JPH08213849A - Audio mute circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] In an audio circuit for applying an output from an audio output circuit to a speaker through a coupling capacitor, an audio mute is applied after the audio output circuit. [Configuration] A capacitor C3 having a DC power supply Vcc applied to one end through a diode D1 and the other end grounded, a Zener diode D2 having a required zener voltage having a DC power supply Vcc applied to a cathode end, and a base connected to the Zener. The first connected to the anode end of the diode, the collector connected to one end of the capacitor C3 via the resistor R1, and the emitter grounded.
Transistor TR1 and the collector voltage of the same TR1 are applied to the base, and the collector is connected to the connection point between the audio output circuit and the coupling capacitors C1 and C2 via resistors R2 and R3.
A second transistor TR2 and a third transistor TR3, whose emitters are grounded, are provided, and when the DC power supply is on or off, the second transistor is turned on to mute the sound. .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio mute circuit, and more particularly to an audio mute in an audio circuit adapted to apply an audio output from an audio output circuit to a speaker via a coupling capacitor.

[0002]

2. Description of the Related Art There are various configurations of audio output circuits, and FIG. 3 shows a circuit having a typical configuration. As shown in the figure, each of the left (L) and right (R) audio signals passes through the volume control circuit 11 and is amplified to a required output by the left audio output circuit 12 and the right audio output circuit 13, respectively, and the coupling capacitors C11, C are provided.
It is applied to the left speaker 14 and the right speaker 15 via 12. The main purpose of the coupling capacitor is to block the DC voltage to the speaker, but on the other hand, it deteriorates the low frequency characteristic. Therefore, it is necessary to increase the capacitance value in order to extend the low frequency characteristic.

However, depending on the audio output circuits 12 and 13, pop sound (shock sound) may be generated at the transition of power ON / OFF. In particular, some IC (integrated circuit) audio output circuits include an audio mute circuit and some do not. Therefore, when the audio output circuits 12 and 13 do not have an audio mute function, a pop sound is produced from the speaker, which gives the listener uncomfortable feeling. In such a case, if the capacitance value of the coupling capacitor is increased, the pop sound (shock sound) when the power is turned on and off is further increased. For this reason, in the past, this coupling capacitor has a size in the range in which the pop sound is inconspicuous (for example, several hundred μm).
The general method is to set the value in the range from F to 1000 μF) and correct the low frequency in the preceding circuit.

[0004]

As described above, although the coupling capacitor (FIG. 3) is obliged to limit the size of the pop sound in the inconspicuous range, the pop sound generated in the audio output circuit is prevented. If possible, the above limitation can be expanded, which is preferable in terms of low frequency characteristics. The present invention has been made from such a point of view, and in an audio circuit in which a coupling capacitor is provided between the output circuit and the speaker, an audio mute circuit for preventing the pop-on when the power is turned on and off due to the coupling capacitor. The purpose is to provide.

[0005]

SUMMARY OF THE INVENTION The present invention is an audio circuit in which an audio output from an audio output circuit is applied to a speaker via a coupling capacitor, a DC power source is applied to one end, and the other end is grounded. A capacitor, a Zener diode of the required Zener voltage with the DC power supply applied to the cathode end, and a base connected to the anode end of the Zener diode, a collector connected to one end of the capacitor through a resistor, and an emitter A first transistor having a grounded terminal and a collector voltage of the first transistor applied to a base, a collector connected to a connection point between the audio output circuit and a coupling capacitor via a resistor, and a second terminal having a grounded emitter. And a second transistor when the DC power source is in a range of a predetermined voltage or less when the power source is turned on and off. There is provided an audio muting circuit so as make a voice mute allowed on the Njisuta.

[0006]

When the voltage of the DC power supply is at a steady value, the Zener diode is turned on, and thus the first transistor is on. Therefore, the second transistor is turned off and does not affect the operation of the audio output circuit. On the other hand, when the voltage of the DC power supply transiently drops due to power on / off, the Zener diode is turned off, and thus the first transistor is turned off. As a result, the base current flows through the second transistor using the charging voltage of the capacitor as a power source, and the collector-emitter is turned on. By this turning on, the connection point between the audio output circuit and the coupling capacitor is grounded via the small resistance. The state in which the second transistor is on is the mute state.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An audio mute circuit according to the present invention will be described below with reference to the drawings. 1 is a circuit diagram of a main part showing an embodiment of an audio mute circuit according to the present invention, and FIG. 2 is a time chart for explaining FIG. In FIG. 1, 1 is a volume control circuit that controls the volume of each of the left (L) and right (R) audio signals, and 2 and 3 are a left audio output circuit and a right that amplifies the audio signal after volume control to a required output. Audio output circuit,
C1 and C2 are coupling capacitors, and 4 and 5 are left and right speakers. Also, Vcc is a direct current power source of the required voltage,
C3 is charged by Vcc and serves as a base power supply for the second transistor, D1 is a diode for preventing the charging voltage of C1 from flowing back to the power supply Vcc, and D2 defines the on / off timing of the first transistor TR1. A Zener diode having a required Zener voltage, TR1 is a first transistor that is turned on and off according to the turning on and off of D2, and a first transistor that turns on and off a second transistor TR2 and a third transistor TR3.
TR2 and TR3 are a second transistor and a third transistor, respectively, which ground the connection point between each audio output circuit and the coupling capacitor via the small resistors R2 and R3 when turned on.

