JP2512951Y2 - Muting circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a muting circuit which is used in audio equipment such as a tape recorder and which suppresses noise generated when power is turned on and off.

(B) Conventional technology Since a high-grade tape recorder called a tape deck is used by being connected to an amplifier with a large output, the noise generated by the tape recorder is amplified by the amplifier and then emitted at a larger level than the speaker. This will give the user an unpleasant feeling. Some of the noises output from the tape recorder are generated when the operation is switched, but the noises generated when the power is turned on and off are particularly large and may damage the speaker. As a circuit for preventing the output operation of such a noise signal, there is a muting circuit as disclosed in, for example, Japanese Utility Model Publication No. 61-45621.

(C) Problems to be solved by the invention According to the technology of the above publication, noise generated at power-on and power-off can be suppressed, but control operation at power-on and muting control operation at power-off However, there is a problem that the circuit configuration becomes complicated because the steps are performed by separate circuits. The present invention is intended to provide a muting circuit that is improved in this respect.

(D) Means for solving the problem The muting circuit of the present invention is a control connected to control the operation of a muting transistor whose collector / emitter path is connected between the signal transmission path and ground. Through a transistor, a first diode for guiding the voltage output to the output terminal of the constant voltage power supply circuit to the emitter of the control transistor, and a second diode at all times when a DC voltage is output to the output terminal of the constant voltage power supply circuit. It is connected between the first capacitor in a charged state, the charging terminal of the first capacitor and the emitter of the control transistor, and the discharge current of the first capacitor is supplied to the emitter of the control transistor when the power is cut off. Is connected to the leading third diode and the base circuit of the control transistor and It is composed of a time constant circuit including a second capacitor.

(E) Action The present invention controls the operation of the control transistor, which is provided to control the operation of the muting transistor, by the time constant circuit provided in the base circuit thereof. The muting operation of is performed.

(F) Embodiment The illustrated circuit is an embodiment of the muting circuit of the present invention. In the figure, (1) is a reproducing magnetic head, (2) is a preamplifier circuit for inputting a signal reproduced by the magnetic head (1) and amplifying the signal.
(3) is a variable resistor which is applied with the signal amplified by the preamplifier circuit (2) and adjusts the level of the signal, and (4) is a signal whose level is adjusted by the variable resistor (3) Is a main amplifier circuit that amplifies the signal at the same time as is input, and (5) is an output terminal that is connected to the input terminal of an amplifier that has a large output and to which the signal amplified by the main amplifier circuit (4) is applied. is there. (6) is a muting transistor whose collector is connected to the signal transmission line (7) and whose emitter is grounded, and which bypasses the signal flowing through the signal transmission line (7) when it is in the ON state. Has an effect. (8) is a power supply terminal connected to a commercial AC power supply, (9) is a power supply switch that is closed when the power is turned on, and (10) is a DC power supply that is input by closing the power supply switch (9). A constant-voltage power supply circuit that outputs a constant-voltage DC voltage to the output terminal (11). In (12), the emitter is connected to the output terminal (11) of the constant voltage power supply circuit (10) through the first diode (13), and the collector is connected through the Zener diode (14) and the resistor (15). A control transistor (16) connected to the base of the muting transistor (6) is an output terminal (1) of the constant voltage power supply circuit (10).
The first capacitor (18) is in a state of being charged by the current supplied through the second diode (7) when the DC voltage is output to 1), and the charging terminal (19) of the first capacitor (16) is And the control transistor (1
The third diode is connected between the emitter of 2) and guides the discharge charge of the first capacitor (16) to the emitter of the control transistor (12) when the power is cut off. ( 20 ) is a time constant circuit connected to the base circuit of the control transistor (12), and includes a second capacitor (21), a charging terminal (22) of the capacitor (21) and the constant voltage power supply circuit (10). ) Connected to the output terminal (11) of the control resistor (23), the charging terminal (22) of the capacitor (21) and the base of the control transistor (12). It is composed of a second resistor (24) and a third resistor (25) connected between the output terminal (11) of the constant voltage power supply circuit (10) and the ground. (26) is a muting signal generating circuit for outputting a high level muting signal during a muting operation, the output signal of which is transmitted through the zener diode (14) and the resistor (15) to the muting transistor (6). Is connected to be applied to the base of the. The muting circuit of the present invention is constructed as described above. Next, the operation of such a circuit will be described.

The illustrated state is a non-power supply state, and when the power switch (9) is closed in this state, a constant DC voltage is output to the output terminal (11) of the constant voltage power circuit (10). When a DC voltage is output to the output terminal (11), power is supplied to the preamplifier circuit (2) and the main amplifier circuit (4) so that the preamplifier circuit (2) and the main amplifier circuit (4) are in an operating state and a control transistor is provided via a first diode (13). The high voltage of the emitter of (12) is applied. Also, the second capacitor (2) connected to the base circuit ( 20 ) of the control transistor (12).
A charging current flows through 1) through the first resistor (23) and the second resistor (24), and the control transistor (12) is biased into a conducting state during the charging operation. When the control transistor (12) is turned on, the collector of the control transistor (12) passes through the Zener diode (14) and the resistor (15) to provide a muting transistor (6).
Since a bias current is supplied to the base of the muting transistor 6, the muting transistor (6) is turned on for a predetermined time. Therefore, the muting transistor (6)
As a result, the signal transmission path (7) is grounded, and no noise signal is output to the output terminal (5). When a predetermined time has passed after the power switch (9) was closed, the charging operation of the second capacitor (21) constituting the time constant circuit ( 20 ) is completed, so that the control transistor (12) is inverted to the non-conduction state, and its inversion is performed. Accordingly, the muting transistor (6) is turned off. When a reproducing operation is performed in such a state, the tape recorder enters a reproducing operation state, and the signal recorded on the magnetic tape is reproduced by the reproducing magnetic head (1). The signal reproduced by the reproducing magnetic head (1) is amplified by the preamplifier circuit (2) and then applied to the variable resistor (3) to adjust its level. The reproduction signal whose level has been adjusted by the variable resistor (3) is input to the main amplifier circuit (4) and amplified, and then output to the output terminal (5).
The reproduction signal output to the output terminal (5) is amplified by an amplifier having a large output and then applied to a speaker to emit a sound. Muting signal generation circuit (26) when performing a muting operation in such a reproduction operation state
A higher level muting signal is output, and the muting signal is passed through the Zener diode (14) and the resistor (15) to the muting transistor (6).
Applied to the base of. As a result, the muting transistor (6) is inverted to the on state to perform the muting operation, so that the reproduced signal or noise is not output to the output terminal (5). In addition, the first capacitor (16) is in a state of being charged by the charging current supplied through the second diode (17) while the above-described operations are being performed.

