Nuts & Volts

■ WITH TJ BYERS
Q & A
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist.
Feel free to participate with your questions,
comments, or suggestions.
You can reach me at: TJBYERS@aol.com
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WHAT’S UP:
APPLICATIONS
This month’s Q&A — some
new and unusual questions
and answers!
● Why loudspeakers are different.
● Phone busy alert.
● Two optical switching circuits.
● T-Bird tail lights.
SO MANY TO
CHOOSE FROM
QI have noticed a large variety
of “impedance” ratings for
speakers; such as four ohms,
eight ohms, 16 ohms, and
others. Why so many Is it just to make
the design of the power amplifier easier
by matching the output impedance
— Jeff Dunker
■ FIGURE 1
20 March 2006
AWell, that’s part of the answer
— but it’s not the whole
story. Let’s begin with the
physical construction of a
loudspeaker, as shown in Figure 1.
Basically, a loudspeaker consists
of a voice coil (electromagnet)
suspended in a magnetic field. When a
current is run through the coil, it
creates a magnetic field that forces the
coil inside or outside the magnetic field
— according to the strength of the current
and its polarity. The stronger the
electromagnetic field, the farther the
displacement (throw) of the voice coil
with respect to the permanent magnet.
If an AC voltage is applied across
the voice coil, it will move in and out
as the field changes polarity. The voice
coil is glued to a paper or plastic cone
that moves in step with the voice coil.
This movement translates the electron
flow into air movement — a.k.a., sound.
Early on in the development of
audio amplifiers, many methods were
used to produce this movement. The
most feasible was and is the voice-coil
loudspeaker described above. The critical
element of this design is the voice
coil itself. It is nothing more than many
turns of copper wire — wire that has
resistance that’s measured by the foot.
For example, a typical eight-ohm,
four-layer woofer voice coil contains
about 120 feet of number 28 solid
copper wire. That’s a lot of wire to
shove into the small gap between the
north and south poles of the permanent
magnet. Consequently, it’s physically
more practical to use a smaller
wire with more resistance than it is to
use larger wire, which is harder to
work with when forming a rigid voice
coil destined for a small space.
But here comes a trade-off. Current
times voltage makes watts. So voice
coils with more resistance require more
voltage to produce the same wattage.
In the days of vacuum tubes, this
wasn’t a problem. They required hundred
of volts on the plate and an output
transformer in their design, so loudspeakers
of that era were typically 16 to
32 ohms because the value best fit
the tube to transformer coupling —
and the materials and manufacturing
methods of the time.
With the advent of semiconductors,
voltages decreased and currents
increased. That coupled with the discovery
of rare-earth permanent magnets
with stronger magnetic fields
(versus Alnico) made it realistic to
use lower resistance coils. For example,
the car stereo market uses almost
nothing but four-ohm speakers due to
voltage limitations available in cars
(specifically 12 volts). More power can
be driven into a four-ohm speaker
than an eight-ohm speaker (assuming
the same driving signal).
Are four-ohm speakers better than
eight-ohm speakers Absolutely not!
The ohm rating of a speaker has
nothing to do with the quality of the
speaker. But it has a lot to do with the
way you wire an array of speakers
together. Is an eight-ohm speaker
really eight ohms Not likely. Due to
the highly complicated nature of a
loudspeaker, its impedance is not a
simple number, but an AC reactive
value that changes with frequency and
loading that can vary between six and
20 ohms. But that’s a story for another
day. For now, when a speaker is said to
be four or eight ohms, this is understood
to be its nominal impedance.