JPS634288Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a tape recorder, and particularly to a muting circuit that suppresses noise generation.

A high-grade tape recorder called a tape deck is connected to an amplifier with a high output, so the noise generated by the tape recorder is amplified by the amplifier and then emitted from the speaker at a high level, making it difficult for the user to hear. It will make you feel uncomfortable. In order to improve this point, this type of tape recorder is equipped with a circuit called a muting circuit. The present invention aims to provide a muting circuit that can suppress noise generated when power is turned on and off in addition to normal muting operations, and will be described in detail below with reference to the drawings. The illustrated circuit is an embodiment of the present invention, and in the figure, 1 is a reproducing magnetic head;
2 is a preamplifier circuit for amplifying the signal reproduced by the magnetic head 1; 3 is a variable resistor for adjusting the level of the output signal of the preamplifier circuit 2; 4 is a variable resistor for adjusting the level of the output signal of the preamplifier circuit 2; 5 is an output terminal to which the output signal of the main amplifier circuit 4 is applied and is connected to an input terminal of an amplifier having a large output. Reference numeral 6 denotes a muting transistor whose collector is connected to the signal transmission path 7 and whose emitter is grounded, and has the function of bypassing the signal flowing through the signal transmission path 7 when in an on state. 8 is a power supply terminal connected to a commercial AC power source, 9 is a power switch that is closed when power is supplied, and 10 is a power supply terminal that rectifies the input AC power when the power switch 9 is closed, and converts the voltage into a constant voltage. This is a constant voltage power supply circuit that outputs. Reference numeral 11 denotes a capacitor that is charged by direct current, which is the output of the constant voltage power supply circuit 10, supplied through the reverse current prevention diode 12. Reference numeral 13 has an emitter connected to the power supply terminal of the capacitor 11, and a collector connected to the resistor 14. a control transistor whose collector is connected to the base of the mutating transistor 6 via a resistor 15 and a Zener diode 16 for level shifting; This is a time constant circuit composed of a resistor 19 connected between the two and a capacitor 20 connected between the base and ground. Reference numeral 21 denotes a control switch which is closed in a stopped state and is opened by a regeneration operation, and is connected in series with a resistor 22 between the base of the control transistor 13 and ground. 23 is a capacitor connected to the base circuit of the mutating transistor 6;
This serves to prevent a sudden reversal of the In such a circuit, the values of resistors 19 and 22 are set to such values that control transistor 13 is turned on when control switch 21 is closed.

The present invention is constructed as described above, and its operation will be explained next. The illustrated state is a power-off state, and when the power switch 9 is closed in this state, a constant voltage is output from the constant voltage power supply circuit 10. As a result, power is supplied to the preamplifier circuit 2 and the main amplifier circuit 4, and the power is supplied to the capacitor 1 through the diode 12.
A charging current flows through the capacitor 11, and the capacitor 11 is charged. Further, in the stopped state, the control switch 21 is in the closed state, so the control transistor 13 is biased by the resistors 19 and 22 and is in the on state. As a result, the collector of the control transistor 13 is connected to the base of the mutating transistor 6. A bias current flows through the mutating transistor 6, and the muting transistor 6 is in an on state. Therefore, the signal transmission line 7 is grounded by the mutating transistor 6, and signals such as noise are transmitted to the output terminal 5.
It is never output to . When a regeneration operation is performed from such a stopped state, the drive mechanism enters a regeneration operation state and the control switch 21 is opened. When the control switch 21 is opened, the control transistor 13 which was in the on state is turned on for a short time, that is, the capacitor 20
After the time required for the charging voltage to reach a predetermined value, the muting transistor 6 is turned off, and the muting transistor 6, which was on, is turned off. Therefore, the reproduction signal reproduced by the reproduction magnetic head 1 is amplified by the preamplifier circuit 2 and applied to the variable resistor 3. The reproduced signal applied to the variable resistor 3 is level-adjusted and input to the main amplifier circuit 4, where it is amplified and output to the output terminal 5. In such a regeneration operating state, when a stop operation is performed, the drive mechanism is switched to a stopped state and the control switch 2 is switched to a stopped state.
1 is closed. When the control switch 21 is closed, the control transistor 13 which was in the OFF state is immediately reversed to the ON state and the muting transistor 6 is turned on, so that the noise generated due to the reversal to the stopped state is reduced. Output terminal 5
It is never output to . In addition, control switch 2
1 is in a closed state in the fast forward and rewind operation states, the muting transistor 6 is in the on state, and no noise or the like is output to the output terminal 5.

