JP2954795B2 - Signal processing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording / reproducing apparatus such as a video tape recorder (VTR), and more particularly to a signal processing circuit for performing insert editing and after-recording of an area-divided audio PCM signal. .

[0002]

2. Description of the Related Art In a conventional VTR, cost reduction,
In order to simplify the assembly, a circuit such as a switching transistor is used, so that the magnetic head is used for both recording and reproduction, and the number of magnetic heads used and the number of channels of the rotary transformer are reduced.

FIG. 8 is a circuit diagram showing an example of a signal processing circuit in such a VTR, wherein 1a is a recording signal input terminal, 1b is a reproduction signal output terminal, 2a and 2b are power terminals, and 3 'is a recording terminal. Amplifier, 4 is a regenerative amplifier, 5a, 5b,
5c is a capacitor, 6a, 6b and 6c are resistors, 7a and 7
b is a switching transistor, 8 is a rotary transformer, and 9 is a magnetic head for recording and reproduction.

In FIG. 8, during recording, the power supply terminal 2a
Supplies a power supply voltage of + Vcc (for example, +5 V) to the recording amplifier 3 ', and the recording signal input from the input terminal 1a is amplified. At this time, no power supply voltage is supplied from the power supply terminal 2b. The power supply voltage is supplied to the base of the switching transistor 7b via the resistor 6b, and is turned on to fix the reproduction-side terminal of the rotary transformer 8 to the reference potential. As a result, the recording current amplified by the recording amplifier 3 'flows through the capacitors 5a and 5c, the fixed coil of the rotary transformer 8, and the switching transistor 7b, and the recording signal is transmitted from the fixed coil of the rotary transformer 8 to the rotating coil. The data is transmitted and supplied to the recording / reproducing magnetic head 9.

At this time, the switching transistor 7a
+ V from the power supply terminal 2a via the resistor 6c
Although a power supply voltage of cc is applied, since no power supply voltage is supplied from the power supply terminal 2b, no voltage is applied to the base of the switching transistor 7a, and the switching transistor 7a remains off. As a result, no distortion occurs in the recording current.

At the time of reproduction, a power supply voltage of + Vcc is supplied to the reproduction amplifier 4 from the power supply terminal 2b. However, the power supply voltage of + Vcc is not supplied from the power supply terminal 2a. The power supply voltage from the power supply terminal 2a is supplied to the base of the switching transistor 7a via the resistor 6a, and is turned on to fix the recording-side terminal of the rotary transformer 8 to the reference potential. Thus, the signal reproduced from the recording / reproducing magnetic head 9 is supplied to the reproducing amplifier 4 via the rotary transformer 8 and the capacitor 5b, amplified by the reproducing amplifier 4, and then outputted from the output terminal 1b.

As described above, in the conventional VTR, the magnetic head can be used for recording and reproduction.

[0008]

However, in the above-mentioned conventional VTR, the switching transistor 7a
Is the collector + emitter saturation voltage (Vce.sat) at the time of recording and + Vcc at the time of reproduction, which is superimposed on the reproduction signal and controls the power supply. And regenerative amplifier 4
Have different input biases. As a result, a transient occurs in the recording current and the reproduction voltage when the recording / reproduction mode is switched. In this transient period, for example, in the case of a reproduced signal, the bias is different from the steady state, so that even if the reproduced signal is demodulated, the original signal cannot be obtained.

On the other hand, VTRs are required to have advanced editing functions such as insert editing and after recording for area-divided audio PCM signals or index signals. In such a case, switching from the playback mode to the recording mode and switching from the recording mode to the playback mode are always performed. However, in order to realize these editing functions by the above-described conventional configuration, the above-described transient is considered. Then, for example, countermeasures such as not reproducing information before and after the editing place have been required. However, according to this, there is a problem that information signals such as video signals and audio signals are lost only for the time required for mode switching of the recording / playback magnetic head (that is, the period during which the transient is settled).

