the audio amplifying device according to the fourth aspect of the present invention, in addition to the above configuration, the signal amplifying means supplies an audio signal to the transmission line via an output transformer, and the specific frequency component passes through the output transformer. The frequency component is higher than the threshold frequency in the band.

  With such a configuration, the phase of the frequency component close to the cutoff frequency of the output transformer rotates, so that the difference between the current detection signal and the voltage detection signal is detected, and the amplification factor of the signal amplification means is erroneously reduced. Can be prevented. Therefore, even in an audio amplifying apparatus connected to a transmission line via an output transformer, it is possible to detect a drop in the impedance of the load to prevent the occurrence of overcurrent and to prevent erroneous detection. In particular, it is possible to prevent erroneous detection due to phase rotation in the vicinity of the cutoff frequency on the high frequency side, so even if the high frequency cutoff frequency of the output transformer is within the band of the audio signal. The occurrence of overcurrent can be prevented.

  The audio amplifying device according to the present invention detects a load impedance decrease based on a signal which is a difference between a current detection signal and a voltage detection signal and attenuates a specific frequency component. For this reason, it is possible to detect a decrease in impedance with a simple circuit configuration and to prevent erroneous detection due to a phase difference between current and voltage. Therefore, an inexpensive and highly reliable audio amplifying apparatus can be provided.

Embodiment 1 FIG.
<Sound system>
FIG. 1 is a diagram showing a schematic configuration example of a loudspeaker system 100 including an audio amplifying device 2a according to Embodiment 1 of the present invention. This loudspeaker system includes a sound source device 1, an audio amplifying device 2a, an output transformer 3, a transmission line 4, and a speaker 5.

  The sound source device 1 is a device that generates an audio signal having an audible frequency, for example, a microphone or a recording / reproducing device that generates an analog audio signal including a frequency component of 20 Hz to 20 kHz. The audio signal generated by the sound source device 1 is amplified by the audio amplifying device 2 a, boosted by the output transformer 3, and sent to the transmission line 4. A plurality of speakers 5 are connected in parallel to the transmission line 4, and the same sound is output from each speaker 5 based on the audio signal sent to the transmission line 4.

  The output transformer 3 is a step-up transformer in which the audio amplifying device 2a is connected to the primary side and the transmission line 4 is connected to the secondary side. The amplified audio signal amplified in the audio amplifying device 2a is boosted in the output transformer 3 and sent to the transmission line 4 as a high-voltage audio transmission signal. For example, a signal having an effective voltage of 100 V can be used as the audio transmission signal. In the present embodiment, the output transformer 3 is externally connected to the audio amplifying device 2a. However, the output transformer 3 may be built in the same casing as the audio amplifying device 2a in advance.

  The speaker 5 incorporates a step-down transformer called a matching transformer, and has an input impedance higher than that of a general speaker. A general speaker has an input impedance of about 4 to 8Ω, whereas the speaker 5 has an input impedance of, for example, 1 kΩ or more.

  In this loudspeaker system, the audio amplified signal output from the audio amplifying device 2a is converted into a higher voltage signal and transmitted. Such a transmission method is called high impedance transmission. In high impedance transmission, transmission loss can be reduced as compared with the case of transmitting an audio signal as it is. For example, in the case of a local broadcasting system of a large-scale facility such as an airport, a school, or a shopping center, the length of the transmission line 4 reaches several kilometers, and the transmission loss cannot be ignored. High impedance transmission is suitable as a transmission system for such a large-scale facility loudspeaker system.

  Further, since the speaker 5 used for high impedance transmission has a high input impedance, a large number of speakers 5 can be connected in parallel to the same transmission line 4. Further, by providing the output transformer 3, the output impedance on the sound source side viewed from the transmission line 4 is high, and the speaker 5 can be connected without considering impedance matching.

  Further, if an insulating transformer is used as the output transformer 3, the audio amplifying device 2a and the transmission line 4 can be galvanically isolated. For this reason, even if the ground of the transmission line 4 or a lightning strike to the transmission line 4 occurs, the audio amplifying device 2a is not easily affected, and is suitable for a loudspeaker system in which the transmission line 4 is laid outdoors. is there.

<Audio Amplifier 2a>
FIG. 2 is a block diagram showing a configuration example of the audio amplifying device 2a of FIG. The audio amplifying device 2a includes an input terminal 10, a voltage control amplifier 11, a digital amplifier 12, a current detector 13, a voltage detector 14, an overcurrent protection circuit 15a, and an output terminal 16. The audio signal generated by the sound source device 1 is input via the input terminal 10, amplified by the voltage control amplifier 11 and the digital amplifier 12, and then output from the output terminal 16 to the output transformer 3.

