Faraday rings

[Puggie]

I fancy having a play at fitting a faraday ring to a speaker (sub) I'm re-coning. My plan is to do a full height (full length of the pole piece) Faraday ring. Would it be ok to machine the pole piece down by 1mm, and electro-plate the pole piece with copper up to 1mm thick and re-assemble the sub (I see no problem in this)? the copper ring does not need to be electrically insulated from the steel pole piece does it?

I know I'll lose BL due to the bigger magnet gap but this is not really an issue for my purpose (or rather I hope the improvements will outweigh this)

what advantages / problems am I likely to encounter?

[Puggie]

Anyone know if the faraday ring has to be electrically insulated from the steel pole piece?

[LineSource]

Hi Puggie,

The Faraday ring should be electrically connected to the steel pole piece to provide the best total current conduction path.

On the topic of copper plating. Lambda Acoustics built some speakers with just a copper plating on the pole piece, their LE series. The plating did help to modestly extend the frequency range and flatten the impedance curve, but the thick copper sleeves used for Faraday rings in their TD series were substantially superior.

[Puggie]

Thanks for the reply, what are you calling thick, I was planning on plating the pole piece with between a 0.5 and 1mm thickness of copper. I know Lambda seem to be producing a lot of drivers with a lot of copper in the motor but I've had problems trying to find much good technical info on them.

[hancock]

Originally posted by Puggie
I know I'll lose BL due to the bigger magnet gap but this is not really an issue for my purpose (or rather I hope the improvements will outweigh this)

Saturating the pole piece is also really important for reducing inductance (and therefore flux modulation). You may be actually increase the inductance by doing this rather than lowering it.

John

[John Sheerin]

Originally posted by Puggie
Thanks for the reply, what are you calling thick, I was planning on plating the pole piece with between a 0.5 and 1mm thickness of copper. I know Lambda seem to be producing a lot of drivers with a lot of copper in the motor but I've had problems trying to find much good technical info on them.

In addition to what John said, reducing the variation in Le with voice coil position will also reduce distortion. This requires that the copper (or some other conductive material) be placed in a certain location, and depends on the rest of the geometry of the motor. In general, though, Le will rise as the coil moves into the motor (more steel in the middle of the coil), and go down as the coil moves out. So you want to start by putting conductive material on the inside of the motor, then through the gap, then above the gap. How thick the conductive material is also effects how effective it is. For a subwoofer, 1mm might not be enough to make much difference at low frequencies. For a ring only below the gap, 10mm or more thickness of copper may not be excessive.

[Puggie]

Thanks guys, this was looking nice and easy untill those last two posts

looks like I'll have to download that FEMM magnetic modeling package and have a play.

So ideally I need to put as much copper in the motor as possible, while retaining the BL.

I have a sub where the motor has come apart, the top plate and pole piece/back plate glue joins have failed so I have 3 pieces, top plate, pole piece and back plate and 3 magnets still bonded to each other.

The voice coil is 2" dia

The pole piece is 50.5mm outer dia with a 19.5mm dia vent through it and protrudes 19mm above the top of the top plate. so not much room for putting much of a copper ring around there, my plan was to turn the pole piece down by about 1mm and electro-plate this with copper to form the faraday ring, I was planning on doing this over the whole length of the pole piece.

The top plate is 16mm thick and the hole in the centre 56.25mm.

The magnet is made from 3 ceramic 'slugs' 21.5mm thick, 165mm outer dia and 61.5mm inner dia. I was planning on fitting a copper ring in the void between the top and back plates that would fit flush against the inside of the magnets with an inner diameter as close as I could get to the hole in the top plate so its going to be about 4.5mm thick.

I have a couple of motors of dead subs which are to fit 3" VCs and magnets are 8.5" outer dia (BIG) I could look at turning new top plates and pole pieces to use these, this would give me the ability to fit a lot more copper round the outside of the VC and I would expect have a higher BL to offset the increase VC gap.

The sub is a very high excursion design (XMAX of 2" peak to peak but the physical limits are a good bit more) so I could lose the pole vent and replace the dust cap with a phase plug to increase BL maybe.


Any ideas advice etc is most welcome, this is really an experimantal excercise for me to see the effects of faraday rings and see if I can improve on this sub I have.

[hancock]

If you really want to get experimental, instead of a shorting ring, try wrapping some magnet wire tightly around the pole piece and run the wire in series with the voice coil, but so that the current runs in the opposite direction to the voice coil--if voice coil runs clockwise from + to -, run the pole piece coil counter-clockwise from + to -. You'll have to experiment with the number of turns, but you should be able to get the inductance down to zero. There is an article in one of the AES Loudspeaker Anthologies on this.

John

[John Sheerin]

Hey John,
Yeah, I have seen that somewhere... Do you happen to remember the author or title of the paper? I just searched and could not find it (I don't have the anthologies but can get any of the papers in them). If I can figure it out I'll build a speaker with and without next week, take some measurements, and give them a listen.

[BassAwdyO]

If you really want to get experimental, instead of a shorting ring, try wrapping some magnet wire tightly around the pole piece and run the wire in series with the voice coil, but so that the current runs in the opposite direction to the voice coil--if voice coil runs clockwise from + to -, run the pole piece coil counter-clockwise from + to -. You'll have to experiment with the number of turns, but you should be able to get the inductance down to zero. There is an article in one of the AES Loudspeaker Anthologies on this.

And BL goes down the drain
And the difference between this and a DVC woofer with the coils wired reverse polarity is?????