This 4 band equalizer circuit serves to improve the quality of audio before the input signal is amplified by an amplifier, especially in the term fidelity of audio. Equalizer circuit can also reduce unwanted audio frequencies, and amplifies the audio frequency desired.
The equalizer circuit work at 12VDC symmetric. So that the resulting sound can be maximized, the power supply used must be regulated properly. You can use a power supply based regulator LM7812 and LM7912.
Capacitors 0.01 uF contained in the power supply circuit are used to remove high frequency and stray currents. Because the equalizer circuit does not require a large current, then a 100mA transformer is more than enough to operate this equalizer circuit. The frequency range of this equalizer circuit is Low frequency: Freq. 35-680Hz; Medium frequency: Freq. 150-3kHz; Medium/semi high frequency: Freq. 450-8.5kHz; High audio frequency: Freq. 750-15kHz.