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JP2001285988

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Notice
This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
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DESCRIPTION JP2001285988
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
low band expansion circuit. Specifically, low-pass band boost processing of a load such as a
speaker connected to a power amplifier is performed to extend the low-pass of the speaker, and
an input signal is injected to the positive phase input terminal of the band boost circuit. By
connecting a low band pass filter to the feedback path, the number of terminal pins can be
reduced when the low band expansion circuit is integrated.
[0002]
2. Description of the Related Art Among electronic circuits that handle audio signals (audio
signals) as input signals, a speaker serving as a load is connected to a power amplifier provided
at an output stage so that the audio signals are reproduced as audible sound. It has become.
[0003]
The reproduction frequency characteristic of this speaker is known to have a limit of around 100
Hz, as shown by a curve La, as the low-frequency reproduction area as shown in FIG.
A low band extension circuit is known as one that can extend the low band reproduction limit
further to the low band side and extend the reproduction characteristics to 60 to 70 Hz.
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[0004]
FIG. 5 shows a conventional example of the low band expansion circuit 10. The input signal
supplied to the terminal 12 is supplied to the first amplifier 14 and is output as a signal of gain
"1". This input signal is further supplied to the positive phase input terminal (+) of the first
positive phase amplifier 16 constituted by an operational amplifier (op amp). The output of the
first positive phase amplifier 16 is supplied to the speaker 19 which is a load via the power
amplifier 18 to reproduce an audio signal.
[0005]
The signal output from the first amplifier 14 is supplied to adjusting means 20 for adjusting the
band boost characteristic. The adjustment means 20 is composed of four resistors 22a to 22d
connected in series and a changeover switch 24. The adjustment means 20 adjusts the amount of
band boost and supplies it to the current limiter 26. The current limiter 26 is provided to limit
the signal of the input stage when the input level of the power amplifier 18 described above
becomes equal to or higher than a specified level.
[0006]
The output of the current limiter 26 is received by the resistor 28 and converted to a voltage
level, and the output is supplied to the band boost circuit 31 through the buffer amplifier 30
whose gain is adjusted to "1".
[0007]
The band boost circuit 31 is configured of a reverse phase amplifier 32 configured by an
operational amplifier, and a low pass band pass filter 34 connected to the negative feedback path
thereof.
As the low pass band pass filter 34, for example, a band pass filter having a bridged T-type
configuration as shown in the figure can be used, so the low pass band pass filter 34 comprises A
resistor 38 connected to both ends of the series circuit and a resistor 40 for signal injection are
provided, and the output of the buffer amplifier 30 is the reverse phase input terminal (-) of the
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reverse phase amplifier 32 through the resistor 40. Injected into the
[0008]
As this low-pass band pass filter 34 has a damping characteristic (deheat characteristic) as is well
known, connecting this to the negative feedback path of the negative phase amplifier 32 results
in a band boost characteristic of low band as a whole circuit. . This band boost characteristic is a
single peak characteristic, and the adjustment of the filter constant is performed so that the peak
is 60 to 70 Hz as shown in the curve Lb of FIG. .
[0009]
The signal with the band boost characteristic is supplied to the negative phase input terminal of
the positive phase amplifier 16 through the resistor 42. Since it is the positive phase amplifier
16, the feedback resistor 44 is connected between the negative phase input terminal and the
output terminal. Since the positive phase amplifier 16 functions as an adding circuit, the output
terminal of the positive phase amplifier 16 has an expanded characteristic of a frequency band of
100 Hz or less, particularly 60 to 70 Hz in this example, of the input signal. Therefore, the lower
limit frequency of the speaker 19 can be further lowered to the lower range.
[0010]
By the way, when integrating the low frequency band expansion circuit 10 configured in this way
(IC integration), the portion surrounded by the dashed line is integrated into an IC. The low band
pass filter 34 is difficult to be integrated because it has a capacitor.
[0011]
Therefore, since the low pass band pass filter 34 is externally attached, at least three terminal
pins 50 a to 50 c must be prepared for the low pass band pass filter 34 in this configuration.
