Equalizer and audio device using the same

An equalizer includes first and second differential amplifiers which have a common output and respective separate current sources, a first negative feed back circuit which feeds back a voltage signal extracted from the common output to the first differential amplifier, a second negative feed back circuit which feeds back the voltage signal to the second differential amplifier, and a filter circuit which is provided at either the first or the second negative feed back circuit and which differentiates frequency characteristics during signal amplification in the first and second differential amplifiers. A frequency characteristic is selected through selection of one of the current values of the respective current sources in response to a sound quality adjusting signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an equalizer and an audio device using the 
same, and more specifically, relates to an equalizer in a portable type 
audio device such as for a portable type magnetic tape reproducing device 
which is composed of a circuit constituted by a limited number of 
elements, is capable of sound quality control and is suitable for forming 
into an integrated circuit. 
2. Background Art 
FIG. 6 is an equalizer in a conventional audio device. The equalizer is 
generally constituted by simulating equivalently of resistors and 
capacitors through a gyrator circuit provided with a multiplicity of 
variable Gm amplifiers, in the present example, three variable Gm 
amplifiers Gm1, Gm2 and Gm3 as illustrated. In the drawing, a control 
circuit which controls the current for the variable Gm amplifiers is 
omitted. 
Since a multiplicity of transistors are used for constituting one variable 
Gm amplifier, when forming such type of an equalizer into an integrated 
circuit, an increase of the circuit scale can not be avoidable. 
When such type of an equalizer is used in a portable type audio device, it 
is difficult to form the equalizer into an integrated circuit together 
with other circuits and further noises are likely generated because of an 
increase of circuit passages. An increase in number of integrated circuits 
causes an obstacle with respect the cost thereof as well as an obstacle 
with respect to the downsizing and thickness thinning thereof. 
For this reason, an equalizer constituted by a combined circuit of a filter 
circuit including a capacitor and an amplifier circuit is generally used 
for the portable type audio device, and moreover in such equalizer is 
employed a constitution in which frequency characteristics can be 
selectively changed-over by a switch. 
FIG. 7 is an example of a stereo audio device of such type driven by a 
battery in which an audio amplifier 20 is primarily illustrated. An output 
circuit 10 is constituted by an output amplifier 11 for left channel (L 
side) composed of an operational amplifier and an output amplifier 12 for 
right channel (R side) composed of an operational amplifier which are for 
reproducing sounds at right and left sides and a voltage follower type 
center amplifier 13 which generates a reference voltage of about Vcc/2 and 
produces a virtual grounding condition. The amplifier characteristics of 
these amplifiers normally show a substantially flat frequency 
characteristic in an audio band. 
The output amplifier 11 at the L side receives output signals from a low 
frequency region mixing amplifier 15 composed of a differential amplifier 
via a variable resistor 14. Likely, the output amplifier 12 at the R side 
receives signals from a low frequency region mixing amplifier 17 composed 
of a differential amplifier via a variable resistor 16. 
The respective low frequency region mixing amplifiers 15 and 17 receive 
from respective preamplifiers 18 and 19 at L and R sides reproduced audio 
signals at respective (+) input terminals and at the same time receive 
outputs from a bass boosting amplifier 21 at respective (-) input 
terminals via respective buffer amplifiers 15a and 17a. 
The bass boosting amplifier 21 receives a sum of L+R signals of both 
channels at L and R sides from the preamplifiers 18 and 19 via a low pass 
filter 22 composed of a resistor and a capacitor (RC) and amplifiers and 
outputs the low frequency region components of the L+R signals. Therefore, 
the amplification rate at the low frequency region of the low frequency 
region mixing amplifiers 15 and 17 increases depending on the output of 
the bass boosting amplifier 21. 
Symbols Ca, Cb, Cc and Cd are respectively coupling capacitors and numerals 
23 and 24 are right and left side head phones connected to the respective 
amplifiers in the output stage 10. A dry battery used as a power source is 
omitted from the drawing. 
In comparison with an audio device such as a component stereo which 
generally has an ample margin or a slight limitation with regard to its 
circuit size and power source voltage, in a portable type audio device 
driven by a lower voltage the sound quality is sacrificed which has been 
allowed until now. However, in these days a high sound quality tends to be 
required as well as an inclination of multi functions for such portable 
type audio device. The requirement of a high sound quality with the low 
voltage drive is difficult to realized in comparison with the general 
audio device. 
