In the previous tutorial, we learned how to design an audio crossover, using high-pass and low-pass audio filters. Next, let's design an audio equalizer.
Equalization is the process of adjusting the frequency elements of sound. An equalizer (EQ) is an audio filter that isolates certain frequency components of an audio signal by boosting or decreasing them.
Equalizers are widely used in audio systems, including during recording or with amplifiers and mixers. They are typically used to correct or adjust the response of speakers, amplifiers, microphone, or even the acoustics of a room. They are the foundation of any DJ's audio system.
A three-band audio equalizer.
In an EQ system, audio signals are divided into different frequency bands. These bands offer greater control over the frequency ranges and each has a separate slider, which controls the gain of the incoming audio signal. A current summer then matches the frequency bands with the updated audio range.
There are two types of equalizers:
1. Parametric Equalizer: Typically used in mixing and recording studios as these equalizers offer more precision. They allow the user to control bandwidth (also known as Q or shift quotient), center frequency, and frequency levels (the gain) via different knobs.
A parametric audio equalizer.
2. Graphic equalizer: has a fixed bandwidth and center frequency, but the gain of a frequency band can be adjusted using different sliders. The greater the number of sliders in this EQ means greater control over a wider range of frequencies.
A graphical audio equalizer.
Equalizers can be designed using audio filters or integrated chips (like an LA-3600, which is a five-band equalizer IC).
For this project, we will design a three-band graphic equalizer circuit using audio filters. Therefore, it will have low-pass, high-pass, and band-pass filter circuits to separate the low, high, and mid frequencies of the audio signal. The circuits will be active filters using an operational amplifier (operational amplifiers).
The gain for each frequency range will be regulated through variable resistors, connected to the input of the filter circuits. The different frequency components will be combined in the filters output stage and connected to an LM386-based power amplifier circuit. The amplifier circuit will boost the combined audio signal and direct it to a speaker.
To properly test this equalizer, we will analyze the frequency response curve of the audio filters. The curve will be drawn by plotting the voltage levels of the audio signal against the frequencies. A function generator will be used as the input source to produce sine waves at different frequencies.
We will use some common terms associated with audio amplifiers or audio filters, such as gain, cutoff frequency, bandwidth, and quality factor. We covered some of them in the previous tutorial: Understanding filters .
Required components
Block diagram
Circuit Connections
Our equalizer is designed to use three audio filters:
- A high-pass filter to separate high-frequency elements
- A bandpass filter to separate mid-frequency elements
- A low-pass filter to separate low-frequency elements
The gain for the different frequency bands is controlled through variable resistors, connected to the filter input. Subsequently, the frequency bands are combined into an audio signal and passed to a power amplifier and speaker.
The equalizer circuit is built by assembling the following components…
One power supply – A dual power supply is used to power the filter circuits, using two 9V batteries. Since a DC source is needed to bias the op amps, the batteries provide the negative and positive voltages.
The positive and negative supply voltages from these batteries are supplied to the operational amplifiers used in all filters.
- For negative voltage, the cathode of one of the batteries is connected to the negative power pin of the op amp and the anode of the battery is connected to common ground.
- For positive voltage, the anode of the other battery is connected to the positive power pin of the op amp and the cathode of the battery is connected to common ground.
A circuit diagram of the dual power supply for the active high-pass and low-pass filters.
Audio source – Audio input is provided by a smartphone, using a 3.5mm jack. The outlet must have three wires: one for ground, one for the left channel, and a third for the right channel. The wires that connect to the channels are used for the stereo systems.
In this system, the audio signal from both channels is transmitted with a phase difference of 180 degrees. The phase-shifted audio signals are combined to produce a noise-free audio signal, which is called a balanced audio system.
In our circuit, however, only one of the channels is used as an audio source. The ground wire of the connector is connected to common ground. Therefore, this audio system will be unbalanced and the audio source will be connected as a single-source or mono channel.
One 3.5mm audio jack.
The audio signal passes through the capacitor. Its impedance is inversely proportional to frequency and capacitance — therefore, the lower the frequency, the higher the impedance and vice versa.
