Abstract— This paper describes audio equalizer design using second order Sallen Key filter. In the construction of this equalizer, we adjust the base, treble and midrange setting on the sound system. This audio equalizer is constructed by using Sallen Key filter. The input can come directly from the microphone or similar device. Then control the gain of the amplification of each channel independently to allow to choose the tone of the signal, and finally add back the signals together with summing amplifier. The Multisim Software is used for simulation of audio equalizer.
Index Terms —Audio equalizer, Sallen Key Filter, Bass, Treble, Midrange.
I. INTRODUCTION
Audio equalizer is the device to adjust the balance between the frequency components within an electronic signal. The equalizer is used in sound recording and many other applications in electronic telecommunication. The audio frequency range for the human ear is commonly accepted between 20 kHz and 20 kHz. The bass, midrange and treble frequency ranges are less than 5 kHz, between 500 to 5 kHz and above 5 kHz respectively. In this paper, we used the Sallen Key filter to filter the require frequencies. An active Sallen Key filter can be cascaded easily to make higher order filters. The opamp provides the buffering between cascade stages. Sallen Key filter makes filter design easy with cutoff frequency and Q by using R, C values and amplifier gain. This device is a simple second order linear filter which permits to obstruct a frequency range.
II. OPERATIONPRINCIPLE
In this research, there are three filters are required for base, treble and midrange adjustment. The bass adjustment circuit requires a low pass filter, the treble adjustment requires highpass filter and the midrange circuit adjustment circuit requires a bandpass filter. The input signal is fed to three different filters at the input. The output of each opamp circuit is fed into summing amplifier circuit. The block diagram of the audio equalizer is shown in Fig.1.[2]
Fig.1 Block diagram of Audio Equalizer
Manuscript received July, 2019.
III. BEHAVIOR OFAUDIOEQUALIZER
In this paper, the Sallen key filter is used to filter the audio signal.The Sallen- Key filter is a very popular active filter which can be used to create second order filter stages that can be cascaded together to form larger order filters. The op-amp provides buffering between filter stages, so that each stage can be designed independently of the others. These circuits are suitable for filters which have complex conjugate poles. The topology of Sallen Key filter is shown in Fig.2. The setting of a highpass and lowpass filters is the order of accommodation of capacitor and resistors. The equivalent impedance of such devices is substituted in the schematic view. [4]
Fig.2 The topology of Sallen Key filter
Apply KCL at Vz,
VIN−VZ Z1 =
VZ−V+
Z2 +
VZ−VOUT
Z4 (1)
And given opamp functionnaly, replacing V+with Vout
VIN−VZ Z1 =
VZ−VOUT Z2 +
VZ−VOUT
Z4 (2)
Apply KCL at V+,
VZ−V+
Z2 =
V+
Z3 (3)
replacing V+with Vout
VZ−VOUT Z2 =
VOUT
Z3 (4)
Finally, we obtain the equation,
Vz= Vout 1 + ZZ23 (5)
Transfer function of the filter is; H s = Vout
Vin =
Z3Z4
Z1Z2+Z4Z1+ Z2+ Z3Z4 (6)
A. Lowpass Filter
In Fig2,we substitute the equivalent values such as Z1= RAb Z2= RBb Z3=1/sCAb Z4= 1/sCBb The structure of lowpass filter is shown in Fig.3.
Audio Equalizer Design Using Sallen Key Filter
Fig.3 Lowpass filter
After substitution of the previous transfer function H(s), the transfer function of lowpass filter is
=
1 t t2 t t 2+ t+ t
t t t + 1 t t t t
(7)
Comparision with the characteristic equation, we can identify the parameters such as
Natural frequency = 1
t t2 t t (8)
Attenuatuion parameter2 = t+ t
t t t (9)
The quality factor Q = 2 (10)
B. Highpass Filter
[image:2.595.78.259.51.170.2]The schematic for a highpass filter is shown in Fig.4. Its looks like the same as the lowpass filter, except the resistor and capacitor are swapped.
Fig.4 Highpass Filter
= t t 2
1 t t+
t
t+ tt+ t t 2
(11)
H s = s2
s2+s CAtRBt1 +CBtRBt1 +CAtCBtRAtRBt1 (12)
= 1
t t2 t t (13)
= t t2 t t
t t+ t (14)
C.Bandpass filter
The schematic for a bandpass filter is shown in Fig.5.
