708 Chapter 24
(commonly from 3 to 1000 W), but they will all mostly have a DC resistance of 5 to 10
ohms and a nominal (AC, 400 Hz) impedance of 8, 15, or 16 ohms.
24.1.1.5 The Ribbon Driver
The ribbon speaker is a fourth kind of electrodynamic drive-unit. Instead of a voice coil
attached to the radiating part, the amplifi er signal is connected across a length of fl at
(planar) conductor foil or “ ribbon, ” which is again placed in a magnetic fi eld like a voice
coil, but also radiates sound like a cone, diaphragm, or dome. Compared to ordinary voice
coils, this arrangement can be lighter and certainly presents a much purer ( “ resistive ” )
impedance to the amplifi er. The classic ribbon had a very low DC resistance and was
transformer coupled. Modern ribbon speakers have longer strips, amounting to 3 or 5 ohms
of near pure resistance, benign to most audio amplifi ers connected to it. When “ built big ”
as apanel loudspeaker , a ribbon drive-unit forms a wide-range loudspeaker in its own right,
that is, no cabinet required. There is little breakup in the ribbons ’ surface to mar the sonic
quality. And, unlike other drive-units, absence of a cabinet means the sound source radiates
as a dipole, that is, from both sides. This can be important to the amplifi er, in far as room
interaction can change the impedance seen byrefl ection. Small ribbon drive-units are used
as tweeters. They may be horn loaded to magnify their rather low output. Sonic quality can
be very high, although naturally favoring the reproduction of stringed instruments.
24.1.1.6 The Electrostatic Source
The electrostatic loudspeaker (ESL) employs the inverse or dual principle of the
electrodynamic or “ motor ” types of drive-units that we have just looked at. The movement
is provided by electrostatic (electric fi eld) force rather than magnetic attraction and
repulsion. The vibrating part is a thin, critically stretched sheet called the diaphragm.
The fi xed part, after the capacitor it mimics, is called a plate. Electrostatic drivers are
commonly made in the form of panels, like ribbon speakers. A power source (usually from
the AC mains) provides the high EHT DC voltage of over 1000 V that is needed to polarize
the plates. A high signal voltage swing is also required. This, together with isolation
from the EHT, is attained by interposing a transformer. In practically sized and costed
electrostatic speakers, the transformer and the diaphragm have a surprisingly limited
capacity for handling high levels at low frequencies. In primitive designs, overdrive in
the bass can cause the diaphragm to short against the opposite plate. In modern ESLs, the
diaphragm is insulated. A well-known electrostatic employs an aggressive crowbar circuit
for protection. If the ESL is subjected to potentially damaging high levels at low enough