The high frequency element of the audio signal will therefore have less impedance and will easily pass through the capacitor and into the non-inverting input of the amplifier. The low frequency element of the signal will contain a higher level of impedance. It is bypassed by the resistor connected to ground.
Capacitor impedance can be determined using this equation:
(Impedance), Xc= 1/ (2π*f*C)
The high-pass filter is designed using a 100 nF capacitor (“C1” in the circuit diagram) and a 3.2 kilo-ohm resistor (“R2”). Using these values for the capacitor and resistor, the filter cutoff frequency can be calculated as follows:
FH= 1/ (2πR2C1)
FH= 1/ (2π*3.2k*100n)
FH= 500 Hz (approx.)
The RC network forms a passive high-pass filter. Through this network, the filtered audio signal – which now carries only high-frequency signals – is passed to the non-inverting pin of the operational amplifier.
A circuit diagram of an LM741 operational amplifier in an active high-pass filter.
For this project, we are using operational amplifier IC LM741. The LM741 is a general-purpose operational amplifier with a low input impedance (in megaohms), compared to a FET operational amplifier, which has a high input impedance (in gigaohms).
The IC LM741.
The output impedance of the 741 should ideally be zero, but is typically around 75 ohms. The maximum supply current of the 741 IC is about 2.8 mA, with a supply voltage of up to +/- 18V.
The IC has the following pin configuration…
The IC has this pin diagram:
The IC has input and output overload protection and has zero latch when the common mode range is exceeded. The IC can receive a positive or negative supply voltage of up to 22 V and an input signal voltage (amplitude) of up to 15 V. Generally, a positive or negative voltage of at least 10 V must be supplied.
The internal circuit diagram of an operational amplifier IC LM741.
The LM741 can be configured as an open or closed loop amplifier and as an inverting or non-inverting amplifier.
In this circuit, IC LM741 was used as a non-inverting amplifier. The input signal of the passive high-pass filter is connected to the non-inverting input pin of the IC (pin 3). A 22 kilo-ohm resistor (“R5” in the circuit diagram) is connected between pins 6 and 2 of the IC, providing negative feedback. The inverting pin (pin 2) is grounded through a 2.2 kilo-ohm resistor (“R3”).
The amplifier gain is defined by these resistors and can be calculated as follows:
Gain = (R5/R3)
= 22/2.2 kilo ohms
= 10
As a result, the high-frequency element of the audio signal is amplified 10 times compared to the input audio signal. The output of the op amp is taken from pin 6 of the IC, which is connected to one of the speaker wires.
Low Pass Filter – An active first order low pass filter is connected to the circuit. For this filter, the audio input is passed to the non-inverting pin of the op-amp via an RC network. The audio signal passes through the resistor, which has a frequency-independent response. The high frequency elements of the audio signal are shunted through a capacitor to ground.
The capacitor's impedance is inversely proportional to the frequency and its capacitance – therefore, the lower the frequency, the higher the impedance and vice versa. The high frequency elements of the audio signal therefore experience less impedance and are easily shunted through the capacitor to ground. The low frequency elements of the audio signal experience more impedance and cannot bypass the capacitor.
The impedance of a capacitor can be given with this equation:
(Impedance), Xc= 1/ (2π*f*C)
The low-pass filter is designed using a 100 nF capacitor (“C2” in the circuit diagram) and a 3.2 kilo-ohm resistor (“R1”). Given these capacitor and resistor values, the filter cutoff frequency can be calculated as follows:
FH= 1/ (2πR2C1)
FH= 1/ (2π*3.2k*100n)
FH= 500 Hz (approx.)
The RC network forms a passive low-pass filter. Through the network, the filtered audio signal – which now carries only low-frequency elements – is passed to the non-inverting pin of the operational amplifier.
A circuit diagram of a 741 operational amplifier used with an active low-pass filter.
In this low pass filter circuit, IC LM741 is used as a non-inverting amplifier. The filter input signal is connected to the non-inverting input pin of the IC (pin 3).