In addition, in the VTR having the conventional configuration, at the time of reproduction, the input capacity of the reproduction amplifier 4 is apparently increased by the switching transistor 7b which is turned off, thereby narrowing the reproduction frequency band. There was a problem. For this reason, for example, the HDTV VT
When a VTR that needs to record and reproduce a wideband signal such as R or digital VTR, or when information immediately after editing is required to perform more accurate editing, the magnetic head is used for recording and reproduction. For example, they had to be separately equipped. However, in such a case, a large number of magnetic heads must be mounted on the rotating cylinder, which causes various problems such as an increase in cost and a complicated assembly of the cylinder.

An object of the present invention is to provide a signal processing circuit which solves such a problem, reduces the influence of a transient caused by switching the recording / reproducing mode by using the magnetic head for recording / reproducing, and realizes a highly accurate editing function. Is to do.

[0012]

According to a first aspect of the present invention, there is provided a rotary transformer having a rotary side coil connected to a recording / reproducing magnetic head and a fixed side coil of the rotary transformer. A recording amplifier connected to one end via a first capacitor and performing an amplifying operation of the recording signal only at the time of recording by a control signal; and an emitter connected to one end of the fixed side coil of the rotary transformer and the first capacitor. And one of the collector and the other are connected to each other, and the other electrode is set to the reference potential. The first switching transistor which is turned on at the time of reproduction by the control signal is connected to the other end of the fixed side coil of the rotary transformer. A first reproducing amplifier connected through a second capacitor and operated at all times, and a predetermined DC voltage is applied by the control signal at the time of recording. A second regeneration amplifier that amplifies the output signal of the first regeneration amplifier during reproduction, and an emitter or a collector connected to a connection point between the other end of the fixed side coil of the rotary transformer and the second capacitor. A second switching transistor that is connected to one of the electrodes and the other electrode is set to a reference potential, and that is turned on at the time of recording by the control signal, and that the first switching transistor is turned off when the first switching transistor is off. The base potential of the first switching transistor is set to a value lower than the collector potential and the emitter potential.

Further, the second invention has at least a first channel in which one end of a rotating coil is set to a reference potential, and a second channel in which one end of a fixed coil is set to a reference potential. A rotary transformer, a recording amplifier connected to a fixed coil of the first channel of the rotary transformer, and amplifying a recording signal only at the time of recording by a control signal, and a rotating side of the first channel of the rotary transformer. A first capacitor connected to the other end of the coil and transmitting a recording current to the magnetic head; an emitter at a connection point between the first capacitor and one end of the magnetic head;
One of the electrodes of the collector is connected, the other electrode is set to the reference potential, the first switching transistor which is turned on at the time of reproduction by the control signal, and the other end of the magnetic head via the second capacitor A first regenerative amplifier that is connected and constantly operates to amplify, and the control signal amplifies an output signal of the first regenerative amplifier during reproduction to output the second phase of the rotary transformer with a positive-phase output and a negative-phase output. And a third reproduction amplifier connected to a fixed-side coil of the second channel of the rotary transformer for driving the second channel of the rotary transformer and stopping the driving of the second channel of the rotary transformer during recording. One of an emitter electrode and a collector electrode is connected to a connection point between the amplifier and the other end of the magnetic head and the second capacitor, the other electrode is set to a reference potential, and the control signal A second switching transistor that is turned on at the time of recording, and when the first switching transistor is in an off state, the control signal causes the base potential of the first switching transistor to be lower than the collector potential and the emitter potential. Is set.

Further, according to a third aspect of the present invention, in each of the above-mentioned inventions, a step-up transformer is provided between the second capacitor and the first regenerative amplifier, and the step-up transformer has a large number of turns. The first regenerative amplifier side and the coil having a small number of turns are referred to as the second capacitor side.

