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The present invention relates to an audio apparatus in which an electrical signal obtained from
mechanical vibration of a speaker is detected and feedback of the detected signal is used to
control the vibration of the speaker to improve reproduction sound quality. The present
invention relates to a motional feedback (hereinafter referred to as MFB) device.
2. Description of the Related Art As a means for controlling the vibration of a speaker, MFB is a
technology that has an old idea in principle. However, the realization is more difficult than
expected. If it is attempted to stably apply a certain amount of MFB, the frequency characteristics
will be undulated if the phase characteristics and amplitude characteristics are not precisely
compensated over 1 KHz or more. Although many MFB systems have been proposed because of
this difficulty, they have not been widely used today.
For example, although there is a means for detection with a pick-up by another voice coil or
piezo element mechanically coupled, the phase or amplitude is disturbed by mechanical
resonance. Also, in the method of picking up sound with a microphone, the phase is also
disturbed due to the delay of air transmission time. For this reason, one of the first problems is
the signal detection sensor itself. Another method is to detect the back electromotive force of the
voice coil with a bridge, but for this purpose, it is necessary to compensate for the irregular rise
of the impedance in the high region due to the troublesome inductance and the eddy current. It is
necessary to do it precisely with multistage time constant circuit which shifted the constant. This
is obtained by carefully analyzing the mechanism of the rise in impedance of the speaker. The
basic concept of this multistage compensation is disclosed in the invention filed by the present
applicant (Japanese Patent Application Laid-Open No. 6-62487).
SUMMARY OF THE INVENTION The present invention is a further improvement based on such a
conventional compensation method. This time, as a further improvement, the equalizer and
subsonic filter are put in one feedback loop, and the device is made to refine the circuit by
devising the function such as giving the equalizer function to the power amplifier itself, and
further improve the sound quality by simplifying the circuit. It is intended to reduce the cause of
deterioration. In addition, the difference in sound quality due to the parts constituting the circuit
is also examined. Furthermore, as a secondary benefit of such improvement, it is intended to
obtain effects such as the increase in the reliability of the device and the reduction in the cost.
SUMMARY OF THE INVENTION A first feature of the present invention is an MFB apparatus for
detecting the back electromotive force component or current component of a speaker and
feeding back this component to control the operation of the speaker. A metal material having a
positive temperature coefficient is used as the material of the resistance for detection inserted
into the. Speaker voice coils are generally made of metal with a positive temperature coefficient,
such as copper, aluminum or silver. When an electric signal enters there, the temperature rises
and the resistance value rises. Especially in the case of loud sound, the temperature rise is also
remarkable, and it is preferable to compensate for this in order to operate the MFB correctly. In
this case, compensation can be performed by properly matching the temperature coefficient, heat
capacity, and heat time constant of the resistance side which is in series with the speaker. When
the detection circuit is a bridge, compensation may be performed on the opposite side, or a
negative temperature coefficient may be used for the corresponding side. Although the principle
is simple, the behavior of temperature change changes depending on the heat radiation
conditions, so it is preferable to actually decide it. Therefore, the inventor conducted many
audition tests. In this experiment, metal oxide film resistance, metal film resistance, nickelchromium winding resistance, and winding resistance due to copper wire were tested, but copper
winding was able to obtain extremely favorable results. As a result of further checking about
other metal materials preferred for sound quality, that is, metal materials used for audio, such as
aluminum, silver and gold, based on experience so far, favorable results were obtained according
to copper. . According to this experiment, it has been found that even if the temperature
conditions are not strictly matched, even if the winding resistance is made of a metal similar to
the voice coil constituent material such as copper, aluminum or silver, a considerable effect can
be obtained in sound quality. It is considered that the difference in the sound due to the
resistance material includes other factors which can be discriminated from the sense of hearing
besides the factor of the temperature coefficient, which will be described later. It is preferable to
Next, according to a second feature of the present invention, in the circuit including the equalizer
means in the feedback loop, the velocity component of the speaker is differentially compensated
for negative feedback, or the current component of the speaker is differentially compensated for
positive feedback. It is to add. Looking at the two embodiments disclosed in Japanese Patent
Application Laid-Open No. 6-62487 filed by the inventor of the present invention, in the case of
the former example, that is, the circuit example in which the equalizer circuit is disposed outside
the feedback loop, the problem always appears as it is. Absent. In the latter case, that is, when the
equalizer circuit is incorporated in the feedback circuit loop of the amplifier, it works well if the
amount of compensation of the equalizer is small, but if this amount is large, the correct velocity
MFB can not be obtained due to the influence of the equalizer. By adding a differential operation
in the sense of compensating for this, a more accurate velocity MFB can be obtained. Therefore,
the differentiation in this part is essentially different from the acceleration MFB which feeds back
the acceleration signal obtained by differentiating the velocity signal. In addition, in order to
suppress low frequency range peaks caused by the acceleration MFB, neither negative feedback
nor positive feedback of frequency selected velocity signals is used in combination. It is a
differential compensation for correcting the offset when there is a large amount of compensation
of the equalizer to ensure an accurate velocity MFB.
