Experiment 29: Filtering Frequencies
250 Chapter 5
Procedure
The purpose of the audio amplifier chip is to provide enough power to get
a decent amount of sound out of your loudspeaker. The purpose of using a
5-inch speaker is to enable you to hear lower-frequency sounds than the baby
speakers that we have used previously. Bass notes have long wavelengths that
small speakers are not able to generate effectively.
Maybe you remember from building the intrusion alarm that a speaker makes
much more noise if you prevent the sound waves from the back of the cone
from cancelling the sound waves from the front of the cone. The obvious way
to achieve this is by enclosing the speaker in a box. I suggest a plastic box,
because they’re cheap, and we don’t care too much about sound quality as
long as we can hear at least some of the low frequencies. Figure 5-39 shows
the speaker bolted into the bottom of a plastic box, and Figure 5-40 shows the
box turned upside-down after snapping its lid into place.
Normally, a speaker should be mounted in a cabinet of heavy, thick material
that has a very low resonant frequency—below the limits of human hearing.
To minimize the resonance of the shoebox, you can put some soft, heavy fab-
ric inside it before you snap the lid on. A hand towel or some socks should be
sufficient to absorb some of the vibration.Adding an Amplifier
Back in the 1950s, you needed vacuum tubes, transformers, and other power-
hungry heavyweight components to build an audio amplifier. Today, you can
buy a chip for about $1 that will do the job, if you add a few capacitors around
it, and a volume control. The TEA2025B that I’m recommending is intended for
use in cheap portable cassette players and CD players, and can work in stereo
or mono mode, from a power supply ranging from 3 to 9 volts. With 9 volts
and the two sides of the chip bridged together to drive one 8Ω speaker, it can
generate 5 watts of audio power. That doesn’t sound much compared with a
typical home theater system rated at 100 watts per channel, but because loud-
ness is a logarithmic scale, 5 watts will be quite enough to irritate any family
members in the same room—and possibly even in other rooms.
If you can’t find the TEA2025B chip, you can use any alternative listed as an
audio amplifier. Try to find one that is designed to drive an 8Ω speaker with up
to 5 watts in mono mode. Check the manufacturer’s data sheet to see where
you attach capacitors around it. Note carefully whether some of the capacitors
have no polarity marked, even though they have fairly high values, such as 100
μF. These capacitors must function regardless of which way the alternating
current is flowing, and I’ve marked them “NP” in my schematic in Figure 5-41,
meaning “nonpolarized.” (You may find them identified as “bipolar” or “BP” in
parts catalogs.) As noted in the shopping list, you can put two 220 μF capaci-
tors in series, negative-to-negative, to get the same effect as a single 100 μF
nonpolarized capacitor.
For this project, it’s essential to include the regular 100 μF electrolytic smooth-
ing capacitor across the power supply. Otherwise, the amplifier will pick up
and—yes, amplify—small voltage spikes in the circuit.Figure 5-39. A resonant enclosure is neces-
sary if you want to hear some bass (lower
frequencies) from your speaker. A cheap
plastic shoebox is sufficient for demo
purposes.
Figure 5-40. Drill some half-inch holes
in the bottom of the box, then bolt the
speaker in place, running a wire out
through a hole in one end. Snap on the lid,
and you’re ready for not-quite-high-fidelity
audio.