JPH0425630B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a tape recorder, and particularly relates to a muting circuit that prevents noise signals from being output.

(b) Conventional technology A high-grade tape recorder called a tape deck is used by being connected to an amplifier with a high output, so the noise generated by the tape recorder is amplified by the amplifier and then emitted at a high level from the speaker. This will cause discomfort to the user. Some of the noise output from a tape recorder is generated when switching operations, but if the noise generated when the power is turned on and off is especially large and severe, it may damage the speaker. In order to prevent such noise signals from being output, tape recorders equipped with a circuit called a muting circuit are becoming popular.
There is one disclosed in Publication No. 58-71215.

(c) Problems to be solved by the invention The above-mentioned gazette discloses a technique that prevents the output of the noise that occurs when the power is turned on and when the power is turned off, but the power supply voltage detection operation in this technique is Since this is performed by detecting the output voltage of the constant voltage power supply circuit, there is a problem that the start time of the muting operation is delayed when the power is turned on and when the power is turned off. The present invention aims to provide a muting circuit that is improved in this respect.

(d) Means for Solving the Problems The present invention provides a muting means for bypassing a signal flowing through a signal transmission path, and a muting means that outputs a signal to operate the muting means when in an operating state and operates for a predetermined time by a trigger operation. an inverting circuit that enters the inverting state, a control circuit that becomes operational for a predetermined period of time when a DC voltage is output from the constant voltage circuit, and keeps the inverting circuit in the operating state during that time; It consists of a trigger switch that triggers the circuit, and the muting operation is started when the power is turned on and off by using the voltage on the input side of the constant voltage circuit.

(e) Effect The present invention provides a constant voltage circuit that converts a DC voltage rectified and smoothed by a rectifying and smoothing circuit into a constant voltage. The muting operation is performed by utilizing the characteristic that the voltage is faster compared to the voltage characteristic.

(F) Embodiment The illustrated circuit is an embodiment of the muting circuit of the present invention, and in the figure, 1 is a magnetic head for reproducing signals recorded on a magnetic tape;
2 is a preamplifier circuit to which a signal reproduced by the magnetic head 1 is applied and which amplifies the signal;
3 is a variable resistor to which the signal amplified by the preamplifier circuit 2 is applied and whose level is adjusted; 4 is a variable resistor to which the signal whose level has been adjusted by the variable resistor 3 is inputted; A main amplifier circuit 5 for amplifying the signal is an output terminal to which the signal amplified by the main amplifier circuit 4 is applied and is connected to an input terminal of an amplifier having a large output. A mutating transistor 6 has a collector connected to the signal transmission path 7 and an emitter grounded, and has the function of bypassing the signal flowing through the signal transmission path 7 when in a conductive state. 8 is a power terminal connected to a commercial AC power supply, 9
10 is a power transformer that steps down the input AC power when the power switch 9 is closed; 11 is a rectifier and smoother that rectifies and smoothes the AC power input through the power transformer 10; This circuit is composed of a rectifier 12 and a smoothing capacitor 13. Reference numeral 14 denotes a constant voltage circuit to which the DC voltage rectified and smoothed by the rectifying and smoothing circuit 11 is applied, and also makes the voltage constant and supplies power to the amplifier circuits 2 and 4.
6, bias resistor 17 and capacitor 18,1
It is composed of 9. 20 is a capacitor to which the current from the rectifying and smoothing circuit 11 is applied through a Zener diode 21 and a backflow prevention diode 22, and is constantly charged when power is supplied. 23 is the capacitor 2
The operating power is supplied from the charging terminal of
The inverting circuit is constituted by a 1NAND circuit 24 and a second NAND circuit 25, and has an inversion operation time set by a capacitor 26 and a resistor 27. circuit 24
The output terminal is a Zener diode 28 and a resistor 2
9 to the mutating transistor 6
connected to the base of. 30 is a trigger switch which is switched in conjunction with a recording/playback switch that switches between recording and playback operations of the tape recorder, and has a playback side terminal P and a recording side terminal R.
are connected to each other and to the anode of the Zener diode 21. Further, the common terminal C of the trigger switch 30 is connected to a resistor 31.
It is grounded via a resistor 32 and connected to the input terminal of the first NAND circuit 24 via a resistor 32. 33 is a backflow prevention diode that applies the voltage regulated by the constant voltage circuit 14 to the charging terminal of the capacitor 20; 34 is in an operating state for a predetermined time when the constant voltage is outputted by the constant voltage circuit 14; This is a control circuit that puts the inverting circuit 23 into operation when the inversion circuit 23 becomes active, and includes a time constant circuit 37 consisting of a capacitor 35 and a resistor 36. A base current is supplied from the time constant circuit 37, the emitter is grounded, and the collector is It is composed of a control transistor 39 and the like connected to the input terminal of the first NAND circuit 24 via a diode 38. A capacitor 40 is connected between the collector of the control transistor 39 and ground to prevent chattering, and a resistor 41 serves to discharge the charge stored in the capacitor 40. Further, a diode 42 connected in parallel to the resistor 27 constituting the inverting circuit 23 functions to discharge the charge stored in the capacitor 26. In such a circuit configuration, the time constant of the time constant circuit 37 constituting the control circuit 34 is set to be longer than the inversion operation time of the inversion circuit 23.

