JPH09172327A - Muting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a muting circuit which can reduce the operation start time of the muting circuit as much as possible after a power switch SW 1 is supplied and which can considerably reduce the noise level of a sound circuit in the excess level state of an audio unit. SOLUTION: Base current IB2 operating a transistor for muting Tr2 which turns on/off the sound circuit is obtained by the accumulation charge of a capacitor C3, in which the charge is always accumulated. Thus, a muting rise circuit Bb in which the capacitor C3 where the charge is always accumulated is connected to an input part, which is immediately conducted after the power switch is supplied, sets the transistor Tr2 in a conductive stage with the accumulated charge of the capacitor C3 in an early stage, turns off the sound circuit, sets the transistor Tr2 in an interrupted state with a command from the transistor Tr1 of a muting release circuit Ba after power voltage is stabilized and turns on the sound circuit and which contains the capacitor C3 and transistors Tr3 and Tr4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to audio equipment having a power supply circuit such as an amplifier in the field of audio equipment.

[0002]

2. Description of the Related Art Generally, when an audio device is turned on,
During the period until the secondary side power supply becomes stable, the internal circuit becomes unstable and in an excessive state, and noise that is extremely unpleasant to the ear is generated at the speaker output terminal or the output terminal for connecting to another amplifier. A muting circuit is usually used to eliminate this noise. Muting uses a relay or transistor to shut off the audio circuit.
A muting waveform generation circuit is required to drive these blocking elements. The operation of the conventional muting circuit will be described with reference to FIG. 3 showing an example of the configuration using the same symbols as those of the embodiment of the present invention.
A sound output cutoff circuit including a power supply circuit T, rectifying diodes D1 and D2, a smoothing capacitor C1, and a constant voltage circuit (IC or the like) IC1 and a muting transistor Tr2 'for turning on and off the sound circuit. A, and a muting waveform generating circuit B'including a transistor Tr1 'for releasing the muting by turning off the transistor Tr2' after generating the muting waveform and stabilizing the transient state of the circuit.

The operation of the above conventional example will be described with reference to FIGS. 4 and 5. When the power switch SW1 of the power circuit C is turned on, the rectifying diodes D1 and D2 and the smoothing capacitor C1 generate a DC voltage V1. Since V1 contains a large amount of ripples, the ripple is removed by the constant voltage circuit (IC or the like) IC1 to become the constant voltage V2. However, ripples are still included until time T1 when V2 is reached. (See Fig. 4)

The base voltage V3 'of the transistor Tr1' for generating a muting waveform is gradually charged to the capacitor C2 'through the resistor R1'. And V3 'is T
Until time T4 when the voltage that makes r1 'conductive is reached T
r1 'is in the cutoff state, during which the power supply voltage V2' supplies the current Ib1 to the transistor Tr2 'through the resistor R2'. Then, from the time T3 when the transistor Tr2 'is turned on to the time T4, the noise in the transient state which appears on the audio output L is erased. Although T3 varies depending on the hfe of the transistor and the rise time of the IC, it is about 10 to 30 mSec according to experiments.

[0005]

In the conventional muting circuit described above, during the period from when the power switch SW1 is turned on to when the time T3 is reached, since the transistor Tr2 'is in the cutoff state, noise leaks as it is. There was a drawback to say. Therefore, there is a problem to be solved that the time from turning on the power switch SW1 to time T3, that is, the time from the time when the transistor Tr2 ′ is brought into the conductive state must be made as short as possible.

Therefore, according to the present invention, by sufficiently lowering the voltage Va when the power source voltage V2 turns on the transistor Tr2 'to Va', the time T3 is reduced to the time T2 and the time T2 is changed. At the time point, a voltage for supplying the base current Ib1 to the transistor Tr2 'is secured together with a capacitor that sufficiently stores and holds electric charges even when the power supply is off, so that the above-mentioned problem can be solved. The purpose is to provide.

