Introduction: 4 Band Equalizer

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About: Sound and Image Engineering student passionate about audio technology and basic analog audio equipment design. I work on projects ranging from DIY audio gear to smart audio systems and IoT devices. Currently e…

This project is a high-quality, Single-Channel 4-Band Equalizer built on a standard prototyping board. While most professional audio equipment requires complex dual-rail power supplies (like +15V/-15V), this circuit is designed to be accessible and compact.

The design is based on a previous design from the book: , we recommend that book if you're interested in audio circuits.

We will be using a "Virtual Ground" architecture. This technique allows us to power high-fidelity Op-Amps (like the TL072) using a standard, single 12V DC adapter. By creating a stable reference point at half the voltage, we get the necessary "swing" for clean audio signals without the bulk of a heavy transformer.

In this Instructable, we will cover:

  1. How to build the specific voltage divider circuit to create a stable Virtual Ground.
  2. Selecting the right coupling capacitors to ensure a full frequency response (deep bass and clear highs).
  3. Wiring techniques using shielded cabling to prevent hum and noise when connecting to an audio interface.
  4. Alternatives in some components depends on the budget.

If you want to start building your own outboard studio gear, this is the perfect place to start (actually is our first project of this kind).

Supplies

We discuss several customization choices throughout this guide. We highly recommend reading the full Instructable before purchasing your parts to ensure you choose the best options for your setup.

Electronic Components (EQ Circuit)

  1. Resistors (Metal Film, for less noise):
  2. 4x 1.6kΩ
  3. 2x 3.3kΩ
  4. 2x 4.7kΩ
  5. 2x 22kΩ
  6. 2x 180kΩ
  7. Capacitors:
  8. 2x 2.2nF (Polyester/Film)
  9. 1x 4.7nF
  10. 1x 22nF
  11. 1x 33nF
  12. 1x 100nF
  13. Potentiometers:
  14. 4x 20kΩ Linear Taper (B20K)
  15. Op-Amp:
  16. 1x TL072 or NE5532 (Dual Op-Amp IC) + IC Socket (8-pin).

Power Supply & Input/Output Stage (Required for 12V Single Supply)(Essential for the Virtual Ground and Audio Coupling)

  1. Virtual Ground Splitter:
  2. 2x 10kΩ or 22kΩ Resistors (to split the voltage).
  3. 1x 47µF Electrolytic Capacitor (Power filtering).
  4. Audio Coupling (DC Blocking):
  5. 1x 1µF or 2.2µF Capacitor (Input).
  6. 1x 10µF Electrolytic Capacitor (Output).
  7. Power:
  8. 1x 12V DC Power Supply Adapter.
  9. 1x DC Barrel Jack Connector (Female/Panel Mount).

Connectors & Wiring

  1. 2x 1/4" (6.35mm) Mono Audio Jacks (Female/Panel Mount).
  2. Shielded Audio Cable (for Input/Output connections).
  3. Hook-up Wire (22 AWG solid core for PCB, 24 AWG stranded for pots).
  4. Prototyping PCB (Perfboard or Stripboard).

Enclosure & 3D Printing

  1. 3D Printer (FDM).
  2. PLA Filament.
  3. (Alternative): If you do not have a 3D printer, you can use a generic project box and knobs or order the printed parts from a 3D printing service.

Step 1: Designing the Power Supply Circuit

To run this circuit on a standard 12V DC adapter (instead of a complex dual supply), we create a Virtual Ground at 6V. This is done using a simple voltage divider (two equal resistors) stabilized by a capacitor.

Key Wiring Rules:

  1. Op-Amp Power: Connect Pin 8 to real +12V and Pin 4 to real 0V.
  2. Audio Grounds: All ground symbols in the schematic must connect to the Virtual Ground (+6V) reference point.
  3. DC Blocking: You must add capacitors at the Input (approx. 2.2µF) and Output (10µF) to block the 6V DC and protect your external equipment.

WARNING: Connect the metal shield (sleeve) of your input/output Audio Jacks to the Real Ground (0V), never to the Virtual Ground.

If you are using an electrolytic capacitor, you must be careful with the polarity of the capacitors. Connecting the positive leg to the virtual ground of this circuit and the negative one to the real ground (also the -6V on our circuit).

Optional:

You can use a 18/24V DC adapter, to get more headroom, but for most scenarios 12V will be enough. If you decide to use a higher voltage just check that the capacitors allows that voltage range (at least 35/50V).

Step 2: Soldering the Main Circuit

Since this is a custom build, the physical layout is up to you, but following a logical order will make soldering much easier and prevent noise issues, you can try to solder it in the same way we did it or any other way.

1. Place the IC Socket First

  1. Never solder the Op-Amp chip directly. Always use an 8-pin DIP socket, if you solder the op-amp directly you could burn it.
  2. Orientation: Mark on your board where Pin 1 (the notch) is facing. Solder the socket corners to anchor it, but do not insert the chip yet.