The muting circuit of the present invention is constructed as described above, and the operation of this circuit will now be described. The illustrated state is a non-power supply state, and when the power switch 9 is closed in this state, the AC power stepped down by the power transformer 10 is applied to the rectifying and smoothing circuit 11 and converted into a DC voltage. The DC voltage rectified and smoothed by the rectification and smoothing circuit 11 is made constant by a constant voltage circuit 14, and then supplied to the preamplifier circuit 2 and the main amplification circuit 4. When the power switch 9 is closed, this operation is performed and power is supplied to the rectifying and smoothing circuit 1.
The voltage at the output terminal A of the constant voltage circuit 14 rises faster than the voltage at the output terminal B of the constant voltage circuit 14. Therefore, when the power switch 9 is closed, a charging current flows from the output terminal A of the rectifying and smoothing circuit 11 to the capacitor 20 through the Zener diode 21 and the reverse current prevention diode 22, and the capacitor 20 is immediately charged. When the capacitor 20 is charged, the charging terminal of the capacitor 20 is connected to the inverting circuit 23.
Operating power is supplied to the 1NAND circuit 24 and the second NAND circuit 25. When power is supplied to the first NAND circuit 24, the level of one input terminal of the first NAND circuit 24 is at the L (low) level, so the output terminal of the first NAND circuit 24 is at the H (high) level.
A level signal is output. The first NAND circuit 2
When an H level signal is output to the output terminal of
Since the output of the 2NAND circuit 25 goes to L level, the charging current of the capacitor 26 runs out.
An H level signal is output to the output terminal of the 1NAND circuit 24. Also, the output terminal B of the constant voltage circuit 14
When the regulated DC voltage is output, a bias current flows through the time constant circuit 37 to the base of the control transistor 39, and the control transistor 39 is maintained in a conductive state for a predetermined period of time. When the control transistor 39 is in a conductive state, the first
Since the input terminal of the 1NAND circuit 24 is held at L level, the output terminal of the first NAND circuit 24 has an H level.
A level signal will be output. Therefore, from the time the power switch 9 is closed, the constant voltage circuit 14
until a predetermined period of time elapses after the constant voltage DC voltage is output from the
An H level signal is output to the output terminal of the 1NAND circuit 24. Therefore, the muting transistor 6 to which the signal from the first NAND circuit 24 is applied via the Zener diode 28 and the resistor 29 is kept conductive during that time, and the signal transmission path 7 is grounded by the mutating transistor 6. Therefore, signals such as noise are not output to the output terminal 5. When a predetermined period of time elapses after the power switch 9 is closed, the control transistor 3
9 is reversed to a non-conducting state, and the level of the input signal of the first NAND circuit 24 becomes H level, so that the corresponding
The output signal of the 1NAND circuit 24 goes from H level to L level. When the output signal of the first NAND circuit 24 becomes L level, the muting transistor 6 which was in a conductive state is reversed to a non-conductive state. When a playback operation is performed in such a state, the tape recorder enters a playback operation state, and the signal recorded on the magnetic tape is played back by the magnetic head 1. The signal reproduced by the magnetic head 1 is amplified by a preamplifier circuit 2 and then applied to a variable resistor 3. The reproduction signal applied to the variable resistor 3 is level-adjusted and sent to the main amplifier circuit 4.
The signal is input to the main amplifier circuit 4, amplified by the main amplifier circuit 4, and then 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.