  The voltage control amplifier 11 is an analog amplifier circuit that amplifies the audio signal from the input terminal 10, and the amplification factor is variably controlled by the overcurrent protection circuit 15a. Here, when the impedance of the transmission line 4 decreases, the overcurrent protection circuit 15a reduces the amplification factor of the voltage control amplifier 11, thereby preventing an overcurrent from flowing through the transmission line 4. Note that the decrease in the amplification factor includes a case where the output is cut off by setting the amplification factor to zero.

  The digital amplifier 12 further amplifies the audio signal supplied from the voltage control amplifier 11 and outputs it from the output terminal 16 as an audio amplified signal. For example, it has a pulse width modulation circuit, class D amplifier and low pass filter (not shown), converts the input audio signal into a PWM (Pulse Width Modulation) signal, amplifies this PWM signal in class D, and then attenuates high frequency components To convert it into an analog signal.

  The current detector 13 is a current detection unit that detects the current of the audio amplification signal output from the output terminal 16 and generates a current detection signal Di. Here, a current transformer is connected between one of the pair of output terminals 16 and the digital amplifier 12, and the current waveform of the audio amplification signal is detected. The current detection signal Di is a voltage signal indicating the current waveform of the audio amplification signal, and this current detection signal is input to the overcurrent protection circuit 15a.

  The voltage detector 14 is voltage detection means for detecting the voltage of the audio amplification signal output from the output terminal 16 and generating the voltage detection signal Dv. Here, a voltage dividing resistor is connected between the pair of output terminals 16 to detect the voltage waveform of the audio amplification signal. The voltage detection signal Dv is a voltage signal indicating the voltage waveform of the audio amplification signal, and this voltage detection signal is also input to the overcurrent protection circuit 15a.

<Overcurrent protection circuit 15a>
The overcurrent protection circuit 15a generates an amplification factor control signal G based on the current detection signal Di and the voltage detection signal Dv, and controls the amplification factor of the voltage control amplifier 11. That is, the current detection signal Di and the voltage detection signal Dv are compared, and the load state is monitored based on the comparison result. If it is detected that the impedance of the load has decreased, the amplification factor of the voltage control amplifier 11 is decreased to prevent the occurrence of overcurrent.

  The overcurrent protection circuit 15a includes a level adjuster 21, full-wave rectifiers 22i and 22v, low-pass filters 23i and 23v, a subtractor 24, a peak hold circuit 25, and a coefficient multiplier 26.

  The level adjuster 21 is a level adjusting unit that adjusts the relative signal levels of the current detection signal Di and the voltage detection signal Dv so that the signal levels match in a desired load state. Here, it is assumed that the level adjuster 21 is an amplifier that adjusts the level of the voltage detection signal Dv, and the amplification factor thereof is a signal of the current detection signal Di and the voltage detection signal Dv when the load state is a rated load. It is assumed that the levels are predetermined so as to match.

  The full-wave rectifier 22i is a rectifier circuit that rectifies the current detection signal Di, and the rectified current detection signal Dri is output to the low-pass filter 23i. The full-wave rectifier 22v is a rectifier circuit that rectifies the level-adjusted voltage detection signal Dv, and the rectified voltage detection signal Drv is output to the low-pass filter 23v. A half-wave rectifier can be used in place of these full-wave rectifiers 22i and 22v, but it is desirable to use full-wave rectification in order to quickly detect the load state.

  The low pass filter 23i is a filter circuit that blocks the high frequency component of the rectified current detection signal Dri and passes the low frequency component. Similarly, the low pass filter 23v is a filter circuit that cuts off the high frequency component of the rectified voltage detection signal Drv and passes the low frequency component. These low-pass filters 23i and 23v have the same characteristics.

  The cut-off frequencies of the low-pass filters 23 i and 23 v are set in advance to a frequency lower than the cut-off frequency of the output transformer 3, that is, a frequency within the pass band of the output transformer 3. The cut-off frequency is preferably within the frequency band of the audio signal. That is, the cutoff frequency of the low-pass filters 23i and 23v is preferably determined according to the characteristics of the output transformer 3, and is configured as a variable filter that can change the cutoff frequency, or can be easily replaced. It is desirable to be.

