Philips 9710 dissasembling
Copyright 2007 Troels Gravesen
       

The Philips 9710 fullrange driver is one of the few drivers that can be taken apart and there's a lesson to be learned from this exercise. One has to take great care in doing so because it is easy to damage the voice coil and with a bumped voice coil there's no way back.
The chassis is attached to the polepiece by three screws and removing these allows you to lift off the chassis including cone and voice coil. Very simple. The chassis fits nicely to the polepiece, which make the guide for correct placement of the voice coil in the magnet gap. Before rushing to do so:

Warning: Don't do this at home!
If you disassemble the magnet structure you will need to have it remagnitised due to the alnico magnet rapidly loosing its strength. I lost 4-5 dB sensitivity from doing so and I had to take the magnet to a driver manufacturer and have them remagnetised. Thanks Per!

First thing I did was removing the dust cap. The dust cap would most likely be sitting close to an undamped centre polepiece and so it did. What we're looking for here are cavities that may produce resonances from the cone moving back and forth. My initial idea was to remove the dust cap and insert a phase plug and with a scalpel the dust cap was gently cut out in increasing circles and eventually the whizzer cone fell off. No problem, it can be glued back if need be.

The motor of the 9710. Alnico magnet with 11 mm of polepiece and copper plated centre polepiece.
The underhung voice coil has an approx. 8 mm winding height, which leaves +/- 2.5 mm cone excursion before the voice coil will start running out of the magnet gap.
The centre polepiece is 34 mm diameter (including copper plating) and the pole piece has a 36 mm internal diameter, thus we have a 1 mm magnet gap. The voice coil including former has a thickness of 0.5 mm and we're left with a clearance of 0.25 mm on each side of the voice coil. This is not a whole lot - and close to the JBL 125A driver!

Voice coil with rear suspension. The voice coil is wound on a paper former to very close tolerances.

Lifting off the polepiece is a bit tricky as the magnet is very strong, but with a little brute force a chisel was inserted between magnet and polepiece and a wooden stick was used to lift the polepiece from the magnet.

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 impedance plot.

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.

This copper structure is a little strange as it forms a basket below the voice coil and produces a cavity for potential resonances. I guess the reason for the basket form is that the pole piece holds the copper plating in place and there's no need for using glue. With a pair of scissors I removed the "basket" and glued the copper cup in place again with a small amount of silicone glue.

Here you can see the copper plating without the basket and the magnet structure was damped with some sheep's wool to reduce resonances in the magnet cavity.

Getting the polepiece in place again is a bit tricky and you have to add small pieces of wood between the magnet and polepiece. Three layers were used and one by one removed while the polepiece was getting closer and closer to the magnet. After removing the first layer of wood chips, teflon spacers were inserted between magnet and polepiece to keep a constant distance between the two. See below.

As said, the magnet gap is 1 mm wide and I had some 0.5 mm teflon sheets and two layers made the necessary spacers to allow the pole piece to gently slide in place before reinserting the screws.

Gently place the magnet structure over the basket and slide it back into place. Take great care not to damage the voice coil!

Voila! Piece of cake - but if you can't remagnitise your magnets, DON'T!

Some other time:
Does it all matter? Does the removal of the whizzer cone enhance sound? Do the modifications to the copper plating and damping of the magnet structure improve the sound?