As described above, the muting operation is performed when the power is turned on. Next, the muting operation when switching between recording and playback will be explained. When the user performs a recording operation, the recording/playback switch is switched to the recording side, and in conjunction with this switching, the trigger switch 30 is switched to the opposite side from the state shown. When the trigger switch 30 is switched to the opposite side from the illustrated state, the common terminal C becomes disconnected from the reproduction side terminal P and the recording side terminal R during the switching. As a result, the 1st NAND
The input signal of the circuit 24 changes from the H level to the L level, and the inverting circuit 23 is triggered. When the triggering operation of the inverting circuit 23 is performed, the inverting circuit 23 is in an operating state for a predetermined period of time, and an H level signal is output to the output terminal of the first NAND circuit 24 during that time. Therefore, during this time, the muting transistor 6 is in a conductive state and the signal transmission path 7 is grounded, so that the noise generated due to the switching of the recording/playback switch is not outputted to the output terminal 5. The operation of muting the noise that occurs when the recording/playback switch is switched from the playback side to the recording side is performed, but the operation of muting the noise that occurs when the switch is switched from the recording side to the playback side is also performed in the same way. Therefore, its explanation will be omitted.

As described above, the muting operation is performed when the power is turned on and when recording/playback is switched.Next, the muting operation when the power is cut off by opening the power switch 9 will be explained. When the power switch 9 is opened, the voltages at the output terminal A of the rectifier and smoothing circuit 11 and the output terminal B of the constant voltage circuit 14 both drop; The voltage will drop later than the voltage. Output terminal A of the rectifying and smoothing circuit 11
When the voltage of
The output of the 1NAND circuit 24 becomes H level. During the time when the H level signal is output to the output terminal of the first NAND circuit 24, the capacitor 20
However, since the capacitor 20 is also charged by the current from the output terminal B of the constant voltage circuit 14, the output of the constant voltage circuit 14 An H level signal is output until a predetermined period of time elapses after the voltage at terminal B drops. Therefore, during that time, the muting transistor 6 is in a conductive state and the signal transmission line 7 is grounded, so that noise caused by power cut-off is not outputted 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 supply was controlled by a timer device while the power switch 9 remained closed. A similar operation can be performed when

(G) Effects of the Invention The muting circuit of the present invention can suppress the noise generated not only when the power is turned on and off, but also when recording/playback is switched, from being output as a signal. This is highly effective when applied to a tape recorder that is used by being connected to an amplifier. Also,
Since the power supply voltage detection operation at power-on and power-off is performed on the input side of the constant voltage circuit, the muting operation can be started earlier and the output operation of noise signals can be reliably suppressed. The mutating circuit of the present invention has advantages.

[Brief explanation of drawings]

The illustrated circuit is one embodiment of the muting circuit of the present invention. Explanation of main figure numbers, 2... Preamplifier circuit, 4... Main amplifier circuit, 5... Output terminal, 6... Muting transistor, 7... Signal transmission line, 9... Power switch, 11... Rectifier and smoothing circuit, 14... Constant voltage circuit, 2
2, 33... Backflow prevention diode, 23... Inverting circuit, 24... First NAND circuit, 25... Second NAND circuit, 30... Trigger switch, 34... Control circuit,
37...Time constant circuit, 39...Control transistor.

Claims (1)

[Claims] 1. A rectifying and smoothing circuit that rectifies and smoothes an AC power source into a DC power source, a constant voltage circuit that applies a DC voltage that has been rectified and smoothed by the rectifying and smoothing circuit, and makes the voltage a constant voltage; a capacitor that is constantly charged with a current obtained from the rectifying and smoothing circuit through a prevention diode, and a muting means that bypasses a signal flowing through the signal transmission path when in an operating state;
an inverting circuit that is supplied with operating power from a charging terminal of the capacitor and outputs a signal for activating the mutating means when in the operative state, and is inverted to the operative state for a predetermined time by a trigger operation; and the constant voltage circuit. a control circuit which is activated for a predetermined period of time when a DC voltage is output from the inverting circuit and which causes the inverting circuit to be activated during that time; a trigger switch that triggers the inverting circuit when the output voltage of the rectifying and smoothing circuit decreases due to power cutoff, and when the power is turned on, the inverting circuit is activated for a predetermined period of time set by the control circuit and recording/playback switching is performed. During operation, the inverting circuit is kept in an operating state for a predetermined period of time set by the inverting circuit, and when power is cut off, the inverting circuit is kept in an operating state until the electric charge stored in the capacitor is discharged. Features a tape recorder muting circuit.