JPH05122549A - Preamplifier for television receiver - Google Patents

Abstract

PURPOSE:To mixedly amplify television broadcasting signals from two near and distant transmitting points without jamming a down side adjacent channel even at a television receiver installed at any place. CONSTITUTION:Variable frequency band pass filters 18 and 32 are provided before and behind amplifying means 24 and 26 to amplify television broadcasting signals supplied to an input terminal 10, and these filters 18 and 32 are equipped with band pass width to pass the television broadcasting signals of one channel and vary this pass band. A mixer 36 mixes the television broadcasting signals of one channel passing through the band pass filters 18 and 32 and the television broadcasting signals supplied to an input terminal 12. A variable frequency audio trap 84 connected to the mixer 36 varies a trap frequency so as to attenuate the audio signals of the clown side channel adjacent to one channel passing through the filters 18 and 32. A variable resistor 110 changes the pass bands of the filters 18 and 32 and the trap frequency of the trap 84 while interlocking them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preamplifier provided in the front stage of a television receiver, and more particularly to a preamplifier for eliminating the influence of adjacent channels.

Generally, a television receiver may receive television broadcasts from two transmitting stations located at different positions. In this case, each television broadcast signal received by the antenna directed to each transmitting station is mixed by the mixer, and this mixed output is supplied to the television receiver.
In this case, the received television broadcast signals are less likely to cause mutual interference due to the improvement of the directivity of the antenna, etc., but there may be a difference in the reception level due to the distance difference from each transmitting station to the receiving point. However, it is necessary to eliminate this level difference in advance.

[0003]

2. Description of the Related Art Conventionally, in order to eliminate such a level difference, the following preamplifier may be used. That is, a received signal at an antenna for receiving a television broadcast signal from a low-level distant transmitting station (this may include a high-level television signal at a short distance) is transmitted to the low level. Level to a bandpass filter configured to pass only television broadcast signals from distant transmission stations to eliminate television broadcast signals from short-distance transmission stations and from low-level distant transmission stations. The output of the band-pass filter containing only the television broadcast signal is amplified by the amplifying means, and the signal level of the television broadcast signal from the low level distant transmitting station is increased. This is mixed with a television broadcast signal from a short-distance transmitting station received by another antenna by a mixer and supplied to a television receiver.

However, when the long-distance television broadcast signal amplified and raised in level and the short-distance television broadcast signal have a relationship of adjacent channels, in the above method, for example, a short-distance lower channel. The voice signal of
The beat signal may occur in the video signal of the far upper channel. In such a case, the voice signal of the lower channel is attenuated by a voice trap to prevent a beat failure.

[0005]

However, the frequencies of the low-level long-distance television broadcast signal and the high-level short-distance television broadcast signal differ depending on the area where the television receiver is installed. When manufacturing, inventory-controlling, and shipping multiple band-pass filters and voice traps for each receiver installation area, you must replace them with band-pass filters and voice traps that match the shipping destination. There was a problem that was troublesome.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a preamplifier that amplifies low level television broadcast signals to a high level and eliminates adjacent channel interference in a television receiver installed anywhere. ..

[0007]

In order to achieve the above-mentioned object, a first television broadcast signal is supplied respectively.
And a second input terminal, a first amplification means for amplifying the television broadcast signal supplied to the first input terminal, and at least one of the first and second stages of the first amplification means. A variable frequency band pass filter having a pass band width that allows a channel television broadcast signal to pass and configured to change the pass band, and a single channel television broadcast signal that has passed through the variable frequency band pass filter. And a mixing means for mixing the television broadcasting signal supplied to the second input terminal, and a lower side of the television broadcasting signal of one channel which is connected to the mixing means and has passed through the variable frequency bandpass filter. A variable frequency voice trap configured to change the trap frequency so as to attenuate the voice signal of the adjacent channel, and And adjusting means for changing in conjunction with the trap frequency of the pass band and the frequency variable sound trap frequency variable band pass filter, it is to provided.

The variable frequency voice trap may be provided with an attenuation amount varying means for varying the attenuation amount at the trap frequency, or the first amplifying means may have a gain adjusting means. Also, a second amplifying means may be provided between the second input terminal and the mixing means,
The second amplifying means may be provided with a gain adjusting means.

Further, the variable frequency bandpass filter may be provided on each of the input side and the output side of the first amplifying means, and the variable frequency bandpass filter includes at least two parallel circuits including the first voltage variable capacitance element. The resonant circuit can be configured to be coupled by the second voltage variable capacitance element.

