JPS5919495B2 - Shock noise prevention circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit that prevents shock noise caused by sudden signal level fluctuations during difficult training or sudden voltage fluctuations when the power is turned on and off.

In general receivers and amplifiers, shock noise may occur due to sudden voltage changes caused by turning the power on or off.

For example, in a bias feedback direct-coupled amplifier, when the power supply voltage drops from a predetermined value to a certain value or rises from zero to a certain value, the transistor in the previous stage suddenly turns from off to on, and the transistor in the next stage suddenly turns on. This causes the direct-coupled transistor to turn from off to on, so it operates as if a large signal had been applied even though there is no signal, and it is output as shock noise. Furthermore, in an FM receiver, when the receiver goes out of tune, the output of the frequency discrimination circuit changes rapidly, and this change is amplified as shock noise. An object of the present invention is to prevent the above-mentioned shock noise from being output.

Examples will be described in detail below. FIG. 1 is a circuit diagram of a main part of an embodiment of the present invention, where ANT is an antenna, RFA
is a high frequency amplifier, MIX is a frequency converter, OSC is a local oscillator, IFA is an intermediate frequency amplifier, DET is a detector such as a frequency discriminator, AFA is an audio frequency amplifier, PA is a power amplifier, and SP is a speaker. shows the configuration of a well-known receiver.

Also, CC is a control circuit, PW is a power supply circuit, Q1 to Q4 are transistors, R1 to R12 are resistors, C1 to C5 are capacitors, D1 to D4 are diodes, SWI and SW2 are switches, and T is a power transformer. Switch SWI, SW2
has terminals a-c, d-f and a movable contact g, which operate in conjunction with each other, and when the receiver is in operation, the movable contact g contacts the terminal e, and the movable terminal contacts the terminal e. Contacts terminal c, voltage is stepped down from the commercial power supply by transformer T, rectified by diodes D3 and D4, and voltage regulator diode D2
This stabilizes the DC voltage by keeping the base potential of the transistor Q4 constant.
The DC voltage output from No. 4 is supplied as a power supply voltage to each part.

The bias is selected so that the NPN transistor Q1 is in an off state in a steady state.
Therefore, transistors Q2 and Q3 are also in an off state.

Therefore, the control circuit CC has no influence on the reception state. If you detune it from the tuned state by dial operation etc.,
The output signal level of the frequency discriminator DET changes suddenly,
This change component passes through the capacitor C1 to the transistor Q.
1 and turns transistor Q1 from off to on.

That is, a sudden change in the detection output level is detected. When the transistor Q1 is turned on, its collector potential decreases, and the PNP transistor Q2 whose base is connected to its collector is turned on as a base current flows, and the collector potential of the transistor Q2 rises. As a result, the transistor Q3 whose base is connected to the collector of the transistor Q2 via the resistor RlO and the diode D1 is turned on from off, and the power amplifier PA
The input of is grounded by transistor Q3 as a bypass means. Therefore, the signal is not transmitted to the power amplifier, speaker SP, etc. in the stage after the audio frequency amplifier AFA.
The shock noise that occurs during detuning is not output from the speaker SP.

FIG. 2 is an explanatory diagram of the configuration of an example of switches SWl and SW2. When movable contacts G and h are in the solid line positions shown in the figure, the power is off, and movable contact h is in contact between terminals A and b. Therefore, the base of transistor Q2 is grounded via resistor R6.

When movable contacts G and h are moved to the dotted line position to turn on the power, the moment movable contact g comes into contact between terminals E and f,
The movable contact g is still in contact between terminals A and b, and therefore, in the process of rising the output voltage of power supply circuit PW, the base of transistor Q2 is grounded via resistor R6, and transistors Q2 and Q3 are grounded. is on, so the power amplifier P
The input of A will be grounded by transistor Q3. When the movable contact g comes into almost complete contact with the terminals E and f, the movable contact h separates from the terminal a.
Transistor Q2 is turned off and transistor Q3
is also turned off.

Also, when turning the power off from on, contrary to the above, before movable contact g leaves terminal e, movable contact h contacts terminal a and turns on transistor Q2, which turns on transistor Q3. , the input of power amplifier PA is grounded.

Therefore, shock noise caused by voltage fluctuations when the power supply is turned on and off is no longer output from the speaker SP, as in the case of detuning. As explained above, the present invention detects the sudden change in the output level of a detector such as a frequency discriminator, which changes greatly between tuning and detuning, and
Occurs when the output signal of the detector DET or the output signal of the audio frequency amplifier AFA that amplifies it is detuned from the tuned state by controlling it with a bypass means so that it is not transmitted to a subsequent stage such as a power amplifier or speaker. This can prevent the output of shock noise.

Also, regarding the shock noise that occurs when the power is turned on and off, a switch SW2 that is linked to the power switch SWl is provided in the control circuit, and the switch SW2 is turned off after the power switch SWl is turned on, and turned on before the power switch SWl is turned off. The switch SW2 is configured in this way, and when the switch SW2 is on, signal transmission is blocked by bypass means such as the transistor Q3, thereby reliably preventing unpleasant shock noise. It should be noted that the present invention is not limited to the above-described embodiments, but can be modified in various ways.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a main part of an embodiment of the present invention, and FIG. 2 is an explanatory diagram of a configuration of an example of a switch. ANT is antenna, RFA is high frequency amplifier, MIX is frequency converter, 0SC is local oscillator, IFA is intermediate frequency amplifier, DET is detector, AFA is audio frequency amplifier, SP
is the speaker, CC is the control circuit, PW is the power supply circuit, Q1~
Q4 is a transistor, D1-D4 are diodes, C1-
C5 is a capacitor, R1 to Rl2 are resistors, SWl, SW
2 is a switch.

Claims (1)

[Claims] 1 detection means for detecting a sudden change in the detection output level between the tuned state and the detuned state of the receiver; bypass means for blocking the transmission of the signal to the subsequent stage of the receiver by the output of the detection means; The control circuit is provided with a switch that operates in conjunction with a power switch to turn off after the power switch is turned on and turns on before the power switch is turned off, A shock noise prevention circuit characterized in that the circuit is configured to forcibly prevent the signal from being transmitted to a subsequent stage of the receiver by the bypass means.