JPS6145620Y2 - - 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 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 present invention, and in the figure, 1 is a magnetic head for reproduction, 2 is a preamplifier circuit for amplifying the signal reproduced by the magnetic head 1, and 3 is a preamplifier circuit for amplifying the signal reproduced by the magnetic head 1. position amplifier circuit 2
4 is a main amplifier circuit into which the signal level-adjusted by the variable resistor 3 is input; 5 is a main amplifier circuit into which the signal amplified by the main amplifier circuit 4 is input; It is an output terminal connected to an input terminal of an amplifier having a large output power. 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 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 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, the emitter is connected to the power supply terminal of the capacitor 12, and the collector is a Zener. diode 15
and a control transistor connected to the base of the mutating transistor 6 via a resistor 16. A bridge circuit 17 is connected between the power supply line 11 and the ground, and is composed of the reverse current prevention diode 13 and resistors 18, 19, 20, and 21. 22 is a detection transistor whose emitter is connected to a balanced terminal A, which is the connection point between the resistors 18 and 19, and whose base is connected to a balanced terminal B, which is the connection point between the resistors 20 and 21; is connected to the base of the control transistor 14. A second capacitor 23 is connected between the emitter and the collector of the control transistor 14, and has the function of supplying a bias current to the base of the mutating transistor 6 when the power is turned on. 24 is a muting signal generation circuit which 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. . In the circuit configured in this manner, the values of the resistors 18, 19, 20, and 21 constituting the bridge circuit 17 are set to values that reverse bias the detection transistor 22 in a steady state where power is supplied.

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 voltage made constant from 0 is output. 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 capacitor 12 is charged. Further, a charging current flows from the reverse current prevention diode 13 to the second capacitor 23, and this charging current is applied to the base of the muting transistor 6.
remains on until the second capacitor 23 is charged. Therefore, since the signal transmission line 7 is grounded by the muting transistor 6, signals such as noise are not outputted to the output terminal 5. When a predetermined period of time elapses after power is supplied, the muting transistor 6 is inverted to the OFF state, and the detection transistor 22 is reverse biased and is in the OFF state, so that the control transistor 14 is in 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 magnetic head for reproduction 1
The signal reproduced by 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. When a muting operation is performed in such a reproducing operation state, a high-level muting signal is output from the muting signal generation circuit 24, and the muting signal is applied to the base of the muting transistor 6 via the Zener diode 15 and the resistor 16. . As a result, the mutating transistor 6 is turned on and performs a muting operation.
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. When the power switch 9 is opened, the potential of the power supply line 11 drops, so that the potential of the balanced terminal A of the bridge circuit 17 , that is, the emitter potential of the detection transistor 22, also drops. On the other hand, the potential of the other balanced terminal B, that is, the base potential of the detection transistor 22 is the first
The charge on capacitor 12 delays its fall. Therefore, when the emitter potential of the detection transistor 22 drops due to power cutoff, the detection transistor 22 changes from a reverse bias state to a forward bias state, and the detection transistor 22 immediately turns on.
When the detection transistor 22 is turned on, the control transistor 14 whose base is connected to the collector of the detection transistor 22 is turned on.
is turned on, and a bias current is supplied from its collector to the base of the mutating transistor 6. Therefore, the muting transistor 6 is turned on and the signal transmission line 7 is turned on.
As a result of grounding, noise caused by power cut-off will not be output to the output terminal 5. The bias current to the mutating transistor 6 is supplied by discharging the charge stored in the first capacitor 12. When the charge stored in the capacitor 12 is discharged, the bias current is supplied to the mutating transistor 6. The control transistor 14 and the muting transistor 6 are 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

As explained above, the muting circuit of the present invention has a simple configuration and can suppress the noise generated not only when the power is turned on but also when the power is turned off. Ideal as a muting circuit.

[Brief explanation of drawings]

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, 12... First capacitor, 13... Backflow prevention diode, 14... Control transistor, 17 ... Bridge circuit, 22
. . . detection transistor, 23 . . . second capacitor, 24 . . . mutating 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 connected to the current supply side of the control transistor; a detection transistor that is biased by the discharge current of one capacitor and turns on to turn on the control transistor; and an emitter of the control transistor.
A muting circuit for a tape recorder comprising a second capacitor connected between the collectors and supplying a current to turn on the mutating transistor when the power is turned on.