5
DISTORTION
Driver motor systems
Unlike some 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. The CS.5
incorporates several unusual features in its woofer to decrease distortion and increase dynamic range.
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 strength must be constant, the length of voice coil wire acted on by the magnetic field must be constant, and the current in
the voice coil must be directly proportional to the applied voltage. In practice, none of these three conditions actually exist but the CS.5
woofer incorporates refinements of design that greatly improve the accuracy of each of these factors.
The first distortion mechanism is that 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 generates the force to move
the diaphragm by creating its own magnetic field that “pushes” against the magnet’s field. The magnet is somewhat demagnetized by the
coil’s magnetic field when current flows in one direction and is 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 greatly
reduce this effect the CS.5 woofer incorporates a copper sleeve around the center pole. With this sleeve any change in the magnet’s
strength induces an electrical current in the sleeve which generates a magnetic field that is opposed to and practically cancels the original
change.
The second distortion mechanism results from the fact that almost all woofers use a long coil/short gap motor system where the long
coil is acted upon not only by the field within the air gap but also by the “fringe” field in front of and behind the gap region. As the coil
moves forward or backward to produce bass energy, the magnetic field acting on the coil becomes less intense because the coil is further
from its rest position where the magnetic field is strongest. This weakening of field strength as the coil moves away from its rest position is
the primary distortion producing mechanism in woofers.
To eliminate this problem the CS.5 woofer uses a very unusual short coil/long gap system where the coil is much shorter than the
magnetic gap. Therefore, even when the coil moves a considerable distance from its rest position, it continues to be acted upon only by the
uniform magnetic field in the air gap and does not experience the changes in magnetic field strength with position as in the conventional
system. As shown below, the distortion produced by the CS.5 woofer’s short coil motor system at normal excursion levels is only one-tenth
that produced by the typical long coil system.
The third distortion mechanism is 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 therefore with conventional magnet
system geometry, inductance changes during the excursions necessary
to reproduce low frequencies. As the diaphragm and coil move back,
more of the coil is around the pole, increasing the inductance and
decreasing the mid-frequency output of the driver. As the coil moves
forward, less of the coil is around the pole, the inductance decreases,
and the mid-frequency response increases. By this mechanism the
frequency response of the speaker is modulated by driver excursion.
This problem has been virtually eliminated in the CS.5 woofer. The
short coil design results in the entire coil surrounding the pole in all
positions and therefore the coil’s inductance does not change with the
diaphragm position. In addition, the problem is further reduced by the
copper sleeve which reduces the inductance of the coil to a fraction of
its normal value by acting as a shorted turn of a transformer secondary
winding.
An additional problem is that the voice coil is an iron-core inductor. Iron-core inductors are not linear and therefore introduce
distortion. For this reason such inductors are avoided in high quality crossover systems. Nonetheless, one iron-core inductor remains in the
signal path—the driver’s voice coil. An additional benefit of the copper sleeve is that since it reduces the coil’s inductance, it also reduces
the associated distortion.
coil
1
2
3
4
peak excursion ±mm
8%
6%
4%
2%
distortion
1
2
3
4
peak excursion ±mm
8%
6%
4%
2%
distortion