Next, the difference between negative impedance drive and velocity MFB by bridge detection will
be described. To explain this with a circuit diagram, FIG. 2 is an example of a circuit of negative
impedance drive by current positive feedback. On the other hand, FIG. 3 shows the speed MFB of
bridge detection, but the difference is that there is a third route, that is, the route indicated by
(III) in the figure. Reference numerals 1 and 2 denote amplifiers, and 16 denotes a speaker
serving as a load. In the figure, the first to third return routes are indicated by (I) to (III),
respectively. In the case of the circuit of FIG. 3, since the transfer functions of the first route (I)
and the third route (III) are the same, they can be combined as one feedback. In other words, by
renewing the circuit constant, the circuit of the speed MFB can be omitted in the same manner as
the circuit of the negative impedance drive of FIG. It is also argued that this is the case that MFB
is the same as negative impedance drive. However, as in the circuit of the specific embodiment of
FIG. 1, the equalizer is in the first route (I), the network and the differential compensation circuit
are in the second (II) and the third route (III), and When the third route (III) contains a circuit that
compensates for the rise in the impedance of the speaker, it is no longer very difficult to replace
it with a negative impedance drive. That is, only the MFB for bridge detection may be able to use
the third route (III) effectively. In other words, the negative impedance drive is part of the bridge
detection speed MFB and may be omitted.
Next, a third feature of the present invention is a device according to the above-mentioned
device, comprising means for detecting the velocity signal of the speaker with a bridge circuit or
a resistor inserted in series in the speaker, and having a time constant of two or more stages. It is
in combination. This is because, as mentioned above, in order to compensate for the irregular rise
of the impedance in the high region due to the inductance and the eddy current, it is necessary to
perform precisely in a multistage time constant circuit in which the time constant is gradually
shifted. In fact, the frequency characteristic is to select a constant that reduces the waviness.
Furthermore, as a fourth feature of the present invention, the power amplifier itself is provided
with an equalizer function to simplify and configure the circuit in the feedback loop. As
compared with the conventional case where the equalizer circuit is separately provided outside
the power amplifier, the deterioration of the sound quality can be reduced and the reliability can
be improved by simplifying the circuit.
Next, as a fifth feature of the present invention, the stability in direct current is improved by
relatively increasing the feedback amount of low frequency components including the direct
current region. In addition, related to this, as the sixth feature, by setting the time constant to two
or more stages, the function of the subsonic filter is enhanced, and resistance is inserted in series
in the capacitor that constitutes the time constant. It combines the structure which suppresses a
peak and oscillation.
One of the problems with MFB is that the amplifier compensates for the drop in the acoustic
output below the speaker's f0. Although this way of thinking of this compensation itself is good,
there are also cases where it is not good. Particularly in the case of the LP record, many low
frequency components such as several Hz to several tens Hz are generated due to the warp of the
record, eccentricity, and resonance caused by the equivalent mass of the arm and the compliance
of the cartridge. If this is combined with the MFB as it is, the speaker is greatly shaken and
causes intermodulation distortion and distortion due to the Doppler effect. In order to avoid this,
a low cut filter that lowers the gain of frequencies below the required audible band is essential.