When investigating the above explained circuit from such aspect, since the 
bass boosting amplifier receives the L+R signals and increases the 
amplification rate at a low frequency region, the separation between right 
and left channels during bass boosting operation is deterionated. In 
addition, since the low frequency region components are amplified by a 
special amplifier, noises thereby are increased. 
SUMMARY OF THE INVENTION 
An object of the present invention is to resolve the problems in the 
conventional art as explained above and to provide an equalizer which is 
constituted by a circuit having a limited number of elements and permits a 
sound quality adjustment. 
Another object of the present invention is to provide an audio device 
having an audio amplifier in which the equalizer is formed into an 
integrated circuit together with other circuits. 
Still another object of the present invention is to provide an audio device 
suitable for using as portable type device and having an equalizer which 
permits to selectively change-over the frequency characteristics by a 
switch. 
Features of an equalizer according to the present invention which achieves 
the above objects are to comprise first and second differential amplifiers 
which have a common output and respective separate current sources; a 
first negative feed back circuit which negative feeds back a voltage 
signal extracted from the common output to the first differential 
amplifier; a second negative feed back circuit which negative feeds back 
the voltage signal to the second differential amplifier; and a filter 
circuit which is provided at either the first or the second negative feed 
back circuit and which differentiates frequency characteristics during 
signal amplification in the first and second differential amplifiers, and 
are to select a frequency characteristic through selection of one of the 
current values of the respective current sources in response to a sound 
quality adjusting signal. 
Namely, outputs of the two differential amplifiers are made common, at 
least one of the amplifiers is provided with a different frequency 
characteristic from that of the other amplifier and the output is negative 
fed back to the respective amplifiers. Further, the current value of one 
current source is selected for the other and through the selection a 
negative feed back amount for one is varied with respect to a negative 
feed back amount for the other. Thereby, a frequency characteristic during 
the signal amplification representing an integration of the two negative 
feed back amounts can be selected. 
As a result, an equalizer circuit is realized with an element structure 
fundamentally constituted by two differential amplifiers and a feed back 
circuit having a filter, and the number of elements used therein is 
reduced in comparison with the conventional circuits. Moreover, through 
the selection of an operating current of the respective current sources a 
frequency characteristic for entire amplifiers can be selected, an 
equalizer suitable for forming into an integrated circuit is realized. In 
particular, for a portable type audio device, such measures contribute for 
the down sizing and thickness thinning thereof as well as the circuit 
therefor is simplified and the sound quality thereof is improved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, an equalizer 1 is constituted by first and second amplifiers 3 
and 4 which have an active load of a current mirror circuit 2 in common, 
an amplifier 5 which extracts an output from the common load and negative 
feed back circuits 6 and 7 having different negative feed back rates which 
negative feed back the output of the amplifier 5 to the respective 
differential amplifiers 3 and 4. The bases of transistors Q1 and Q3 in the 
first and second differential amplifiers 3 and 4 are connected in common 
to an input terminal 8b and the output of the amplifier 5 is connected to 
an output terminal 8a. The frequency characteristic of the equalizer 1 is 
adjusted by a control circuit 8, which generates control signals in 
response to a voltage of the variable resistor 80 by sound quality 
manipulation from an external member. 
The first differential amplifier 3 is constituted by transistors Q1 and Q2 
and a current source 3a therefor and includes at the respective collector 
sides thereof transistors Q5 and Q6 in the current mirror circuit 2 as a 
load. The second differential amplifier 4 is constituted by transistors Q3 
and Q4 and a current source 4a therefor and likely includes at the 
respective collector sides thereof transistors Q5 and Q6 in the current 
mirror circuit 2 as a load. 
The negative feed back circuit 6 is a circuit for dividing the voltage at 
the output terminal 8a and is constituted by a series connection of 
resistors R1 and R2 connected between the output terminal 8a and a bias 
line V1. The voltage at a voltage divided point N is fed back to the base 
of the transistor Q2. The negative feed back circuit 7 is constituted by a 
series circuit composed of a resistor R3 and a CR filter 7a connected 
between the output terminal 8a and ground GND, a CR filter 7b connected in 
parallel with the CR filter 7a and a CR filter 7c connected in parallel 
with the resistor R3, and the voltage at a voltage dividing point M 
determined by the resistor R3 and the CR filter 7a is fed back to the base 
of the transistor Q4. Further, the resistances of the respective resistors 
and capacitances of the respective capacitors are so selected that the 
negative feed back rate of the negative feed back circuit 7 is lower than 
the negative feed back rate of the negative feed back circuit 6. 