Still further, a fourth aspect of the present invention provides the above-mentioned first and second aspects.
In each of the second and third inventions, the base potential of the second switching transistor is set lower than a collector potential and an emitter potential by the control signal, and
Are turned off.

[0016]

According to the first and second aspects of the present invention, the DC potential on the fixed side of the rotary transformer and the DC potential of the magnetic head are different from the reference potential set for the electrodes of the first and second switching transistors by the first and second switching transistors. Alternatively, it is determined by the collector-emitter saturation voltage of the second switching transistor. That is, the amount of change in the potential superimposed on the reproduction signal due to the switching of the recording and reproduction modes is the difference between the collector and emitter saturation voltages of the respective switching transistors. This is achieved by using the same type of transistors as these switching transistors. Can be smaller. Further, since the first regenerative amplifier is always in the operating state and its input bias does not change due to the mode switching, the transient of the reproduced signal amplified and output by the first regenerative amplifier at the time of the mode switching. This can reduce the size of the information, thereby reducing the time period during which reproduction information is lost.

Also, the leakage of the recording signal amplified by the first reproducing amplifier during recording is caused by the second
By controlling the output of the regenerative amplifier, the signal does not appear in the output of the second regenerative amplifier, so that malfunction of other circuits does not occur.

Further, at the time of recording, the base potential of the first switching transistor is made lower than the potentials of the collector and the emitter, so that the first switching transistor can be always kept in the off state, and no distortion occurs in the recording current.

According to the third aspect of the present invention, since the step-up transformer is provided immediately before the input side of the first regenerating amplifier, the effect of the increase in capacity due to the addition of the switching transistor is reduced, and The frequency band can be widened.

In each of the above inventions, the base voltage is set to a low value in the off state of the switching transistor. Therefore, the collector-base voltage can be increased to reduce the collector output capacitance in the off state. Thus, the increase in capacitance due to the addition of the switching transistor can be reduced, and the reproduction frequency band can be widened.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a signal processing circuit according to the present invention, wherein 2c is an input terminal of a control signal, 3 is a recording amplifier, 4a is a reproducing amplifier which always performs an amplifying operation, 4b
Is a reproduction amplifier whose output is set to a predetermined DC voltage at the time of recording by a control signal. Reference numeral 10 denotes an inverter, and portions corresponding to those in FIG.

In FIG. 1, during recording, the input terminal 2c
, A control signal of a negative voltage (for example, −5 V) is applied.
This control signal is supplied to the recording amplifier 3 and the reproducing amplifier 4b. The recording amplifier 3 performs an operation of amplifying the recording signal from the input terminal 1a, and the reproducing amplifier 4b performs an operation of outputting a predetermined DC voltage. The control signal is supplied to the base of the switching transistor 7a via the resistor 6a, and the potential thereof is set to, for example, -5 V to turn off the switching transistor 7a. Further, the polarity of the control signal is inverted by the inverter 10, and the control signal is, for example, a + 5V control signal via the resistor 6b and the switching transistor 7b.
Supplied to the base. As a result, the switching transistor 7b is turned on.

The recording current amplified by the recording amplifier 3 flows through the capacitor 5a, the fixed coil of the rotary transformer 8, and the switching transistor 7b.
The reproduction side terminal of the rotary transformer 8 is set to the reference potential. As a result, the recording signal is transmitted from the fixed side coil of the rotary transformer 8 to the rotating side coil, and the recording current also flows through the recording / reproducing magnetic head 9 to perform recording.

At the time of reproduction, a control signal of a positive voltage (for example, +5 V) is applied from the input terminal 2c. This control signal is supplied to the recording amplifier 3 and the reproducing amplifier 4b. The recording amplifier 3 stops the amplification operation, and the reproducing amplifier 4b performs the operation of amplifying the output signal of the reproducing amplifier 4a and outputting it from the output terminal 1b. The control signal of the positive voltage is supplied to the resistor 6
The switching transistor 7a is supplied to the base of the switching transistor 7a via a, and the switching transistor 7a is turned on by setting the base potential to be positive. Further, the polarity of the control signal is inverted by the inverter 10 and a negative voltage (for example, -5
V), which is supplied to the base of the switching transistor 7b via the resistor 6b to make the base potential negative and turn off the switching transistor 7b.

