You folks are using terms that might confuse some readers, if they were not
versed in terms that are still heard, but used with slightly different
meanings. In a modern context, a "dynamic" speaker is any speaker that uses a
cone,
a permanent magnet, and a voice coil. Among collectors of audio gear from
the so-called "Golden Age" of Hi-Fi, dynamic speakers with large, permanent,
alnico magnets are very desirable. Many, if not most of those speakers were
shipped off to the pacific rim, while the Japanese sold us rack-system junk.
I think the speakers you are referring to as being "dynamic" speakers are
now referred to as "Electro-dynamic" speakers, or "Field coil" speakers. They
use a magnet that is powered by an external power supply. This allows those
speakers to have an enormous advantage in efficiency, especially woofers,
which require long, heavy throws of the voice coils.
Amplifiers used with "dynamic" speakers have to generate all of the power
required to move the cones in modern dynamic speaker systems. That is one of
the reasons I have a dedicated circuit for my stereo. I use two stereo
amplifiers that are large enough, if driven to full power, as a pair, to trip
a 15
amp circuit breaker in my house's main fuse panel.
Field coil speakers do not need amplifiers with that kind of power. They
require amplifiers that are only powerful enough to excite, or modulate the
magnetic field that is generated by the power supply. I have not run the
numbers,
but I would guess that this can mean the difference between a speaker
needing 5 watts for a given SPL (sound pressure level), and 500 watts.
Field coil speakers were abandoned, mostly because they were expensive to
make, and relatively complicated. As permanent magnet technology improved,
field
coil speakers were phased out. However, a small, but highly vocal, and
dedicated fringe of modern Audiophiles still seek out old field coil speakers,
and
get a kick out of what those speakers can do with miniscule amounts of
amplifier power. They are especially popular with fans of amplifiers that used
single-ended, triode output tubes, like 2A3's 300B's, etc.
I believe there is one company left in the world that makes uses
high-quality, field-coil drivers. I think it is a French company, but I'm not
sure.
Google it, if you are curious.
One easy way to spot a field coil speaker is to look at the number of wires
going to the unit. If there are more than four wires connected to the speaker,
it is probably a field coil unit.
Randy Minor