Fig.5 bandpass filter
Making C1m= C2m= C yield,
fc =21 1Ϡ+ 3Ϡ
1Ϡ 2Ϡ 3Ϡ (15)
1Ϡ=2 (16)
2Ϡ= (17)
3Ϡ=2 2 2− (18)
Q = 2πfoCA0R1m (19)
Q = πfoCR2m (20)
2πfoCA0R1m= πfoCR2m
A0= R2m/2 R1m (21)
BW = (22)
D. Non-inverting Amplifier
In audio equalizer, this circuit consists of non-inverting opamp circuit characterized by the voltage gain of 1+ Rf/R1.[8]
Fig.6 Non-inverting Amplifier
E. Summing Amplifier
[image:2.595.45.267.393.643.2]The summing amplifier is needed to add incoming signal to the three amplifiers. The potentiometer can be adjusted to enable the full spectrum of sound. In this case, the summing amplifier works as non-inverting amplifier. The structure of summing amplifier is shown in Fig.7.[6]
Fig.7 Non-inverting Amplifier
IV. SYSTEM DESIGN
[image:2.595.337.523.587.713.2]the summing amplifier is required to sum the currents from the three filter circuits. The circuit diagram is shown in Fig.8.
Fig.8. Circuit Diagram for Audio Equalizer
The system will essentially consist of three filters, three amplifiers, and a signal summing amplifier. Including also an operational amplifier (opamp), device which main purpose is to increase or reduce the input’s amplitude. So the RC design will regulate the frequency while the opamp, will modulate the intensity of the output. Opamps produce control systems which are self-regulating through the feedback impulse, which makes them fairly stable and simple to use. Opamps consist of three active terminals (the ones which will actually carry the signal and modify it): v+, v−, vout. And two passive ends which are simply the opamp’s voltage source: vcc+, vcc−.
A. Bass Adjustment
The base adjustment circuit will be required a lowpass filter. The base frequency for human ear is accepted less than 500 Hz. So we select the base cutoff frequency is 500Hz. Select RAb= RBb=R2b= 1 kΩ
From equation(8), fc=21 = t Hz CAb=CBb=C = 0.318 μF
We choose standard values for capacitors with 0.33 μF.
B. Treble Adjustment
The highpass filter is needed to adjust the treble. The treble frequency for human ear is accepted greater than 5 kHz. So we select the treble cutoff frequency is 5 kHz.
Select RAt= RBt=R2t= 1 kΩ
fc=21 = tk Hz
CAt=CBt=C = 0.0318μF
We choose standard values for capacitors with 0.033 μF.
C. Midrange Adjustment
The bandpass filter is needed to adjust the midrange. The midrange for human ear is 1k Hz to 5kHz . We choose center frequency 2.5kHz.
Select C1m= C2m= 0.01 uF, R1m=10KΩ, R2m=20kΩ From equation (15),
fc=21 1Ϡ+ 3Ϡ 1Ϡ 2Ϡ 3Ϡ
Q = πfcCR2m = 1.57
A0= R2m/2 R1m= 1
3Ϡ=2 2 2− = 2 Ϡ2tt Ϡ t 1 Ϡ 1tt1tt 2−1 = th
BW = = 1t 䁗
V. TEST ANDRESULTS OFAUDIOEQUALIZER
In this section, the testing results of step by step are shown with multisim [3]. The simulation and hardware results of lowpass filter for bass adjustment is shown in Fig.9.
(a) Bode Plot for lowpass filter
(b) Simulation result of lowpass filter Fig.9. The result of lowpass filter
(b) Simulation result of highpass filter Fig.10. The result of highpass filter
The simulation and hardware results of bandpass filter for midrange adjustment is shown in Fig.11.
(a) Bode Plot for bandpass filter
(b) Simulation result of bandpass filter Fig.11. Simulation and Hardware result of bandpass filter
The results for overall circuit are shown in following figures. In figures, the green line shows the results for base adjustment, the red line for treble adjustment and the black line for midrange adjustment. For base adjustment, the cutoff frequency is below 500 Hz. At that frequency, there is no filter output for highpass and bandpass.the result is shown in Fig.12.
Fig.12.The result for base adjustment
For treble adjustment, the cutoff frequency is above 5k Hz. The result is shown in Fig.13.
Fig.13. The result for treble adjustment
For midrange adjustment, the cutoff frequency is 1 kHz to 5k Hz. The result is shown in Fig.14.
VI. CONCLUSION
In this paper, we construct audio amplifier with Sallen Key filter. By using this filter, we can easy adjustable corner frequency and simple implement with reusable parts. As no inductor is used, the circuit is compact and less weight. As the output impedance is low, it can drive the low impedance load.
REFERENCES
[1] Carter, Bruce, etal. Opamps for Everyone. Second Edition, Texas Instruments, MA 2003.
[2] Engineering Circuit Analysis. Eighth Edition, William Hayt,Javk E. Kemmerly, Steven M.Durbin.
[3] https://www.ni.com/multisim
[4] https://en.wikipedia.org/wiki/Active_filter
[5] R o b e r t L.Boylestad Louis Nashelsky, Electronic Devices and Circuit theory,Tenth Edition
[6] Theodore F.Bogart Jr.Jeffrey S.Beasley Guillermo Rico, Electronic Devices and Circuit, Sixth Edition
[7] http://www.arc.id.au/FilterDesign.html
[8] J.B.Gupta, An Integrated Course in Electronics Engineering
Khin Yu Yu Hlaing, Lecturer, pursuing Master of Electronic Engineering,