This is because a total of three external terminals for the capacitors 36a and 36b and one
external terminal for injecting the input signal to the resistor 40 are prepared as external pins.
When the low-range expansion circuit 10 is used for stereo, it is necessary to prepare a total of
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six terminal pins.
[0012]
An increase in the number of pins of the IC circuit leads to an increase in manufacturing cost and
also limits the circuit elements that can be integrated.
[0013]
Therefore, the present invention solves such a conventional problem, and proposes a lowfrequency expansion circuit which can suppress the number of terminal pins to the minimum
number of pins even when integrated into an IC. is there.
[0014]
SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, in the low band
expansion circuit according to the present invention described in claim 1, a first positive phase
amplifier in which an input signal is supplied to a positive phase input terminal. A band boost
circuit composed of a second positive phase amplifier whose input signal is supplied to its
positive phase input terminal and a low pass band pass filter connected to its negative feedback
path; And a subtracting circuit that subtracts the output of the band boost circuit, and the output
of the subtracting circuit is supplied to the negative phase input terminal of the first positive
phase amplifier, and the load connected to the first positive phase amplifier It is characterized in
that the low-pass characteristic of is extended.
[0015]
In the present invention, the injection of the input signal to the low pass band pass filter is
stopped.
Therefore, an input signal is injected into the positive phase input terminal of the second positive
phase amplifier constituting the band boost circuit, and a subtraction circuit is provided which
subtracts the input signal from the band boost output.
[0016]
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By doing this, the number of external terminals of the externally mounted low-pass band pass
filter becomes only two terminals for a pair of capacitors, and the number of terminal pins can be
reduced by one.
The two-channel configuration for stereo can reduce two-terminal pins.
As a result, it is possible to suppress an increase in cost when the low band expansion circuit is
integrated into an IC.
[0017]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of a low
band expansion circuit according to the present invention will be described in detail with
reference to the drawings. Also in this case, a speaker is used as a load, and the case where it is
applied to the output stage is shown.
[0018]
FIG. 1 shows an embodiment of a low band expansion circuit 10 according to the present
invention, which has a first amplifier 14 and a first positive phase amplifier 16 as in the prior art,
and an input signal supplied to a terminal 12 Is supplied to the first amplifier 14 and output as a
signal of gain "1". This input signal is further supplied to the positive phase input terminal (+) of
the first positive phase amplifier 16 constituted by an operational amplifier (op amp). The output
of the first positive phase amplifier 16 is supplied to the speaker 19 which is a load via the
power amplifier 18 to reproduce an audio signal.
[0019]
The signal output from the first amplifier 14 is supplied to adjusting means 20 for adjusting the
band boost characteristic. As in the prior art, this adjusting means 20 is composed of four
resistors 22a to 22d and a changeover switch 24 in this example connected in series, and after
the band boost amount is adjusted, the current limiter 26 is used. Supplied. The current limiter
26 is for limiting the signal of the input stage when the input level of the power amplifier 18
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described above becomes equal to or higher than a specified level.
[0020]
The output of the current limiter 26 is converted into a voltage signal by the resistor 28, and the
output is supplied to the band boost circuit 60 through the buffer amplifier 30 whose gain is
adjusted to one.
[0021]
The band boost circuit 60 is composed of a second positive phase amplifier 62 composed of an
operational amplifier and a low pass bandpass filter 34 connected to its negative feedback path.
Then, a signal is injected into the positive phase input terminal of the second positive phase
amplifier 62.
[0022]
As the low pass band pass filter 34, a band pass filter with a bridged T configuration can be used
as described above. The low pass band pass filter 34 comprises a pair of capacitors 36a and 36b
connected in series, a resistor 38 connected across the series circuit, and a resistor 40 for signal
injection. The other end is grounded. The other end of the resistor 40 can be grounded because
the input signal is injected into the positive phase input terminal of the second positive phase
amplifier 62.
