ELEC 101
Prof. Rich Kozick
March 26, 1998

Laboratory 8
Design and Construction of a Three-Band Audio Equalizer

Problem: Design and construct a three-band equalizer circuit. Your circuit should include three filters: a low-pass filter that passes frequencies up to about 500 Hz, a band-pass filter that passes frequencies centered around 1073 Hz, and a high-pass filter that passes frequencies above about 2000 Hz. You should be able to adjust the gain of each frequency band independently. You will demonstrate the operation of your equalizer by passing music through your circuit and playing the "equalized" music through a speaker.

System Design: Use the "subsystem" circuits shown on the back of this page to design your equalizer. Which circuit is the low-pass, band-pass, and high-pass filter? How can you tell the "cutoff" frequency of each filter? How should you connect the subsystems to form your equalizer? Do you need "buffer" circuits in your design?

Groups: The morning lab will build one equalizer, and the afternoon lab will build two equalizers. Each equalizer will be built by three groups, which are specified in the table below. Each group will build and test one of the filters (low-, band-, or high-pass), and then you will connect your individual filters to form the equalizer at the end.

AM Lab

DiBello, Glover, Condor
Coppin, Hoskinson, Maroon
Fields, Goldschmidt, Veitas
PM Lab 1

Becker, Evans
Hansberger, McBride, Eisenstein
Caldwell, Sulick, Torres
PM Lab 2

Durst, Phillips, McCaffrey
Braun, Smith, Tumsuden
Hughes, Tees


  1. As a group, complete the system design, and draw the total circuit diagram in your lab notebook.
  2. Decide which group will build the low-, band-, and high-pass filter.
  3. Each group should set up the filter and then pass sine waves with various frequencies through the filter. Observe the filter input and output sine waves on the oscilloscope, and verify that the circuit is "filtering" in the desired way. In particular, determine the "cutoff frequency" of each filter. Record your approach and observations in your lab notebook. If you finish early, help the other groups with their testing.
  4. When all of the individual filters are working properly, connect the total equalizer circuit.
  5. Demonstrate the equalizer operation by processing music.
  6. Write a short summary of your observations in your lab notebook, and discuss with the instructor before you leave.

Pre-Lab: Study the notes from class, and please arrive to lab on time!