Background

Most good quality audio products, such as radios and, tape and cassette  players, provide some frequency control to tailor the sound to the listeners' liking.

In more expensive equipment, a frequency equalizer allows the listener to increase  or decrease the loudness of a number of selectable narrow frequency bands in the  20 Hz to 20 KHz audio range.

Another very common set of controls are the bass and treble controls. These  will boost or attenuate a broad range of frequencies compared to the more selective equalizer.

The objective of this project is to design, construct, and test a circuit to provide bass, treble, and volume control.

Specifications:

a. the range of frequencies from 300Hz to 1 KHz is to be continuously  controlled over a 12 dB range, i.e. over a 4:1 range to provide for

bass control.

b. the range of frequencies from 2 KHz to 15 KHz is to be continuously  controlled over a 12 dB range to provide treble control.

c. for equal settings for the bass and treble controls, the filter bandpass  is to be flat within +/- 2 dB from 400 Hz to 10 KHz.

The initial design will be verified using Electronic Workbench. The response characteristic can be displayed using the Bode Plotter available in  the Workbench instrument group.

Designing Bandpass Filters

A bandpass filter can be thought of as a cascade of a high pass and low pass filter:
 

 

The cut off frequecy selection is the most important part in designing a bandpass filter.

Consider the output amplitude response, Vo with respect to a constant amplitude input frequency, Vi, shown in Figure 1. Between the frequencies f1 and f2 The output amplitude remains constant at a maximum value. For frequencies below f1 and greater than f2 the output amplitude decreases. The frequency region between f1 and f2 is termed the filter bandpass, i.e. the filter output voltage in this frequency range remains constant at some maximum level. The lower frequency, f1, is commonly termed the "roll-over" frequency; the higher frequency, f2, the "cut-off " frequency. At these frequencies, the output voltage is 0.707 times the output voltage in the bandpass region; the significance of this value is that the filter output will only supply one-half of the power it is capable of supplying to a load in the bandpass frequency range.

The filter characteristic shown in Figure 1 can be realized by the op amp configuration

shown in Figure 2

The rollover frequency, f1, is determined by;

f1 = 1 / (2*pi* R1C1)

The cutoff frequency, f2, is determined by;

f2 = 1 / (2*pi *R2C2)

The amplitude of Vo in the bandpass region can be determined by

evaluating:

Vo = (R2C1*w*1 / 1.4) Vi

Amplification or attenuation of the input is dependent on the values selected for R2 and C1.

Thus, the design flow should be following: