Thiel ss2 参考指南
5
The next graph shows the distortion produced by each of
these systems. In both cases the Xmax value is 15 mm,
meaning the coil should be able to move 15 mm each way
without excessive distortion. You can see that the normal
system produces 17% distortion at this excursion whereas
the long gap system produces less than 2% — only one-tenth
as much! Also, at the slightly more normal excursion of 12
mm, the normal system produces 10% distortion whereas the
short coil system produces less than 0.5%— a twenty fold
improvement.
meaning the coil should be able to move 15 mm each way
without excessive distortion. You can see that the normal
system produces 17% distortion at this excursion whereas
the long gap system produces less than 2% — only one-tenth
as much! Also, at the slightly more normal excursion of 12
mm, the normal system produces 10% distortion whereas the
short coil system produces less than 0.5%— a twenty fold
improvement.
The second 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.
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 SS2 drivers incorporate
a thick copper ring around the center pole. With this ring any
0
5
10
15
20
0
2
4
6
8
10
12
14
16
Xmax = 15 mm in both cases
Long coil / short gap
Short coil / long gap
Peak excursion —
±
mm
THD — %
Copper pole sleeve and ring used in the SS2 woofers to stabilize the magnetic field strength
for reduced distortion.
for reduced distortion.
changes in the magnet’s strength induces an electrical current
in the ring which generates a magnetic field that is opposed
to and practically cancels the original change.
in the ring which generates a magnetic field that is opposed
to and practically cancels the original change.
The third distortion mechanism is that the voice coil
current is dependent not only on the driving voltage and the
coil resistance but also on the voice 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, the 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 higher frequency response increases. By this
mechanism the frequency response of the speaker is
modulated by driver excursion. This problem has been
practically eliminated in all THIEL drivers. 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 a copper pole 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.
coil resistance but also on the voice 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, the 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 higher frequency response increases. By this
mechanism the frequency response of the speaker is
modulated by driver excursion. This problem has been
practically eliminated in all THIEL drivers. 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 a copper pole 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 coil is an iron-core
inductor. Since iron is not magnetically linear, the coil’s
inductance changes with current in the coil and for this
reason such inductors are avoided in high quality crossover
systems. An additional benefit of the copper sleeve is that
since it reduces the coil’s inductance it also reduces the
distortion associated with changing inductance.
inductance changes with current in the coil and for this
reason such inductors are avoided in high quality crossover
systems. An additional benefit of the copper sleeve is that
since it reduces the coil’s inductance it also reduces the
distortion associated with changing inductance.
One of two SS2 drivers