As there is no feedback circuit, the OPAM gain will be unity. However, there is a variable resistor (“RV1” in the circuit diagram), which is connected to the input of the low-pass filter. By adjusting this variable resistor, the voltage level of the input audio signal increases or decreases.
The low frequency element of the audio signal will be proportional to the voltage level of the input signal since the gain of the op amp is unity. As such, the gain or strength of the low frequency band can be adjusted using the variable resistor RV1, which is typically mounted to a knob on the equalizer.
The output of the operational amplifier is taken from pin 6 of the IC, which is connected to the input of the power amplifier.
Bandpass Filter – An active second order bandpass filter is connected to the circuit. It is designed with a center frequency of 1 kHz and a bandwidth of 1.5 kHz.
A band-pass filter consists of a high-pass filter connected in series with a low-pass filter. Observation:
- The high-pass cutoff frequency is the lowest cutoff frequency of the bandpass filter.
- The low-pass cutoff frequency is the highest cutoff frequency of the bandpass filter.
- Only frequencies between the two cutoff frequencies high-pass and low-pass can pass through the output.
This bandpass filter has a frequency range between 500 Hz and 1.5 kHz.
As seen in the circuit diagram, a low-pass filter with a 1.5K ohm resistor (“R2”) and a 100 nF capacitor (“C3”) are connected in series with the high-pass filter. It uses a 100 nF capacitor (“C2”) and a 3K ohm resistor (“R3”).
IC LM741 is used for the op amp as a non-inverting amplifier. The passive bandpass filter input signal is applied to the non-inverting input pin of the IC (pin 3).
A circuit diagram of the 741 operational amplifier-based active bandpass filter, with an adjustable volume feature.
This is the second order called key bandpass filter. It only passes the frequency that is in its cutoff frequency range.
The bandpass filter has two cutoff frequencies – the lower and upper cutoff frequencies. The center frequency and bandwidth provide these two frequency bands as shown in the curve below…
A graph showing a typical frequency curve of a band-pass filter.
In this circuit, the capacitance of capacitors “C2” and “C3” is the same. The resistance of “R2” is also equal to the feedback of resistor “R4”.
The center frequency (fc) of this band-pass filter can be calculated as follows:
Fc= 1/ (2πRC)
Fc = 1/ (2π*1.5k*100n)
Fc=1kHz (approx.)
The resistor network (“R6” and “R7”) is responsible for the amplifier gain. For the non-inverting amplifier configuration, the gain can be calculated using this equation:
G = (1+R7/R6)
G = (1 + 1k/1k) = 2
The quality factor for this configuration is calculated as follows:
Q =1/(3-G)
So, Q = 1
The bandwidth of this bandpass filter can be calculated as follows:
Bandwidth =fc/Q
So bandwidth = 1.5 kHz
In the frequency curve graph, it can be seen that the lower and upper cutoff frequencies of this filter are:
- Lower cutoff frequency, fL = 500Hz
- Upper cutoff frequency, fH = 1.5 kHz
The frequency (between 500 Hz and 1.5 kHz) passes through the bandpass filter with a voltage gain of two. There is a variable resistor (“RV2”), which is connected to the input of the band-pass filter. By adjusting this variable resistor, the voltage level of the input audio signal increases or decreases.
The mid-frequency component of the audio signal will be twice the voltage level of the input signal since the gain of the op-amp is two. As such, the gain or strength of the mid frequency band will be amplified by two and can be adjusted using the variable resistor “RV2”.
This variable resistor is typically mounted on an equalizer knob. The output of the operational amplifier is taken from pin 6 of the IC, which is connected to the input of the power amplifier.
Power Amplifier – IC LM386 is used as summer power amplifier and not inverter in this circuit. Summer is responsible for adding the input signal and providing the output.
The three frequency bands are combined and amplified by the 386 IC. It has an internal gain of 26 dB when pins 1 and 5 are left open, meaning the input signal is amplified by a factor of 20.
The IC LM-386.
Speaker – A speaker with a power of 10 watts and an impedance of 8 ohms is used as a load at the output of the amplifier.
