Definition of Engineering Phrases
"A" "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Resonant Equalizer

This topic covers a resonant equalizer which is a different style circuit than the common audio equalizer, although both produce the same result. The difference here is that the frequency response of both the high and low frequency equalizer covered on the previous page overlap each other.

Notice how the capacitor placement for the high-pass equalizer and the capacitor placement for the low-pass equalizer are used in this circuit design. This circuit is also sometimes called a universal equalizer, because by changing or adding the capacitors any of the three equalizer responses can be produced. Although this schematic only shows one possible example, the other two were shown in the previous design.

Operational Amplifier Usage

The circuit design is almost identical to the unity gain 741 circuit used before. The only difference is the addition of the second capacitor to the circuit schematic. As with all these circuits, the general purpose LM741 is only used to represent almost any operational amplifier. Also see Manufacturers of Op Amps.

This circuit basically combines the low and high pass filters, similar to circuits found on the Passive Tone Control page, or the Active Tone Control page. The low and high frequency points are combined and placed symmetrically above and below center resonant frequency.

Resonant Equalizer Design

The resonant equalizer functions around a particular center frequency and than adds gain or attenuation above or below that frequency. However the response only effects a certain range above or below the center frequency, as shown in the graphic. Which appears just like a bandpass filter, with the additional characteristic of both gain and attenuation control.

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

Component Value Calculations

As shown in the graphic to the right the boost an cut are symmetrical. The high frequency [Fh] 3dB point and the low frequency [Fl] 3dB points are also symmetrical around the center frequency. So the first calculation to be perform is to determine the number of octaves the equalizer functions over:
F/Fh = Fl/F = n [number of octaves].
F = 2n * Fh = Fl/2n
The capacitor values are selected as follows:
C1 = 1/(4 * n * 3.14 * R1 * F)
C2 = n/(3.14 * R2 * F)

Using the component values shown in the schematic, and a desired center frequency of 1KHz: [Common Capacitor Values]
C1 = 7940pF which is normalized to 8200pF.
C2 = 3180pF which is normalized to 3300pF.

Equalizer Control

So the equalizer circuit above adjusts the frequency response of an audio signal slightly above or below a particular center frequency. Additional circuits could be designed that are centered on other frequencies resulting in a number of frequency controls each operating over a different frequency band. Each of the different outputs from the individual equalizers could be applied to an Audio Mixer so their combined effect could be felt over a single audio channel. Using the ratio [octave] selection in the equation each equalizer could be made as wide or narrow as desired.

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