Further, the trap frequency is selected between the first input terminal and the first amplifying means so as to attenuate the video signal of the upper channel of the television signal of one channel that has passed through the variable frequency bandpass filter. It is also possible to provide a video trap, and switch means for switching the video trap between a state in which it is connected between the first input terminal and the first amplifying means and a state in which it is not connected,
It can be provided.

[0011]

According to the present invention, the television broadcasting signal of one channel corresponding to the pass band of the frequency variable band pass filter among the television broadcasting signals supplied to the first input terminal is transmitted by the first amplifying means. It is amplified and supplied to the mixing means. Each of the television broadcast signals supplied to the second input terminal is also supplied to this mixing means, and the audio signal of the lower adjacent channel of the television signal that has passed through the variable frequency bandpass filter among them is also supplied to this mixing means. Is attenuated by the variable frequency voice trap. The pass band of the variable frequency band pass filter and the trap frequency of the variable frequency voice trap are changed by the adjusting means in conjunction with each other. Therefore, the preamplifier of the present invention can be used as a preamplifier for a television receiver installed in any area.

Further, if the attenuation variable means is provided in the frequency variable voice trap, the attenuation at the trap frequency can be adjusted to an arbitrary value. Furthermore, when the gain adjusting means is provided in the first amplifying means, the gain in the first amplifying means of the television broadcast signal of one channel passing through the variable frequency bandpass filter can be arbitrarily adjusted.
Further, when the second amplifying means is provided between the second input terminal and the mixing means, it is possible to amplify the television broadcast signal supplied to the second input terminal. When the gain adjusting means is provided in the amplifying means, the gain of the television broadcast signal supplied to the second input terminal can be adjusted.

Further, the frequency variable bandpass filter may be a first filter.
When provided on both the input and output sides of the amplifying means, only the television broadcast signal of one channel in the pass band of the frequency variable band pass filter is supplied to the mixing means more reliably. Furthermore, the frequency variable band pass filter is connected to at least two parallel resonant circuits including the first voltage variable capacitance element,
With the configuration in which the voltage variable capacitance elements are coupled, the pass band width and the pass loss can be made constant even if the pass band frequency is changed, depending on how the capacitance of the second voltage variable capacitance element is changed.

Further, a trap frequency is provided between the first input terminal and the first amplifying means so as to attenuate the video signal of the upper channel of the television signal of one channel that has passed through the variable frequency bandpass filter. The provision of the selected video trap makes it possible to eliminate the interference caused by the high level short-distance television broadcast signal supplied from the first input terminal, which is present in the upper channel. Further, when the switch means is provided and the video trap is switched between the state in which it is connected between the first input terminal and the first amplifying means and the state in which it is not connected, the above-mentioned upper channel exists. It can handle both cases and cases where it is not.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment shown in FIG. 1 is used in Japan, for example.
A low level television broadcast signal of one of the two channels is received by an antenna (not shown), and input terminal 1
0 of the high-level 4 to 12 channels transmitted from a short-distance transmitting station, and the input terminal 10
Some television broadcast signals other than the television broadcast signal supplied to are received by an antenna (not shown),
It is supplied to the input terminal 12. It should be noted that the television broadcast signals of the vertically adjacent channels of the television broadcast signal supplied to the input terminal 10 are always supplied to the input terminal 12, and the television broadcast signals of these vertically adjacent channels are It is assumed that an output is also generated at the input terminal 10 because of the high level.

Each television broadcast signal supplied to the input terminal 10 is supplied to the frequency variable band pass filter 18 via the capacitor 14 and the switch 16. The frequency variable band pass filter 18 has a pass band width of 6 MHz according to the channel of the television broadcast signal, and its pass band, that is, the lower cut-off frequency and the upper cut-off frequency can be varied. Of the television broadcast signals supplied to the input terminal 10, the pass band is selected so as to pass the low level television broadcast signal from the distant transmitting station.
The output of the frequency variable bandpass filter 18 is supplied to the high frequency amplifier 24 through the capacitor 22, amplified, and the gain is adjusted by the gain adjusting circuit 26, and then further amplified by the high frequency amplifier 28, and the capacitor 30 is supplied. It is supplied to the frequency variable band pass filter 32 via the. The frequency variable bandpass filter 32 has the same structure as the frequency variable bandpass filter 18. The output of the variable frequency bandpass filter 32 (amplified low level television broadcast signal from a distant transmission station) is supplied to a mixer 36 via a capacitor 34.

