JPH06343011A - Muting circuit - Google Patents

Abstract

PURPOSE:To surely prevent the ripple component produced by the application of a power supply from entering the signal line of a power amplifier and also to prevent the noises caused by the ripple component from being produced by the power amplifier by switching off the current source of the power amplifier at the time of application of the power supply and then supplying no current to the power amplifier. CONSTITUTION:A transistor Tr2 constructing a muting control circuit 3 is connected between a resistor R3 and a constant current source 7. When the muting control voltage VCT is set to an L level, the muting Tr2 is turned on and a power supply Tr1 is turned off. Therefore a drive current ID is not supplied to the power amplifiers 4 and 5 nor to a current supply circuit 2. Then no capability is secured any more to amplify an input signal SIN and no signal voltage is produced at the output side. Thus it is possible to eliminate the capacitor charging noises connected to the output and also produce no output POUT even with the ripple component added to the signal current when the revolution of a motor is started in a muting state caused at the time of application of a power supply.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a muting circuit, and more particularly, it is suitable for use in a muting circuit for preventing a noise component generated when power is turned on from being output from a power amplifier.

[0002]

2. Description of the Related Art Some audio output devices such as tape recorders perform muting to prevent noise components generated when a power supply voltage is applied from being output to the outside. FIG. 3 is a circuit configuration diagram showing an example of a conventional muting circuit used in such a case. This muting circuit includes a power amplifier 20,
A circuit to prevent noise from being output from 21
It is composed of a current source 22, a current mirror circuit 23, a muting transistor 24, and the like.

The muting control voltage VCT is applied between the muting transistor 24 and the current source 22 through the diode 25.
The muting control voltage VCT is "H"
The muting transistor 24 is turned on when a "H" level voltage is applied, and turned on when an "L" level voltage is applied.

When the muting transistor 24 is turned on, a current I ₁ having the same magnitude as the current flowing through the current source 22 flows through the transistor Q1 forming the current mirror circuit 23, and the current I The mirror current I _{2 of 1} flows through the transistor Q2. Therefore, when the muting control voltage VCT is at “L” level, the current I ₂ flows into the bias circuit of the power amplifier 21.

When the current I ₂ flows in, the bias voltage of the power amplifier 21 becomes, for example, the ground potential GND.
The audio signal does not appear on the output side of the power amplifier 21. That is, muting of the power amplifier is performed. On the other hand, when the muting control voltage VCT becomes "H" level, a high voltage is applied to the emitter of the muting transistor 24 and the transistor 24 is turned off. As a result, the bias circuit of the power amplifier 21 receives the current I
_{Since 2} does not flow in, the power amplifier 21 starts normal operation. That is, the muting state of the power amplifier is released and input, and the input signal is amplified and appears on the output side.

[0006]

Conventionally, the bias voltage of the power amplifier has been manipulated to perform muting and cancellation of muting as described above. Therefore, when canceling the muting state, the potential of the power amplifier 21 is changed. Will return from the ground potential GND to a predetermined bias potential. Therefore, when the muting state is released, the potential changes abruptly, which causes a step-like voltage to appear on the output side of the power amplifier 21.

The output of the power amplifier 21 is supplied to a sound producing headphone, a speaker or the like via a capacitor. At this time, the step signal is differentiated into a pulse signal. Since the electric potential of the capacitor is lowered to the ground electric potential GND during muting, if a pulsed voltage whose electric potential changes abruptly is given when muting is released, it will be output from the above-mentioned headphones or speaker as considerably large noise. There is an inconvenience.

Further, when the power is turned on, the tape running motor starts to rotate. Since a considerably large current flows at the start of rotation of this motor, the current flowing at this time has a considerably large ripple. It will be included. Therefore, as shown in the characteristic diagram of FIG. 4, the mute current I _{M including} ripples flows into the bias circuit of the power amplifier 21 when the power supply voltage rises.

Even if the mute current I _M containing such a ripple flows in, no ripple component appears on the output side when muting is performed.
There is no inconvenience that the noise due to the ripple is output. Further, since the mute current I _M does not flow into the power amplifier 21 when the muting is released, the inconvenience that the noise due to the ripple is output does not occur in this case as well.

However, the effect of the ripple current becomes noticeable from the beginning to the end of the muting cancellation. That is, when the muting is released, the bias voltage of the power amplifier 21 rises toward a predetermined potential, but the bias voltage is swung up and down by the mute current I _M containing ripples. Will end up. Therefore, as shown in FIG. 4, the output voltage POUT of the power amplifier 21 pulsates until the muting control voltage VCT completely rises to a predetermined voltage and the muting is released. The problem that it is output as a motor sound occurs.

In view of the above problems, it is an object of the present invention to prevent motor noise, abnormal noise, etc. from being produced when the power is turned on.

