Tone control or active equalizer circuits, especially those based on bass, treble, and MID controls, are important circuits in audio amplifier design. Typically, a three-stage active equalizer filter requires three controls for bass, treble, and MID. The bass control allows low frequencies to pass but blocks high frequencies, the treble control allows high frequencies to pass but blocks low frequencies, and the MID control balances high and low frequencies. In this project we will design an active tone control circuit powered by an op amp with a PCB design. It will work with a 12V power supply and has bass, treble and midrange controls so that the output audio can be adjusted as needed.

required components

The components required to build this tone control circuit using an op amp are given below.

100k- Potentiometers – 2

470k- Potentiometer – 1

TL072 Operational Amplifier

12V power supply

.1uF 35V capacitor

1.2nF 63V capacitor




22k resistor

22nF 63V capacitor

270R resistor

33pF capacitor

4.7nF 63V Capacitor – 2


1.8k – 2

10uF, 25V – 2

3.3k – 2

47k – 2

10k – 5

A printed circuit board

Audio Equalizer Circuit Diagram

The complete bass and treble circuit diagram is shown in the figure below. The main component of this circuit is the operational amplifier. The op amp TL072 is a popular op amp with two independent op amps in a single monolithic package.

The circuit is explained below, but you can also skip to the video at the end of this page, which also explains how the circuit works. The figure below shows the pinout of the TL072P Op-Amp. The two op amps are depicted in the schematic as IC1A and IC1B.

Operational amplifier buffer circuit:

IC1A is configured as an inverting buffer amplifier. This buffer amplifier provides a buffered output of the input signal for filtering or equalization by a tri-band filter. Capacitor C4 is a DC blocking capacitor that blocks DC signals and only allows AC signals to pass through.

Resistors R3 and R4 need to be accurate and matched. It is recommended not to change these two values ​​at this stage. The output 2.2uF, C6 capacitor will pass the signal from the buffered output.

Mid, Bass and Treble Control Circuits:

In the next stage, IC1B is the actual active filter with a three-pass filter connected across a negative feedback loop. Here’s the actual pitch filtering that’s happening –

The negative input comes from a 2.2uF capacitor. Op-amp IC1B is again configured as an inverting amplifier, which takes the inverting input from IC1A and inverts again at the output.

The three-band filters are all RC filters. Since the capacitance value cannot be changed, a variable potentiometer is used here to change the resistance value. Here, resistor R12 and capacitor C11 are used as gain settings. Changing the value of R12 will also change the gain.

In the first filter is the bass filter (low pass). The first network circuit is R8, the bass potentiometer, R9 is the total resistance of the filter, and the capacitor is C7. To determine the cutoff frequency, the following formula can be used −

fc = 1 / 2piCR

fc is the cutoff frequency, C is the capacitance value, and R is the total resistance of the network. So changing a different potentiometer value or changing the C7 capacitor will change the frequency response of the bass filter (low pass filter).

Calculate the cutoff frequencies of the bass and treble circuits:

For example, in the above circuit, the value of the potentiometer is 100k. So total resistance, 100k (bass pan) + 10k (R8) + 10k (R9) = 120k. Therefore, according to the formula, the bass control can handle frequencies up to 28 Hz.

The same thing happens with MID filters. But instead of using a low-pass or high-pass filter, it uses a bandpass filter structure.

The cutoff frequency can be obtained using the same formula fc = 1/2piCR. The highest frequency band can be calculated using resistor R6 and capacitor C8 (according to the schematic value it is 10.2 kHz) and the lowest frequency band can be calculated using – MID pot value + R10 as the total resistance and capacitor C9 (according to the schematic value it is 70 Hz) .

In the last filter band it is a treble control with a high pass filter. The formula hasn’t changed, it’s the same fc = 1/2piCR. The total resistance is the tweeter resistance, and the R11 and capacitor are C10. When the treble is completely low, that means the pot is exactly 470k using the schematic value and the filter has a cutoff frequency of -71 Hz. But in full treble mode, when the potentiometer is fully on, the resistance of the potentiometer becomes negligible, and only the resistor R11 works. In this case, the cutoff frequency becomes -18 kHz. The output is obtained from C12.

Bias/Offset Circuits:

Since this is a single rail supply voltage that does not use the negative rail, the input signal needs to be offset. This is due to the inability of the op amp to amplify the negative peaks of the input signal in single-rail supply mode.

To create the offset, place a voltage divider across the positive feedback of the op amp. The voltage divider will cancel the signal by half the supply voltage. Since it uses a 12V supply, the input signal is offset by 6V DC. C1 and C2 are filter capacitors, R1 and R2 are used to make the voltage divider and an additional filter capacitor C3.

Active Audio Filter PCB Design

The PCBs of our active audio filter circuits are designed for double-sided boards. I use Eagle to design my PCBs, but you can use any design software of your choice. A 2D image of my board design is shown below.

Sufficient ground fill vias are used to properly create ground paths throughout the board. Input signal and input voltage sections are created on the left and outputs on the right for better usability.

Assemble and Test Active Audio Filter Circuits

The top and bottom layers of the board are shown in the image below. We chose red for the solder mask simply because it’s attractive and PCBway offers all mask colors for the same price, so why not have fun with PCB colors.

As you can see from the image above, the quality of the PCB is very good. Tracks, pads, vias, and other gaps are perfectly fabricated. I started assembling my board as soon as I received it. You can see the assembled board below.

However, for a few capacitors, the voltage rating is not as accurate as required, but will not have any effect on the circuit output. Also, the op-amp TL072 was replaced with JRC4558 due to the unusable IC. Other op amp ICs will work, but the pin mapping must match the standard op amp pin mapping.

The circuit was tested using an audio input from a laptop, a 12V power supply, and a 15W 2.1 speaker output system.

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