Further, each television broadcast signal supplied to the input terminal 12 is supplied to a band pass filter 38 having a pass band for allowing all of them to pass therethrough, and after removing unnecessary signals, a gain adjusting circuit 40 is supplied. It is supplied to the high frequency amplifier 42 via. The output of the high frequency amplifier 42 is supplied to the mixer 36 via the band pass filter 44.
In the mixer 36, the television broadcast signal from the distant transmission station and the television broadcast signal from the short-distance transmission station are mixed, and the mixed output is amplified by the high frequency amplifier 46 and then passed through the band pass filter 48. Are supplied to the output terminal 50. The output terminal 50 is connected to a television receiver (not shown).

The variable frequency bandpass filter 18 is provided with a series circuit of a resonance inductor 52 and a DC blocking capacitor 54, a variable capacitance diode 56 in parallel, and a capacitor 53 for correcting the variable capacitance diode 50 connected in parallel. To form a parallel resonance circuit, and a similar parallel resonance circuit is formed by using a resonance inductor 52a and a DC blocking capacitor 54.
a, a variable-capacitance diode 56a, and a capacitor 53a, which are coupled by a series circuit of a variable-capacitance diode 58 and a capacitor 60, and connected to the switch 16 and the capacitor 22 in the preceding stage by coupling inductors 62 and 64. .. Variable capacitance diode 56,
The capacitance of 58 is changed by changing the value of the voltage supplied via the coupling inductor 62, the high frequency blocking coil 66, and the voltage adjusting resistor 68. Similarly, the capacitance of the variable capacitance diode 56 a is changed by the coupling inductor 62 a. 64,
It can be changed by changing the value of the voltage supplied via the high frequency blocking coil 70 and the voltage adjusting resistor 72. Similarly, the frequency variable bandpass filter 32 also includes high frequency blocking coils 66a and 70a and a voltage adjusting resistor 68.
It is changed by the voltage supplied via a and 72a. Reference numeral 74 denotes a high frequency blocking coil for flowing the current flowing through the variable capacitance diode 58 to the reference potential point,
Reference numerals 76, 76a, 78, 78a denote bypass capacitors.

Although the pass band is changed by changing the capacitance of the variable capacitance diodes 56 and 56a, the pass band width is kept constant over the entire use band (4 to 12 channels), and the insertion loss is also the use band. Since the capacitance is constant over the entire region, the capacitance of the variable capacitance diode 58 that acts as a coupling capacitance is also variable capacitance diodes 56 and 5.
Along with 6a, the impedance is changed so as to exhibit a constant impedance over the entire used band. That is, when only the capacitor having a constant capacitance is used without using the variable capacitance diode 58, the impedance of the capacitor is reduced when the pass band is increased, and the two parallel resonant circuits are in a tightly coupled state. In order to prevent this, the capacitance of the variable capacitance diode 58 is reduced as the pass band is increased. The capacitor 60 is for correcting the coupling capacitance.

The mixer 36 includes a core coil 80 for mixing two inputs and a transformer core coil 82 for impedance conversion.
And a variable frequency audio trap 84 is provided between the connection point between the core coil 80 and the transformer core coil 82 and the reference potential point. This trap 8
4 is a resonance inductor 86 and a DC blocking capacitor 8
8. A parallel resonance circuit including a variable capacitance diode 90 is a capacitive coupling type in which a variable resistor 92 and a coupling capacitor 94 are connected in series. The capacitance of the variable capacitance diode 90 can be changed by changing the voltage supplied via the high frequency blocking coil 96 and the voltage adjusting resistor 98, and by changing the capacitance of the variable capacitance diode 90, The trap frequency of the frequency variable voice trap 84 changes, but the trap frequency always changes so as to attenuate the voice signal of the channel adjacent to the lower side of the channel passing through the frequency variable bandpass filters 18 and 32.

The amount of attenuation can be changed by operating the variable resistor 92. Since the frequency variable audio trap 84 is provided at the connection point between the core coil 80 and the transformer core coil 82 of the mixer 36, the television broadcast signals of the lower adjacent channels are mixed from both the input terminals 10 and 12. Even if the audio signal is supplied to the device 36, the audio signal can be surely attenuated, and the impedance at the connection point is the impedance Zo at the input sides a and b of the core coil 80 and the output side c of the transformer core coil 82. Since it is just 1/2, if a trap is provided at this connection point, it is possible to obtain a trap characteristic with excellent steepness.