The signal reproduced by the recording / reproducing magnetic head 9 is supplied to the rotary transformer 8 and the capacitor 5.
b, the signal is supplied to the regenerative amplifier 4a which is always in operation, amplified there, further amplified by the regenerative amplifier 4b, and output to the output terminal 1b.

FIG. 2 shows an example of the waveform of each electrode of the switching transistor 7a. As shown in FIG.
The base potential of the switching transistor 7a is always lower than both the emitter and collector electrodes during recording. Therefore, during recording, the switching transistor 7a is always off, and the collector voltage, that is, the recording current is distorted. Will not give. Further, when switching from the reproduction mode to the recording mode, the charge remaining in the base can be rapidly reduced, so that the switching operation is performed quickly and reliably.

FIG. 3 shows a transient when the mode is switched from the recording mode to the reproducing mode. FIG. 3A shows the envelope of the waveform of the collector of the switching transistor 7b, and FIG. 3B shows the reproducing amplifier 4a. 5C shows an envelope of the output waveform of the regenerative amplifier 4b. In addition, these FIG.
In (a), (b) and (c), the scale in the amplitude direction is changed for easy understanding, and the dotted lines indicate the bias potentials in the respective waveforms.

As shown in FIG. 3A, at the time of recording, there is leakage of a recording signal with the collector / emitter saturation voltage of the switching transistor 7b as a central bias in the reproducing amplifier 4a. The signal reproduced from 9 is transmitted with the center bias as the collector-emitter saturation voltage of the switching transistor 7a. On the other hand, in the output signal of the reproduction amplifier 4a, as shown in FIG. 3B, a transient appears when the mode is switched from the recording mode to the reproduction mode because the DC component of the signal is removed by the capacitor 5b. As described above, the reproduction amplifier 4b can obtain a constant output voltage during recording as described above.
As shown in FIG. 3C, a signal starting with the transient shown in FIG. Here, T is the time required for this transient to settle, and since the original signal cannot be reproduced at this time T, this portion immediately after switching from the recording mode to the reproduction mode is removed as described above. In order to reduce the loss of information due to the removal of the part, it is necessary to shorten the transient settling time T. This is achieved by reducing the bias fluctuation superimposed on the signal shown in FIG.

In this embodiment, the bias fluctuation is caused by the difference in the collector-emitter saturation voltage between the switching transistors 7a and 7b because there is only the fixed side coil of the rotary transformer 8 between the switching transistors 7a and 7b. However, by using transistors having the same characteristics as the switching transistors 7a and 7b, this difference can be extremely reduced. Thus, in this embodiment, the bias fluctuation has a very small value compared to the prior art. Therefore,
Even if a transient occurs, it becomes very small, and the time T required to settle the transient can be shortened, and the loss of information immediately after switching from the recording mode to the reproduction mode can be reduced.

When the reproducing amplifier 4b is of a feedback type, for example, the leakage signal at the time of recording is normally saturated by amplification, and the resulting distortion deteriorates the transient. By fixing the output level to a predetermined voltage at the time of recording, the feedback of the distortion component can be prevented, and the transient can be stably reduced.

Next, the frequency band at the time of reproduction will be described. The reproduction frequency band is determined by the inductance component viewed from the input side of the reproduction amplifier 4a and the capacitance existing in the signal path to the input terminal. Also, this capacitance value is
It is determined by the input capacitance of the regeneration amplifier 4a, the collector output capacitance of the switching transistor 7b, and the stray capacitance of the signal path. Here, the relationship between the collector output capacitance of the switching transistor 7b and their base-collector voltages is as shown in FIG.

