JPS62188597A - Voice output circuit - Google Patents

Abstract

PURPOSE:To protect a speaker connected to a sound amplifier circuit from being excessively inputted by detecting the signal level of the voice output of the sound amplifier circuit, and lowering the sound volume set value of a sound volume control circuit when the detected signal level exceeds a set allowable input level of the speaker. CONSTITUTION:When a protection circuit 17 detects the signal level of the voice output of the sound amplifier circuits 15a, 15b and the detected signal level exceeds the preset allowable input level of the internal speakers 6a, 6b, a voice output circuit 11 automatically lowers the sound volume set value in the sound volume control circuit 13. Thereby, the internal speakers 6a, 6b connected to the sound amplifier circuits 15a, 15b are protected from being excessively inputted. When a source voltage is reduced, the protection level of the protection circuit 17 is lowered interlockingly therewith and when the source voltage is increased, the zener diode Zd of a safety circuit controls the allowable input level to a fixed value and prevents a band influence from being applied to the protection function of the protection circuit 17, so that a cumulative protection can be applied to the internal speakers 6a, 6b.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an audio output circuit that protects a speaker from excessive input.

[Prior Art] In the projection type television receiver l shown in FIG. 3, a projector 4 incorporating three projection tubes 3 connected to a video output circuit 2 projects an enlarged image onto a screen 5 in front. This allows for a powerful video experience that cannot be enjoyed with a direct-view television receiver.

This type of projection television receiver 1 usually has internal speakers 6a with little directivity on the left and right sides of the front of the projector 4.
, 6b are incorporated, and these internal speakers 6a
, 6b alone may not provide sufficient acoustic effects, external speakers 8a and 8b can be connected to the audio output circuit 7 by switching. In general, external speakers 8a, 8b are smaller than internal speakers 6a, 6b.
Although a large speaker with a large diameter sounding body is used, the internal speakers 6a and 6b have to be smaller than the external speakers 8a and 8b due to constraints on the installation space. Therefore, the rated input of the internal speakers 6a, 6b and the maximum input, which is a safety limit against momentary excessive input, are normally smaller than those of the external speakers 8a, 8b, and from the viewpoint of suppressing audio distortion, , external speaker 8a.

The impedance of the internal speakers 6a and 6b is set higher than that of the internal speakers 8b.

[Problems to be Solved by the Invention] The conventional audio output circuit 7 described above has, for example, an internal speaker 6.
When using speakers 6a and 6b, if you turn up the volume too much or excessive audio output
When the internal speakers 6a and 6b have a large impedance, distortion occurs more easily than the external speakers 8a and 8b, and the user does not notice that the internal speakers 6a and eb are strained. There have been problems in that the internal speakers 6a, 6b may be damaged or their lifespans may be shortened due to long-term operation due to excessive input.

[Means for Solving the Problems] The present invention solves the above problems, and includes an audio amplifier circuit that amplifies an audio signal and supplies it to a speaker, and a level of the audio signal that is supplied to the audio amplifier circuit. a volume control circuit that controls the volume by varying the volume; and a volume control circuit that detects the signal level of the audio output of the audio amplifier circuit, detects the original signal level, and divides the power supply voltage of the audio amplifier circuit to set the speaker permissible human power. a protection circuit that lowers the volume setting value by the volume control circuit to protect the speaker from excessive input when the signal level exceeds the allowable input level; The device is characterized by comprising a safety circuit that limits the permissible human power level to a predetermined value when the power supply voltage exceeds a certain level.

[Function] According to the present invention, the protection circuit detects the signal level of the audio output of the audio amplifier circuit, and when the detected signal level exceeds a preset permissible input level of the speaker, the volume setting value in the volume control circuit is adjusted. This protects the speakers connected to the audio amplifier circuit from excessive input, and when the power supply voltage increases, the safety circuit limits the allowable input level to a constant value, adding a layer of protection.

[Example] Hereinafter, an example of the present invention will be described with reference to FIG. 1.2. FIG. 1 is a circuit diagram showing an embodiment of the audio output circuit of the present invention, and FIG. 2 is an output voltage characteristic diagram of the resistive voltage divider circuit shown in FIG.