The CR filter 7a and the CR filter 7c are filters for setting a low 
frequency range gain, the CR filter 7a is constituted by a series 
connection of a resistor R4 and an electrolytic capacitor C1 and the CR 
filter 7c is constituted by a series circuit of a resistor R6 and a 
capacitor C3. Further, the CR filter 7b is a filter for setting a high 
frequency range gain and is constituted by a resistor R5 and a capacitor 
C2. 
The current values of the current source 3a and 4a are respectively 
controlled in response to current values in DC current signals A and B 
from the control circuit 8 and the increase and decrease of the current 
values in the DC current signals A and B are controlled in opposite 
directions each other with respect to a sound quality adjusting voltage 
signal of a variable resistor 80. Namely, when the voltage of the 
adjusting voltage signal is high, the current signal A reduces and the 
current signal B increases. Contrary, when the voltage of the adjusting 
voltage signal is low, the current signal A increases and the current 
signal B reduces. 
The control circuit 8 which generates such current signals A and B can be 
realized, for example, by simply providing an inverting amplifier for a 
signaling line at the side of the current signal A which performs the 
opposite operation with regard to the adjusting voltage signal by the 
variable resistor 80. Then, the respective voltage signals are converted 
into the corresponding current values via a voltage/current converting 
circuit. Thus, the above explained current signals A and B are obtained. 
In the equalizer 1 having amplifiers using the common load as explained, 
when the voltages are negative fed back to the respective differential 
amplifier 3 and 4 from the common output terminal 8a, the negative feed 
back amount to the first differential amplifier 3 is determined by the 
ratio of the resistance values of the resistors R1 and R2 with respect to 
the output voltage. Accordingly, the amplification rate determined by the 
differential amplifier 3 and the amplifier 5 is expressed as; 
EQU Gain=A/(1-A.beta.).apprxeq.(R1/R2)+1 
wherein, A is the open loop gain thereof and .beta.=R1/(R1+R2). 
Further, the negative feed back amount of the second differential amplifier 
4 is determined by the radio of an impedance Z1 corresponding to the 
frequency characteristic of the parallel circuit of the resistor R3 and 
the CR filter 7c to an impedance Z2 corresponding to the frequency 
characteristic of the parallel circuit of the CR filters 7a and 7b with 
respect to the output voltage and the amplification rate determined by the 
differential amplifier 4 and the amplifier 5 is expressed as; 
EQU GainG=A/(1-A.beta.).apprxeq.{R3.parallel.(R6+1/.omega.C3)}/R4+1 
wherein A is the open loop gain thereof, .beta.=Z1/(Z1+Z2) and 
R3.omega.(R6+1/.omega.C3) is a parallel resistance value of the resistor 
R3 and a series circuit of the resistor R6 and the capacitor C3. 
Now, at first the amplification characteristic of a combined amplifier 
consisting of the differential amplifier 3 and the amplifier 5 with 
respect to the input signals is discussed. Since the negative feed back 
circuit 6 is constituted by the resistors R1 and R2, the frequency 
characteristic thereof shows a flat frequency characteristic determined by 
the equation of Gain G.apprxeq.(R1/R2)+1. As a result, the characteristic 
do as shown in FIG. 2 is obtained through this combined amplifier. 
Secondly, the amplification characteristic of another combined amplifier 
consisting of the differential amplifier 4 and the amplifier 5 with 
respect to the input signals is discussed. Since the negative feed back 
circuit 7 has a frequency characteristic, the negative feed back circuit 7 
shows the lowest negative feed back rate determined by the resistors R3 
and R4 in a frequency band in which the resistance value of the 
electrolytic capacitor C1 can be neglected among the low frequency region 
components in the input signals, and the gain G of the combined amplifier 
is represented as; 
Maximum gain G.apprxeq.(R3/R4)+1. When frequencies of the input signals 
exceed the above frequency band the resistance value of the electrolytic 
capacitor C1 increases to an innegligible value and the negative feed back 
voltage increases depending on the resistance value of the resistor R4 and 
the electrolytic capacitor C1. As a result, the gain G of this combined 
amplifier decreases. When the frequencies of the input signals reaches to 
an intermediate region of comparatively high frequencies, the resistance 
value of the electrolytic capacitor C1 increases, further the resistance 
value of the series circuit of the resistor R6 in the CR filter 7c and the 
capacitor C3 decreases, thus the negative feed back voltage increases. 