[0023]
The low pass band pass filter 34 has an attenuation characteristic (deheat characteristic) as is
well known, but by connecting this to the negative feedback path of the second positive phase
amplifier 62, the band boost characteristic of the low pass as a whole circuit It becomes. This
band boost characteristic is a single-peak characteristic as shown in FIG. 2B, and adjustment of
the filter constant is performed so that the peak thereof becomes 60 to 70 Hz. FIG. 6A shows the
level (dB) of the input signal, which is a flat characteristic.
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[0024]
The signal subjected to the band boost characteristic is subtracted from the input signal by a
subtraction circuit (inversion addition circuit) 66. An operational amplifier is also used for
subtraction circuit 66, and the input signal output from buffer amplifier 30 is divided by a pair of
resistors 64a and 64b, and the divided input signal is supplied to the positive phase input
terminal of subtraction circuit 66. Ru. On the other hand, the band boost output outputted from
the band boost circuit 60 is supplied to the negative phase input terminal of the subtraction
circuit 66 through the resistor 68. A resistor 69 for the negative feedback path is connected to
the subtraction circuit 66.
[0025]
The subtractor 66 subtracts the band boost output from the input signal to obtain an output
signal as shown in FIG. 2C. The output signal is supplied to the negative phase input terminal of
the first positive phase amplifier 16 to perform addition processing with the input signal.
Therefore, as shown in FIG. 2D, the first positive-phase amplifier 16 obtains a frequency
characteristic in which the low region is emphasized. When the output signal whose low range is
emphasized in this way is supplied to the speaker 19, the frequency characteristic of the speaker
19 is a frequency band 60 which is lower than the reproduction limit frequency band (around
100 Hz) as shown by the curve Lb in FIG. The reproduction frequency band can be extended to
70 Hz.
[0026]
Here, as described above, the second positive phase amplifier 62 is prepared as the band boost
circuit 60, and the signal output from the buffer amplifier 30 is injected to the positive phase
input terminal side thereof. One end of the resistor 40 provided in the band pass filter 34 can be
grounded.
[0027]
As a result, although it is necessary to externally connect the low band pass filter 34 when the
low band expansion circuit 10 is integrated as in the prior art, even in this case, only the
terminals related to the pair of capacitors 36a and 36b can As long as it is sufficient, the number
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of external terminal pins is only two.
Therefore, even when the L and R2 channels are integrated into an IC, only four terminal pins are
required.
[0028]
Therefore, the number of terminal pins can be reduced as compared with the conventional case,
so that the IC manufacturing cost can be suppressed, and a circuit element capable of realizing
more functions can be incorporated.
[0029]
The band boost processing is performed so that, even when the band boost characteristic is
changed by the adjustment circuit 20, the gains in frequency bands other than the frequency
band to be band boosted do not change.
[0030]
FIG. 3 shows another embodiment of the present invention. In this embodiment, the number of
amplifiers is devised to be the same as in the prior art.
In FIG. 1, the output of the current limiter 26 is converted to a voltage level by a resistor 28, and
then supplied to a band boost circuit 60 via an amplifier 30 of gain "1".
[0031]
In the case of FIG. 1, since the pair of resistors 64a and 64b are connected to the positive phase
input terminal of the subtraction circuit 66, these resistors 64a and 64b can be substituted by
the resistor 28 described above.
Therefore, as shown in FIG. 3, the limiter output can be directly supplied to the second positive
phase amplifier 62 and the pair of resistors 64a and 64b, and the buffer amplifier 30 and the
resistor 28 can be omitted. This can simplify the circuit configuration.
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[0032]
As described above, according to the present invention, the input signal is injected into the
positive phase input terminal of the second positive phase amplifier constituting the band boost
circuit, and the input signal is generated from the band boost output. A subtraction circuit for
subtracting is provided.
[0033]
By doing this, the number of external terminals of the externally mounted low-pass band pass
filter is only two for the pair of capacitors, and the number of terminal pins can be reduced.
In the two-channel configuration for stereo, the number of terminal pins can be reduced by twice
that number. As a result, it is possible to suppress an increase in cost when the low band
expansion circuit is integrated into an IC.
[0034]
Therefore, the low frequency band expansion circuit according to the present invention is
extremely suitable for application to a circuit system of an output stage connected to a load such
as a speaker.
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