From this point of view, it is better to cut below the necessary band as rapidly as possible. There
are two ways of cutting, one is a method of passing a subsonic filter through orthodox. The other
is a method of cutting the low band by devising the NFB circuit. Either method can be used, but
the present invention recommends the latter. The reason is that there are two merits that the
sound quality can be improved by simplifying the signal path, and the stability of the amplifier
such as the drift can also be improved. From this point of view, it is preferable that the number of
time constant stages be large. However, when the time constant is two or more, the phase
approaches or exceeds 180 degrees. This causes peaks and oscillations. A filter that makes the
NFB circuit have multistage inverse characteristics is a means that has not been used since it is
usually difficult to design. Therefore, no technology has been established. The reason is that
there are many other common circuits that are easy to design as a filter. Therefore, when the
analysis was conducted, it was possible to achieve the purpose by inserting a resistor with Q
dump function to suppress this peak and oscillation in the circuit. This makes it possible to
obtain simple, high-quality sound, and control the peak and oscillation to achieve stable
the most stable velocity MFB among acceleration MFB, velocity MFB and amplitude MFB.
Therefore, the basic idea is to minimize the rise and fall time delays in the low range by damping
the resonance of this system. This method is also resistant to disturbances. Those using
resonance increase the efficiency, but on the other hand, the rising and falling envelopes of the
sound are also theoretically delayed. Although there is an afterglow as a property of the
resonance body, this does not mean that the resonance body produces the energy, but merely
lags out what absorbed the energy of the rising part of the sound. This is the cause of so-called
motility in the low region. Therefore, the cabinet to be combined with this device is not a bath
reflex system utilizing resonance, but in principle a closed box is desirable. The idea is also to
dampen the resonances that occur between the compliance of the air inside the box and the mass
of the vibrating system. However, the application of the device of the present invention does not
have to be a completely sealed structure, but it may be a kind of using a dump material in the
sense of providing a hole for relief of back pressure and suppressing the resonance noise
generated by this hole. The reason is that, in the case of a small sealed box, the air inside may
interfere with the movement of the cone paper and may be cramped, losing the freedom of
sound. As described above, the present invention is based on the idea that although the braking
action is excellent, the lowering of the low band, which is a drawback of the speed MFB, is
compensated by the equalizer.
In addition, the combination of so-called bass reflex type box and MFB or negative impedance
drive and equalizer has a relatively small box because the efficiency rises near the resonant
frequency, but on the frequency axis that the bass can be sufficiently emitted despite being a
relatively small box. There is a big merit of However, there is a serious musical defect that when
viewed on the time axis, bass time delays occur. This is because the audio engineer customarily
emphasizes the characteristics on the frequency axis and does not emphasize the characteristics
on the time axis. Also, it is known that, although audio does not generally appear in
measurement, there are many things that affect sounds that can be determined by human
hearing. It is said that differences in amplification elements cause differences in sound due to
capacitors, contact materials, conductive materials, insulation materials, and the like. The
difference of the sound by the current resistance material mentioned above has the possibility.
As an embodiment of the present invention, an equalizer circuit is incorporated in a feedback
loop of an amplifier for driving a speaker. This raises the low range in advance. This output is
connected to the speaker through the network circuit if necessary. And it has a bridge circuit
which detects the back electromotive force of a speaker, and an impedance compensation circuit,
and the compensation circuit of the speaker's impedance is contained in the other side of this
bridge. After this output is differentially compensated, it enters into a differential amplifier
composed of another amplifier to detect an accurate velocity component to obtain a feedback
signal. Furthermore, a multistage subsonic filter circuit is added to increase the amount of
feedback in the unnecessary low band. Since such a circuit configuration is a very simple circuit,
the operation of the amplifier can be stably driven without degrading the sound quality. This is
refined while repeating the experiment repeatedly based on the applicant's previous application,
it is simple, stable under many conditions, and high sound quality can be obtained. Moreover,
since it can apply also to a multiway by using the network together, the MFB apparatus rich in
versatility is obtained.