  The subtracter 24 is a difference calculation circuit that calculates a difference between the current detection signal Dfi and the voltage detection signal Dfv after passing through the low-pass filter as the error signal E. Here, it is assumed that a value obtained by subtracting the output of the low-pass filter 23v from the output of the low-pass filter 23i is output as the error signal E. The error signal E increases as the load impedance decreases, and decreases as the load impedance increases. Note that a value obtained by subtracting the output of the low-pass filter 23i from the output of the low-pass filter 23v may be used as the error signal E.

  The peak hold circuit 25 is a circuit that holds the peak value Ep of the error signal E output from the subtracter 24. That is, the peak hold circuit 25 holds the maximum value of the error signal E output by the subtractor 24 in the past, and outputs this maximum value as the peak value Ep. The reset of the peak hold circuit 25 may be performed based on, for example, a reset operation by the user, or the peak hold is reset after a certain period of time using a charge / discharge characteristic of a capacitor (not shown). It may be.

  The coefficient multiplier 26 is conversion means for converting the error signal E into an amplification factor control signal G for the voltage control amplifier 11. Here, the amplification factor control signal G is generated by multiplying the peak value Ep of the error signal output from the peak hold circuit 25 by a predetermined coefficient. Note that a constant adder for adding constants may be further provided as necessary.

<Operation of Overcurrent Protection Circuit 15a>
Next, the operation of the overcurrent protection circuit 15a will be described. For example, when the transmission line 4 is short-circuited or when a low-impedance speaker is connected to the transmission line 4, the impedance of the load decreases. At this time, in the audio amplification signal output from the audio amplifying device 2a, the current increases and the voltage decreases. That is, the level of the current detection signal Di increases, the level of the voltage detection signal Dv decreases, and the error signal E obtained by the subtractor 24 increases. The coefficient multiplier 26 converts the error signal E into an amplification factor control signal G, and lowers the amplification factor of the voltage control amplifier 11. Thus, when the impedance of the load is reduced, the amplification factor of the audio amplifying device 2a can be automatically reduced. Therefore, it is possible to prevent an overcurrent from flowing due to a drop in the impedance of the load and destroying the audio amplifying device 2a. In addition, each circuit constituting the overcurrent protection circuit 15a can be realized as a relatively simple analog circuit. Therefore, it is possible to protect the audio amplifying device 2a from overcurrent while suppressing a significant increase in cost for overcurrent protection.

  Further, since the peak hold circuit 25 holds the peak value Ep of the error signal E, it is possible to reliably prevent the occurrence of overcurrent even in an unstable state in which the load impedance increases or decreases. it can. That is, if the error signal E output from the subtractor 24 is increased, the amplification factor of the audio amplifying device 2a is reduced. However, even if the error signal E is temporarily reduced, the amplification factor of the audio amplifying device 2a is decreased. Will not increase again. Therefore, the occurrence of overcurrent can be reliably prevented.

  Further, the low frequency filters 23i and 23v corresponding to the frequency characteristics of the output transformer 3 are used to cut off the high frequency band, thereby preventing erroneous detection due to the frequency characteristics of the output transformer 3. Prevention of this erroneous detection will be described in detail below.

  FIG. 3 is a diagram illustrating an example of frequency characteristics of the output transformer 3 illustrated in FIG. 1, where the horizontal axis represents frequency and the vertical axis represents attenuation (dB). The output transformer 3 has a substantially flat frequency characteristic in a frequency band including 1 kHz. Further, the cut-off frequencies f1 and f2 at which the attenuation amount is 3 dB are lower than the frequency range (20 Hz to 20 kHz) of the audio signal, while the lower frequency f1 is less than 20 Hz, whereas the higher frequency. f2 is 18 kHz and is within the frequency range (20 Hz to 20 kHz) of the audio signal.

  The output transformer 3 desirably has a flat frequency characteristic over the entire frequency band (20 Hz to 20 kHz) of the audio signal. However, a transformer having a high-frequency cut-off frequency f2 exceeding 20 kHz is extremely expensive and increases the manufacturing cost. On the other hand, when an inexpensive transformer is used, the cutoff frequency f2 on the high frequency side exists in the frequency band of the audio signal. In general, a transformer has a property in which a capacitance component increases with respect to a frequency component close to a cutoff frequency and rotates its phase. Therefore, when such an inexpensive transformer is used as the output transformer 3, the phase of the frequency component close to the cutoff frequency f2 in the audio amplification signal output from the audio amplifying device 2a rotates. As a result, the current detection signal Di and the voltage detection signal Dv are out of phase with each other, and even if the signal levels of the current detection signal Di and the voltage detection signal Dv are the same, the error signal E is output from the subtractor 24. Is output. As a result, erroneous detection occurs.

