The Hong Kong University of Science and Technology Department of Electronic and Computer Engineering ELEC240 BASIC ELECTRONICS SPRING 203 LAB 5a Audio Equalizer (PSpice Simulation). Objective The objectives of this experiment are to design and build a simple analog audio equalizer using operational amplifiers and to provide experience with the applications of operational amplifier. 2. Component and Instrumentation a. Four 74 Op Amp ICs b. Dual power supply c. Waveform generator d. Dual channel oscilloscope 3. Background Information An audio equalizer is an electronic device to alter the frequency response characteristics of an audio system (e.g. MP3). The equalizer can be implemented in analog domain using passive and active electronic elements or in digital domain with some digital signal processing algorithms. In this experiment, you will design, build and test a simple analog audio equalizer using operational amplifiers. Peaking equalizer is one of the most popular implementations of audio equalizer. Peaking equalizer can raise or lower the level of a range of frequencies around a central frequency in a bell shape. Some peaking equalizers can control the gain, bandwidth and center frequency. In this experiment, a peaking equalizer will be designed which can adjust the gain (level) of each frequency band. Figure below shows one implementation of the audio equalizer system with three frequency bands and its frequency characteristic. Band-Pass Filter Audio in Band-Pass Filter 2 Mixer Audio out v i Band-Pass Filter 3 v o v v o i BPF BPF 2 BPF 3 Figure : An audio equalizer system Frequency (log scale) p.
There are several configurations may be used to realize a band-pass filter. A multiple-feedback band-pass filter is used in this experiment and is shown in Figure 2 which uses one operational amplifier and five resistors and capacitors. C4 v fi R R2 C3 R5 v fo Figure 2: A band-pass filter The voltage transfer function is given by ( / 4) ( )( ) ( ) v s RC fo = 2 v fi s + s R5 C3+ C4 + R5C3C4 R+ R2 / / / / (/ / ) () In terms of the band-pass filter function, H o = ( R / R )( + C / C ) 5 4 3 (2) ω o = + RCC R R 5 3 4 2 /2 C3 C 4 = + Q R5( / R+ / R2) C4 C3 (3) (4) where H o is the gain of the pass-band filter at the center frequency ω o and Q is the Q factor of the band-pass filter. Equations (2) to (5) can be used to design the gain, center frequency and the bandwidth of the band-pass filter. However, the equations are rather complicated and several assumptions can be made to simplify the design. First of all, the gain of the band-pass filter is not very important as the overall gain each frequency band can be adjusted by the tunable amplifier in series with the filter. By setting C 3 = C 4 = C and assuming R 2 >> R. Equations (3) and (4) can be simplified to ω o = (5) R RC 5 R 2 Q = R (6) 5 Hint: For a 3-band audio equalizer, the Q value of the filter should be smaller than. p. 2
The complete schematic of the 3-band audio equalizer system is shown in Figure 3. The tunable amplifiers and the mixer can be realized by an operational amplifier adder together with 3 variable resistors. Rm(s) are used to set the minimum resistances at the input of the adder and thus limit the maximum gain of the mixer. C a R a R 5a v a R 2a C a v i R b C b R 5b v b VR VR 2 R G R 2b C b VR 3 v o C c R c R 5c v c R 2c C c Figure 3: A 3-band equalizer p. 3
4. Experiment You are required to design your own equalizer. You need to determine the center frequencies and the Q values of the band-pass filters. You may consider the lower frequency filter corresponds to the Bass of your equalizer, the middle frequency is the vocal frequency and the higher frequency filter is the Treble of your equalizer. For your information, human can hear around 20 Hz to 20 khz. Human voice is between 80 Hz to 800 Hz and piano ranges from 30 Hz to 4 khz. In the other words, you need to find out the resistor and capacitor values of your equalizer. Assume there is only one turn 00 kω VRs are available in the laboratory. Please note that there are only a certain standard resistance and capacitor values available in the laboratory shown in the section 5 (Appendix). You should use the PSpice circuit simulator (PSpice student release) to simulate your equalizer to obtain the frequency characteristic to confirm the resistor and capacitor values. You should refer to the session 4.46-4.48 of the lab#2 manual for reviewing how to obtain the frequency characteristic (20Hz-20KHz). Q. Write down all the calculated component values. R a R 2a R 5a R b R 2b R 5b VR 00K Ω VR 2 00K Ω VR 3 00K Ω R c R 2c R 5c R G C a C b C c f o_middle f o_lower f o_higher Q2. Show the simulated frequency characteristic of your equalizer to TA. TA signature : p. 4
5. Appendix Available Standard Resistance Values of 5%-tolerance resistors (/4W) in the laboratory. 0 00 330 680 k.5k 2.2k 4.7k 6.8k 8.2k 0k 5k 8k 22k 27k 33k 39k 47k 56k 68k 82k 00k 50k 220k 330k 470k 560k Available capacitance Values of ceramic capacitor in the laboratory. n 2.2n 4.7n 8.2n 0.0µ 0.05µ 0.022 µ 0.033µ 0.047µ 0.068µ 0.082µ 0.µ Available variable-resistor (VR) in the laboratory. 00k Available operational-amplifier in the laboratory. UA74CN p. 5