In FIG. 1, the audio output circuit 11 is a volume control circuit 13 connected to the audio input terminals 12a and 12b of the left and right channels.
controls the volume according to the volume setting value set by the volume circuit 14. Between the volume control circuit I3 and the left and right audio amplifier circuits 15a and 15b, volumes 16a and 16b for independently varying the left and right volumes are set at +1i. The outputs of the audio amplifier circuits 15a, 15b are supplied to left and right internal speakers 6a, 6b or external speakers 8a, 8b, respectively.

The volume circuit 14 outputs, for example, O~! by a volume 14c connected to a DC power source 10B. A volume output of 2 volts is supplied to the volume control circuit 13, and the volume level is controlled according to this volume output.

17 is a protection circuit whose rated input is external speaker 8a.
, 8b.

This is provided mainly for the purpose of avoiding excessive input to 6b. This protection circuit 17 includes the audio amplifier circuit 1
Diodes Da and D rectify the audio outputs of 5a and 15b.
b is commonly connected to the emitter of a transistor Qc for level comparison, and the emitter voltage of the transistor Qc corresponds to the average level of the audio output. transistor Q
The internal speaker 6 is connected to the base of c by dividing the power supply voltage V.
a.

Resistor voltage divider circuit I for setting the allowable input level of 6b
8 is connected. The resistive voltage divider circuit 18 includes a backup capacitor C for stabilizing the divided voltage output at the connection point between the voltage dividing resistors Ra and Rh, and a capacitor C that determines the allowable input level when the power supply voltage V increases beyond the rating. A Zener diode Zd is connected in parallel as a safety circuit to limit the value.

In addition, a parallel connection circuit of a resistor Rt and a capacitor Ct is connected to the collector of the transistor Qc, and the terminal voltage of the capacitor Ct is supplied to the base of the output transistor Qo, so that the audio level detection signal is transmitted to the volume control circuit. It is fed back to the 13th side.

That is, the volume control circuit 13 and the protection circuit 17 are connected by a signal line 20, and an output transistor Qo corresponding to a shunt switch is connected in series to this signal line 20 via a diode Dc. In this output transistor QO, the emitter resistor Re serves as a voltage dividing resistor that divides the output of the volume circuit 14.
When the output transistor Qo becomes conductive, protection by the protection circuit 7 is performed.

Here, when the audio level detection signal detected via the diodes Da and Db increases beyond the base voltage of the transistor Qc, the transistor Qc, which had been non-conductive, becomes conductive. As a result, the capacitor Ct is charged with the conduction of the transistor Qc, and when the terminal voltage of the capacitor Ct exceeds a certain voltage necessary for the conduction of the output transistor QO, the output transistor QO becomes conductive.

As a result, the volume output of the volume circuit 14, that is, the volume setting value of the volume control circuit 13 is controlled by the emitter resistor R.
The voltage is divided by e, and the apparent effect is the same as lowering the volume setting value of the upper volume circuit 14.

By the way, the resistance voltage divider circuit 18 that sets the allowable input level for the level comparison transistor Qc divides the power supply voltage V of the audio amplifier circuits 15a and 15b by a voltage division ratio Rb/R.
Since the voltage is divided by a + Rb, if the power supply V fluctuates below the rated voltage, the divided voltage output of the resistor voltage divider circuit 18 (
Rb/Ra+Rh)V will also vary. but,
In such a case, in order to prevent the risk that the audio output of the audio amplifier circuits 15a and 15b themselves will be distorted as the power supply voltage V decreases, the protection circuit I7 will also operate at a relatively low level as the divided voltage output of the resistor voltage divider circuit 18 decreases. Since the protection operation starts from the audio output, it is the same as when the power supply voltage V is kept constant.
It is possible to effectively prevent the inconvenience of highly distorted voices being emitted from the speakers 6a and 6b.

Furthermore, when the power supply voltage V increases beyond the rated value, the divided voltage output of the resistor voltage divider circuit 18 also increases until the constant voltage Vzd shown in FIG. Since it is suppressed to the Zener voltage Vzd of the diode Zd, the allowable input level set by the resistive voltage divider circuit 18 is also fixed to Vzd. Therefore, it is possible to reliably prevent inconveniences such as the permissible human power level increasing extremely as the power supply voltage V increases and the protection circuit 17 being unable to protect the speakers 6a and 6b.