  FIG. 4 is an explanatory diagram showing a situation in which erroneous detection of the load state occurs due to the phase difference between the current detection signal Di and the voltage detection signal Dv, both of which have a normal load impedance and an overcurrent flows. The main signal waveforms in the overcurrent protection circuit 15a when there is no fear are shown.

  (A)-(c) in the figure shows an example in which the frequency of the audio amplification signal is in a region where the frequency characteristic of the output transformer 3 is flat, for example, 1 kHz. The phase of this frequency component does not rotate, and there is no deviation in the phase of the current and voltage of the audio amplification signal. Therefore, the waveforms of the current detection signal Di and the voltage detection signal Dv match ((a) in the figure), and the rectified current detection signal Dri and the voltage detection signal Drv also match ((b) in the figure). ). Therefore, the error signal E obtained by the difference circuit 24 becomes a constant signal of zero, and the amplification factor of the voltage control amplifier 11 does not decrease.

  On the other hand, (d) to (f) in the figure show examples in which the frequency of the audio amplification signal is a frequency near the cutoff frequency f2 of the output transformer 3, for example, 18 kHz. The phase of the audio amplification signal is affected by the frequency characteristics of the output transformer 3, particularly the capacitance component, and the current and voltage phases are shifted. Therefore, the waveforms of the current detection signal Di and the voltage detection signal Dv are out of phase ((d) in the figure), and the rectified current detection signal Dri and the voltage detection signal Drv are also out of phase. ((E) in the figure). Therefore, the error signal E obtained by the difference circuit 24 is not a constant signal of zero. At this time, the peak hold circuit 25 holds the peak value Ep of the error signal E, and the amplification factor of the voltage control amplifier 11 is reduced based on the peak value Ep.

  For this reason, in the audio amplifying apparatus 2a according to the present embodiment, the low-pass filters 23i and 23v are used to block the frequency component in the vicinity of the high-frequency cutoff frequency f2 of the output transformer 3. For this reason, it is possible to prevent erroneous detection as shown in (d) to (f) of FIG.

  In order to quickly detect a change in load impedance, it is desirable that the current detection signal Dfi and the voltage detection signal Dfv input to the subtractor 24 include all the frequency components of the audio signal. However, as described above, the frequency region whose phase is rotated by the output transformer 3 needs to be blocked by the low-pass filters 23i and 23v.

  For this reason, the cut-off frequency fc of the low-pass filters 23i and 23v needs to be determined in advance to be lower than at least the high-frequency cut-off frequency f2 of the output transformer 3. The cut-off frequency fc is preferably in the frequency band of the audio signal. Therefore, it is desirable that it is within the pass band of the output transformer 3, that is, the frequency band between the cutoff frequencies f1 and f2.

Embodiment 2. FIG.
In the above embodiment, the example of the overcurrent protection circuit 15a in which the low-pass filters 23i and 23v are provided in the previous stage of the subtractor 24 has been described. In contrast, in the present embodiment, an overcurrent protection circuit 15b in which a low-pass filter 23 is provided at the subsequent stage of the subtractor 24 will be described.

  FIG. 5 is a diagram showing a configuration example of the audio amplifying device 2b according to the second embodiment of the present invention. When this audio amplifying device 2b is compared with the audio amplifying device 2a (Embodiment 1) of FIG. 2, a low-pass filter 23 is provided between the subtractor 24 and the peak hold circuit 25 instead of the low-pass filters 23i and 23v. It differs in that it is provided.

  As the low-pass filter 23, a filter circuit having the same characteristics as the low-pass filters 23i and 23v is used. The frequency component included in the output of the subtractor 24 shown in FIG. 4F is the same as the frequency component of the current detection signal Dri and the voltage detection signal Drv input to the subtractor 24, and the output signal of the subtractor 24. 2 is attenuated by the low-pass filter 23, the same result as in the case of the overcurrent protection circuit 15a in FIG. 2 can be obtained.