Accordingly, the gain of the combined amplifier is represented as; 
EQU Gain G.apprxeq.(R6/R4)+1 
wherein R6&lt;R3. On the other hand, with respect to frequencies in high 
frequency region components among the input signals the resistance value 
of the series circuit of the resistor R5 and the capacitor C2 is added in 
parallel to the resistance value of the resistor R4. Thereby the negative 
feed back voltage is limited to some extent, and the gain in the high 
frequency region is suppressed low. Accordingly, the gain of the combined 
amplifier at this instance is expressed as; 
EQU Gain G.apprxeq.(R6/R4.omega.R5)+1. 
As a result of the above discussion, when the current value of the current 
source 4a is set near its maximum current value, the characteristic dm as 
shown in FIG. 2 is obtained through the amplifier. 
In the present embodiment, the amplifier 5 constitutes a common amplifier 
which receives output voltage signals via the common load in the form of 
the current mirror for the differential amplifiers 3 and 4. Accordingly, 
the output of the amplifier 5 shows a voltage representing substantially 
the logical sum of the respective outputs of the differential amplifiers 3 
and 4. Therefore, when the operating currents of the respective 
differential amplifiers 3 and 4 are equal, a priority is given to the 
operation of an amplifier in a circuit showing a larger negative feed back 
amount among the negative feed back circuits connected to the output. 
This is because that when obtaining a voltage output via a common load 
composed of a wired OR, an amplifier receiving a larger negative feed back 
amount with respect to the same output voltage is subjected to a larger 
amplification rate effect, thereby the entire output reduces and a 
negative feed back circuit having a larger negative feed back amount is 
largely affected with respect to the reduced output voltage in comparison 
with the negative feed back amount to the other amplifier to perform again 
a negative feed back. Accordingly, the characteristic of an amplifier 
disposed in the negative feed back circuit having a larger negative feed 
back amount appears with priority in the entire amplifier characteristic 
by the rate of the negative feed back amount at the side of the instant 
amplifier to the negative feed back amount to the other amplifier. 
Therefore, through controlling the respective negative feed back amounts 
in response to the operating currents of the two amplifiers the frequency 
characteristics of the respective amplifiers are combined with a 
predetermined rate and the combined frequency characteristic can be used 
as that of the equalizer 1. 
In the present embodiment, because of the difference in frequency 
characteristics of the negative feed back circuits 6 and 7 the frequency 
characteristics of the differential amplifiers 3 and 4 are differentiated. 
The differential amplifier 3 is provided with a flat frequency 
characteristic and the differential amplifier 4 is provided with a 
frequency characteristic having a high gain at low and high frequency 
regions. As a result, a synthesized characteristic of the characteristics 
of the differential amplifiers 3 and 4 determined depending on the 
respective negative feed back amounts is obtained as the frequency 
characteristic of the entire amplifiers. 
Since the negative feed back rate of the negative feed back circuit 7 is 
set lower in advance than the negative feed back rate of the negative feed 
back circuit 6, the flat frequency characteristic of the differential 
amplifier 3 receiving a larger negative feed back amount is given 
priority, in particular, with respect the characteristic in the medium 
frequency region. Thereby, a range of the flat frequency characteristic is 
enlarged in the intermediate frequency range wherein a sound quality 
emphasis is seldom required. The respective actual negative feed back 
amounts are determined by the operating currents of the respective 
differential amplifiers 3 and 4. The operating currents of the respective 
differential amplifiers can be determined by the current values of the 
current sources 3a and 4a. These current values are controlled by the 
control current values in the DC current signals A and B from the control 
circuit 8. 
