Analysis of the working principle of audio equalizer circuit

Equalization is a technique widely used in studio sound production and recording. Using an audio equalizer, we can control different frequency bands in the audio spectrum using linear filters. Simply using an equalizer, you can adjust the frequency range that is allowed and the frequency range that is rejected from the audio signal. This will modify the rhythm, tone, and other different aspects in the audio signal.

The most commonly used equalizers have 3 knobs to control the frequency range within bands like bass, mids, and treble. But some high-end equalizers have dedicated controls for each specific frequency band for more precise control. This equalizer comes with three knobs where you can modify the audio signal to your liking.

Working principle of audio equalizer circuit:

It is very simple to make an audio equalizer at home. We only need an audio amplifier and a few passive components to make a good sounding equalizer. This equalizer circuit consists of a four-section high-pass filter, a band-pass filter, a low-pass filter for filtering out signal frequencies, and an audio amplifier for boosting the strength of the output audio signal.

Signal filter:

In the first stage of the circuit, the input signal is fed into our circuit using the RCA connector. Then, the signal is filtered through a series of filters high pass, band pass and low pass. The filters are used to cut off the corresponding frequencies of the source signal within the allowed range. For example, the high pass filter only allows frequencies above the set frequency (fc) to pass. This fc is determined by the formula given below.

fc = 1/(2*pi*R*C)

Let's see how to choose the component values. As we all know, the audible frequency range is 20 Hz to 20Khz. So, in order to get 3 different EQ-bands, we need to split this range into 3 different bands. This gives us an approximate frequency value of 6.6kHz. For the sake of convenience, let's take 7kHz. Now we need to fix the center frequency of all these three filters.

Low pass filter:

The center frequency of the low pass filter is 7kHz, which means the filter should block signals above this frequency. Using this center frequency, we need to find the value of R & C. First, let's choose the value of the capacitor. Here we are using a 0.1uf capacitor. So we only need to calculate the value of R.

So we put these values ​​into the equation to determine the value of the resistor, R.

fc = 1/(2*pi*R*C)

= 1/(2*3.14*R*0.1uF)

R = 1 / (4.396 * 10-3)

R = 220 (We choose the closest standard value resistor)

High Pass Filter:

Let us calculate the fc value of the high pass filter. The centre frequency of the high pass filter is fixed to 14Khz, which means the signals with a frequency exceeding 14Khz will pass through this filter. Now calculate the resistor used in the high pass filter, fix the value of the capacitor C to 0.1uF and apply in the formula gives

fc= 1/(2*pi*R*0.1uF)

14Khz = 1 / (2 * 3.14 * R * 0.1uF)

R = 110 (select the closest standard value)

Bandpass filter:

Now what is left is the band pass filter. This is quite tricky. So in order to have the filter set to a mid frequency between 7khz and 14khz we use a band pass filter. This band pass filter is built using a low pass filter in series with a high pass filter. Remember that the low pass filter used here should have a center frequency greater than the high pass filter in series with it. We want to cover the frequency range between 7.4kHz and 14.4kHz. To do this we should set the low pass filter center frequency to 14.4kHz and the high pass filter to 7.4kHz. Notice that the resistor values ​​are swapped for the low pass and high pass sections of the band pass filter.

Finally, we connect potentiometers VR1, VR2, and VR3 in parallel to each filter. This potentiometer will allow us to adjust the gain of each signal individually the way we like it. The sum of these signals will go into the audio amplifier to amplify the output stage.

LM386 Audio Amplifier:

We use LM 386 audio amplifier IC in this audio amplifier. It is cheap and easily available. It is used in low power audio power amplification applications. Its internal gain is set to 20 but it can be adjusted using a capacitor between the gain pins (pin 8 and pin 1). A variable resistor can also be used along with the capacitor to fine tune the gain.

As you can see in the circuit diagram, there is a capacitor CP1 and a variable resistor VR8 between pin 1 and pin 4. To adjust the gain of the IC. The output signals from the three filters will be added to the op-amp input. The gain of the summed signal will be amplified at the output of the LM386. This will be fed to the output speaker connected to the LM386. Usually using capacitors and potentiometers VR4 and CP1, we can adjust the overall signal gain from 26dB to 46dB.

Notice:

By building this circuit, you can only get a mono audio signal. If you want stereo output, you just need to make another equalizer similar to this one. Then, you can pass the left channel of the stereo audio signal to one equalizer and the right channel to another equalizer. In this way, you will be able to control these two audio channels individually and get great audio/music.

Parts List:

Audio source (Audio or RCA jack)

5 to 12V battery

Capacitor 0.1uF (5), 0.005uF (1),

Polarized capacitor 10u (1), 250u (1)

Integrated Circuit LM386 Operational Amplifier

Speaker (8 to 32 ohms)

Resistor 110 ohm (2), 220 ohm (2), 10 ohm

Potentiometer 10k (4)