Audio Equalizer Circuits


Posted on Feb 4, 2014

This discussion continues on from the audio tone control topic, which introduced both passive and active tone controls. The follow on topics might be a 2-Band Active Tone Control, or the 3-Band Active Tone Control. An audio equalizer is used to adjust the amplitude of one or more frequency bands. The actual circuit adds equalization just as the


Audio Equalizer Circuits
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tone controls did. The circuits are similar and use the same terms to indicate an amplification [boost] or attenuation [cut]. In fact there isn`t much difference between the two classes of circuits. Both of the circuits below indicate that they use a LM741 operational amplifier, but like all of these Op-Amp circuits any amplifier could be used instead of the 741. The LM741 is just used to indicate that any general purpose operational amplifier might be used. In addition, no pin numbers are provided in the circuit diagrams so a single, dual or quad package Op-Amp could be used. Two operational amplifier circuit schematics are provided below. Both circuits are based on a non-inverting amplifier design. The LM741 [uA741] are configured as a unity gain amplifier, as Rf and Ri are shown as the same value. The gain of an inverting amplifier is Rf/Ri, or in this case R2`/R2 [ignoring the other passive components for the moment]. A Shelving Equalizer gets its name from the frequency response curve the circuit achieves. The characteristic curves resemble stacked shelves. There are two types; a high-pass Shelving Equalizer (which passes all frequencies above a certain frequency), and a low-pass Shelving Equalizer (which passes all frequencies below a certain frequency). Both low and high pass equalizers are shown below. The first circuit uses a component placement that configures it as a high frequency equalizer. The equalizer is...




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