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  • 10 Band Stereo Graphic Equalizer

  • Created: Dec 04, 2020

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The LME49720 is a device that has two operational amplifiers designed for high fidelity applications. It can be powered by a split-type power supply with a minimum of +/-2.5V and a maximum of +/-17V. The device has an extremely low noise density (2.7nV/√Hz) and a very low total harmonic distortion plus noise percentage (0.00003%), which makes it ideal to be used in high fidelity audio applications. It can be used in preamplifiers, equalizers, crossover networks, line drivers, line receivers, active filters, etc. In this reference design, the LME49720 is used in a 10 band stereo graphic equalizer.

A graphic equalizer is an audio equipment that is used to compensate for irregularities in the listening environment or to match the sound to the listener’s taste. It is commonly available as 2-octave (5 bands), 1-octave (10 or 11 bands), or ⅓-octave (30 bands). This reference design features a 1-octave or 10 band stereo graphic equalizer that utilizes the LME49720. The whole design is composed of the power supply and two identical circuits for the graphic equalizer. In order for the LME49720 device to perform according to its specified electrical characteristics, the power supply was designed to be a split-type that outputs a positive and a negative 15V level. In the graphic equalizer circuitry, U3A and U9A op amps act as preamplifiers, while U3B and U9B act as unity gain buffers. The rest of the op amps in the whole circuit act as simulated inductors. These simulated inductors are part of the low-Q band-pass filters attached to the wiper terminal of the 20kΩ potentiometers. The band-pass filters are centered at 32Hz, 64Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz, and 16kHz frequencies. By sliding down or up their respective 20kΩ slide potentiometers, the gain of the sound’s frequencies are cut or boosted.

The gain of this stereo graphic equalizer is dependent on the gain of the preamplifiers U3A and U9A. It can be adjusted through the potentiometers RGAINL and RGAINR. If the resistances across these potentiometers are set to zero, the preamplifiers will just act as buffers. The potentiometers RMASTERL and RMASTERR at the output of U3B and U9B respectively, serve as master volume of the graphic equalizer. It controls the output signal sent by the graphic equalizer to the power amplifier or any audio equipment connected at its output. This 10 band stereo graphic equalizer can be used as equalizer for audio players or musical instruments.