Building a Three Channel EQ on a Breadboard





Introduction: Building a Three Channel EQ on a Breadboard

This Instructable will show you how to build a three-channel audio equalizer on a breadboard   An audio equalizer is a device used to regulate the tone or frequency response of an audio system to reach the desired sound quality.  Equalizers are used in home stereo systems, loudspeakers, microphones, etc. They can be used to cancel out unwanted sounds. This Instructable will show you how to wire an audio equalizer using resistors, capacitors, and potentiometers on a breadboard, along with how to test your finished product. This simple, easy-to-understand guide to building an audio equalizer is great for any expertise level, beginner to guru.  The project can take anywhere from 20 minutes to a few hours based on the builder's experience level.  Digi-key or Mouser are good sites to find all these parts. The parts themselves are actually fairly inexpensive.
Good Luck!

Step 1: Parts List

Five 10k ohm resistors
Two 3.3k ohm resistors
Two 1.8k ohm resistors
One 270 ohm resistor
Two 100k ohm potentiometers
One 500k ohm potentiometer
One 33 uF capacitor
One 0.1 uF capacitor
One 22 nF capacitor
One 47 nF capacitor
Two 4.7 nF capacitors
One 0.47 nF capacitor
One 1 nF capacitor
*One LF351 op amp
12V Power supply
Several wires of assorted lengths
One breadboard

*The design calls for an LF351, but but a UA741, LM356, or any other standard op amp will work.

To understand the electronic color code for the resistors, please visit the website

Step 2: The Breadboard

A breadboard is a tool to set up circuits for testing. Breadboards have many tiny holes arranged on a 0.1" grid. The holes in the top two rows and bottom two rows are linked horizontally across as shown between red and blue lines on the diagram. The other holes are linked vertically. Each column of five holes is connected with no link across the center.

Note: When you are trying to put parts on breadboard, please do not connect their positions depending on the circuit diagram, because the arrangement of components on breadboard will look quite different from the circuit diagram.

Step 3: Placing Resistors

Now that you have all of the parts, we’ll start with the resistors. A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element.

R1 – 10K (brown, black, orange)
R2 – 10K (brown, black, orange)
R4 – 3.3K (orange, orange, red)
R5 – 3.3K (orange, orange, red)
R7 – 1.8K (brown, grey, red)
R8 – 1.8K (brown, grey, red)
R10 – 10K (brown, black, orange)
R11 – 10K (brown, black, orange)
R12 – 10K (brown, black, orange)
R13 – 270Ω (red, purple, brown)

Find the right resistor and put the leads into the breadboard holes.

Step 4: Placing Capacitors

Now let’s put on the capacitors.  A capacitor is a passive two-terminal electrical component used to store energy in an electric field. (Wikipedia)
The breadboard associated with this step has both the resistors and capacitors attached.

C1 – 47 nF
C2 – 4.7 nF
C3 – 0.47 nF
C4 – 22 nF
C5 – 4.7 nF
C6 – 33 µF (Be sure to place the leads of this capacitor into the breadboard correctly.)
C7 – 1 nF
C8 – 0.1 µF

Find the right capacitor and place it in the corresponding breadboard holes from the pictures.

Step 5: Placing Potentiometers

Next are the potentiometers. A potentiometer (or pot) is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. (Wikipedia)  
R3 – 100K
R6 – 100K
R9 – 500K
Although the pots look similar, there are numbers on the sides that coorespond to the value of each pot.  Be sure you understand how to read these so they can be placed properly.  It is also worth mentioning how the pot's terminals are layed out.  The middle offest terminal is where the resistance changes, so be sure to place that accordingly.

Step 6: OP AMP

In this audio equalizer, we use the LF351* op amp. It is an integrated circuit and it uses an 8-pin dual in-line package, which can easily plug into the breadboard. There is no specific ground connection. For our circuit, use op amp power supply voltages of 12V for the positive terminal (pin 7), and ground the negative power terminal (pin 4). The pin arrangements for the op amp are shown below. Be sure to look at the completed breadboard to see that the op amp is placed in its correct position.

*Or 356. or 741, or whatever might be available to you.

Step 7: Testing

Checking and Testing:
- Make sure all wires’ connections are correct.
- Check that potentiometers and capacitors placed properly (the only capacitor where direction matters is the 33uF, which is C6).
- Check that no leads are touching.
- Connect the breadboard to a 12V supply and test the circuit with an audio input (connect + to the input terminal, connect - to ground).
- If your circuit does not work, turn off the power supply and re-check every connection very carefully based on the Figure.
- The reasonable output will be a sound wave with low/mid/high frequencies attenuated based on the turn of each potentiometer.
- Be sure to refer back to the picture (the MS paint version) of the breadboard with all of the parts placed properly.  The photo may be a bit confusing because when it was taken; a few elements (capacitors) were hidden behind the resistors. 

When you've got it working, Congratulations!  You have just built yourself an audio equalizer, capable of adjusting the frequencies to your choosing.  If you want to tone down of some of those annoying higher-pitch notes in a song, just turn down the high frequency knob.  If that bass line is just a little too much for you, then voila! the music bows to your command.  You, my friend, are now Master of the Melodies.



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    Hi, I was wondering about the schematic and the friz file. In schematic you have f.ex. 1.8k resistors and 0.47nF cap going to the 500k pot, but in friz 10k resistors and 47nF cap. Also, I don't really understand why both V+ (pin7) and Input + (pin3) would be going to voltage + input. In the schematic V+(pin7) goes to ground. There might be simple explanation and I might be completely wrong, but when I tried to build the circuit following the schematic my breadboard started to melt... Maybe the circuit you're showing on breadboard might be the one that works? Gonna try that one now.

    will this need a pre amp to work? with an audio signal?


    How have you chosen the resistor and capacitor values?
    Is there any equations or formula behind that?

    Can you replace the lf351 with lm386? I tried that but i can hear only noise. I changed pins 6 and 7 (Vcc and out) for 6 and 5. But it did not work.

    Very nice, I favorited it. I'm going to build it when I finish the projects I'm working on.

    Is there any need to add power amplifier in the circuit?

    And if there is then how can i add it?

    Would this circuit work well with 5v instead of 15v?

    no, supply voltage range is 6 to 32v


    How can i ad a 3.5 mm input and output, or how can i connect phone and speakers.


    you could try something like this or you could just find an old pair of headphones and cut the wire off that. You should find 3 wires inside, one red, one green and one black. Red and green are right and left (not sure which way round) and black is common.