embodiment of the MFB apparatus of the present invention will be described. FIG. 1 is a circuit
diagram showing a circuit of this embodiment. In the figure, the amplifier 1 incorporates an
equalizer circuit 3 in a feedback loop. This raises the low range in advance. This output is divided
through the network circuit 4 and the high frequency side is supplied to the tweeter 5 and the
low frequency side is supplied to the woofer 6. This portion constitutes a bridge circuit of signal
detection. The impedance compensation circuit 7 in the network 4 is to make the operation of
the network more accurate. A speaker impedance compensation circuit 9 is included in the
opposite side of the bridge. Although this compensation may be performed on the speaker side, a
capacitor with a large value is required, so it is performed on this side where the impedance can
be increased. The current detection resistor 8 inserted in series to the woofer 6 is formed of a
resistor wound with a copper wire. After this output is differentially compensated by the
differential compensation circuit 13, an accurate velocity component is detected by entering the
differential amplifier configured by the amplifier 2. The two capacitors 14, 15 in this part also
constitute part 10 of the impedance compensation. The + side input of the amplifier 2 has a
function of a subsonic filter by feeding back a large amount of unnecessary low frequency
components through a two-stage time constant circuit 11 containing a Q dump resistor 12. Note
that SW1 is additionally provided because it is possible to optionally select on / off of the
subsonic filter circuit and SW2 on / off of the MFB itself according to the use situation.
The graph of the frequency characteristics according to the present invention configured as
described above will be described schematically. FIG. 4 is a graph showing this frequency
characteristic, in which the reproduction frequency characteristic of the original speaker is
shown. A characteristic is obtained by combining this with an equalizer. Furthermore, it becomes
the characteristic by adding speed MFB, and extends to the low region far compared with the
original characteristic. Since the characteristic of the equalizer circuit when working the subsonic
filter circuit becomes a curve of this, the curve of the frequency characteristic will be obtained if
the velocity MFB is added to this. Is effective for preventing harmful effects caused by
inadvertently extending the low region as described above for the curve.
An embodiment of negative impedance drive by current positive feedback will be described with
reference to FIG. The amplifier 1 raises the low band by the equalizer circuit 3. This output drives
the speaker 16 and flows to the resistor 8 for current detection. As the impedance of the high
region of the speaker 16 rises gradually, the generated voltage decreases accordingly. The
impedance compensation circuit 9 compensates for this decrease in voltage. At this time, by
adding one resistor 17, the capacitor of the circuit 9 for impedance compensation can be made a
small value. By feeding back this signal differentially compensated with 13 capacitors, an effect
similar to that of the previous embodiment can be obtained. As shown in FIG. 5, the effect of the
subsonic filter is also fed back to the + side input of the amplifier 2 through the two-stage time
constant circuit 11 including the Q dump resistor 12 to feedback a lot of unnecessary low
frequency components. In some cases, the function of the subsonic filter can be provided.
As described above, according to the present invention, in the MFB apparatus, the most stable
speed MFB is used as the basis of the circuit, and a specific material is selected for the detection
resistance, or one of the equalizer and the subsonic filter is used. By devising the circuit by
devising the circuit by putting it in the feedback loop and devising the equalizer function to the
power amplifier itself, the damping of the speaker is optimized, and the rise and fall time delays
in the low range are minimized You can get good playback sound quality. Moreover, the
simplification of the circuit can reduce the cause of the sound quality deterioration, and at the
same time, the secondary effects such as the improvement of the reliability of the device and the
reduction of the cost can be obtained.
Brief description of the drawings
1 is a circuit diagram showing an embodiment of the present invention.
2 is a circuit diagram illustrating a circuit of negative impedance drive by current positive
3 is a circuit diagram illustrating a circuit of speed MFB signal generation of the bridge detection.
4 is a graph showing the frequency characteristics to be reproduced.
5 is a circuit diagram showing an example of negative impedance drive by current positive
Explanation of sign
1, 2 amplifier 3 equalizer circuit 4 network circuit 5 tweeter 6 woofer 7, 9, 10 impedance
compensation circuit 8 detection resistor 11 subsonic filter circuit 12 Q dump resistor 13
differential compensation circuit 14, 15 capacitor 16 speaker 17 resistor
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