The normal operation is carried out as described above, but next, the operation when the power is cut off by opening the power switch 9 will be explained. First, a case will be described in which the power switch 9 is opened when the power switch 9 is in a stopped state, that is, when the control switch 21 is in a closed state. If the power switch 9 is opened while the motor is in a stopped state, the potential at the output terminal of the constant voltage power supply circuit 10 will drop;
The potential of the emitter of the control transistor 13 gradually drops because the capacitor 11 is connected thereto. Therefore, even if the power supply from the constant voltage power supply circuit 10 is cut off by opening the power switch 9, the control transistor 13 that is in the on state will not be immediately turned off, but will be turned off after a predetermined time delay. Invert. Therefore, during that time, the muting transistor 6 is also held in the on state and the signal transmission path 7 is grounded, so that noise caused by power cut-off is not output to the output terminal 5. As described above, the operation is performed when the power switch 9 is opened when the power switch 9 is in the stopped state, but the next case when the power switch 9 is opened when the power switch 9 is in the playback operation state, that is, when the control switch 21 is in the open state, will be described. explain. In the reproducing operation state, the control transistor 13 is reverse biased and in the OFF state as described above, and when the power switch 9 is opened in such a state, the potential of the output terminal of the constant voltage power supply circuit 10 drops, so that the control transistor 13 The base potential also drops. On the other hand, since the capacitor 11 is connected to the emitter of the control transistor 13, the emitter potential gradually drops. Therefore, when the current supply from the constant voltage power supply circuit 10 is cut off by opening the power switch 9, the control transistor 13 is quickly turned on, and the muting transistor 6 is turned on, causing the signal transmission path to be turned on. 7 is grounded, so that no noise is output to the output terminal 5. When the charge stored in the capacitor 11 is discharged, the control transistor 13 and the muting transistor 6 are turned off.

The operation when the power is cut off is performed as described above. Next, the operation when the power switch 9 is closed while the tape recorder is set to the playback mode will be described. control switch 21
When the power switch 9 is closed while the power supply switch 9 is set to the regeneration state in which the power supply is in the open state, a predetermined voltage is outputted from the constant voltage power supply circuit 10. As a result, the capacitor 11 connected to the emitter of the control transistor 13 is immediately charged by the current flowing through the diode 12, so that
The charging potential of the capacitor 11, ie, the emitter potential of the control transistor 13, immediately rises. On the other hand, the base potential of the control transistor 13 gradually increases due to the action of the time constant circuit 17 .
As a result, the control transistor 13 remains on until the charging potential of the capacitor 20 constituting the time constant circuit 17 reaches a predetermined value, causing the muting transistor 6 to remain on for a predetermined time. Therefore, during that time, the signal transmission line 7 is grounded by the muting transistor 6, and the noise generated when power is supplied is not output to the output terminal 5.

In this embodiment, the operation was explained in which the power was cut off and supplied by opening and closing the power switch 9, but the power was controlled by a timer device while the power switch 9 remained closed. A similar operation can also be performed in this case.

As explained above, the muting circuit of the present invention not only grounds the signal transmission path when the tape recorder is not in playback mode, such as when it is stopped, to prevent signals such as noise from being output to the output terminal, but also This invention can suppress the noise that occurs when switching from the playback state to the stop state and from the playback operation state to the stop state, and can also suppress the noise that occurs when power is supplied and cut off. It is ideal as a muting circuit for a tape recorder connected to an amplifier with a

[Brief explanation of the drawing]

The illustrated circuit is one embodiment of the muting circuit of the present invention. Explanation of main drawing numbers, 1...Magnetic head for recording and playback, 2...Preamplifier circuit, 4...Main amplifier circuit, 6
... Muting transistor, 7 ... Signal transmission line, 9 ... Power switch, 10 ... Constant voltage power supply circuit, 13 ... Control transistor, 17 ...
...Time constant circuit, 21...Control switch.

Claims (1)

[Scope of utility model registration request] a mutating transistor having a collector-emitter path connected between the signal transmission path and ground; and a muting transistor connected to control the operation of the muting transistor and turning on the mutating transistor when in the on state. a control transistor; a control switch that is closed when in a stopped state and turns on the control transistor and is opened by a regeneration operation; a control switch that is connected to a base circuit of the control transistor and opens the control switch; a time constant circuit that keeps the control transistor in the on state for a predetermined time when power is supplied in the state, and a capacitor that is constantly charged in the power supply state and supplies a current that keeps the control transistor in the on state for a predetermined time when the power is cut off. A muting circuit for a tape recorder, characterized in that the control transistor is held in the on state for a short time by the action of the time constant circuit when the control switch is opened due to switching from a stop state to a playback state. .