Design Project: Audio Tone Control

Ask your students to describe what type of filter circuit a series-connected capacitor forms: low-pass, high-pass, band-pass, or band-stop? Discuss how the name of this filter should describe its intended function in the sound system.

Regarding the follow-up question, it is important for students to recognize the practical limitations of certain capacitor types. One thing is for sure, ordinary (polarized) electrolytic capacitors will not function properly in an application like this!

The most important answer to this question is how your students arrived at the correct potentiometer identifications. If none of your students were able to figure out how to identify the potentiometers, give them this tip: use the superposition theorem to analyze the response of this circuit to both low-frequency signals and high-frequency signals. Assume that for bass tones the capacitors are opaque (Z = ∞) and that for treble tones they are transparent (Z = 0). The answers should be clear if they follow this technique.

This general problem-solving technique - analyzing two or more “extreme” scenarios to compare the results - is an important one for your students to become familiar with. It is extremely helpful in the analysis of filter circuits!

A good source of audio signal is the headphone output jack of 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 familiar with.

The higher-impedance the headphones are, the better this circuit works, since the combination of 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 following component values:

C₁ = 0.1 μF

L₁ = 200 mH (actually two 100 mH inductors in series)

R₁ = R₂ = 1 kΩ

R_pot1 = R_pot2 = 10 kΩ

Some students with limited hearing range have difficulty detecting the changes in tone using 10 kΩ potentiometers. You may wish to use 100 kΩ potentiometers instead for added attenuation. Operating such a circuit is akin to operating a water faucet with “hot” and “cold” water valves: the two settings together determine temperature and flow (tone and volume, respectively, for the metaphorically challenged).

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.

If you plan to use this exercise as a troubleshooting assessment, I recommend against inducing the following component failures, as they are difficult to detect when the signal source is music rather than a constant tone of known frequency and amplitude:

Shorted capacitor (C₁)

Shorted inductor (L₁)

Shorted fixed-value resistors (R₁ or R₂)