Has anyone investigated converting a PM speaker to field coil? Could one - as a practical matter - remove the PM from a driver and add an electromagnet in its place? Maybe using an older driver with the smaller magnets housed in U shaped frame (easier to remove than modern designs?)...
OK. I get some odd thoughts at 1:45 AM... Maybe I should go to bed.
mike
OK. I get some odd thoughts at 1:45 AM... Maybe I should go to bed.
mike
I spent some 'thought experiment' time on this and while I think it's possible in theory, in practice it may not be the ideal way to go. Reasons
- tough to get the magnet off without wrecking the cone
- voice coil design probably should be different for a field coil unit (not an issue if you're just trying to make something work)
On the other hand, I'm not sure how else you'd go about experimenting. You either need to remove a magnet from a working speaker, OR be able to get the cone/spider out without damaging it to serve as the basis for the field coil unit. Maybe the way to go is to look for drivers that are easily disassembled - some pro units may fall into this category.
- tough to get the magnet off without wrecking the cone
- voice coil design probably should be different for a field coil unit (not an issue if you're just trying to make something work)
On the other hand, I'm not sure how else you'd go about experimenting. You either need to remove a magnet from a working speaker, OR be able to get the cone/spider out without damaging it to serve as the basis for the field coil unit. Maybe the way to go is to look for drivers that are easily disassembled - some pro units may fall into this category.
Peavey makes "pro" drivers with a magnet structure that can be unbolted. You can buy the metal "basket" with cone and suspension attached as a spare part. Might be an easy way to get started, but I suppose that you were thnking of using a driver that is already considered among the best. Idon't know how the Peaveys sound.
Someone is doing just that...
http://www.shindolabs.com/pages/5/page5.html?refresh=1112334014765
http://www.shindolabs.com/pages/5/page5.html?refresh=1112334014765
Now this guy is serious about field coils. Poke around on his site for a minute.
http://home5.inet.tele.dk/f-hammer/index.htm
http://home5.inet.tele.dk/f-hammer/index.htm
Maybe it would be more practical to somehow add a wound electromagnet in parallel with an existing permanent magnet? Would not have to risk destroying a good speaker and if it is not a success then you could restore it back to how it was.
Don't forget that if you increase the flux density in the gap you wil raise the speaker impedance proportionally. This will reduce the output dB for a given drive voltage, perhaps the opposite of what you expected!
I have wondered in the past how much effect there would be if the base of the centre pole of the magnet structure was wound with an auxiliary coil of equal number of turns to the voice coil and phased such that increasing voice coil current that tends to buck the main magnet field also causes this aux coil to boost main mag field by the same amount. Make sense?
Edit --> hey Variac, good avatar you have there ;-)
Don't forget that if you increase the flux density in the gap you wil raise the speaker impedance proportionally. This will reduce the output dB for a given drive voltage, perhaps the opposite of what you expected!
I have wondered in the past how much effect there would be if the base of the centre pole of the magnet structure was wound with an auxiliary coil of equal number of turns to the voice coil and phased such that increasing voice coil current that tends to buck the main magnet field also causes this aux coil to boost main mag field by the same amount. Make sense?
Edit --> hey Variac, good avatar you have there ;-)
Interresting thread this.. I have some questions on the matter:
How will a bobbin in aluminum affect the calculated magnetic field of a field coil magnet (calculated with a core mu=4pi*10^-7 a.k.a empty space)? Do I have to take the permability of aluminum into account when calculating the magnetic field? I envision the tube surrounding the air core (supporting the windings) would have to be around 2mm thick to be rigid enough to support the construction.. Can such an aluminum tube mathematically be concidered as "air" for practical purposes? and will it in any way "shield" the voice coil inside from the magnetic field produced?
How will a bobbin in aluminum affect the calculated magnetic field of a field coil magnet (calculated with a core mu=4pi*10^-7 a.k.a empty space)? Do I have to take the permability of aluminum into account when calculating the magnetic field? I envision the tube surrounding the air core (supporting the windings) would have to be around 2mm thick to be rigid enough to support the construction.. Can such an aluminum tube mathematically be concidered as "air" for practical purposes? and will it in any way "shield" the voice coil inside from the magnetic field produced?
Just to check my own calcs here, what AWG og wire diameter would one want to use to run 5A DC through (in a 1.3T field coil)? The alu thing I mentioned in last post sounds stupid, guess there are other more suitable composites for the job, just forget I mentioned it.
in my notebook, both math and copper seem to get very heavy
in my notebook, both math and copper seem to get very heavy
Sorry for flooding the thread, I've realized a field coil actually is quite more complex than I first suspected. Also it is hard sourcing good information on the matter online. Doing the math, it seems there is no getting around a pole piece here, to increase the permeability. Is this correct? What metals am I looking for in that position? I attached some equations on magnetic field, wire length and DCR. Seems to be way out of range if no core material is used..
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