  In each of the above-described embodiments, the configuration in which the low-pass filters 23, 23i, and 23v are provided after the full-wave rectifiers 22i and 22v has been described. However, the present invention is not limited to such a case. That is, if the error signal input to the peak hold circuit 25 does not include a frequency component exceeding the cut-off frequency fc of the low-pass filter, the phase rotation of the audio amplification signal appears in the miscalculation signal E, and the load impedance decreases. Is not detected by mistake. For this reason, a low-pass filter can also be arranged before the full-wave rectifiers 22i and 22v. For example, the current detector 13 and the voltage detector 14 may be configured as a detection circuit that does not output a frequency component exceeding the cut-off frequency fc of the low-pass filter.

  In the above embodiment, the example of the audio amplifying apparatus 2a that amplifies the audio signal using the voltage control amplifier 11 and the digital amplifier 12 has been described. However, the present invention is not limited to such a case. That is, the signal amplifying means for amplifying the audio signal only needs to be able to control the gain based on the gain control signal G from the overcurrent protection circuits 15a and 15b, and the specific configuration of the amplifier is not limited. For example, it may be configured by one signal amplifier or a combination of three or more amplifiers regardless of a digital method or an analog method.

In the above embodiment, an example of an audio amplifying apparatus for high impedance transmission has been described. However, the present invention can also be applied to an audio amplifying apparatus for low impedance transmission. That is, the present invention can also be applied to an audio amplifying apparatus that does not include the output transformer 3 and is connected to the transmission line 4 without the output transformer 3 interposed therebetween. In this case, it is possible to prevent the phase difference between the voltage and current generated by the capacitive load and the inductance load of the transmission line 4 from being erroneously detected as a drop in impedance.
This is also effective for the overcurrent protection function.

It is the figure which showed the example of schematic structure of the loudspeaker system to which the audio amplifier 2a by Embodiment 1 of this invention is applied. FIG. 2 is a block diagram showing a configuration example of an audio amplifying device 2a in FIG. It is the figure which showed an example of the frequency characteristic about the output transformer 3 of FIG. 1, and has represented the frequency on the horizontal axis | shaft and the attenuation amount on the vertical axis | shaft. It is explanatory drawing which showed the mode when the erroneous detection of a load state generate | occur | produces with the phase difference of the electric current detection signal Di and the voltage detection signal Dv. It is the figure which showed one structural example of the audio amplifier 2b by Embodiment 2 of this invention. It is the figure which showed one structural example of the audio amplifier 2c proposed in patent document 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sound source device 2a, 2b, 2c Audio amplifier 3 Output transformer 4 Transmission line 5 Speaker 10 Input terminal 11 Voltage control amplifier 12 Digital amplifier 13 Current detector 14 Voltage detector 15a, 15b, 15c Overcurrent protection circuit 16 Output terminal 21 level adjusters 22i, 22v full wave rectifiers 23, 23i, 23v low pass filter 24 subtractor 25 peak hold circuit 26 coefficient multiplier Dfi filtered current detection signal Dfv filtered voltage detection signal Di current detection signal Dri after rectification Current detection signal Drv Voltage detection signal Dv after rectification Voltage detection signal E Error signal Ep Peak value f1 of low frequency side f1, f2 Cutoff frequency fc of output transformer Cutoff frequency G of low pass filter Amplification rate control signal

Claims (4)

Signal amplifying means for amplifying the audio signal and supplying it to the transmission line;
Current detection means for generating a current detection signal indicating a current waveform supplied to the transmission line;
Voltage detection means for generating a voltage detection signal indicating a voltage waveform supplied to the transmission line;
And an overcurrent protection unit that controls the amplification factor of the signal amplification unit based on a signal that is a difference between the current detection signal and the voltage detection signal and attenuates a specific frequency component. Audio amplifier. The overcurrent protection means includes a first low-pass filter that attenuates a frequency component higher than a threshold frequency from the current detection signal;
A second low-pass filter that attenuates a frequency component higher than the threshold frequency from the voltage detection signal;
2. The audio amplifying apparatus according to claim 1, further comprising difference calculating means for obtaining a difference between the current detection signal that has passed through the first low-pass filter and a voltage detection signal that has passed through the second low-pass filter. The overcurrent protection means includes a difference calculation means for obtaining a difference between the current detection signal and the voltage detection signal;
The audio amplifying apparatus according to claim 1, further comprising: a low-pass filter that attenuates a frequency component higher than a threshold frequency from the output signal of the difference calculation means. The signal amplifying means supplies an audio signal to the transmission line via the output transformer,
2. The audio signal amplifying apparatus according to claim 1, wherein the specific frequency component is a frequency component higher than a threshold frequency in a pass band of the output transformer.

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