[0012]

A muting circuit according to the present invention comprises a current source whose current value rises following the rise of the power supply voltage when the power supply voltage is turned on, and a current supplied from the current source. A current supply circuit that supplies a current according to the value to the output power amplifier, and a muting control circuit that performs on / off control of the current supply operation of the current supply circuit according to a muting control voltage externally applied is doing.

[0013]

[Function] When the power is turned on, the current is not supplied from the current source of the power amplifier to the muting at the time of turning on the power to prevent the ripple component generated at the time of turning on the power from the signal line. Prevent the noise due to the ripple component from being output.

[0014]

1 is a block diagram of a muting circuit showing an embodiment of the present invention. As shown in FIG. 1, the muting circuit of this embodiment is composed of a current source 1, a current supply circuit 2, a muting control circuit 3 and the like. When performing muting when the power is turned on, the power amplifier 4, The current source 5 is turned off.

In the current source 1, the current value rises following the voltage applied when the power supply voltage is turned on, and the current I _O output from the current source 1 is the transistor T _r1 forming the current supply circuit 2. Of the base and the base of T _r3 , provided to the collector. The current supply transistor T _r1
The emitters of T _r3 and T _r3 are connected to the power supply Vcc via resistors R ₃ and R ₄ , respectively. Further, the muting control circuit 3 is provided between the resistor R ₃ and the constant current source 7.
Is connected to the transistor T _r2 .

The transistor T _r2 is provided to turn off the transistor T _r1 of the current supply circuit 2 when the power is turned on, and the bias resistor R ₁ ,
A bias voltage is applied from R ₂ . The muting control voltage VCT is applied to the emitter of the muting transistor T _r2 through the diode 25. The muting control voltage VCT is
As described above, the muting transistor T _r2 changes to the “H” level and the “L” level, and is turned off when the “H” level voltage is applied, and the “L” level voltage is applied. Turn on when.

When the muting transistor T _r2 is turned on, the current supply transistor T _r1 is turned off, so that the drive current I _D is not supplied from the current supply circuit 2 to the power amplifiers 4 and 5. On the contrary, when the muting transistor T _r2 is turned off, the current supply transistor T _r1 is turned on, so that the drive current I _D is supplied from the current supply circuit 2 to the power amplifiers 4 and 5. When the drive current I _D is no longer supplied from the current supply circuit 2, the power amplifiers 4 and 5 have no ability to amplify the input signal S _IN , so that no signal voltage appears at the output side.

The muting circuit of this embodiment performs muting when the power is turned on by turning off the current sources of the power amplifiers 4 and 5 as described above. When the muting is performed, the electric charge of the capacitor is not forcibly discharged. Therefore, when the muting is released, the voltage of the capacitor is near the voltage before the muting, so that noise caused by charging the capacitor can be eliminated.

As shown in the characteristic diagram of FIG. 2, ripples generated at the start of motor rotation also appear in the current sources of the power amplifiers 4 and 5. However, in the case of the muting circuit of this embodiment, since the current including the ripple is not added to the signal current, the noise due to the ripple hardly appears at the output POUT. That is, in the conventional audio equipment, it is possible to effectively prevent the motor sound generated when the power is turned on. The power is determined by the ripple rejection of the power amplifiers 4 and 5. In the above embodiment, the case of muting the tape recorder has been described, but the present invention can be well applied to other audio equipment such as a disc player such as a compact disc or a video disc.

[0020]

As described above, according to the present invention, when the power is turned on, the current source of the power amplifier is turned off, and the muting is performed so that no current is supplied to the power amplifier. Ripple can be surely prevented from entering the signal line of the power amplifier, and noise due to the ripple component can be prevented from being output from the power amplifier. Further, since the voltage of the signal line is not operated, even when the signal from the power amplifier is output by capacitor coupling, the capacitor is not forcibly charged / discharged but the capacitor is charged / discharged. Noise can be prevented.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a muting circuit showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram for explaining the operation of the circuit of this embodiment.

FIG. 3 is a configuration diagram showing an example of a conventional muting circuit.

FIG. 4 is a characteristic diagram for explaining the operation of a conventional muting circuit.

[Explanation of symbols]

1 Current Source 2 Current Supply Circuit 3 Muting Control Circuit 4,5 Power Amplifier I _O Current Source Output Current I _D Drive Current T _r1 Current Supply Transistor T _r2 Muting Transistor VCT Muting Control Voltage

Claims (1)

[Claims] 1. A current source whose current value rises following the rise of the power source voltage when the power source voltage is turned on, and a current corresponding to the current value given from the current source to a signal output power amplifier. A muting circuit comprising: a current supply circuit to be supplied; and a muting control circuit for controlling on / off of a current supply operation of the current supply circuit according to a muting control voltage externally applied.