Because of the above reason, when the operating currents of these current 
sources 3a and 4a are set equal, the frequency characteristic of the 
differential amplifier 3 is given priority and the signal amplification 
characteristic of the equalizer 1 is gradually narrowed depending on the 
increase of frequency emphasis in the flat frequency characteristic in the 
intermediate frequency region as illustrated in FIG. 2. Further, when only 
the current source 3a is operated and the current of the current source 4a 
is interrupted, a completely flat frequency characteristic do as 
illustrated in FIG. 2 is realized. Contrary, when the current of the 
current source 4a becomes larger than that of the current source 3a and 
the negative feed back amount at the side of the differential amplifier 4 
becomes larger than that at the side of the differential amplifier 3, the 
frequency characteristic at the side of the differential amplifier 4 is 
given priority. At such instance the frequency characteristic determined 
by the respective CR filters 7a, 7b and 7c is provided. In this instance a 
frequency characteristic which continuously varies in a range of d1, d2, . 
. . , dm as shown in FIG. 2 depending on the operating current set by the 
current source 4a is obtained. 
Therefore, when the variable resistor 80 is out of intensifying operation 
condition of low and high tone, for example, the movable contact of the 
variable resistor 80 locates at the lower most position, the control 
circuit 8 generates a control current signal A which maximizes the current 
value of the current source 3a and the operating current of the current 
source 4a is rendered zero. At this instance, the amplifier in the 
equalizer 1 operates as an amplifier having a flat frequency 
characteristic with gaih G.apprxeq. (R1/R2)+1. When the movable contact of 
the variable resistor 80 is moved upward to provide a low and high tone 
intensifying operation condition, the control circuit 8 generates the 
current signals A and B which respectively reduce the operating current of 
the current source 3a and increase the operating current of the current 
source 4a depending on the operation amount of the movable contact. At 
this instance, a frequency characteristic such as d1 and d2 is, for 
example, selected among the frequency characteristics from do to dm. 
Further, the movable contact of the variable resistor 80 reaches to the 
upper most position, the operating current of the current source 3a is 
rendered zero and the operating current of the current source 4a is 
maximized. At this instance, the gains at the lower frequency region, at 
the intermediate frequency region and at the high frequency region are 
respectively given as gain G.apprxeq.(R3/R4)+1, gain G.apprxeq.(R6/R4)+1 
and gain G.apprxeq.(R6/R4.omega.R5)+1. As a result, the amplification 
characteristic of the equalizer 1 at this instance assumes the frequency 
characteristic dm. 
In FIG. 3 embodiment, the differential amplifier 3 in FIG. 1 embodiment is 
eliminated, instead, a differential amplifier 40 having the same structure 
as of the differential amplifier 4 is provided independently, and the 
output sides of these differential amplifiers are connected in common to 
the output terminal 8a and the voltage of the output terminal 8a is fed 
back to the respective differential amplifiers. A current source 40a 
corresponds to the current source 4a and is a current source for the 
differential amplifier 40. 
The control of the current values of the respective current sources 4a and 
40a is performed by the control circuit 8. The frequency characteristics 
of the respective CR filters 7a', 7b' and 7c' for the differential 
amplifier 40 are different from those of the respective CR filters 7a, 7b 
and 7c. 
As a result, when the operating currents of these current sources are set 
substantially the same, the combined amplifiers show a combined frequency 
characteristic, and further when the operating current of the differential 
amplifier 4 is increased and the operating current of the differential 
amplifier 40 is decreased, the gain of a predetermined frequency band of 
the differential amplifier 4 can be intensified and when the operating 
current of the differential amplifier 4 is decreased and the operating 
current of the differential amplifier 40 is increased, the gain of a 
predetermined frequency band of the differential amplifier 40 can be 
intensified. 
FIG. 4 shows an example of portable type audio devices in which numeral 100 
is the portable type audio device, 50 is an audio amplifying circuit at L 
side and 60 is an audio amplifying circuit at R side. Since these two 
audio amplifying circuits 50 and 60 have a same circuit structure, only 
the audio amplifying circuit 50 at L side is shown in detail. Further, the 
same constitutional elements as in FIG. 7 are assigned the same reference 
numerals and the explanation thereof is omitted. 
Numeral 9 is an output amplifying circuit of the audio amplifying circuit 
50 and corresponds to the output amplifier 11 at the L side in FIG. 7, and 
in the present embodiment no center amplifier 13 in FIG. 7 is used which 
is an independent circuit from both the output amplifiers at the L and R 
sides. However, the center amplifier 13 can be added so as to take the 
same constitution as in FIG. 7. In such instance, the output amplifier 11 
is replaced by the output amplifying circuit 9 and likely the output 
amplifier 12 is replaced by the output amplifying circuit 9. 
