JPH0416286Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Industrial application field This invention is an ALC used in tape recorders.
The present invention relates to a recording amplifier equipped with an automatic level control (automatic level control) circuit, and particularly relates to a recording amplifier in which the ALC circuit is prevented from being adversely affected by a recording bias circuit.

(b) Prior Art A recording amplifier equipped with an ALC circuit that keeps the level of the signal to be recorded produced at the output end of the amplifier constant regardless of the magnitude of the input signal level was disclosed in Japanese Patent Laid-Open No. 58
It is well known as seen in Japanese Patent No.-178615. Additionally, technology for supplying an AC bias signal to the magnetic head when recording onto magnetic tape was introduced in June 1973.
As can be seen on page 154 of ``Tape Recorders and Their Effective Use'' published on the 20th, this is well known. Figure 2 shows a recording amplifier including the ALC circuit and recording bias circuit.
2 is a recording amplifier; 2 is an ALC circuit for detecting the output signal level of the recording amplifier 1 and attenuating the input signal of the recording amplifier 2; 3 is an ALC circuit for performing recording according to the output signal of the recording amplifier 1; magnetic head, 4
5 is a recording bias circuit for supplying an AC bias signal to the magnetic head 3, and 5 is a trap circuit for preventing the output signal of the recording bias circuit 4 from leaking to the recording amplifier 1 side.

Thus, the input signal applied to the input terminal 6 is amplified by the recording amplifier 1 and delivered to the output terminal of the recording amplifier 1, but when the output signal level of the recording amplifier 1 reaches a predetermined value, the ALC circuit 2 operates to attenuate the input signal, so the output signal level of the recording amplifier 1 is maintained at a constant value. Then, the output signal of the recording amplifier 1 and the output AC bias signal of the recording bias circuit 4 are applied to the magnetic head 3 to accomplish recording. At this time, in order to prevent the output AC bias signal of the recording bias circuit 4 from leaking to the output side of the recording amplifier 1, a trap circuit 5 whose center frequency is the frequency of the output AC bias signal is provided. This prevents malfunctions caused by bias signals. Usually, the AC bias signal frequency is 50 to 200 KHz, which is 4 to 5 times the signal frequency, so the center frequency of the trap circuit 5 is also
Set to 50~200KHz.

(c) Problems to be solved by the invention Under normal operating conditions, the output impedance of the recording amplifier 1 is sufficiently small, so the trap circuit 5 is fully effective and the recording bias circuit 4
The output signal is cut off, and there is no adverse effect on the ALC circuit 2. However, during a transient period such as immediately after the power is turned on, the output impedance of the recording amplifier 1 does not become sufficiently small, the output signal of the recording bias circuit 4 leaks, the smoothing capacitor of the ALC circuit 2 is abnormally charged, and the recording amplifier 1 The disadvantage is that the input signal is attenuated more than necessary.

(d) Means for solving the problem The present invention was developed in view of the above points, and has the feature that it is equipped with a stop section for stopping the operation of the comparison section of the ALC circuit for a certain period of time after power is turned on. Features.

(e) Effect According to the present invention, the ALC circuit is made inactive when the power is turned on, and the ALC circuit is returned to the operating state after a predetermined period of time has elapsed, so that the smoothing capacitor of the ALC circuit is not charged abnormally. .

(F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
7 is a recording amplifier that amplifies the input signal applied to the input terminal 8; 9 is a comparator section 12 consisting of a pair of differentially connected transistors 10 and 11; and a rectifier transistor 1 that rectifies the output signal of the comparator section 12 .
3, a smoothing circuit 16 consisting of a resistor 14 and a capacitor 15 that smooth the output signal of the rectifying transistor 13, and an ALC circuit consisting of a drive transistor 17 and an attenuation transistor 18 that operate according to the output signal of the smoothing circuit 16 ; 19 is a ripple filter consisting of a transistor 20 whose collector-emitter path is inserted into the power supply line, and a capacitor 21 connected to the base of the transistor 20; 22 is a pair of differentially connected transistors 2;
3 and 24, the base of one transistor 23 is connected to one end of the capacitor 21, the base of the other transistor 24 is connected to the reference potential point, and 25 is the stop circuit of the other transistor 24 of the stop circuit 22. This is a diode-connected transistor that turns on in response to the collector current and turns on the comparatively 12 current source transistors 26 of the ALC circuit 9 .

