Here we have all the
parts of the magnet system. Now, what's that
copper "thing"? Philips has made a lot
of nice things over the years without bragging
too much about it, and I'm sure they never
bragged about using "symmetric drive",
because this is what it is. And long before this
was "invented" by other companies. The
copper plating of the centre polepiece reduces
eddy currents generated by the coil moving in a
strong magnetic field and the result is a nice
flat impedance profile and reduced distortion.
One of the reasons for some people
preferring alnico magnets is said to be due to
the fact that alnico demagnitises more rapidly
compared to ceramic magnets. When the voice coil
moves in the magnet gap, it generates a magnetic
field of its own trying to demagnetise the
magnet, which it may do locally and temporarily.
This may cause a minor compression and slow the
transient response, thus a smoother and less edgy
sound compared to ceramic magnets.
The "symmetric drive"
feature basically works in the opposite direction
due to the copper plating reducing "eddy
currents", i.e. reduces the inductance
produced by the voice coil moving in the magnet
gap and produces a consistent impedance vs.
frequency. And so it does as seen from the
So is the best of both worlds
combined in the 9710 driver? Well, there's more
to a speaker than the magnet structure and the
cone and suspensions are serious factors in
designing good drivers.