As shown in FIG. 4, by increasing the base-collector voltage, the collector output capacitance can be reduced, and the reproduction frequency band can be expanded. In this embodiment, when the switching transistor 7b is turned off, the base potential is set to, for example, -5 V without changing the collector potential, thereby increasing the base-collector voltage and deteriorating the transient. , The collector output capacity can be reduced, that is, the reproduction frequency band can be widened.

FIG. 5 is a circuit diagram showing another embodiment of the signal processing circuit according to the present invention. Reference numeral 11 denotes a step-up transformer, and portions corresponding to those in FIG. 1 are denoted by the same reference numerals.

In this embodiment, as shown in FIG. 5, the configuration other than that the step-up transformer 11 is provided between the capacitor 5b and the regenerating amplifier 4a, and the basic operation is shown in FIG. Since this embodiment is the same as the embodiment described above, a detailed description of these points will be omitted. The step-up transformer 11 has a winding ratio of 1: n (n> 1), and a winding having a large number of turns is on the regenerative amplifier 4a side.

Since the step-up transformer 11 is provided between the switching transistor 7b and the regenerating amplifier 4a, the capacitance value viewed from the input side of the regenerating amplifier 4a is such that the capacitance value of the signal path and the switching transistor is 1 / n ² . Since it can be seen, the size is reduced as compared with the case where the step-up transformer 11 is not provided. Also, when the level of the signal that is stepped up by the step-up transformer 11 and input to the regenerating amplifier 4a is the same as the case where the step-up transformer 11 is not provided, that is, the same as the case of the embodiment shown in FIG. The inductance seen from the input side of the regenerative amplifier 4a is the same as that of the embodiment shown in FIG. Therefore,
In this embodiment, the reproduction frequency band is widened by an amount corresponding to the decrease in the capacitance value without causing a signal loss, and a wider band signal can be handled.

FIG. 6 is a circuit diagram showing still another embodiment of the signal processing circuit according to the present invention, wherein 4c and 4d are amplifiers, 8a is a recording channel of a rotary transformer, and 8b
Denotes a reproduction channel, and 12 denotes a slip ring. Parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

This embodiment is suitable for recording and reproducing signals of a wider band, and is a case where a reproducing amplifier is mounted on a rotary cylinder.

In FIG. 6, amplifiers 4a and 4c, capacitors 5a and 5b, resistors 6a and 6b, switching transistors 7a and 7b, and an inverter 10 are mounted on a rotary cylinder (not shown) together with a recording / reproducing magnetic head 9. Therefore, the rotary transformer has a recording channel 8a for transmitting a recording signal from the recording amplifier 3 to the rotary cylinder side, and a reproducing channel 8b for transmitting a reproduction signal from the rotary cylinder side to the amplifier 4d which is always operating. Are provided. Further, a control signal input from the input terminal 2c is transmitted to the rotary cylinder side by the slip ring 12.

In this configuration, at the time of recording, the signal input from the input terminal 1a is amplified by the recording amplifier 3 and then transmitted to the rotary cylinder via the recording channel 8a of the rotary transformer. Further, a negative voltage control signal (for example, −5 V) input from the input terminal 2 c is also transmitted to the rotary cylinder via the slip ring 12. On the rotating cylinder side, this control signal turns off the switching transistor 7a and turns on the switching transistor 7b as in the previous embodiment. Thereby, at the time of recording, the recording signal current transmitted via the recording channel 8a of the rotary transformer flows through the capacitor 5a, the recording / reproducing magnetic head 9, and the switching transistor 7b.

Here, the amplifier 4c has a positive-phase output and a negative-phase output.
Control is performed so that the differential components of these outputs become zero.
Therefore, no signal is transmitted on the reproduction channel 8b.