[0007]

A muting circuit according to the present invention for achieving the above object will be described with reference to the reference numerals used in the embodiment shown in FIG. 1. The first invention is a voice circuit. Charge is always accumulated in the audio output cutoff circuit A including a muting transistor Tr2 for turning on and off, a muting release circuit Ba including a transistor Tr1 for releasing the muting by turning off the transistor Tr2 and an input portion. The connected capacitor C3 is connected to conduct immediately after the power switch is turned on, the transistor Tr2 is brought into the conductive state early by the accumulated charge of the capacitor C3 to turn off the voice circuit, and the command from the transistor Tr1 is issued after the power supply voltage is stabilized. To turn off the transistor Tr2 and turn on the audio circuit. A muting circuit, characterized by comprising a muting waveform generating circuit B comprising muting rise circuit Bb including transistors Tr3 and the transistor Tr4.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a muting circuit according to the present invention will be described with reference to FIGS. C is a power supply circuit, which includes a power supply transformer T, rectifying diodes D1 and D2, a smoothing capacitor C1, and a constant voltage circuit (IC or the like) IC1 as in the conventional example.

Reference numeral A is an audio output cutoff circuit including a muting transistor Tr2 for turning on / off the audio circuit. The transistor Tr2 is connected between the audio circuit and ground, and a base current Ib2 flows through the transistor Tr2 by a voltage V4. And the audio signal flowing through the audio circuit is cut off.

Reference numeral B denotes a muting waveform generating circuit, which includes a muting releasing circuit Ba including a transistor Tr1 for releasing the muting by turning off the transistor Tr2 and a muting rising circuit Bb for raising the muting. And two parts.

A muting releasing circuit Ba which is in charge of a portion for releasing muting in the muting waveform.
Consists of a transistor Tr1 and a base circuit having a predetermined time constant, which is composed of a series circuit of a resistor R1 and a capacitor C2 connected to the power supply circuit C, and the base of the transistor Tr1 is the capacitor C2 of the base circuit. It is connected to the hot side, and the collector is also connected to the muting rising circuit Bb described in the next section.

Bb is a muting rising circuit,
It is composed of a transistor Tr4 and a transistor Tr3 which is connected in series to the base of the transistor Tr4 and whose emitter is grounded. The output of the muting release circuit B is connected to the base of the transistor Tr3 and the collector output of the transistor Tr4. Is connected to the audio output cutoff circuit A via a resistor R6.

A capacitor C3 is connected between the emitter input circuit of the transistor Tr4 and the ground point. The capacitor C3 has a large capacity and is used for holding a memory of a microprocessor. For example, a capacitor having a low leak current and an extremely small self-discharge, such as an electric double layer capacitor, is used to normally hold the charge for a considerably long time. However, in this embodiment, a diode D4 is further provided as shown in the figure. Connected to the backup power source D via the power switch SW1
It is considered that sufficient electric charge is accumulated even in the state of.

The operation of this embodiment will be described with reference to FIGS. 1, 4 and 5. When the power switch SW1 is turned on, the transistor Tr3 is turned on by the power supply voltage V2, and at the same time the current Ib4 flows. Transistor Tr
4 becomes conductive. Then, the electric charge accumulated in the capacitor C3 flows to the base of the transistor Tr2 through the resistor R6, and the transistor Tr2 is turned on to remove the noise included in the audio output L. Next, at time T4 when the capacitor C2 is gradually charged, the transistor Tr1 becomes conductive, and the transistor Tr1 becomes conductive.
3 is in a cutoff state, and the base of the transistor Tr4 is raised by the resistor R4, and the transistor Tr4 is in a cutoff state. And the base current Ib of the transistor Tr2
2 becomes 0, the transistor Tr2 is turned off, and the muting is released.

The voltage V2 (= Vb) when the transistor Tr3 is in the conductive state (that is, the power supply at the rising time) is obtained. The base and emitter when all the transistors in FIGS. 1 and 3 are in the conductive state. If the inter-voltage is approximated to the equal value Vbe, the h parameter of the transistor Tr3 is set to hf.
e1 and the h parameter of the transistor Tr4 is hfe2, in the present embodiment of FIG. 1, V2 = Vbe + R2.Ib3 Ib3 = Ib4 / hfe1 Ib4 = Ib2 / hfe2 In the conventional example of FIG. , V2-V2 ′ = R2 · Ib2 / [hfe1 · hfe2]. When Ib1 = Ib2 and R2 = 10 · R2 ′ are set, generally hfe1 ≧ 100, hfe2 ≧ 1
Since it is 00, V2-V2 '=-R2'Ib1 = -V2∴V2 <<V2'

That is, the voltage V2 is sufficiently lower than the voltage V2 '. The timing in this case is as shown in FIG.
The base voltage of the transistor Tr2 rises at time T2. According to the experiment, this time T2 is 5 to 10 m.
Sec, and the muting waveform rises at time T2, which is sufficiently shorter than time T3 in the above-mentioned conventional example, and the effect of the present invention occurs.

