Definition of Engineering Phrases
"A" "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
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Equalizer Circuits

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 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.

Operational Amplifier Design

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].

Shelving Equalizer

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.

Active Equalizer Circuit

The first circuit uses a component placement that configures it as a high frequency equalizer. The equalizer is configured to pass all signals with out adding or reducing gain up to a particular frequency. The default gain of the amplifier is set at unity gain (1). The frequency at which the equalizer with either add gain [Boost] to the signal, or attenuation [cut] to the signal is determined by the following equation: 1 / (2 * p * R2 * C1). The values called out in the equation set the frequency at 2kHz.

High Frequency Equalizer Circuit Design using a 741 Op-Amp
High Frequency Equalizer Circuit

This second circuit operates as the first, but at the opposite end of the frequency range. The circuit will pass all low frequencies up to a cut-off frequency (basically a low-pass filter). The equation that calculates the cut-off frequency is the same as before; 1 / (2 * p * R2 * C1). The component values used in the circuit will produce a cut-off frequency of 200Hz.

Low Frequency Equalizer Circuit Design using a 741 Op-Amp
Low Frequency Equalizer Circuit

Potentiometer Selection

A standard linear potentiometer should be used to control the equalizer. This differs from other audio controls such as a volume control which would use an audio taper which is logarithmic; however it's the same style adjustable resistor that would be used as a bass or treble control.

Normally when used on a front panel the potentiometer has a linear control, that is it moves up and back in a straight line, not left or right in a circular movement. The linear movement of the control should not be confused with the linear value change of a taper.

The resistor value ratio should be kept constant: The amount of change [boost or cut] is determined by the ratio of input and feedback resistance; R2, R3 / R1, R2.

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