A frequency variable image trap 96 is provided between the switch 16 and the reference potential point. The video trap 96 is also of a capacitive coupling type in which a coupling capacitor 104 is connected to a parallel resonant circuit including a resonance inductor 98, a DC blocking capacitor 100, and a variable capacitance diode 102. This frequency variable video trap 9
Switch 6 is switched by switch 16 between a state in which it is connected between the input side of frequency variable bandpass filter 18 and capacitor 14 and a state in which it is not connected. The capacitance of the variable capacitance diode 102 is equal to the high frequency blocking coil 10
6. It can be changed by changing the voltage supplied through the voltage adjusting resistor 108, and the trap frequency of the frequency variable video trap 96 changes according to the capacitance change of the variable capacitance diode 102.
The trap frequency greatly attenuates the video signal of the upper adjacent channel of one channel passing through the frequency variable bandpass filters 18 and 32 at that time.

Each variable capacitance diode 56, 56
a, 58, 90, 102 for simultaneously changing the capacitance of each voltage adjusting resistor 68, 68a, 72, 72a, 9
Reference numerals 8 and 108 are connected to a variable resistor 110 for varying the voltage. Therefore, by operating the variable resistor 110, the pass bands of the frequency variable band pass filters 18, 32 are changed, and the voice trap frequency of the frequency variable voice trap 84 is then set by the frequency variable band pass filters 18, 32. The trap frequency of the variable frequency video trap 96 is changed to the audio frequency of the lower adjacent channel of the television broadcasting signal of one channel passing therethrough.
At that time, the video frequency of one channel of the television broadcast signal passing through the variable frequency bandpass filters 18 and 32 changes to the video frequency of the upper adjacent channel.

FIG. 2 shows a change in the attenuation amount at the trap frequency by operating the variable resistor 92 in the frequency variable voice trap 84 in this embodiment.
The curves indicated by reference characters A and B show the frequency-gain characteristics at the output terminal 50 with respect to the signal supplied from the input terminal 10, and the curve indicated by reference sign A operates the variable resistor 92 to reduce the amount of attenuation. The curve indicated by the symbol B shows the state in which the variable resistor 92 is operated to increase the attenuation amount. Similarly, the curves indicated by the symbols C and D show the frequency-gain characteristics at the output terminal 50 with respect to the signal supplied from the input terminal 12, and the curve indicated by the symbol C shows the state in which the attenuation amount is reduced, and the curve indicated by the symbol D. The curve shown shows the state where the amount of attenuation is increased. From FIG. 2, it is shown that the frequency variable voice trap 84 shows a large attenuation at the trap frequency with respect to the signal input from either the input terminal 10 or 12, and the attenuation amount at this trap frequency is variable resistor. It is clear that adjustment can be made by operating 92.

In FIG. 3, the variable resistor 110 is operated to change the pass bands of the frequency variable band pass filters 18 and 32, the voice trap frequency of the frequency variable audio trap 84, and the video trap frequency of the frequency variable video trap 96. Gain characteristics vs. frequency characteristics E, F and G of the output signal of the output terminal 50 with respect to the signal from the input terminal 10 and gain characteristics vs. frequency characteristic H of the output signal of the output terminal 50 with respect to the signal from the input terminal 12, I and J show the gain vs. frequency characteristic E by changing the pass bands of the frequency variable band pass filters 18 and 32, the voice trap frequency of the frequency variable audio trap 84 and the video trap frequency of the frequency variable video trap 96. Gain vs. frequency characteristic H when obtained
When the gain vs. frequency characteristic F is also obtained,
When the gain-frequency characteristic I is obtained and the gain-frequency characteristic G is also obtained, the gain-frequency characteristic J is obtained. As is clear from this figure, even if the pass bands of the variable frequency band pass filters 18 and 32 are changed, their bandwidths do not change and the pass loss is constant. The voice trap frequency is always the variable frequency band pass filter 1 at that time.
It is the audio frequency of the lower adjacent channel of one channel passing through 8 and 32, and the video trap frequency is always the variable frequency band pass filter 1 at that time.
It is the video frequency of the upper adjacent channel of one channel passing through 8 and 32.

