Advertisement
Driver Motor Systems
Unlike the other sources of distortion, motor system distortion is very dependent
on volume level, being low during quiet playback levels but increasing rapidly as
volume levels increase. At moderate to loud playback levels it is the major source of
distortion and can therefore be considered a dynamic range limit. The CS3.6
incorporates several unusual features in its drivers to decrease distortion and increase
dynamic range.
Excursion: The most basic limit to dynamic range of a dynamic driver is the
limit on diaphragm movement caused by the driving voice coil leaving the magnetic
gap beyond a certain excursion. The solution of using a longer coil requires that the
magnet be much larger and is therefore usually avoided. The CS3.6 woofer has a very
long coil, 13 mm longer than the magnetic gap, which provides output twice as high
as typical "long excursion" woofers.
The midrange driver uses a short coil/long gap design that provides very low
levels of distortion. As shown in Figure 20, the distortion produced by the short coil
Figure 20 Distortion comparison
10
8
6
10 mm coil
4
2 mm coil
2
0
0.0
0.5
1.0
peak excursion — ±mm
Woofer magnet system: The purpose of the driver's motor system is to apply a
force to the diaphragm that is directly proportional to the voltage supplied by the amplifier as modified by the electrical network. In order
for the force to be directly proportional to the voltage applied, as desired, the magnetic field must be constant, the length of voice coil wire
acted on by the magnetic field must be constant, and the current must be directly proportional to the applied voltage. In fact, none of these
conditions actually exist but the CS3.6 woofer incorporates refinements of design that improve the accuracy of each of these factors.
The strength of the magnet's field is not actually constant in operation but is changed by the current from the amplifier through the coil.
This change occurs because the amplifier current through the coil creates its own magnetic field that "pushes" against the magnet's field,
generating the force to move the diaphragm. Magnets are somewhat demagnetized by the coil's magnetic field when current flows in one
direction and are remagnetized when current flows in the opposite direction. Therefore, since the magnet's field strength is not constant, the
force generated is not in the desired direct proportion to the current in the coil. To drastically reduce this effect the CS3.6 woofer
incorporates heavy copper rings around the center pole. With these rings any changes in the magnet's strength induces an electrical current
in the rings which generates a magnetic field that is opposed to and practically cancels the original change.
A second problem is that the field strength is not symmetrical in front of and behind the gap but is stronger behind the gap than in front
of the gap. Therefore, when the diaphragm moves outward, the coil experiences less magnetic field than when it moves inward. This
mechanism is the major cause of even harmonic distortion . The woofer in the CS3.6 utilizes a specially shaped pole piece to reduce this
problem. Figure 21 and figure 22 show the standard and improved field strength symmetry and distortion. At most output levels distortion
is reduced by 75%.
The third problem stems from the fact that the coil current is dependent not only on the driving voltage and the coil resistance but also
on the coil inductance. The problem is that the coil inductance varies with the amount of iron inside the coil and, with conventional pole
piece geometry, changes during the excursions necessary to reproduce low frequencies. As the diaphragm and coil move back, more of the
Figure 21 Field strength and
distortion of standard pole geometry
1.0
0.8
0.6
0.4
0.2
0.0
-10
0
10
position from gap center – mm
10
8
6
4
2
0
0
1
2
3
4
5
peak excursion — ±mm
motor system at normal excursion levels
( 1 mm or less) is only one-tenth that
produced by the typical long coil
system.
The tweeter has been specially
designed to achieve a dynamic range 15
dB greater than standard tweeters. This
performance is achieved by utilizing an
unusual wide roll surround and a short
coil/long gap magnetic system which
1.5
2.0
allows much greater linear excursion
( 1.5 mm).
Figure 22 Field strength and distortion
of CS3.6 woofer
1.0
0.8
0.6
0.4
0.2
0.0
-10
0
10
position from gap center – mm
10
8
6
4
2
0
0
1
2
3
4
6
peak excursion — ±mm
coil is around the pole, increasing the inductance and decreasing the
output of the driver by about 1 dB. As the coil moves forward, less of
the coil is around the pole, the inductance decreases and the response
increases approximately 1 dB. By this mechanism the frequency
response of the speaker is modulated by driver excursion. This
problem has been drastically reduced in the CS3.6 woofer. The same
pole geometry which makes the field symmetrical also results in a
constant amount of iron inside the coil regardless of coil position. In
addition, the problem is further reduced by the copper rings around
the pole which reduce the inductance of the coil to a small fraction of
its normal value by acting as a shorted turn of a transformer
secondary winding.
All of these design details contribute to the CS3.6's exceptional
technical performance, its accurate reproduction of all musical
5
6
information, and its ability to faithfully reproduce the musical
experience.
CS3.6 midrange
Cast magnesium frame
for rigidity and clearer
Double cone diaphragm
reproduction
for extremely transparent
reproduction
Heavy copper sleeve
for exceptional attack
Phase plug for more
Short coil / long gap motor
uniform response
for very low levels of distortion
6
Advertisement