The noise prevention operation and the normal muting operation are performed when the power is turned on as described above. Next, the operation when the power is cut off by opening the power switch (9) will be described. When the power switch (9) is opened, the potential of the output terminal (11) of the constant voltage power supply circuit (10) drops, so the electric charge stored in the second capacitor (21) forming the time constant circuit ( 20 ) is charged. Are discharged through the first resistor (23) and the third resistor (25). When the charge stored in the second capacitor (21) is discharged, the control transistor (1
The base potential of 2) drops, but its emitter potential is delayed by the charge stored in the first capacitor (16). Therefore, when the potential of the output terminal (11) of the constant voltage power supply circuit (10) drops due to power interruption, the control transistor (12) is turned on for a predetermined time by the discharge charge of the first capacitor (16), A bias current is supplied to the base of the muting transistor (6). Therefore,
As a result of turning on the muting transistor (6) and grounding the signal transmission path (7), a noise signal generated due to power interruption is not output to the output terminal (5). Since the bias current to the muting transistor (6) is supplied by discharging the electric charge charged in the first capacitor (16), it is controlled when the charged electric charge in the first capacitor (16) is discharged. The transistor (12) and the muting transistor (6) are turned off.

In the present embodiment, the operation when the power is cut off and supplied by opening and closing the power switch (9) has been described, but the power supply is controlled by the timer device while the power switch (9) is closed. The same operation can be performed in the case of performing.

(G) Effect of the Invention The muting circuit of the present invention is provided in the base circuit of the control transistor for the operation of the control transistor connected to control the operation of the muting transistor when the power is turned on and off. Since it is configured to be controlled by the time constant circuit, that is, the same time constant circuit is used to perform the muting operation at power-on and power-off, the circuit configuration is very simple. Has the advantage that

In addition, the present invention is to apply the DC voltage output to the output terminal of the constant voltage power supply circuit when the power is turned on to the emitter of the control transistor directly through the first diode, and to supply the discharge current as the operating power when the power is cut off. Since the third diode is connected between the charging terminal of the provided first capacitor and the emitter of the control transistor, the emitter potential of the control transistor is turned on without being affected by the charging operation of the first capacitor. Sometimes it can rise rapidly. Therefore,
The present invention can instantly perform the muting operation when the power is turned on, and the muting circuit of the present invention is most suitable as a muting circuit of a tape recorder used by being connected to an amplifier having a large output. is there.

[Brief description of drawings]

The circuit shown is an example of the muting circuit of the present invention. Description of main drawing numbers (6) ... Muting transistor, (7) ... Signal transmission line, (9) ... Power switch, (10) ... Constant voltage power supply circuit, (12) ... Control transistor, (13) ... First diode, (16) ... First capacitor, (17) ... Second diode, (18) ... Third diode, ( 20 ) ... Time constant circuit, (21) ... Second capacitor.

Claims (1)

(57) [Scope of utility model registration request] 1. A muting transistor which performs a muting operation when a collector-emitter path is connected between a signal transmission path and ground and is in an on state.
A collector is connected to the base of the muting transistor to control the operation of the muting transistor, and a control transistor for turning the muting transistor on when the muting transistor is in an on state, and power is supplied. A constant voltage power supply circuit that outputs a predetermined DC voltage to the output terminal when the output terminal is in the open state; and a first diode whose anode is connected to the output terminal of the constant voltage power supply circuit and whose cathode is connected to the emitter of the control transistor. , The anode is connected to the output terminal of the constant voltage power supply circuit, and the cathode is the first
A second diode connected to the charging terminal of the capacitor and charging the first capacitor when a DC voltage is output to the output terminal; and an anode connected to the charging terminal of the first capacitor The cathode is connected to the emitter of the control transistor, and is connected to a third diode that supplies the discharge current of the first capacitor to the emitter of the control transistor as an operating power when the power is cut off, and to the base circuit of the control transistor. A time constant circuit including a second capacitor that holds the base voltage of the control transistor at a low potential for a predetermined time when the power is turned on, and discharges the charge when the power is turned off to quickly reduce the base potential of the control transistor. And the output terminal of the constant voltage power supply circuit when the power is turned on. When the flowing voltage is output, the DC voltage is applied to the emitter of the control transistor through the first diode, and the base potential of the control transistor is kept low by the time constant circuit to turn on the control transistor for a predetermined time. The muting transistor is turned on for a predetermined time, and when the voltage of the output terminal of the constant voltage power supply circuit is lowered by shutting off the power, the discharge current of the first capacitor is passed through the third diode. Is applied to the emitter of the control transistor, and the base potential of the control transistor is lowered by the time constant circuit to turn the control transistor on for a predetermined time, thereby turning on the muting transistor for a predetermined time. Characterized by doing Muting circuit.