At the time of reproduction, a positive voltage control signal (for example, +5 V) input from the input terminal 2c is applied to the slip ring 1
2 to the rotating cylinder side. On the rotating cylinder side, this control signal turns on the switching transistor 7a and turns off the switching transistor 7b as in the previous embodiment. Accordingly, the signal reproduced by the recording / reproducing magnetic head 9 is
The signal b is supplied to the regenerative amplifier 4a via b, where it is amplified and supplied to the regenerative amplifier 4c. The reproduction amplifier 4c drives the reproduction channel 8b of the rotary transformer with the positive and negative phase outputs, and transmits a signal to the reproduction amplifier 4d via the reproduction channel. This regenerative amplifier 4
d amplifies the supplied signal and outputs it from the output terminal 1b.

In this embodiment as well, the transient associated with switching from the reproduction mode to the recording mode is the same as in the previous embodiment, and therefore it is clear that the same effects as in the previous embodiment can be obtained. The same applies to the transient accompanying the switching from the recording mode to the reproduction mode. In this embodiment, however, the reproduction channel 8b of the rotary transformer is driven by the positive and negative phase outputs of the reproduction amplifier 4c and the reproduction amplifier 4d is driven. Since the signal is transmitted, the output of the reproduction amplifier 4c does not necessarily have to be a fixed voltage at the time of recording, and the output of the positive-phase and negative-phase differential components of the reproduction amplifier 4c is blocked. For example, the transistor at the output stage is turned off. And the signal output from the output terminal 1b does not fluctuate in the center bias at the time of recording and at the time of reproduction, and the DC component can be removed by the signal processing at the subsequent stage. For example, the degree of freedom in design is increased. Further, in this embodiment, since the reproducing amplifier 4a is mounted on the rotary cylinder, the stray capacitance existing from the recording / reproducing magnetic head 9 to the reproducing amplifier 4a can be reduced, and a wider band signal can be reproduced. Becomes

FIG. 7 is a circuit diagram showing still another embodiment of the signal processing circuit according to the present invention, in which parts corresponding to those in the preceding drawings are denoted by the same reference numerals. In this embodiment, as shown in the drawing, a step-up transformer 11 is provided between the capacitor 5b and the amplifier 4a as in the embodiment shown in FIG. It is designed to be able to cope with the reproduction of a wider band signal.

In this embodiment, it is apparent that the same effects as those of the previous embodiments can be obtained. The stray capacitance can be reduced by mounting the regenerative amplifier 4a on the rotating cylinder, and the switching by the step-up transformer 11 can be achieved. The effect of the collector capacitance of the transistor 7b can be reduced, and a very wide band signal can be reproduced.

In the above embodiment, the respective base potentials are set to -5 V in order to turn off the switching transistors 7a and 7b. However, the present invention is not limited to this. May be set to a lower voltage than the collector and the emitter. Further, in the embodiment shown in FIGS. 6 and 7, the slip ring is used to transmit the control signal onto the rotary cylinder. However, the present invention is not limited to this, and the control signal is transmitted by other means such as a rotary transformer. Alternatively, the control signal may be generated again on the rotating cylinder.

[0046]

As described above, according to the present invention,
The transient caused by the mode switching can be reduced, and no distortion occurs in the recording signal, and the signal can be reproduced over a wide band. Therefore,
Advanced editing functions such as after recording of area-divided signals can be supported, and extremely excellent recording and reproduction can be performed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a signal processing circuit according to the present invention.

FIG. 2 is a diagram showing waveforms of respective electrodes of a switching transistor in FIG.

FIG. 3 is a waveform diagram showing a transient of a reproduction signal.

FIG. 4 is a diagram showing characteristics of a collector output capacitance of a switching transistor.

FIG. 5 is a circuit diagram showing another embodiment of the signal processing circuit according to the present invention.

FIG. 6 is a circuit diagram showing still another embodiment of the signal processing circuit according to the present invention.

FIG. 7 is a circuit diagram showing still another embodiment of the signal processing circuit according to the present invention.