[0017]

Embodiments of the muting circuit of the present invention will be described below. Since the circuit configuration is the same as that of the embodiment of the present invention described above, detailed description is omitted, and specific numerical values such as elements are entered in FIG. 2 showing the configuration as an example. In the figure, C is a power supply circuit, which is similar to the conventional example and includes a power supply transformer T, rectifying diodes D1 and D2, a smoothing capacitor C1, and a constant voltage circuit (I
C) IC1. The output of the BK power supply circuit C is not only the power supply of the muting circuit described below but also the power supply of the other circuit S constituting the device body.

In the audio output cutoff circuit A, an audio output cutoff transistor Tr2 is connected between the audio circuit and ground.
The base current Ib is applied to the transistor Tr2 by the voltage V4.
When 2 flows, it becomes conductive and the audio signal flowing through the audio circuit is cut off.

The muting waveform generation circuit B includes a muting releasing circuit B including a transistor Tr1 for releasing the muting by turning off the transistor Tr2.
a and a muting rising circuit Bb for raising muting.

The muting release circuit Ba is composed of a transistor Tr1 and a base circuit having a predetermined time constant composed of a series circuit of a resistor R1 and a capacitor C2 connected to a power supply circuit C, and a base of the transistor Tr1. Is connected to the hot side of the capacitor C2 of the base circuit, and the collector is also connected to the muting rising circuit Bb, which is also described in the next section.

The muting rising circuit Bb is composed of a transistor Tr4 and a transistor Tr3 connected in series to the base of the transistor Tr4 and having an emitter grounded. The base of the transistor Tr3 has the output of the muting releasing circuit B. The collector output of the transistor Tr4 is connected to the audio output cutoff circuit A via a resistor R6.

A capacitor C3 is connected between the emitter input circuit of the transistor Tr4 and the ground point. The capacitor C3 has a capacitance of about 0.1 to 1F as shown in FIG. , Which has a large capacity and is used for holding memory of the microprocessor M, such as an electric double layer capacitor, uses a capacitor with low leakage current and very little self-discharge, and normally retains the charge for a considerably long time. However, in the present embodiment, as shown in the figure, it is further connected to the backup power source D through the diode D4 so that sufficient electric charge is always accumulated even when the power switch SW1 is OFF. Is taken into consideration.

Although the examples considered to be representative of the present invention have been described above, the present invention is not necessarily limited to the structures of these examples, and is provided with the above-mentioned constitutional requirements of the present invention, and The present invention can be carried out by appropriately modifying it within a range that achieves the object of the present invention and has the following effects.

[0024]

As is apparent from the above description, the muting circuit according to the present invention utilizes the electric charge accumulated in the capacitor provided in the circuit when generating the muting waveform. Since the switching transistor of the audio circuit is turned on and off, it is possible to perform muting sufficiently early in the rise of the power supply voltage. Therefore, it is possible to almost cut off the noise in the transient state when the power switch is turned on by simply adding a charge storage capacitor, a backflow prevention diode, and two transistors to the conventional muting circuit. It can be expected that the use interval will be improved and the commercial value of high-grade audio equipment will be increased.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of an embodiment of the present invention.

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

FIG. 4 is a voltage waveform diagram of each part of the muting circuit of the example and the conventional example.

FIG. 5 is a time relationship diagram of the muting operation of the example and the conventional example.

[Explanation of symbols]

 A Audio output cutoff circuit B Muting waveform generation circuit Ba Muting release circuit Bb Muting rising circuit C Power supply circuit D Backup power supply M Microprocessor, etc. S Other circuits of device body

Claims (1)

[Claims] 1. An audio output cutoff circuit including a muting transistor Tr2 for turning on and off the audio circuit.
(A), a muting releasing circuit (Ba) including a transistor Tr1 for releasing the muting by turning off the transistor Tr2, and a capacitor C3 in which an electric charge is always stored in the input part are connected, and a power switch is turned on. Immediately after that, the transistor Tr2 is brought into the conducting state early by the accumulated charge of the capacitor C3 to turn off the voice circuit, and after the power source voltage is stabilized, the transistor Tr2 is turned off and the voice circuit is turned on by the command from the transistor Tr1. A muting waveform generating circuit (B) composed of the capacitor C3, a transistor Tr3, and a transistor Tr4, and a muting waveform generating circuit (B).