Next, the operation of the present invention will be described. During normal operation, the left and right audio signals controlled to the required level by the volume control circuit 1 are amplified to the required output by the output circuits 2 and 3 and applied to the left and right speakers via the coupling capacitors C1 and C2. . On the other hand, the Zener diode D2 is in the ON state during normal operation, so the first transistor
TR1 is also on. The resistor R1 is a load resistor of the TR1. When the same TR1 is turned on, there is no base current of each of the second transistor TR2 and the third transistor TR3, and the collector-emitter is off. As a result, TR2 and TR3 have no effect on the audio output circuits 2 and 3. Now, assuming that the power supply is turned off from the operating state (at T1 in FIG. 2A), the DC power supply Vcc starts to fall. Due to this drop, the Zener diode D2 is turned off at time T2.
That is, the Zener voltage of the diode D2 has such a value.

When D2 turns off, the base voltage of TR1 disappears, so that TR1 turns off. When this TR1 is turned off, a base current flows to the bases of TR2 and TR3 using the capacitor C3 as a power source, and the collector-emitter is turned on. This TR2
And when TR3 is turned on, it becomes a mute state (FIG. 2B).
Further, when this is turned on, the connection points between the audio output circuits 2 and 3 and the coupling capacitors C1 and C2 are small resistors R2 and R3 (for example, 1
Ω) to ground and bypass noise and the like from each audio output circuit to ground. Therefore, noise (pop sound) that generally occurs within a certain period after the power is turned off (FIG. 2C).
Even if there is, muting is performed by turning on TR2 and TR3, and the pop sound from the speaker is suppressed. Note that FIG.
C is for the case where this mute circuit is not provided, and is actually suppressed by the mute.

On the other hand, when the power is turned on from the power-off state (T3), the power supply Vcc rises. TR1 is in the OFF state until the Zener diode D2 is turned on. Therefore, the base current flows to the bases of TR2 and TR3 through the diode D1 while gradually increasing, which causes TR2 and TR3 to turn on (however, Saturation is not on) and is in mute. Furthermore, if the power supply Vcc increases,
Zener diode D2 of T4 turns on. When D2 turns on, TR1 turns on and TR2 and TR3 turn off. At this point, mute is released (FIG. 2B). Therefore, noise (pop sound) (FIG. 2C) generated when the power source Vcc rises when the power source is turned on is also muted in the same manner as when the power source is turned off. Hereinafter, the above-mentioned normal operation state is set. In addition,
The diode D1 is effective in preventing the charging voltage of the capacitor C3 from flowing back to the Vcc side when the power is turned off. Further, the power supply Vcc is shared with the power supply used for the audio output circuit, so that the mute timing can be made appropriate.

[0011]

As described above, according to the present invention, in the audio circuit in which the audio output from the audio output circuit is applied to the speaker through the coupling capacitor, the audio mute is performed at the audio output end side. Since it is applied, it is possible to suppress the pop sound due to noise or the like generated in the audio output circuit. Therefore, in the past, the capacitance value of the coupling capacitor was limited in order to make the pop sound inconspicuous, whereas it is possible to expand the limitation frame and select a large capacitance value, which reduces the voice The region characteristic can be improved. Further, in recent years, the audio output circuit has been integrated into an IC (integrated circuit), but some of them do not include an audio mute circuit. In such a case, the pop sound can be suppressed by applying the present invention.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a main part showing an embodiment of an audio mute circuit according to the present invention.

FIG. 2 is a time chart for explaining FIG.

FIG. 3 is a main part circuit diagram showing an example of a conventional audio circuit.

[Explanation of symbols]

 TR1 1st transistor TR2 2nd transistor TR3 3rd transistor D1 Diode C1 Coupling capacitor C2 Coupling capacitor C3 Capacitor D2 Zener diode R1 Resistor R2 Resistor R3 Resistor 1 Volume control circuit 2 left Audio output circuit 3 Right Audio output circuit 4 Left Speaker 5 Right speaker

Claims (4)

[Claims] 1. An audio circuit configured to apply an audio output from an audio output circuit to a speaker via a coupling capacitor, wherein a DC power source is applied to one end and a grounded other end, and a cathode end is provided with the capacitor. A Zener diode having a required Zener voltage to which a DC power source is applied, a base is connected to an anode end of the Zener diode, and a collector is connected to one end side of the capacitor through a resistor,
A first transistor having an emitter grounded, a collector voltage of the first transistor applied to the base, a collector connected to a connection point between the audio output circuit and a coupling capacitor through a resistor, and an emitter grounded An audio mute circuit comprising two transistors, wherein the second transistor is turned on to perform audio mute when the DC power supply is in a range of a predetermined voltage or less when the power is turned on and off. 2. The audio mute circuit according to claim 1, wherein a diode is provided between one end of the capacitor and the DC power supply so that the anode side is the same DC power supply side. 3. The audio mute circuit according to claim 1, wherein the DC power supply is shared with a DC power supply used for the audio output circuit. 4. A stereo audio circuit in which an audio output circuit and a speaker are provided on the left and right sides and are respectively connected by a coupling capacitor, and the second transistor is provided at a connection point between one audio output circuit and the coupling capacitor. And a collector voltage of the first transistor is applied to the base, the collector is connected through a resistor to a connection point between the other audio output circuit and the coupling capacitor, and a third transistor having an emitter grounded is provided. The audio mute circuit according to claim 1, wherein: