JP2668891B2 - Input level adjustment circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording signal input level adjusting circuit for a tape recorder or the like. 2. Description of the Related Art In recent years, many input level adjustment circuits of this type which employ an electronic volume as level adjustment means have been commercialized. Hereinafter, an example of the above-described conventional input level adjustment circuit will be described with reference to the drawings. FIG. 3 is a connection diagram of main parts of a conventional input level adjustment circuit. In FIG. 3, reference numeral 1 denotes a tuner, or an input terminal to which a music signal or the like from a compact disc player or the like (not shown) is inputted via a stereo amplifier or the like (not shown), and R1 and R2 are resistors R1. Is connected to the input terminal 1, the other end is connected to one end of R2, and the other end of R2 is grounded. C has one end connected to the connection point between the resistors R1 and R2, and the other end connected to the signal input end 3 of the level adjusting means 11 described later.
2 is a signal input end 3, a signal level adjusting part 4, a control voltage control part 5, a reference power supply part 6, a signal output end 7, a power supply input end 8, a ground end 9, a volume control voltage input end. A level adjusting means including 10 and the like, and 11 is an amplifying means for amplifying a signal from the signal output terminal 7. The operation of the input level adjusting circuit configured as described above will be described below. First, when the user operates a tuner, a compact disc player, a stereo amplifier, or the like, the input terminal 1
Is supplied to the level adjusting means 2 through a capacitor C, and is attenuated to a predetermined level by resistors R1 and R2.
Is supplied to the input end 3 of the. Then, a volume control voltage from a microcomputer (not shown) having a PWM (pulse width modulation) output function or the like is applied to the volume control voltage input terminal 10 of the level adjusting means 2, so that the reference power supply section 6 The control voltage controller 5 driven by the power source controls the signal level adjuster 4 to adjust the music signal or the like supplied to the input terminal 3 to a predetermined level and output it to the signal output terminal 7. Then, the signal is supplied from the signal output terminal 7 to the amplifier 11, amplified to a predetermined level, and then output. Problems to be Solved by the Invention However, in the above configuration, if it is applied to an input level adjusting circuit such as a tape recorder and system-connected to a stereo amplifier, a tuner, a compact disc player, etc., the system used by the user Conditions (Temporarily turn off the power switch because no tape recorder is used)
When the power supply of the level adjusting means 2 is cut off by the input terminal 1, the input terminal 1 is always connected to the signal source (amplifier, etc.), so that the input terminal 3 constitutes a non-linear circuit. , When the level of the signal supplied to the input terminal 1 becomes relatively high (about 0.6 V to 0.7 V or more), a current starts to flow in the non-linear circuit, causing distortion, and the signal source connected to the input terminal 1 ( Amplifier, etc.). Therefore, the signal supplied to the input terminal 1 is divided according to the division ratio of the resistors R1 and R2, the level is lowered and then supplied to the input terminal 3, and after the level adjustment, the amplification means 11 again amplifies the signal to a predetermined level. Therefore, there are various problems such as deterioration of S / N ratio, increase in the number of parts and complexity, and increase in cost. In view of the above problems, the present invention prevents the signal source (stereo amplifier or the like) from being distorted by the level adjusting means having an input terminal section that constitutes a non-linear circuit when the power is cut off, and S / An object of the present invention is to provide an input level adjusting circuit configured so as not to deteriorate the N ratio. Means for Solving the Problems In order to solve the above problems, the input level adjusting circuit of the present invention includes a signal level adjusting unit including a non-linear circuit and a control voltage control unit for controlling the signal level adjusting unit. And a reference power supply unit for driving the control voltage control unit, and a P-channel field effect transistor. The positive power supply has a power source input terminal of the reference power supply unit of the level adjustment unit and a P-channel field effect transistor. The gate line of the channel field effect transistor is connected, and either the source or the drain of the P channel field effect transistor is connected to the input line of the level adjusting means, and the other is grounded. Be prepared. According to the present invention, with the above-described configuration, even if the power supply of the level adjusting means is cut off and the input terminal portion thereof constitutes a non-linear circuit, the input line is formed by the P-channel field effect transistor which is turned on when the power supply is cut off. Is shunted, and the current does not flow from the signal source (stereo amplifier or the like) to the non-linear circuit, so that the occurrence of distortion can be prevented. Embodiment An input level adjusting circuit according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a connection diagram of main parts of a level adjusting circuit according to an embodiment of the present invention, and FIG. 2 shows an attenuation amount-volume control voltage characteristic of a signal level adjusting part. In FIG. 1, 15 is an input terminal to which a tuner or a music signal from a compact disc player or the like (not shown) is input after passing through a stereo amplifier or the like (not shown), and R3, R4 and R5 are resistors. Thus, one end of R3 is connected to the input terminal 15, the other end is connected to one end of R4, and the other end of R4 is grounded. R5 has one end connected to the gate of a P-channel field effect transistor Q described later, and the other end connected to a power input terminal described later. One end of Q is grounded, the other end is connected to the connection point of resistors R3 and R4, and the gate is connected to the power supply input end 22 via the resistor R5. A P-channel field effect transistor (hereinafter simply referred to as P-FET). , C1 has one end connected to the connection point between the resistors R3 and R4 and one end of the