In this way, the audio output circuit 11 adjusts the signal level of the audio output from the audio amplifier circuits 15a and 15b to the protection circuit 17.
When the detected signal level exceeds the preset allowable input level of the internal speakers 6a, 6b, the volume setting value in the volume control circuit I3 is automatically lowered, and the volume control circuit I3 is connected to the audio amplifier circuit 15a, 15b. Internal speaker 6a.

Since the configuration protects the speaker 6b from excessive input, if the volume is turned up too high or excessive audio output is caused by the speaker 6a.
, 6b, the protection circuit 17 automatically operates to protect the internal speakers 6a, 6b, and the internal speakers 6a, 6 are more susceptible to distortion than the external speakers 8a, 8b.
It is possible to reliably prevent the inconvenience of damaging the internal speakers 6a, 6b or shortening their lifespan due to long-term operation due to excessive input without noticing that force is being applied to the internal speakers 6a and 6b. In addition, the power supply voltage V
When this decreases, the protection level of the protection circuit 17 decreases, and the audio amplifier circuit 15a is in a state where distortion is likely to occur.

15b's own audio output to the internal speaker 6a.

It is possible to prevent the inconvenience that the output of 6b is distorted, and when the power supply voltage V increases, the Zener diode Zd, which is a safety circuit, limits the allowable input level to a constant value, which adversely affects the protection function of the protection circuit 17. Therefore, the reliability of the protection operation of the protection circuit I7 is doubly improved, and multilayered protection can be added to the internal speakers 6a and 6b.

The protective operation of the protection circuit 17 for the internal speakers 6a and 6b can be varied as appropriate by changing the voltage division ratio of the resistor voltage divider circuit 18, and the lower the voltage division ratio, the better the protection against relatively low-level excessive input. The circuit 17 can be activated.

Further, in the above embodiment, mainly the internal speakers 6a, 6
Although I explained that the purpose was to protect internal speaker 6a,
, 6b are switched and connected to external speakers 8a, 8b, the same protection is still possible.

[Effects of the Invention] As explained above, in the present invention, the protection circuit detects the signal level of the audio output of the audio amplifier circuit, and when the detected signal level exceeds the preset permissible human power level of the speaker, By lowering the volume setting level in the volume setting circuit, the speaker connected to the audio amplifier circuit is protected from excessive input, so if the volume is turned up too much or excessive audio output is supplied to the speaker. The speaker is automatically protected by a protection circuit, eliminating the inconvenience of damaging the speaker or shortening its life due to long-term operation due to excessive input.
This can be prevented reliably and in advance. Furthermore, when the power supply voltage decreases, the protection level of the protection circuit decreases, causing distortion to occur due to the audio output of the audio amplifier circuit itself, which is prone to distortion. , it is possible to prevent the inconvenience of distorting the output of the speaker, and when the power supply voltage increases, the safety circuit limits the allowable input level to a certain value and prevents the protection function of the protection circuit from being adversely affected. Therefore, the reliability of the protection operation of the protection circuit is doubled, and excellent effects such as being able to add multi-layered protection to the speaker are achieved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the audio output circuit of the present invention, Fig. 2 is an output voltage characteristic diagram of the resistor voltage divider circuit shown in Fig. 1, and Fig. 3 is a conventional audio output circuit. 1 is a schematic configuration diagram showing an example of a projection type television receiver using a circuit. 6a, sb, 8a, 8b, 0. Speaker, 11°1.
Audio output circuit, 13. ,,Volume control circuit. 15a, 15b, , audio amplifier circuit, 17. ,. Protection circuit, Zd, Zener diode.

Claims (1)

[Claims] an audio amplifier circuit that amplifies an audio signal and supplies it to a speaker; a volume control circuit that controls the volume by varying the level of the audio signal supplied to the audio amplifier circuit; and a volume control circuit that controls the audio output signal level of the audio amplifier circuit. The detected signal level is compared with the permissible input level of the speaker set by dividing the power supply voltage of the audio amplifier circuit, and when the signal level exceeds this permissible input level, the volume control circuit a protection circuit that lowers the volume setting value and protects the speaker from excessive input; and a protection circuit provided in this protection circuit;
An audio output circuit comprising a safety circuit that limits the allowable input level to a predetermined value when the power supply voltage exceeds a certain level.