Equalizer Circuit Operating on Three Bands

Figure 1 

The circuit was designed to illustrate the use of tone control circuit by which the audio signals are adjusted before being carried out to any output devices such as headphones, speakers, or recording devices through the use of an amplifier.

• Equalizer – a passive or an active electronic element or circuits (like tone control) used for the reason of altering or flattening the frequency response characteristics of a system
• Op-Amp – a differential amplifier having a large voltage gain, very high input impedance and low output impedance
  Tone control permits the circuit to operate in 3 different frequencies. Low frequencies may lie around 50 Hz, mid frequencies around 1 kHz, and high frequencies around 10 kHz. The op-amp used was LF351 wide bandwidth JFET input operational amplifier which is low cost and has an internally trimmed input offset voltage. The use of this op-amp enables the circuit to adjust the base, middle (balance) and treble controls. The tone controls were normally being used in a preamplifier for music production. Bass is lying in the lower part of the audio frequency band while treble is the highest part in musical harmony.

Within the frequencies mentioned, there will be a presence of around +/- 20 dB of increase in sound. This is made possible with the supply voltage reaching the maximum value of 18 V, although a range of 6 V to 30 V may be applied to the circuit. 20 dB symbolizes a ratio of ten to one for voltage, meaning 10 times the voltage. Decibel is a logarithmic unit of measure of the ratio between two numbers.

Equalizers have been used in several industries such as in multitrack recording and sound reinforcement systems, in live events where microphones and speakers operate simultaneously, in all audio records or vinyls where equalization is applied to the sound waveform, in early telephone systems where it was used to correct for the reduced level of high frequencies in long cables, in picturephone circuits, in modern digital telephone systems, and in individual channels of a mixing board and the sound of electric instruments where it was used to manipulate the timber of musical instruments and sounds. The different kinds of equalizers depending on the pattern of attenuation or boost include a peaking equalizer, a pass filter, a shelving type, and variable equalizers.

Tone control circuits may bring several advantages to listeners of recorded or broadcast music by modifying the audio as they prefer it to be. The modification is essential due to the inadequacy of recording, playback equipment, hearing, or room acoustics for listening.

The LF351 op-amp may be used in applications such as high speed integrators, sample and hold circuits, fast D/A converters, and other circuits requiring wide bandwidth, high slew rate, high input impedance, low input bias current and low input offset voltage.

Source:www.zen22142.zen.co.uk/Circuits/Audio/3band.htm