Design Project: Audio tone control This worksheet and all related iles are licensed under the Creative Commons Attribution License, version 1.0. To view a copy o this license, visit http://creativecommons.org/licenses/by/1.0/, or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanord, Caliornia 94305, USA. The terms and conditions o this license allow or ree copying, distribution, and/or modiication o all licensed works by the general public. Your project is to research the schematic diagram and component values or an audio tone control circuit (bass and treble controls), then build one in a nice enclosure. There are plenty o resources on the internet or you to research, as well as many good textbooks on audio circuitry or you to read. I you are a musician, you may want to consider building a tone control or your electric instrument (electric guitar, electric violin or viola, etc.). I you like to listen to portable music devices, you may want to consider building a stereo tone control (using double-ganged potentiometers so both channels have the same tone) or your portable music player. Deadlines (set by instructor): Project design completed: Components purchased: Working prototype: Finished system: Full documentation: 1
Question 1 Suppose you were installing a high-power stereo system in your car, and you wanted to build a simple ilter or the tweeter (high-requency) speakers so that no bass (low-requency) power is wasted in these speakers. Modiy the schematic diagram below with a ilter circuit o your choice: Ampliier let right "Wooer" "Wooer" Hint: this only requires a single component per tweeter! ile 00613 Answer 1 C Ampliier C let right "Wooer" "Wooer" Follow-up question: what type o capacitor would you recommend using in this application (electrolytic, mylar, ceramic, etc.)? Why? Notes 1 Ask your students to describe what type o ilter circuit a series-connected capacitor orms: low-pass, high-pass, band-pass, or band-stop? Discuss how the name o this ilter should describe its intended unction in the sound system. Regarding the ollow-up question, it is important or students to recognize the practical limitations o certain capacitor types. One thing is or sure, ordinary (polarized) electrolytic capacitors will not unction properly in an application like this! 2
Question 2 Examine the ollowing schematic diagram or an audio tone control circuit: Source Determine which potentiometer controls the bass (low requency) tones and which controls the treble (high requency) tones, and explain how you made those determinations. ile 03506 Answer 2 Source Treble Bass Notes 2 The most important answer to this question is how your students arrived at the correct potentiometer identiications. I none o your students were able to igure out how to identiy the potentiometers, give them this tip: use the superposition theorem to analyze the response o this circuit to both low-requency signals and high-requency signals. Assume that or bass tones the capacitors are opaque (Z = ) and that or treble tones they are transparent (Z = 0). The answers should be clear i they ollow this technique. This general problem-solving technique analyzing two or more extreme scenarios to compare the results is an important one or your students to become amiliar with. It is extremely helpul in the analysis o ilter circuits! 3
Question 3 Competency: Tone balance control circuit Schematic Version: L 1 R 1 Headphones R pot1 R pot2 C 1 R 2 Given conditions L 1 = C 1 = R pot1 = R pot2 = (Use audio source or signal, voltage adjusted or ample volume) Parameters R 1 = R 2 = Z headphones = Predicted Measured Bass control Treble control (Identiy which pot is which) (maximum bass, minimum treble) (minimum bass, maximum treble) Note: when testing the requency response o the tone control circuit, you may need to replace the headphones with a non-inductive resistor o equivalent impedance, and measure across it. ile 02022 4
Answer 3 Notes 3 Use circuit simulation sotware to veriy your predicted and measured parameter values. A good source o audio signal is the headphone output jack o almost any radio, media player, or other portable audio device. Students like being able to do a lab exercise that directly relates to technology they re already amiliar with. The higher-impedance the headphones are, the better this circuit works, since the combination o potentiometers and mixing resistors tends to result in a relatively high output impedance. I have used cheap headphones (32 ohm) with some success, given the ollowing component values: C 1 = 0.1 µf L 1 = 200 mh (actually two 100 mh inductors in series) R 1 = R 2 = 1 kω R pot1 = R pot2 = 10 kω Some students with limited hearing range have diiculty detecting the changes in tone using 10 kω potentiometers. You may wish to use 100 kω potentiometers instead or added attenuation. Operating such a circuit is akin to operating a water aucet with hot and cold water valves: the two settings together determine temperature and low (tone and volume, respectively, or the metaphorically challenged). An extension o this exercise is to incorporate troubleshooting questions. Whether using this exercise as a perormance assessment or simply as a concept-building lab, you might want to ollow up your students results by asking them to predict the consequences o certain circuit aults. I you plan to use this exercise as a troubleshooting assessment, I recommend against inducing the ollowing component ailures, as they are diicult to detect when the signal source is music rather than a constant tone o known requency and amplitude: Shorted capacitor (C 1 ) Shorted inductor (L 1 ) Shorted ixed-value resistors (R 1 or R 2 ) 5
Question 4 Competency: Tone balance control circuit Schematic Version: L 1 R pot1 R 1 T 1 Speaker R pot2 C 1 R 2 Given conditions L 1 = C 1 = R pot1 = R pot2 = R 1 = R 2 = Z speaker = T 1 = (Use audio source or signal, voltage adjusted or ample volume) Parameters Predicted Measured Bass control Treble control (Identiy which pot is which) (maximum bass, minimum treble) (minimum bass, maximum treble) Note: when testing the requency response o the tone control circuit, you may need to replace the transormer/speaker assembly with a non-inductive resistor o equivalent impedance, and measure across it. ile 02023 6
Answer 4 Notes 4 Use circuit simulation sotware to veriy your predicted and measured parameter values. A good source o audio signal is the headphone output jack o almost any radio, media player, or other portable audio device. Students like being able to do a lab exercise that directly relates to technology they re already amiliar with. I have experienced good success with the ollowing component values: C 1 = 0.1 µf L 1 = 200 mh (actually two 100 mh inductors in series) R 1 = R 2 = 1 kω T 1 = 1000:8 ohm audio output transormer R pot = R pot2 = 10 kω Speaker = small 8 Ω unit (salvaged rom an old clock radio or other inexpensive audio device) An extension o this exercise is to incorporate troubleshooting questions. Whether using this exercise as a perormance assessment or simply as a concept-building lab, you might want to ollow up your students results by asking them to predict the consequences o certain circuit aults. 7