JPS6128251B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a muting circuit that prevents noise generated when the power switch is turned on and off from appearing in a speaker in an audio device having a power switch on the secondary side of a power transformer. By using terminals, noise can be reliably and inexpensively prevented.

Figure 1 shows a conventional muting circuit, where 1 is a power plug connected to AC100V,
T is a power transformer, S ₁ is a power switch, a, b,
c is its off terminal, common terminal, and on terminal; 2 is a rectifying and smoothing circuit composed of a diode D ₁ and a capacitor C ₁ ; 3 is a transistor Q ₁ and a capacitor
Regulator circuit consisting of C ₂ , resistor R ₁ , Zener diode D ₂ , 4 DC output terminal, 5 diode D ₃ , capacitors C ₃ , C ₄ , transistor
Q ₂ , Q ₃ , a mutating control circuit composed of resistors R ₂ to _{R 5} ; L and R denote left and right audio signal transmission lines; 6 a mutating drive circuit composed of transistors Q ₅ to _{Q 7} and a resistor R ₆ . , 7 and 8 are power amplifiers, and 9 and 10 are speakers.

In the above configuration, the constants of resistors R ₂ , R ₃ and capacitor C ₃ are set so that when power switch S ₁ is on (the state shown in Figure 1) and in a steady state, transistor Q ₂ is off. Therefore, at this time, transistor Q ₃ is turned on, and the voltage obtained through resistor R ₅ is grounded through transistor Q ₃ .
Therefore, the muting drive circuit 6 does not operate,
The signals on the transmission lines L and R are transmitted as they are to power amplifiers 7 and 8 and speakers 9 and 10.

At the moment when the power switch is turned from off to on, transistor Q ₂ is turned on for a short period of time until the small capacitor C ₃ is charged, but then transistor Q ₂ is turned off and resistor R ₄ is turned on. The base voltage of transistor _Q3 rises with a time constant determined by capacitor _C4 . Since the transistor _Q3 is off for a period until the base voltage reaches a predetermined level, the muting drive circuit 6 operates during this period, and the transmission lines L and R
is blocked. Therefore, muting is applied for a while after the power is turned on to prevent noise from appearing. When the base voltage of the transistor _Q3 reaches a predetermined level, the transistor _Q3 is turned on, and at the same time, the muting drive circuit 6 stops operating. Therefore, the signals on the transmission lines L and R are transmitted directly to the power amplifiers 7 and 8 and the speakers 9 and 1.
Transmitted to 0. This state is maintained during the steady state period when the power switch _S1 is turned on.

On the other hand, at the moment power switch _S1 is turned from on to off, there is a period of time until the voltage across capacitor _C3 applied to the base of transistor _Q2 attenuates to a predetermined level (as described above,
(Since C ₃ is chosen to have a small capacitance, this period is short.) Transistor Q ₂ remains off, and then transistor Q ₂ is turned on, discharging the voltage across capacitor C ₄ and discharging the voltage across transistor Q _3.
is turned off. Therefore, the voltage across the smoothing capacitor _C1 is applied to the mutating drive circuit 6 via the regulator ₃ and the resistor _R5 , and the transmission line Cut off L and R.

Therefore, even in the conventional muting circuit shown in FIG. 1, muting is applied for a certain period of time when the power switch is turned on and off, thereby making it possible to eliminate the noise that accompanies the turning on and off of the power supply.

However, in the case of Figure 1, the power switch S ₁
When S1 is turned on, no muting is applied until the voltage across capacitor _C3 rises to the predetermined level, and even when power switch _S1 is turned off, the voltage across capacitor _C3 remains at the predetermined level. Muting is not applied until it is attenuated. Therefore, in order to completely prevent noise from appearing during this period, it becomes extremely difficult to set constants. There is also the problem that when the voltage decreases due to fluctuations in the power supply voltage, transistor _Q3 turns off due to variations in circuit constants, resulting in erroneous muting. Furthermore, as is clear from FIG. 1, it is necessary to use a very complex structure with a large number of elements as the muting control circuit 5, which is disadvantageous in terms of circuit structure and cost.

The present invention provides a muting circuit that solves these problems.

