Such a thing as electromagnetic headphones?

I understand that there were loudspeakers like that back before even alnico magnets were readily available, but I've never heard of a headphone being built that way.

I think even the original beyer designs from the 30's used permanent magnets.

Neophyte?  Nope, that's a good question.  What you're talking about is usually referred to as a field coil speaker.  They were used back in the day before permanent magnets of good strength were reasonably available.  The design has some benefits - as you pointed out, you can have a very powerful electromagnet.  Also, I've heard that field coils do not suffer from the temporary instability and magnetic field collapse from powerful transients the way permanent magnets do.  They also have some interesting properties with the magnetic gap, but I don't recall the specifics.

I've never heard of a field coil being used in headphones.  The very early headphones used moving iron drivers.  Moving iron drivers, by the way, sound terrible.  They were used mostly in headphones for radio sets and telephones.  If you've ever heard them, the fidelity isn't so hot.

One issue would be that you would need a separate DC power supply to power the field coils if you put them into headphones.  Output from an amplifier would not be enough, you'd have yo have another box delivering DC.

Field coils have made something of a comeback.  You can buy speaker drivers from Fertin, Supravox and a handful of others.

One factor that does not work in it's favor is heat.

One of the jobs of the permanent magnet (other than

to move the voice coil) is to absorb heat. If you have

an electro magnet in there as well as the voice coil

then you have two heat sources an no way to get

it out.

Call it a guess:

In the few speakers where MFRs do offer field coils in place of permanent magnets the sales of these units border on "lunatic fringe". The manufacturers who offer field coil speakers are already building speakers a pair at a time, basically to order. Building it with a different magnet is no trouble for them.

Headphone drivers on the other hand are ALL mass produced devices. This is good because it allows very high quality parts to be made at an affordable price, but if you want something just a little bit special your out of luck. Due to the way that most headphone drivers (actually most speaker drivers too) are made removing the magnet after the thing is built is a daunting task. 

I think the easiest drivers to tweak if you have a serious desire to do it would be Grados. If you heat up the magnet with a heat-gun it pulls out of the frame with very little deformation of the plastic. After that its "just" a matter of constructing a coil form, winding a coil, installing it, and building a power supply. 

Regarding "another box to drive the headphones":

The power supply for the field coil could EASILY tap off of the power supply for the amplifier, but dont forget that we on this site are the lunatic fringe who even use amps. To add to that, even amongst head-fiers there are some who freak out with STAX requiring a special amp, never allowing the fall back of "what if I want to plug my $2200 headphones into my computer"? Its the stupidest thing ever, but its reality. Off of head-fi the majority of people will take a good quality headphone like the Grado RS-1 or Senn HD650 and run them straight off of the ipod or computer, never thinking of an amp. The fact that the headphones would force the use of an amp would be a real bummer to most people.

electromagnets are horribly inefficient. Besides which they need a core that adds to the

weight. OK for large bass drivers. Just awful for headphones.

Here is a picture of a 3.5 tesla electromagnet. (35000 gauss)

http://gilmore.chem.northwestern.edu/epr.jpg

It weighs a little over 12 thousand pounds.

It consumes 8 gallons of cooling water per minute.

It consumes 225 amperes at 3.5 tesla.

Here is a  Neodymium magnet that is 13800 gauss, put 3 of

them together and you get over 3.5 tesla.

Weight about 21 lbs total.

http://www.magnet4less.com/product_info.php?cPath=1_11&products_id=871

Just for a non-scientific reference.

These are full-range transducers.

The first one has neodymium permanent magnet.

The second has Alnico permanent magnet.

The third has a huge field coil.

Quote:

Originally Posted by kevin gilmore 

electromagnets are horribly inefficient. Besides which they need a core that adds to the

weight. OK for large bass drivers. Just awful for headphones.

Clearly what we need are superconducting field coils, preferably cooled with liquid helium.

I'll have to ask my pal at UTSA if he has any pics / specs for his 11T magnet.

actually what we need are room temperature superconductors.

liquid helium gets to be a pain in the neck after a while.

And even the latest generation (i have a 14T magnet) are quite huge.

However you can make really good sounding audio with only

one turn of wire.

Quote:

Originally Posted by kevin gilmore 

actually what we need are room temperature superconductors.

I am anxiously trying to find audio applications for graphene.