How to Build Your Own Audio Equalizer
An equalizer is a device that allows you to adjust the gains of individual frequencies. You may want to lower bass, so on an equalizer, you would go to the frequency range of bass, which may be around 500Hz and turn down the volume. If you want to increase treble, you may go to, say, 2KHz, and turn the volume up.
In order for an equalizer to know which frequency range to adjust, it operates on the concept of filtering, meaning it must filter the individual frequencies so they can be individually adjusted. So equalizers are made up of filters. An equalizer consists of one or or filters which can cut or boost the amplitude of frequencies in various ranges.
A simple equalizer circuit is shown below. This is the same one we will build:
This circuit is known as a Boxandall tone control circuit and it has treble and bass filters in the feedback loop of an op amp.
The treble control cuts or boosts the response above 1KHz to a maximum of about 20dB at 10KHz. The bass control cuts or boosts the response below 1KHz to a maximum of about 20dB at 50Hz.