FIG. 4 shows a pre-amplifier for receiving a television broadcasting signal in a foreign country having a bandwidth of 170 MHz to 230 MHz of 7 MHz, which is operated by the variable resistor 92 when the present invention is carried out. The change of the attenuation amount of 84 is shown. In this case, the switch 16 is opened, and the variable frequency image trap 96 is deactivated. A1 and B1 are input terminals 10
The frequency vs. gain characteristics at the output terminal 50 with respect to the signal supplied from the above are shown. The curve indicated by the symbol A1 shows the state in which the attenuation amount is reduced by operating the variable resistor 92, and the curve indicated by the symbol B1 is The state where the attenuation amount is increased by operating the variable resistor 92 is shown. Similarly, the code C
The curves indicated by 1 and D1 show the frequency-gain characteristics at the output terminal 50 with respect to the signal supplied from the input terminal 12, and the curve indicated by the symbol C1 shows the state in which the variable resistor 92 is operated to reduce the amount of attenuation. A curve indicated by reference numeral D1 shows a state in which the variable resistor 92 is operated to increase the amount of attenuation.

FIG. 5 shows a preamplifier for receiving a television broadcast signal in a foreign country having a bandwidth of 170 MHz to 230 MHz of 7 MHz, and operating the variable resistor 110 in the case of implementing the present invention to change the frequency. Band pass filter 1
In the state where the pass bands of 8 and 32, the audio trap frequency of the frequency variable audio trap 84 and the video trap frequency of the frequency variable video trap 96 are changed, the attenuation amount of the audio trap frequency is increased to increase the frequency variable video trap 96.
Gain vs. frequency characteristics E1, F of the output signal of the output terminal 50 with respect to the signal from the input terminal 10 when the switch is deactivated.
1, G1 and the gain vs. frequency characteristics H1, I1, and J1 of the output signal of the output terminal 50 with respect to the signal from the input terminal 12 are shown. It should be noted that the attenuation amount of the audio trap frequency is increased while the pass bands of the frequency variable band pass filters 18 and 32, the audio trap frequency of the frequency variable audio trap 84 and the video trap frequency of the frequency variable video trap 96 are changed, Variable frequency video trap 9
When gain 6 vs. frequency characteristic E1 is obtained with 6 not operated, gain vs. frequency characteristic H1 is obtained.
When 1 is obtained, the gain-frequency characteristic I1 is obtained, and when gain-frequency characteristic G1 is obtained, the gain-frequency characteristic J1 is obtained.

In the above embodiment, the variable frequency image trap 96 is provided, but this can be eliminated in some cases. Further, in the above embodiment, the variable frequency bandpass filters 18 and 32 are provided on both the input side and the output side of the amplifiers 24 and 28, but either one may be removed. In addition, although the amplifiers 24, 28, 42 and 46 are provided,
In some cases these can be removed.

[0029]

As described above, according to the present invention, the pass band of the frequency variable band pass filter and the voice trap frequency of the frequency variable voice trap are changed in conjunction with each other, and at the same time, the frequency variable band pass filter is always passed. Since the audio frequency of the lower adjacent channel of one of the channels is changed to the trap frequency of the frequency variable audio trap, what position is the installation position of the television receiver used with this preamplifier? Even if there is, it is possible to prevent beat failure due to the lower adjacent channel. Also,
This variable frequency voice trap can change the amount of attenuation at the trap frequency arbitrarily, so that it reliably attenuates the high level TV adjacent channel of the lower adjacent channel from the short distance transmission station at the installation position. Can be made Further, the television broadcast signal from the distant transmitting station is amplified by the first amplifying means, and since the amplifying means has the gain varying means,
It is possible to amplify a television broadcast signal from a distant place to an optimum level. Further, the television broadcast signal from the short-distance transmitting station is also amplified by the second amplifying means having the gain adjusting means, so that the level can be adjusted to an appropriate level. Further, since the variable frequency band pass filter is provided at both the input and output of the first amplifying means, it is possible to more reliably extract and amplify only the low level television broadcast signal from a long distance. Furthermore, since the frequency variable bandpass filter has a configuration in which at least two parallel resonant circuits including the first voltage variable capacitance element are coupled by the second voltage variable capacitance element, the capacitance of the second voltage variable capacitance element is reduced. By changing it, the coupling amount of both parallel resonant circuits can be made constant at all times, the bandwidth of the frequency variable bandpass filter can be made constant, and the pass loss can be made constant. Further, since the frequency variable video trap in which the video frequency of the upper adjacent channel of one channel that has passed through the variable frequency bandpass filter is always used as the trap frequency, the beat failure due to the upper adjacent channel can be prevented. Further, since the frequency variable video trap is switched between the operating state and the non-operating state by the switch means, the frequency variable video trap can be stopped when it is unnecessary.

