Headphones 737Moving-coil headphones are essentially medium- to low-impedance devices with
impedances between eight and a few hundred ohms. They are usually operated via the
normal amplifi er headphone socket, which simply takes the loudspeaker signal and
diverts it through a resistor network to reduce it to a level more suitable for the high
sensitivity of the headphones. Alternatively, some high-quality amplifi ers provide a
separate amplifi er to drive the headphones directly.
Many cassette decks also provide a headphone outlet, and some headphones of lower than
average sensitivity sometimes do not work very well in this situation due to the limited
output available.
25.3.3 Electrodynamics Orthodynamic
This type of headphone is essentially in the same family as the moving-coil type, except
that the coil has, in effect, been unwound and fi xed to a thin, light, plastics diaphragm.
The annular magnetic gap has been replaced by opposing bar magnets, which cause the
magnetic fi eld to be squashed more or less parallel to the diaphragm. The “ coil ” is in fact
now a thin conductor zig-zagging or spiraling its way across the surface of the diaphragm,
oriented at right angles to the magnetic fi eld so that sending a constant direct current
through the conductor results in a more or less equal unidirectional force, which displaces
the diaphragm from its rest position. An alternating music signal therefore causes the
diaphragm to vibrate in sympathy with it, creating a sound-wave analogue of the music.
The great advantage of the electrodynamic, or fl at diaphragm type of headphone, is
that the conductor moves the air almost directly, without requiring a cone to carry
the vibrations from a coil at one point, to the air at another, with the very real risk of
introducing colorations, break-up, distortion, and uneven frequency response.
Unlike a cone, the fi lm diaphragm does not have to be very stiff, although it is sometimes
pleated to give it a little more rigidity because the force on it is not entirely uniform. As
a result, it can be very thin and light, which results in a lack of stored energy and a very
great reduction of the other problems inherent in cones as outlined earlier.
Consequently, good headphones of this type tend to sound more like electrostatics, with
an openness and naturalness that eludes even the best moving-coil types.
Electrodynamic headphones tend not to be quite so sensitive as their moving-coil
counterparts and are often best used at the output of amplifi ers, rather than with cassette
decks. Impedance is usually medium to low and is almost entirely resistive.