FIG. 8 is a circuit diagram illustrating an example of a conventional signal processing circuit.

DESCRIPTION OF SYMBOLS 1a Input terminal of recording signal 1b Output terminal of reproduction signal 2c Input terminal of control signal 3 Recording amplifier 4a-4d Reproduction amplifier 5a, 5b Capacitor 6a, 6b Resistance 7a, 7b Switching transistor 8 Rotary transformer 8a Recording channel 8b Reproduction channel 9 Magnetic head for both recording and reproduction 10 Inverter 11 Step-up transformer 12 Slip ring

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takashi Furihata 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Pref. 58) Field surveyed (Int.Cl. ⁶ , DB name) G11B 5/09

Claims (4)

(57) [Claims] 1. A magnetic recording / reproducing apparatus having a recording / reproducing magnetic head mounted on a rotary cylinder, comprising: a rotary transformer having a rotary side coil connected to the magnetic head; and a first end provided at one end of a fixed side coil of the rotary transformer. A recording amplifier which is connected via a capacitor of the type described above, and which amplifies the recording signal only during recording by a control signal; and a connection point between one end of the fixed side coil of the rotary transformer and the first capacitor, which is either an emitter or a collector. One of the electrodes is connected, the other electrode is set to a reference potential, and the first switching transistor is turned on at the time of reproduction by the control signal. The second switching transistor is connected to the other end of the fixed side coil of the rotary transformer. A first regenerative amplifier connected via a capacitor and constantly performing an amplifying operation; A second regenerative amplifier that outputs a voltage and amplifies the output signal of the first regenerative amplifier during reproduction, and an emitter and a collector at a connection point between the other end of the fixed side coil of the rotary transformer and the second capacitor. And a second switching transistor that is connected to one of the electrodes and the other electrode is set to a reference potential, and is turned on during recording by the control signal. A signal processing circuit, wherein a base potential of the first switching transistor is set to a value lower than a collector potential and an emitter potential by a signal. 2. A magnetic recording / reproducing apparatus in which a recording / reproducing magnetic head is mounted on a rotary cylinder, a first channel having one end of a rotating coil set to a reference potential, and one end of a fixed side coil set to a reference potential. A rotary transformer having at least a set second channel, a recording amplifier connected to a fixed coil of the first channel of the rotary transformer, and performing a recording signal amplifying operation only during recording by a control signal; A first capacitor connected to the other end of the rotary side coil of the first channel of the rotary transformer and transmitting a recording current to the magnetic head; and a connection point between the first capacitor and one end of the magnetic head. Either the emitter or the collector is connected, the other electrode is set to a reference potential, and a first signal which is turned on during reproduction by the control signal. An etching transistor, a first reproduction amplifier connected to the other end of the magnetic head via a second capacitor, and always performing an amplification operation, and an output signal of the first reproduction amplifier during reproduction by the control signal. A second reproducing amplifier for amplifying and driving the second channel of the rotary transformer with the positive and negative phase outputs and stopping the driving of the second channel of the rotary transformer during recording; A third regenerative amplifier connected to a fixed coil of a second channel of the transformer, and one of an emitter electrode and a collector electrode connected to a connection point between the other end of the magnetic head and the second capacitor; The other electrode is set to the reference potential, and the control signal
A second switching transistor that is turned on at the time of recording; and when the first switching transistor is off, the control signal sets the base potential of the first switching transistor to a value lower than the collector potential and the emitter potential. A signal processing circuit. 3. The step-up transformer according to claim 1, wherein a step-up transformer is provided between the second capacitor and the first regeneration amplifier, and the step-up transformer has a large number of turns. A signal processing circuit, wherein a coil having a small number of turns on the amplifier side is used as the second capacitor side. 4. The control signal according to claim 1, wherein a base potential of the second switching transistor is set lower than a collector potential and an emitter potential by the control signal, and the second switching transistor is turned off. A signal processing circuit.