When the power (+V _cc ) is turned on, the base of the other transistor 24 of the stop circuit 22 becomes the reference voltage almost immediately. Ripple filter 19
Due to the presence of the capacitor 21, the base voltage of one transistor 23 is low, and the short circuit 22 is
One transistor 23 is turned on and the other transistor 24 is turned off. As a result, diode-connected transistor 25 is turned off and the ALC
The current source transistor 26 of the comparator 12 of the circuit 9 is turned off, and the comparator 12 does not start operating. If the comparator 12 does not operate, the rectifier transistor 13 will not conduct, and the smoothing capacitor 15 will not be charged, so the attenuation transistor 18 will not perform the attenuation operation and no ALC operation will be performed.

As time passes, charging of the capacitor 21 of the ripple filter 19 progresses, and the terminal voltage of the capacitor 21 changes to the other transistor 24 of the short circuit 22.
When the reference voltage applied to the base of the transistor 23 is exceeded, one transistor 23 of the short circuit 22 is turned off;
The other transistor 24 is turned on, and the diode-connected transistor 25 is also turned on. Therefore, the current source transistor 26 of the comparator 12 becomes normally conductive, and the comparison operation of the comparator 12 is started.

A reference voltage is applied to the base of the other transistor 11 of the comparator 12 , and one transistor 1
Since the output signal of the recording amplifier 7 is applied to the base of the recording amplifier 7, one transistor 10 is turned off and the other transistor 11 is turned on while the output signal of the recording amplifier 7 is smaller than the reference voltage.
No ALC operation is performed. When the output signal of the recording amplifier 7 becomes large and exceeds the reference voltage, one of the transistors 10 is turned on, and a collector current flows according to the difference voltage. Therefore, the rectifier transistor 13 becomes conductive, and the capacitor 1 of the smoothing circuit 16
5 charging is performed, the drive transistor 17 and the attenuation transistor 18 are turned on, and an ALC operation is performed. Therefore, the output signal of the recording amplifier 7 is kept constant, and a recorded signal of a constant level is generated at the output terminal 27.

The short circuit 22 stops generating an output signal after a predetermined period of time has elapsed after the power is turned on, and this time is determined by the charging time constant of the capacitor 21 of the ripple filter 19 and the other transistor 24 of the short circuit 22.
The reference voltage applied to the base of the Therefore, malfunction of the ALC circuit 9 can be prevented by setting the time of the short circuit 22 so that the output signal is generated after the recording amplifier 7 is stabilized and its output impedance becomes sufficiently small. .

(g) Effects of the invention As described above, according to the invention, it is possible to prevent the malfunction in which the capacitor of the smoothing circuit is charged by the AC bias signal immediately after power is turned on, and the ALC circuit is activated. Furthermore, according to the present invention, the above malfunction can be prevented simply by providing a short circuit, and the output signal of the short circuit becomes the bias current of the current source transistor of the comparison section, so it is easy to integrate the circuit and the configuration is simple. Therefore, an inexpensive recording amplifier can be provided.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit block diagram showing a conventional recording amplifier with an ALC circuit. Explanation of main drawing numbers, 7...Recording amplifier, 9 ...
ALC circuit, 12 ...comparison section, 16 ...smoothing circuit,
19...Ripple filter, 22 ...Short circuit.

Claims (1)

[Scope of utility model registration request] In a recording amplifier equipped with an ALC circuit, a comparison section detecting an output signal level of the recording amplifier;
a smoothing section that rectifies and smoothes the output signal of the comparison section; an attenuation section that attenuates the input signal of the recording amplifier according to the output signal of the smoothing section; and a ripple filter that includes a capacitor and is inserted into a power supply line. a constant current transistor whose base is connected to a predetermined potential point and which supplies a predetermined operating current from its collector to the comparison section; and whose emitter is commonly connected to the emitter of the constant current transistor, and whose base connects the ripple filter. and a stop transistor connected to a constituting capacitor, and turns off the constant current transistor by turning on the stop transistor for a certain period of time after power is turned on,
A recording amplifier characterized in that the operation of the comparison section is stopped to prevent a capacitor of the smoothing section from being charged by a recording bias circuit.