An equalizer la in the output amplifying circuit 9 is a circuit 
corresponding to the equalizer 1 in FIG. 1, however, in the present 
embodiment the equalizer 1a constitutes an input stage in the output 
amplifying circuit 9. The selection of the frequency characteristic 
therefor, in that the sound quality adjustment, is performed by the 
switches 3c, 3d and 4b in place of the control by the variable resistor 80 
and the control circuit 8. Further, in order to prevent saturation of the 
output waveform a maximum amplitude detection circuit 25 is provided at 
the output terminal 8a of the equalizer 1a through which the output of the 
equalizer 1a is limited so as not to exceed a predetermined value. 
Further, since the frequency characteristic is selected by the switches in 
the equalizer 1a, a constant current source 3b which is connectable via 
the switch 3c is provided separately as a current source for the 
differential amplifier 3 so as to select a flat frequency characteristic. 
When the switch 3c is turned ON, the common emitters of the differential 
transistors Q1 and Q2 are connected to the current source 3a via the 
switch 3c and the first differential amplifier 3 is grounded, further when 
the switch 3d is turned ON, the common emitters are connected to the 
constant current source 3b via the switch 3d and the first differential 
amplifier 3 is grounded. When the switch 4b is turned ON, the common 
emitters of the differential transistors Q3 and Q4 are connected to the 
current source 4a via the switch 4b and the second differential amplifier 
4 is grounded. 
To the output terminal 8a the maximum amplitude detection circuit 25 is 
further connected and the detection signal of the maximum amplitude 
detection circuit 25 is applied to the current source 3a as well as 
applied to the current source 4a via an inverting amplifier 26. The 
maximum amplitude detection circuit 25 is constituted by a serise circuit 
of a diode D25 and a capacitor C25 connected between the output terminal 
8a and ground GND and a resistor R25 connected in parallel with the 
capacitor C25. When a voltage is generated at the terminal 8a which is 
sufficient to render the diode D25 conductive, the diode is turned ON and 
a predetermined electric charge is charged into the capacitor C25 to 
generate the detection signal. The electric charge in the capacitor C25 is 
applied to the current source 3a in a form of current so as to increase 
the operating current, thereby the negative feed back amount is increased 
and the saturation of the output waveform is suppressed. Likely, the 
detection signal is applied to the current source 4a via the inverting 
amplifier 26 to reduce the operating current, thereby an decrease of the 
negative feed back amount at low and high frequency region is suppressed 
so that the saturation of the output waveform is also suppressed. 
The electric charge in the capacitor C25 is discharged at a predetermined 
time constant .tau. which is determined by the resistance of the resistor 
R25 and the capacitance of the capacitor C25, and after a certain 
predetermined time the suppression is released. 
The respective current values of the current sources 3a and 4a are set at 
predetermined values except for the instance when the detection signal is 
generated from the maximum amplitude detection circuit 25. When the 
switches 3d and 3c is turned OFF and the switch 4b is turned ON (as 
illustrated in the drawing), only the differential amplifier 4 is rendered 
operative via the current source 4a and the current value of the current 
source 4a is set so that the frequency characteristic of the amplification 
rate thereby assumes the characteristic dm among the characteristics shown 
in FIG. 2. 
When the switch 3d is turned OFF and the switches 3c and 4b are turned ON, 
both the differential amplifiers 3 and 4 are rendered operative via the 
current source 3a and the current value of the current source 3a is set so 
that a frequency characteristic of the amplification rate thereby is 
selected representing an intermediate frequency characteristic between do 
and dm among the frequency characteristics as shown in FIG. 2, for 
example, the frequency characteristics such as d1 and d2 is selected. 
Further, as explained previously when the switches 3c and 4b are turned 
OFF and the switch 3d is turned ON, the constant current source 3b is 
activated and the current of the constant current source 3b is set so that 
the frequency characteristic do is selected among the frequency 
characteristics as shown in FIG. 2. 
Therefore, when a frequency characteristic dm as shown in FIG. 2 in which 
low and high tone regions are intensified is required, only the switch 4b 
is turned ON and the other switches are turned OFF, and when a frequency 
characteristic in which low and high tone regions are properly boosted is 
required, the switches 4b and 3c are turned ON and the others are turned 
OFF. Further, when a flat frequency characteristic is required, the switch 
3d is turned ON and the others are turned OFF. 
