JPS634293Y2 - - 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 generated when power is turned on and when power is turned off. A high-grade tape recorder called a tape deck is used by being connected to an amplifier with 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, causing no harm to the user. It will give you a sense of pleasure. Some of the noise output from tape recorders is generated when switching operations, but the noise generated when power is turned on and off is particularly large, and various circuits have been developed to suppress such noise. The present invention aims to provide a muting circuit that can suppress noise generated when power is turned on and off, and will be described in detail below with reference to the drawings. The illustrated circuit is an embodiment of the muting circuit of the present invention, and in the same figure,
1 is a magnetic head for reproduction; 2 is a preamplifier circuit for amplifying the signal reproduced by the magnetic head 1; 3
4 is a variable resistor for adjusting the level of the output signal of the preamplifier circuit 2; 4 is a main amplifier circuit into which the signal whose level has been adjusted by the variable resistor 3 is input; 5 is a main amplifier circuit for the main amplifier circuit 4; This is the output terminal to which the amplified signal is applied and which is connected to the input terminal of an amplifier having a large output. A mutating transistor 6 has a collector connected to the signal transmission line 7 and an emitter grounded, and has the function of bypassing the signal flowing through the signal transmission line 7 when in an on state. 8 is a power terminal connected to a commercial AC power source; 9 is a power switch that is closed when power is supplied; 10 is the power switch 9;
This is a constant voltage power supply circuit that rectifies the input AC power by closing the circuit and outputs a constant DC voltage to the power supply line 11. Reference numeral 12 denotes a first 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 13, and 14 has an emitter connected to the power supply terminal of the first capacitor 12 and a collector. is a control transistor connected to the base of the mutating transistor 6 via a Zener diode 15 and a resistor 16.
A time constant circuit 17 is connected between the base of the control transistor 14 and ground, and is composed of a resistor 18 and a second capacitor 19. 2
0 is a discharging diode whose anode is connected to the connection point A of the resistor 18 and the second capacitor 19 and whose cathode is connected to the power line 11, and when the voltage of the power line 11 drops due to power cutoff, It is turned on and has the effect of discharging the electric charge stored in the second capacitor 19. Reference numeral 21 denotes a muting signal generation circuit that outputs a high-level muting signal during a muting operation, and is connected so that the output signal is applied to the base of the mutating transistor 6 via a Zener diode 15 and a resistor 16. . The muting circuit of the present invention is constructed as described above, and its operation will be explained next. The illustrated state is a non-power supply state, and when the power switch 9 is closed in this state, the constant voltage power supply circuit 1
A DC voltage made constant from zero is output to the power supply line 11. As a result, power is supplied to the preamplifier circuit 2 and the main amplification circuit 4 and they become operational, and a charging current flows to the first capacitor 12 through the backflow prevention diode 13, so that the first capacitor 12 is charged. Further, a charging current flows through the backflow prevention diode 13, the emitter-base of the control transistor 14, and the resistor 18 to the second capacitor 19 constituting the time constant circuit 17 for a predetermined period of time, during which the control transistor 14 is turned on. When the control transistor 14 is turned on, a bias current is supplied from its collector to the base of the muting transistor 6 through the Zener diode 15 and the resistor 16. Therefore, as the muting transistor 6 is turned on for a predetermined period of time after the power is turned on and the signal transmission line 7 is grounded, no signal such as noise is output to the output terminal 5. When a predetermined period of time has elapsed after power was supplied, the control transistor 14
is reversed to the off state, so that the muting transistor 6 is also reversed to the off state. In such a state, when a playback operation is performed, a playback operation state is entered, and the signal recorded on the magnetic tape is played back by the playback magnetic head 1. The signal reproduced by the reproducing magnetic head 1 is amplified by a preamplifier circuit 2 and applied to a 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. The reproduced signal outputted to the output terminal 5 is amplified by an amplifier having a large output, and then applied to a speaker to emit sound. The reproducing operation is performed in this manner, but if a muting operation is performed while in the reproducing operation state, a high-level muting signal is output from the muting signal generation circuit 21, and the muting signal is transmitted through the Zener diode 15 and the resistor 16. is applied to the base of the muting transistor 6. As a result, the muting transistor 6 is turned on and performs a muting operation, so that no reproduced signal or noise is output to the output terminal 5. As mentioned above, the noise prevention operation when the power is turned on,
A reproducing operation and a normal muting operation are performed, but next, the operation when the power is cut off by opening the power switch 9 will be explained. The power switch 9
When the second capacitor 19 is opened, the voltage of the power supply line 11 drops, so that the discharging diode 20, which was in the off state, is turned on, and the charge stored in the second capacitor 19 is rapidly discharged. When the charge stored in the second capacitor 19 is discharged, the base potential of the control transistor 14 drops, but the emitter potential of the control transistor 14 decreases due to the charge stored in the first capacitor 12. is maintained and will not fall. Therefore, as the base potential drops, the control transistor 14 becomes forward biased and turns on. When the control transistor 14 is turned on, a bias current is supplied from its collector to the base of the mutating transistor 6, so the muting transistor 6 is turned on and the signal transmission path 7 is grounded, resulting in power cutoff. No accompanying noise is output to the output terminal 5. The bias current to the mutating transistor 6 is supplied by discharging the electric charge stored in the first capacitor 12. Therefore, when the electric charge stored in the first capacitor 12 is discharged, the bias current to the mutating transistor 6 is supplied to the control transistor 14.
and the muting transistor 6 are both turned off. 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 be performed when Further, although a case has been described in which a diode is used as a switching element to discharge the charge stored in the second capacitor 19 when the voltage drops due to power cut-off, other elements such as a transistor can also be used. As explained above, the muting circuit of the present invention has a simple circuit configuration and can suppress the noise generated not only when the power is turned on but also when the power is turned off, so it can be used by being connected to an amplifier with a large output. Ideal as a muting circuit for tape recorders.

[Brief explanation of the drawing]

The illustrated circuit is one embodiment of the muting circuit of the present invention. Explanation of main drawing numbers, 4... Main amplifier circuit, 5... Output terminal, 6... Muting transistor, 7... Signal transmission path, 9... Power switch, 1
0... Constant voltage power supply circuit, 11... Power supply line, 12
...First capacitor, 13... Backflow prevention diode, 14... Control transistor, 17 ...
Time constant circuit, 19...Second capacitor, 20...
...Discharge diode, 21... Muting signal generation circuit.

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 first capacitor that is constantly charged through a reverse current prevention diode in a power supply state and is connected to the current supply side of the control transistor; and a first capacitor that is connected between the base of the control transistor and ground; A muting circuit for a tape recorder comprising a time constant circuit comprising a resistor and a second capacitor, and a switching element that is turned on when the voltage drops due to power cutoff and rapidly discharges the charge in the second capacitor.