[Brief description of drawings]

FIG. 1 is a circuit diagram of one embodiment of a preamplifier according to the present invention.

FIG. 2 is a diagram showing changes in the amount of attenuation at a trap frequency in the case of a television broadcast signal having 4 to 12 channels and a channel width of 6 MHz in the embodiment.

FIG. 3 is a diagram showing a pass band of a frequency variable band pass filter, a voice trap frequency of a frequency variable voice trap, and a video of a frequency variable video trap in the case of a television broadcasting signal having 4 to 12 channels and a channel width of 6 MHz in the embodiment. It is a figure which shows the state which changed the trap frequency.

[FIG. 4] In the same embodiment, 170 MHz to 230 M
It is the figure which showed the change of the amount of attenuation in the trap frequency in the case of the overseas television broadcast signal whose channel width is 7 MHz at 7 MHz.

[FIG. 5] In the embodiment, 170 MHz to 230 M
It shows a state in which the pass band of the frequency variable band pass filter, the voice trap frequency of the frequency variable voice trap, and the video trap frequency of the frequency variable video trap are changed in the case of an overseas television broadcasting signal with a channel width of 7 MHz at 7 Hz. It is a figure.

[Explanation of symbols]

 10 First Input Terminal 12 Second Input Terminal 18 Frequency Variable Bandpass Filter 24 First Amplifying Means 26 Gain Adjusting Means 28 First Amplifying Means 32 Frequency Variable Bandpass Filters 36 Mixer 40 Gain Adjusting Means 42 Second Amplification means 84 Frequency variable audio trap 96 Frequency variable video trap

Claims (8)

[Claims] 1. A first and a second input terminal to which a television broadcast signal is respectively supplied, a first amplifying means for amplifying the television broadcast signal supplied to the first input terminal, and the first amplifying means. A variable frequency band pass filter having a pass band width for passing a television broadcast signal of one channel and configured to change the pass band, the frequency variable band pass filter being provided in at least one of the front stage and the rear stage of the amplifying means, Mixing means for mixing the television broadcast signal of one channel that has passed through the bandpass filter and the television broadcast signal supplied to the second input terminal, and the mixing means connected to this mixing means and passing through the frequency variable bandpass filter. The trap frequency can be changed so as to attenuate the audio signal of the adjacent lower channel of the television broadcast signal of one channel. And a variable frequency voice trap configured so that it is possible to change the pass band of the variable frequency band pass filter and the trap frequency of the variable frequency voice trap in association with each other. amplifier. 2. The preamplifier for a television receiver according to claim 1, wherein the variable frequency voice trap comprises attenuation amount varying means for varying the attenuation amount at the trap frequency. Preamplifier for receiver. 3. The preamplifier for a television receiver according to claim 1 or 2, wherein the first amplifying means has a gain adjusting means. 4. The preamplifier for a television receiver according to claim 1, 2 or 3, wherein a second amplifying means is provided between the second input terminal and the mixing means, and the second amplifying means is provided. A preamplifier for a television receiver, characterized in that the means is provided with a gain adjusting means. 5. The preamplifier for a television receiver according to claim 1, 2, 3 or 4, wherein the frequency variable bandpass filter is provided on each of an input side and an output side of the first amplifying means. Preamplifier for television receivers characterized by. 6. The preamplifier for a television receiver according to claim 1, 2, 3, 4, or 5, wherein the frequency variable bandpass filter includes at least two parallel resonance elements including a first voltage variable capacitance element. A preamplifier for a television receiver, characterized in that the circuits are connected by a second voltage variable capacitance element. 7. The preamplifier for a television receiver according to claim 1, 2, 3, 4, 5 or 6, wherein the variable frequency band is provided between the first input terminal and the first amplifying means. A preamplifier for a television receiver, comprising a video trap whose trap frequency is selected so as to attenuate the video signal of the upper channel of the television signal of one channel that has passed through the pass filter. 8. The preamplifier for a television receiver according to claim 7, wherein the video trap is connected between the first input terminal and the first amplifying means, and is not connected. A preamplifier for a television receiver, characterized in that a switch means for switching to is provided.