In the above for the sake of explanation convenience a separate operation 
of three discrete switches is explained, however when the above switching 
elements are constituted by a switching circuit, the ON/OFF operations of 
the respective switching elements can be easily associated each other. 
Further, the output of the amplifier 9a which serves as the output for the 
head phone 23 can be directly connected to the input of the maximum 
amplitude detection circuit 25. 
FIG. 5 is a specific circuit which permits a low voltage drive by 
eliminating the respective switches for selecting the frequency 
characteristics provided at the bottom side of the differential amplifier 
circuit. 
In the present embodiment, the current values set by ON/OFF of the 
respective switches are transmitted to the differential amplifiers 3 and 4 
via current mirror circuits to thereby set the current values of the 
respective current sources. 
The current values of the current sources 3a and 4a are selected by current 
sources 26, 27, 28 (28') and switches 3c, 4b and 3d. A series circuit of 
the current source 26 and the switch 3c and a series circuit of the 
current source 27 and the switch 4b are respectively connected between the 
power source line Vcc and ground GND via input side transistors Q7 and Q8 
of current mirror circuits of 29 and 30. A series circuit of the current 
sources 28 and 28' and a double throw switch 3d is connected between the 
base of the transistor Q7 and ground GND. As a result, the series circuits 
of the respective switches and current sources are arranged in parallel 
with respect to the differential amplifiers 3 and 4 between the power 
source line Vcc and ground GND. 
The current mirror circuit 29 is connected in a manner of current mirror to 
the base of the transistor of the current source 3a (3b) via output side 
transistors Q9 and Q11, and the current mirror circuit 30 is also 
connected in a manner of current mirror to the base of the transistor of 
the current source 4a via output side transistors Q10 and Q12. With this 
measure, the operating current values set by the selected current sources 
26, 27 and 28 (28') are transmitted to the current sources 3a (3b) and 4a 
and these current mirror circuits 29 and 30 serve as an operating current 
value setting circuit for the current sources 3a (3b) and 4a. 
In the drawing, NOR represents a condition normal) of a flat frequency 
characteristic, BB represents a condition (bass boosting) wherein low and 
high frequency regions are boosted, and MID represents a condition wherein 
the frequency characteristic at the intermediate frequency region is 
intensified. Further, in place of the diode D25 in the maximum amplitude 
detection circuit 25 in FIG. 4 embodiment a transistor Q13 is provided in 
the present embodiment. When the transistor Q13 is turned ON, the 
transmitting current through the mirror circuit 30 decreases, as a result, 
the current value of the current source Q4 reduces. 
As will be understood from the above description the equalizer 1 shown in 
FIG. 1 can be disposed in the input stage of the output amplifier circuit 
like the equalizer la as shown in FIG. 4. 
Further, the switches 3c, 3d and 4b as shown in FIG. 4 can be provided 
between the current sources 4a and 40 for the respective amplifiers in the 
circuit shown in FIG. 3 so as to permit the frequency characteristic 
selection through the switching operation. 
When forming the equalizer shown in FIG. 5 into an integrated circuit, the 
capacitor C1 constituting the low pass filter or the series circuit of the 
resistor R4 and the capacitor C1 has to be connected to the integrated 
circuit from the outside, however capacitors which have very small 
capacitance, and resistors can be incorporated into the integrated 
circuit. Moreover a conventional boosting amplifier exclusively used for a 
low frequency region is dispensed with and the circuit scale is reduced. 
Further, in the embodiments three switching circuits are used, however when 
the switching circuit 3c and the current source 3d are eliminated and the 
constant current source 3b is directly connected to the common emitter of 
the differential amplifier 3 without routing the switching circuit 3a, the 
flat frequency characteristic produced only by the operation of the 
differential amplifier 3 or the low and high frequency region intensified 
characteristic produced by the operation of the differential amplifiers 3 
and 4 can be selected with only one switching circuit, in that the 
switching circuit 4a. 
For the sake of explanation convenience these switching circuits in the 
embodiments are disposed between the current sources and the common 
emitter of the differential amplifiers, however, the disposition thereof 
can be modified so long as the function of these switching circuits is 
fulfilled which are required to turn ON/OFF the currents of the respective 
current sources or set the current values thereof at predetermined values 
as shown in FIG. 5.