Introduction

An equalizer (or tone control) allows to an audio system frequency response to be adjusted to compensate the response of the speakers in the room or auditorium in which they are being listened, or simply to customize as preferences by the listener.

The equalizer in this project consists of a high-fidelity tone controler circuit that offers cut, flat, and boost characteristics for the three typical audio frequency bands (bass, midrange, and treble).

Circuit Diagram

Basically, circuits from an equalizer are formed by a filters network for different audio frequency bands. These filters can be passive, which use coils, capacitors, and resistors only; or they can be active, in addition to the components mentioned, use transistors and integrated circuits that add amplification functions.

In the construction of equalizers, an active filter design is often chosen over a passive filter design, since active filter circuits have the frequency-response components located in the feedback loop of the filter amplifiers, providing much lower THD, and a symmetrical frequency response about the axis over in cut and boost, compared with most passive designs.

The circuit, described here, consists of an active filter built around an operational amplifier. This circuit also includes an input buffer amplifier (section indicated as U1A) to provide some gain and isolation from source impedance variations. A simplified block diagram of the equalizer circuit is shown in Figure 1.

Figure 1. Equalizer Block Diagram

The equalizer schematic diagram circuit is shown in Figure 2. The two internal stages from LM358N operational amplifier, indicated as U1A and U1B in the circuit, are responsible for amplification and filtering of the audio frequency signal.

Figure 2. 3-Tone Equalizer Circuit Diagram

The input buffer amplifier, U1A, offers a gain of approximately 2 (RF/RIN) with the specified resistor values. The input capacitor C1 (with the value of 100nF) blocks the input DC voltage. The tone adjusting action of the circuit is provided by the equalized amplifier (or active filter). This filter is implemented by a frequency-dependent negative feedback network linked to the op-amp stage U1B.

The rotary potentiometers P1, P2, and P3 allow the frequency response of bass, midrange, and treble to be adjusted to approximately -20 dB of cut, flat response, and +20 dB of boost. A flat frequency response (no cut or boost) is obtained overall when all tone controls are in their center-point position. The output level can be adjusted via volume potentiometer P4.

Frequency Response

Almost any overall gain-vs-frequency characteristic can be defined by the design of feedback network. The composite frequency response curves shown in Figure 3 are provided by the component values ​​indicated in the circuit diagram (Figure 2).

Figure 3. Equalizer Frequency Response Curves

PCB Design

Figure 3. PCB Dimensions and Details

Source: https://blogtronika.blogspot.com/2018/01/ecualizador-monofonico-tres-tonos.html