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OzMikeH 26th September 2007 09:06 AM

Has anyone DIY a Field coil?
Has anyone machined and wound their own field coils for those drivers with replaceable magnets? (Lowther, AER etc)

I fail to see how they justify such high prices for what is essentially an electro-magnet with a soft iron cover.

Can anyone please direct me to a thread or webpage that shows in detail how these are put together?

Geoff H 26th September 2007 11:21 AM

Yeah, your right, sucks don't it. You must bear in mind the drivers you are referring to aren't thrown together by robots.

The was an article in EA May 1990, page 114, detailing a little on the Rice and Kellogg 6" EM driver.

It's not that hard to remove the backplate / pole piece assy and magnet from modern drivers. Dropping one can break the glue.

Older drivers had screws to keep the works together. They would be easier to convert, though I don't know if it would be worth the effort.

Many modern drivers have the front polepiece riveted to the basket.

This is where it all started:

If I can find a source of soft iron, I'll let you know. Until I do, I'm stuck with steel. It might be possible to melt down old gal waterpipe. That's not got much, if any carbon in it.

Voice coils can be sourced from speakerbits in Melbourne.


PS. here's another.

AndrewT 26th September 2007 11:38 AM

cast iron has lots of carbon in it and in big crystals that make cast iron brittle.
Soft iron has most of all the impurities removed. It's the removal of all the impurities than makes it soft iron and makes it good for passing flux.
Adding zinc to the melt, from galvanising, will probably make an alloy that is worse than soft iron, but I'm guessing on that one.

There are a couple of threads in this Forum on manufacturing a coil magnet for a big speaker and another on test/ listening results for a commercial coiled electromagnet driver.

Geoff H 26th September 2007 12:58 PM

"cast iron has lots of carbon in it"

That would be cast steel? It might be possible to separate the zinc.

The gal waterpipe I am referring to is malleable, not the cast pipes used for drainage. I have thought of using iron from old drivers. That means getting rid of cadmium. I'd prefer to mess with zinc.

Cast steel is better than cast iron, though not as good as pure iron or armco, on a b-h curve, according to my reference.

Edit: Andrew, you could be right about the zinc, there is no mention of it in the various mixes. And I don't recall seeing it when I worked for a magnet producer, and they were mixing high grades of alnico.

AndrewT 26th September 2007 01:23 PM


Originally posted by Geoff H
"cast iron has lots of carbon in it"

That would be cast steel? ....

I don't think so.
Cast steel is steel that is cast and has a reduced carbon content, compared to cast iron, but probably just as important in a different crystalline form.

Geoff H 26th September 2007 02:43 PM

OK, they both appear to have similar quantities of carbon. Wrought iron seems to have the least. So cast iron isn't cast fe. It's an alloy.

One a side note, see the reference to the speed of sound in iron compared to ferrite.

AndrewT 26th September 2007 03:03 PM

back to a suitable iron for passing flux around the magnetic circuit.

Very low content of any alloys would be essential. But particularly the alloys that interfere with the magnetic properties.
Some types of steel may be second best after soft iron.

I wonder how close wrought iron gets to fulfilling the need? Problem may be getting blanks big enough to turns poles and face plates from it.

Tubehead 26th September 2007 09:28 PM

What about using transformer iron? It is used to pass flux, but it doesnt look like pure iron to me? Also its in the form of sheet. Is it an alloy?

OzMikeH 27th September 2007 07:41 AM

The thin plates from transformer iron would be easy to melt down.
What about pig iron briquettes? Old radar magnetrons have easily accessible thick iron plates. You'd need quite a few. Wrought iron has extra lead added to make it more malleable so I don't think it's suitable.

I mentioned those drivers because they usually have their magnets retained with screws.

Given a billet of the right material I think I could turn a pair up on a lathe in a day easily. It might be possible to use a whole roll of magnet wire, spool and all - no winding. I see no reason why a lump of metal and a roll of wire should cost a couple of thousand euro.

AndrewT 27th September 2007 09:01 AM


Originally posted by OzMikeH
....... It might be possible to use a whole roll of magnet wire, spool and all - no winding.
I don't think so.
You want the maximum volume of copper in there to minimise the heat build up for the necessary AT (ampere turns) to generate our field strength.
If we can't get above 1T then we are wasting our time.
I believe some of the cheaper grades of steel allow around 1.9T to 2T to be generated.
2.1T to 2.2T should be possible with soft iron. I wonder if we can get higher?
Do we need to get higher?
Or even as high as 1.9T?

What are the T/S benefits from high flux?
What are the sound quality characteristics accruing from high flux.
Gilbert Briggs did some work on this?
Does anyone have links to ANY papers on this flux strength advantage?

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