Tutorial on Designing Active Tone Control Circuits

Tone control or active equalizer circuits, especially equalizers based on bass, treble and mid control are important circuits in audio amplifier design. Usually, a three-stage active equalizer filter requires three controls 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 between high and low frequencies. In this project, we will design an active tone control circuit powered by an operational amplifier with a PCB design. It will work with a 12V power supply and have bass, treble and mid controls so that the output audio can be adjusted as required.

Required Components

Given below are the components required to build this tone control circuit using an operational amplifier.

100k- Potentiometer - 2 pcs

470k - Potentiometer - 1 pc

TL072 Operational Amplifier

12V power supply

.1uF 35V Capacitor

1.2nF 63V Capacitor

100uF, 35V

10uF, 35V

2.2uF, 63V

22k resistor

22nF 63V capacitor

270R resistor

33pF capacitor

4.7nF 63V Capacitor - 2 pcs

47nF

1.8k - 2

10uF, 25V - 2 pcs

3.3k - 2

47k - 2

10k - 5

Printed Circuit Board

Audio equalizer circuit diagram

The complete bass tweeter circuit diagram is shown in the figure below. The main component of this circuit is the operational amplifier. The operational amplifier TL072 is a popular operational amplifier that has two independent operational amplifiers in a single monolithic package.

An explanation of the circuit is below, but you can also jump to the video at the end of this page which also explains how the circuit works. The diagram below shows the pinout for 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 through a three-band filter. Capacitor C4 is a DC blocking capacitor that blocks DC signals and allows only AC signals to pass.

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

Midrange, bass and treble control circuits:

In the next stage, IC1B is the actual active filter, which has three pass filters connected across the negative feedback loop. This is the actual tone filtering that is happening -

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

The three-band filters are all RC filters. Since the capacitor value cannot be changed, a variable potentiometer is used here to change the resistor value. Here, resistor R12 and capacitor C11 are used for gain setting. Changing the R12 value 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 cut-off frequency, the following formula can be used −

fc = 1 / 2piCR

fc is the cut-off frequency, C is the capacitor value and R is the total resistance of the network. So changing different potentiometer values ​​or changing the C7 capacitor will change the frequency response of the bass filter (low pass filter).

Calculate the cutoff frequencies for the bass and tweeter circuits:

For example, in the above circuit, the value of the potentiometer is 100k. Therefore, the total resistance, 100k (bass pot) + 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 a MID filter. But instead of using a low-pass or high-pass filter, it uses a band-pass 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 values, it is 10.2 kHz) and the lowest frequency band can be calculated using -MID potentiometer value + R10 as the total resistor and capacitor C9 (according to the schematic values, it is 70 Hz).

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

Bias/Offset Circuit:

Since this is a single rail supply voltage and the negative rail is not used, the input signal needs to be offset. This is because the op amp cannot amplify the negative peaks of the input signal in single rail supply mode.

To create the offset, a voltage divider is placed across the positive feedback of the op amp. The voltage divider will offset the signal by half of 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 along with an additional filter capacitor C3.

Active Audio Filter PCB Design

The PCB design for our active audio filter circuit is for a double sided board. I used Eagle to design my PCB, but you can use any design software of your choice. A 2D image of my board design is shown below.

Sufficient ground filled vias are used to properly create ground paths throughout the board. The input signal and input voltage sections are created on the left and the output is created on the right to improve usability.

Assembling and testing 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 is attractive and PCBway offers all mask colors at the same price, so why not have some fun with PCB colors.

As you can see from the picture above, the quality of the PCB is very good. The tracks, pads, vias, and other clearances are all manufactured perfectly. 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 was not exactly as required, but it did not have any effect on the circuit output. Also, the op amp TL072 was replaced with a JRC4558 due to unavailability of the IC. Other op amp ICs may also work, but the pin mapping must match the standard op amp pin mapping.

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