In this article, we'll be creating a simple 3-band equalizer that you can use for any audio project. It's very module and is a little box that can sit between any audio source and speaker so that you can boost that bass up for better sound!
We'll be covering the basics of how to filter out specific frequencies using RC networks (resister capacitor circuits) so if you are new to low pass, high pass and band pass filters, this is a great introduction for you. With this knowledge, you could easily go on to create more complicated 10-band equalizers.
The human audible range is from about 20Hz to 20K Hz. So when you are listening to music, you are hearing the sound waves somewhere within that frequency range.
With a 3 band equalizer we break up the spectrum in three sections
For each of these 3 bands, we create filters in our circuit to control their respective audible volumes. You can pump up that low frequency bass sound or increase those treble high sounds.
For this project, you'll need:
For the capacitors and resistors, if you don't have those specific values, you could substitute them for different values by calculating your target frequency. Don't worry, we'll go through exactly how to do this.
Unlike some of the other articles, this particular project should be consider more as a module than a complete project. Here is what we'll be building.
Our 3 band equalizer will accept:
We'll have 3 potentiometers (10K), one for each range (bass, mid and treble) to control the volume of each frequency range.
The outputted audio would normlaly be passed on to an audio amplifier such as the LM386. The audio amplifer would then be responsible for amplifying the signal so it can be heard on a speaker.
The reason we need an amplifer between the equalizer and speaker is because the direct signal from a microphone or audio source is typically too small to directly fed into a speaker.
If you've never set up an amplier, learn more about how to setup your own amplifier in our LM386 article.
Also keep in mind that this is a rather crude and simple 3 band equalizer but it helps to demonstrate the ideas of RC filters.
Here is the full schematics. Don't worry we'll be breaking down each portion in the following sections.
Additionally, here is birds eye view of a breadboard prototype. You'll notice that there is are three lines, Audio In, GND and Audio Out.
Each piece of this circuit is really formulated through three types of RC circuts.
RC circuits are a circuits created with a resistor (R) and capacitor (C) and depending on it's configuration can block specific frequencies and allow other frequencies to pass through.
The key to understanding why RC circuit work is because of capacitive reactance in which the capacitor can't instantly change its charge. As a capacitor is being charged or discharged, the current flow is restricted by the internal impedance of the capacitor.
Let's look at three types of filters, the high pass, low pass and band filter.
This results in this circuit cutting off higher frequencies and only allowing low frequencies to get through.
That is, a low pass filter allows any frequency less than its frequency cut off to get through.
The exact frequency cut off depends on the capacitor and resistor values you use. We'll get to how to calculate the frequency cut off below.
Now the question is, what value capacitors and resistors should you use to cut off specific frequencies?
This can be calculated using this formula.
For example, let's say we select 10ohm resistor (R) and a 10uf capacitor(C).
frequency cutoff = 1 ÷ ( (2 * 3.1415) x R x C)
frequency cutoff = 1 ÷ ( (2 * 3.1415 x 10 x 0.00001))
frequency cutoff = 1 ÷ (0.0006283)
frequency cutoff = 1591Hz
So if you used, a 10ohm resistor and a 10uf capacitor:
Actually calculating the values can be a cumbersome so feel free to use our RC circuit calculator.
Now let's look at our final type of filter which is the band pass filter.
A band pass is created by a combination of a high-pass filter and a low pass filter. The band pass filter only allows a certain range of frequencies to pass by using the combination of a high pass and low pass filter.
In the first stage, a high pass filter is used to say, I want only frequencies of this and above to pass through.
In the second stage, a low pass filter is used to say, I want frequencies up to this value to pass. The low-pass cut off frequency would have to be larger than the high pass cut off frequency for this to work.
Now that you have an understanding of high, low and band pass filters, let's take another look at the equalizer circuit.
For the treble filter, I've used a high pass filter connected to a potentiometer. Capacitor (C6) at 0.47uf and Resistor (100R). This results in letting through >3386Hz for this treble filter.
The output connects to a 10K potentiometer (R3) to control the volume of this specific treble range.
For the bass filter, I've used a low pass filter. Capacitor (C8) at 10uf and Resistor (220R) calculates let's any frequency <72Hz through the low pass filter.
Again the output connects to a 10K potentiometer (R7) to control the volume of this specific bass range.
The final mid range is a band pass filter that let's through any range from 72Hz ~ 3386Hz. Basically the range that isn't covered by the treble and bass filters.
You'll notice that the high pass filter portion of the band filte is configured to let through any frequency above 72Hz by using a 10uf capacitor and 220R resistor. The low pass filter is set to cut off any frequency above 3386Hz.
As with the other filters, the output connects to a 10K potentiometer to control the volume of this specific mid range.
As a last step of the project, I designed a PCP circuit based on the schematics above, printed a 3d case for it and put everything together.
I'm a senior front-end engineer by day and electronics inventor by night. I setup this site to share my explorations, discoveries and learnings with you.
Feel free to mail me if you have any questions or feedback.
