EE100 Spring 2005 Bharath MuthuswamyEE100 Final Project GuideBefore I begin, many thanks to the EE40 team (Prof. White, Michael Case, Abhijit Davare, MatthewLeslie, Cheuk Chi Lo, Marghoob and Bruno) from Fall 2004 for designing this project. They wrote anexcellent guide for this project. This guide for EE100 is based on part 1 and part 2 of their guide. 1. Objective In this project, you get to design a stereo tone-controller. In other words, the circuit willamplify the base and/or treble for a two-channel stereo system. 2. Prelab1. Please bring a real audio source – a discman, walkman, iPod, MP3 player, laptop etc.to the lab so you can test your design. Try to place as many components on thebreadboard BEFORE coming to lab.2. Read the material on Active Filters from the reader.3. WORD OF ADVICE: BUILD CLEAN, NEAT CIRCUITS. Use the shortest wirepossible, use the power rails on your board instead of stringing 12 V and groundeverywhere, and be methodical when you wire so that you (and your GSI) can makesense of it. For instance, use red for power, black for ground and blue for signal.Figure 1 shows one half of my tone controller. Notice how all the wires are almostparallel to the breadboard. Also notice how I have added user interface elements– theswitch and LED.Figure 1. One half (right speaker) of my tone controller circuitEE100 Spring 2005 Bharath Muthuswamy3. System Block DiagramA block diagram of your tone controller is shown in figure 2.Figure 2. System block diagramThe power supply for your system is a 9 V battery than you can buy from any electronicsstore/supermarket. The power supply system is to provide a constant 5 V supply to theelectronics. This is because a real 9 V battery does not provide a steady 9 V supply. Theregulator tries to make sure the 5 V supply is constant. Of course, as time goes on, thebattery starts dying and the regulator cannot maintain the voltage. In my case, when theLED starts dimming, that means I need a new battery! A non-mathematical explanationof how the regulator works is given in section 4.The tone controller is the heart of the above circuit. The left tone controller is the exactsame circuit as the right. The tone controller is an op-amp voltage follower followed byan op-amp active filter that can filter the base or the treble regions of the frequencyspectrum. A non-mathematical explanation of how this circuit works is given in section5.Why do you need the power amplifier stage? The answer: your op-amp tone controllercannot provide enough current to drive any speaker. Speaker resistances are usually 8 or32 ohms. Your op-amps may be able to provide 10 mA of current on a good day! This isnot enough to drive the speakers at all and you will end up blowing your op-amps. This is where the power amplifier comes in. The power amplifier simply transfers theinput from the op-amp tone controller to the speakers, but it can generate a lot of currentfor driving the speakers. Overall, the cost of the above system is around $10. You can find the datasheets for allthe chips (ICs) in your reader.EE100 Spring 2005 Bharath Muthuswamy4. Block Diagram Component 1: Power SupplyFigure 3. The LM2940CT-5.0 9.0 V to 5.0 V voltage regulator. The 5.0 V suffix indicates the regulatoroutputs 5 V.Figure 3 shows the top view of the voltage regulator you will be using. A linear voltageregulator can be thought of as two variable resistors. Voutis obtained from Vinthrough avoltage divider. Refer to figure 4.Figure 4. A simple voltage divider between Vout and Vin.However, the regulator adjusts Vout when Vin changes using internal feedback.5. Block Diagram Component 2: Tone ControllerThe tone control circuitry is shown in figure 5. NOTE: I HAVE NOT INCLUDEDTHE VOLTAGE FOLLOWER IN FIGURE 5. The input from your male headphonejack should go to the voltage follower. The output of the voltage follower should go tothe tone controller. Details can be found in section 7. Important parts of the circuit arehighlighted (source: http://sound.westhost.com/dwopa2.htm)EE100 Spring 2005 Bharath MuthuswamyFigure 5. The tone controllerYou will be using the LMC6482 opamp, the pinout is given in figure 6 for yourconvenience.Figure 6. The LMC6482 pinout (top view)The tone control circuitry is rather complicated. However, a “feel” for the circuit is morethan enough. As a rule of thumb, it is always a good idea to have a “feel” for anythingbefore you build it! This gives you an intuition for what to look at if something goeswrong and you need to debug. It also makes the whole process much more fun. The positive input to the LMC6482 is simply half the power supply due to the 11kresistors. The 10 uF capacitor seen at the positive input merely keeps the node stable(free of AC variations). The purpose of the 1 uF capacitor at the input is to block any DCcomponent of the audio signal . DC voltages do not contain any information about soundand therefore are unnecessary. The rest of the passive components are involved in thefeedback path. The key to understanding the bass and treble gains lie in thepotentiometers.EE100 Spring 2005 Bharath MuthuswamyIf our input signal is very low in frequency (bass), the top potentiometer controls the gainbecause the 22 nF capacitor appears as an open (use the impedence formula for acapacitor to verify this). Therefore our input signal simply divides inside thepotentiometer. Note that for low frequencies, the 560 pF capacitor is effectively an openand does not feed the signal through to the bottom potentiometer.If our input signal is very high in frequency (treble), no voltage appears across the toppotentiometer because the 22 nF capacitor appears as a short. However, the bottompotentiometer divides the input signal and feeds it through the 560 pF capacitor (whichnow appears as a short) to the input. So for high frequencies the gain is controlled by thebottom potentiometer.6. Block Diagram Component 3: Amplifier StageFigure 7. The gain stage + power amplifierBefore the signal goes to the power amplifier, we add an extra gain stage. This isbasically your “volume control” knob. The block diagram of the power amplifier isshown in figure 8. The pinout (top-view) is shown in figure 9.Values for the components in the power amplifier are: Ci= 20 uF, Rf= Ri= 20 k, Cb=0.1 uF, Co= 220 uF. If you are interested in knowing why we chose these values, pleasetalk to your TA.EE100 Spring 2005 Bharath MuthuswamyFigure 8. A block diagram view of the
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