A first embodiment of the present invention will be described below with reference to FIG. In FIG. 2, parts having the same functions as those in FIG. 1 are given the same symbols and their explanations will be omitted. S ₂ is a power switch connected after the rectifying and smoothing circuit 2, and this power switch changes from the on state (common terminal b and on terminal c are connected) to the off state (common terminal b and off terminal a are connected). This is a so-called switching type switch in which all terminals a, b, and c are connected at the moment the switch is switched to the connected state. The common terminal b is connected to the output terminal of the rectifying and smoothing circuit 2, and the on terminal c is connected to the transistor of the regulator 3.
The off terminal a is connected to the collector of _Q1 , and the off terminal a is connected to the muting drive circuit 6 via a time constant circuit composed of a capacitor _C5 and a resistor _R7 .

In the above configuration, when the power switch _S2 is in the off state, the smoothing capacitor _C1 and the capacitor _C5 forming the time constant circuit are connected in parallel via the power switch _S2 , and the capacitor C5 is connected to both ends of the capacitor _C5 . The same voltage as that of the smoothing capacitor _C1 is charged. Therefore, the muting drive circuit 6 is operated by the output of the time constant circuit, and the transmission lines L and R are cut off.

When the power switch S ₂ is turned on from this state, the voltage across the capacitor C ₅ is applied to the muting drive circuit 6 via the resistor R ₇ from the moment it is switched on, so that the voltage across the capacitor C ₅ reaches a predetermined level. Muting is applied for a period of time until the level attenuates, making it possible to eliminate noise when the power is turned on.

On the other hand, when the power switch _S2 is turned off again from this state, the voltage across the smoothing capacitor _C1 connected to the secondary side of the power transformer is applied to the muting drive circuit 6 from the moment the power switch S2 is turned off. Therefore, when the power is turned off, muting is applied continuously from the moment the switch is switched, and noise when the power is turned off can be eliminated. Note that if a switching type switch is used as the power switch S ₂ as in this embodiment, the off terminal a and the on terminal c will remain separated from the common terminal b for a long time, causing damage to the mutating drive circuit 6. It is possible to reliably prevent a situation where the operation stops.

FIG. 3 shows a second embodiment of the present invention, in which the on-terminal c of the power switch _S3 is connected to the base of the transistor _Q1 constituting the regulator 3, and the output terminal of the rectifying and smoothing circuit 2 and the transistor _Q1
The collector is directly connected.

In the embodiment shown in Fig. 2, all the rectified and smoothed output flows through the power switch _S2 , so a switch with a large current capacity must be used as the power switch _S2 . switch
When S ₃ is turned off, the base circuit of transistor Q ₁ is cut off, transistor _{Q 1} is turned off, and the rectified and smoothed output can be cut off. things can be used.

As described above, according to the present invention, muting can be applied from the moment the power switch is turned on and off. That is, when the power switch is turned on, the voltage across the capacitor provided at the OFF terminal of the power switch is applied to the muting drive circuit. When the power supply transistor is operated and the power switch is turned off, the voltage at the output terminal of the power transistor operates the muting drive circuit, so that a reliable muting operation can be performed without having to set difficult constants. Furthermore, in a steady state after power is turned on, the capacitor of the time constant circuit is completely isolated from the power supply line, so there is no problem of the muting circuit erroneously operating when the voltage is reduced. Furthermore, since there is no need to use a complicated muting control circuit as in the prior art, it is extremely advantageous in terms of circuit configuration and cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional example, and FIGS. 2 and 3 are circuit diagrams of first and second embodiments of the present invention. 1...power plug, 2...rectifier smoothing circuit, 3...
...Regulator, 4 ... DC output terminal, 6 ... Muting drive circuit, 7, 8 ... Power amplifier,
9, 10...Speaker, L, R...Signal transmission line, T...Power transformer, _S2 , _S3 ...Power switch, a...Off terminal, b...Common terminal, c...
On terminal, C ₅ , R ₇ ... Capacitor and resistor that make up the time constant circuit.

Claims (1)

[Claims] 1. Connect the signal transmission line to a switching type power switch that connects the output end of the DC power circuit connected to the secondary side of the power transformer to the common terminal and the power end of the signal processing circuit of the device to the on terminal. a resistor for connecting the muting drive circuit to the off terminal of the power switch so that the muting drive circuit operates with the output of the power supply circuit when the power switch is in the off state, and a resistor for connecting the muting drive circuit to the off terminal of the power switch, and A muting circuit equipped with a capacitor provided with a time constant circuit so that the muting drive circuit operates for a while at the off terminal of the power switch.