The speaker is connected to pin 5 of the IC, which is the output pin of the LM386. The speaker ground wire is connected to common ground. An RC network includes a 10 ohm resistor (“R8” in the circuit diagram) and a 100 nF capacitor (“C6”), which is connected before the speaker.
This is called the “Zobel network”. It ensures that the speaker impedance provides constant resistance to the amplifier after output. This stabilizes the frequency and oscillations in the output.
If capacitor “C6” C6 and resistor “R1” were swapped, it would no longer form a Zobel network, but the output impedance would still remain constant.
Pin 7 of the LM386, which is the “bypass terminal pin”, is grounded with a capacitor (“C5”), which is important to improve the stability of the amplifier output.
The following precautions must be taken when assembling this circuit:
1. Use only speakers equivalent to the amplifier output – or of high power.
2. Avoid clipping the output signal as this may damage the speakers.
3. Always place components as close together as possible to reduce noise in the circuit.
4. IC 386 starts to heat up when it provides high power, so it is recommended to first apply thermal paste to its surface, which will also increase its service life.
5. The breadboard produces a lot of noise and loose components, so it is recommended to make this circuit on PCB to obtain clear noise without distortion.
A prototype three-band audio equalizer.
How the circuit works
An audio signal is obtained from a smartphone connected via a 3.5 mm jack to a single channel. This signal is passed through low-pass, high-pass, and band-pass filter circuits, each with variable resistors at its input. Resistors adjust input signal levels and filters adjust gain for each frequency band accordingly.
For example, the high-pass filter separates high-frequency elements (above 1.5 kHz) and delivers them to the power amplifier with unity gain. The low-pass filter separates low-frequency elements (below 500 Hz), delivering them to the power amplifier with unity gain. The band-pass filter separates the mid-frequency elements (from 500 Hz to 1.5 kHz) and passes them to the power amplifier with a voltage gain of two.
All frequency elements are then combined at the input of the LM386 and amplified by 20 due to the IC's programmed set gain of 26 dB. The output of the power amplifier is directed to a speaker to regenerate the sound.
By adjusting the variable resistance at the input of the filter circuits, the gain of the three frequency bands can be changed. This can be used to reduce noise or to create special sound effects from the original audio.
A prototype active high-pass, band-pass and low-pass filter.
Testing the circuit
EQ can be tested by examining the performance of the filter circuits using the function generator as the input source. It generates a sine wave of constant amplitude and variable frequency.
Because an audio signal is essentially a sine wave, a function generator can be used instead of a microphone or other type of audio source.
Note: A speaker is not used as a load because it is resistive and inductive. At different frequencies, its inductance changes, which in turn changes the impedance (the “R” and “L” combination). Therefore, using a speaker as a load on the amplifier output may give false or non-standard results.
Instead, we will use a purely resistive dummy load. Since the resistance does not change with frequency, it can be considered a reliable load regardless of the frequency of the input audio signal.
The peak-to-peak amplitude of the function generator signal should be set to 100 mV, with a 10 ohm resistive load connected to the output (instead of the speakers). The cutoff frequency of the high-pass filter must be 1.59 kHz and that of the low-pass filter must be 500 Hz. The center frequency of the band-pass filter must be 1 kHz, with a bandwidth between 500 Hz and 1 .5 kHz. The power amplifier signal gain must be 20 (26 dB).
Based on our testing, the following frequency responses of the high-pass, band-pass and low-pass filters were observed:
This table is used to plot the frequency curve for the three filters. The frequency curve of the low-pass filter was:
The frequency response of an audio low-pass filter.
The frequency response of a band-pass audio filter.
The frequency response of an audio high-pass filter.
The frequency curve for high, low, and bandpass filters can be plotted more accurately by taking voltage level readings for a larger number of frequencies.
In this tutorial, we designed a three-band audio graphic equalizer that allows you to boost or change audio signals at high (above 1.5 kHz), low (below 500 Hz), and mid (from 500 Hz to 1.5 kHz) frequencies. kHz).
The output power of this equalizer is 700 mW and the voltage gain is 26 dB. This equalizer circuit can be used in DJ systems and for mixing soundtracks.
In the next tutorial, we will learn how to design an audio mixer.
