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Design Project Two

# Design Project Two - Design Project Two Audio Equalizer...

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Design Project Two: Audio Equalizer Omar English December 13, 2005

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Abstract We designed and built a three-band audio equalizer using three bandpass filters (BPFs) and a summing amplifier. We were able to achieve an adjustable gain, ranging from 1 to 10, for each bandpass filter. We were also able to input a an audio signal, from the output of a computer sound card, to our circuit and output that signal to a power amplifier and a speaker. Introduction It is often necessary to compensate for the non-ideal amplitude response in devices such as speakers, amplifiers, or other components in an audio system. In order to do this an audio equalizer can be used. An audio equalizer is a device with an adjustable frequency response curve. In designing a practical equalizer a number of BPFs are typically used, each with adjustable gain and fed to a summing amplifier. In our design, three BPFs were used, which were fed into a summing amplifier. Theory The following equations were used to design our BPFs (Fig.1) for use in our audio equalizer. Figure 1: Multiple Feedback Bandpass Filter
The center frequency ( 0 f ) for a bandpass filter is given by the equation 2 1 0 2 1 R R C f π = (where 1 R and 2 R are the two resistors used in the design of the BPF and C is the capacitor used). The gain at the center frequency or the resonant gain is r A (where 1 2 2 R R A r - = ). The quality factor for each BPF is Q and is given by the equation 1 2 2 1 R R Q = . The general form for the transfer function of a second-order bandpass filter is ) ( f H , where + - = ) / )( / ( ) / ( 1 ) / )( / ( ) ( 0 2 0 0 f f Q j f f f f Q j A f H r . For the design of the summing amplifier (Fig. 2) used in our circuit the following equation was used 3 3 2 2 1 1 V R R V R R V R R V f f f out × - × - × - = , where 1 R , 2 R and 3 R are the resistors connected to the inverting input of the summing amplifier and f R is the feedback resistor. Figure 2: Summing Amplifier The theory behind the overall audio equalizer is in the bandpass filters. Each filter only lets a limited range of frequency to pass through the filter. One filter lets the low frequency through, the second lets the middle frequency through, and the third lets the high frequency through. When a music or CD input is used for the input for the

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filters, if you add variable resistors, one can adjust the level of voltage coming out of each filter. So in theory, one can adjust the bass levels (low frequencies), mid frequencies, and the treble levels (high frequencies). Then, sum up all the voltages into a
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