2. Create the Power Rails Before adding audio components, establish your power lines using the cut-off legs of resistors or solid-core wire on the bottom of the board:

  1. Run a line for V+ near Pin 8.
  2. Run a line for V- near Pin 4.
  3. Run a Ground bus (a thick trace or wire) through the center or edge of the board. Connect Pins 3 and 5 of the socket directly to this Ground bus now.

3. Solder "Stage 1" (Left Side of Schematic) Build the circuit around the first half of the Op-Amp (Pins 1, 2, 3) to avoid confusion. Also is recommended to get the pins that you have to connect to the potentiometers near to the bottom side of the perfboard to avoid long flying cables which cause noises and may be annoying during the rest of the assembly. During the supplies step, we recommended using metal film resistor but common resistors will work as well.

  1. Resistors R1, R2, R5, R6: Place these close to the input side. Note that R1/R2 and R5/R6 form voltage dividers; solder their junction points together on the bottom of the board.
  2. Capacitors C1 & C2: Place these near the area where you will attach the "LO MID" potentiometer wires.
  3. Feedback Loop: Getting the Op-Amp physically close to R5, R3, R4, and C3 will reduce interferences.

4. Solder "Stage 2" (Right Side of Schematic) Now move to the second half of the Op-Amp (Pins 5, 6, 7).

  1. The Bridge: Connect the output of Stage 1 (Pin 1) to the input resistor network of Stage 2 (R8/R7 junction) using a short jumper wire or solder bridge.
  2. Components: Install R7, R8, C4, R9, R10, C5, R11, R12, and C6. Mirror the layout logic you used for Stage 1.

5. Install "Flying Lead" Headers or Pins We do not recommend soldering the potentiometers directly to the board. Instead, leave empty holes labeled for wires at these specific points:

  1. IN (Input signal)
  2. OUT (Output signal)
  3. LO MID Connections: (To R1/R2 junction, to C1/C2 junction, and Ground).
  4. HF, LF, HI MID Connections: (Repeat for the other 3 control sections).

Pro-Tip for Perfboard: Use the "Low to High" rule. Solder the lowest profile components first (resistors), then the socket, then the small film capacitors. This ensures the board stays flat on your work surface while you solder.

Step 3: Connecting Potentiometers

Potentiometers have 3 legs. Wiring them correctly ensures that turning the knob "up" (clockwise) boosts the frequency, rather than cutting it.

Looking at the schematic arrows labeled CW (Clockwise):

  1. Pot Orientation: Hold the pot with the shaft facing you and the 3 legs pointing down. know you should connect the right leg to the side pointed by the arrow named CW on the schematic.
  2. Pin 2 is always the Wiper (the arrow in the schematic).
  3. The Schematic Logic:
  4. If you turn the potentiometers counterclockwise, the signal should be attenuated.
  5. If you turn the potentiometers clockwise, the signal should be amplified.

If the circuit doesn't work like that, then you should change the cable connected to pin 1 with the one connected to pin 3.

Step 4: Mounting in the Case

1. Drill the Box or use the attached design

  1. Create a template to mark positions for the 4 Potentiometers, Input/Output Jacks, and Power Socket.
  2. Drill the holes.


2. Mount the Hardware

  1. Install the Potentiometers and Jacks into the holes and tighten the nuts before soldering wires to them.
  2. WARNING: Ensure that the power socket have the same format that the power supply. Also you have to check the polarity of the charger, every power supply have a draw like the one below.

1. If your power supply matches the Left Symbol (Center Positive)

Most common standard for general electronics.

  1. +12V Wire: Connects to the Center Pin.
  2. Ground Wire: Connects to the Outer Sleeve (Barrel).

2. If your power supply matches the Right Symbol (Center Negative)

Common for guitar pedals and some musical equipment.

  1. Ground Wire: Connects to the Center Pin.
  2. +12V Wire: Connects to the Outer Sleeve (Barrel).
  3. Orient the potentiometer legs inward so they are easy to reach with the soldering iron.


3. Wire the Connections ("Flying Leads")

  1. Cut wires to reach from the board to the components.
  2. Potentiometers: Solder 3 wires per pot. Twist these 3 wires together like a braid to cancel out noise. Connect them to the corresponding points on your Perfboard.
  3. Jacks: Connect the Tip to Signal (IN/OUT) and the Sleeve to Ground.


4. Insulate and Secure the Board

  1. Crucial: If your box is metal, glue a piece of plastic or thick cardboard to the bottom of the box.
  2. Place the Perfboard on top of this insulation to prevent short circuits. Secure it with adhesive foam tape or hot glue.
  3. Also you could solder the ground cable to the case to create a faraday cage.

Attachments