P-FET, and the other end connected to the signal input end 16 of the level adjusting means 17 described later, and 17 indicates the signal input end. 16, a signal level adjusting unit 18, a control voltage control unit 19, a reference power supply unit 20, a signal output end 21, a power supply input end 22, a grounding end 23, a volume control voltage input end 24, etc. . The input level adjusting circuit configured as described below will be described below with reference to FIGS. 1 and 2 assuming that the operation is applied to the input level adjusting circuit of a tape recorder or the like. First, when the user operates a tuner, a compact disc player, a stereo amplifier, etc. (none of which is shown) in a normal use state in which power is supplied, a music signal or the like is supplied to the input terminal 15 and the resistor R3 , And is attenuated to a predetermined level by R4 and supplied to the input terminal section 16 of the level adjusting means 17 via the capacitor C1. Then, when a volume control voltage from a microcomputer (not shown) having a PWM (pulse width modulation) output function is applied to the volume control voltage input terminal 24 of the level adjusting means 17 as shown in FIG. ,
A control voltage control unit 19 driven by a power supply from the reference power supply unit 20 controls the signal level adjustment unit 18,
An attenuation characteristic as shown in FIG. 2 is obtained. As a result, the music signal or the like supplied to the input terminal unit 16 is adjusted to a predetermined level and output from the signal output terminal 21. Then, after performing necessary signal processing, it is recorded by recording means (not shown). Next, a case where the user does not use the tape recorder and turns off the power switch to use a stereo amplifier, a tuner, a compact disc player or the like will be described. When the power supply of the tape recorder is turned off and the power supply of the level adjusting means 17 is cut off, the input terminal 15 is always connected to the signal source (stereo amplifier etc.). In the case where 16 constitutes a non-linear circuit, when the level of the signal supplied to the input terminal 15 becomes relatively high (about 0.6 V to 0.7 V or more), current begins to flow in the non-linear circuit, causing distortion. Although it should occur, the P-FET connected between the input line and ground turns on and shunts the input line when the power supply is cut off, so no current flows in the non-linear circuit and distortion occurs. Does not happen. At this time, the resistor R3 is set so as not to overload the signal source. The P-FET turns off when V _GS (gate-source voltage) exceeds the pinch-off voltage and is positively biased,
Turns on when V _GS = 0V. As described above, according to the present embodiment, the signal level adjusting unit configured by the non-linear circuit, the control voltage controlling unit for controlling the signal level adjusting unit, and the reference power supply unit for driving the control voltage controlling unit are provided. And a P-channel field effect transistor, wherein the power source input terminal of the reference power source section of the level adjusting means and the gate line of the P-channel field effect transistor are connected to the positive power source. By connecting either one of the source and the drain of the P-channel field effect transistor to the input line of the level adjusting means and grounding the other, the power supply of the level adjusting means is cut off and the input of this is input. P-channel field-effect transistor that turns on when power is cut off even if the terminal section forms a non-linear circuit Therefore, the input line shunted eliminates flows from the signal source (stereo amplifier or the like) of the current to the non-linear circuit, it is possible to prevent the occurrence of distortion. Although the P-channel field effect transistor Q is discretely provided in one embodiment, it may be provided inside the level adjusting means 17, and naturally, it is integrated into an IC together with the level adjusting means 17. You may try. EFFECTS OF THE INVENTION As described above, according to the present invention, a signal level adjusting unit configured by a non-linear circuit, a control voltage control unit for controlling the signal level adjusting unit, and a reference power supply for driving the control voltage control unit. And a P-channel field effect transistor.
The power source input terminal of the reference power source section of the level adjusting means and the gate line of the P-channel field effect transistor are connected to the positive power source, and either the source or the drain of the P-channel field effect transistor is connected to the level adjusting means. Since it is connected to the input line of and the other is grounded, the level adjustment means that has an input terminal section that forms a non-linear circuit when the power is cut off has the effect of distortion on the signal source (stereo amplifier, etc.). It is possible to provide an input level adjusting circuit which has a simple structure and which is inexpensive and which does not deteriorate the S / N ratio.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a connection diagram of main parts of an input level adjusting circuit in an embodiment of the present invention, and FIG. 2 is an attenuation-volume control voltage characteristic diagram for explaining the operation of FIG. FIG. 3 is a connection diagram of main parts of a conventional input level adjusting circuit. 15 …… input terminal, R3, R4, R5 …… resistor, C1 …… capacitor, Q …… P junction field effect transistor, 16
…… Signal input terminal section, 17 …… Level adjustment means, 18 …… Signal level adjustment section, 19 …… Control voltage control section, 20…
... Reference power supply unit, 21 ... Signal output terminal, 22 ... Power supply input terminal,
23: Ground terminal, 24: Volume control voltage input terminal.

Claims (1)

(57) [Claims] A level adjusting unit including a signal level adjusting unit configured by a non-linear circuit, a control voltage controlling unit for controlling the signal level adjusting unit, and a reference power supply unit for driving the control voltage controlling unit, A P-channel field-effect transistor, a power source input terminal of the reference power source section of the level adjusting means and a gate line of the P-channel field-effect transistor are connected to a positive power source, and the source of the P-channel field-effect transistor. An input level adjusting circuit, characterized in that either one of the drains is connected to the input line of the level adjusting means and the other is grounded.