Electromagnetic Speakers

[planet10]

Re: Purty!

I suppose it's in your auctions?

Where do you find these gems, Dave?!

Yes, these went in one of my auctions. And fetched a very nice profit.

These i found at a 2nd hand furniture store (seems that this area is rich for mining old hifi). They had bought up an estate, and they got an old mouse & spider infested hifi in the lot. They cabinet literally fell apart while i was getting the useful parts out. The most notable thing about this piece was that it showed me that old hifi badges have value on eBay. Just as i was tossing the last piece of wood into the truck to go off to the bonefire pile, i noticed this little badge at the bottom -- made of pressed paper & paint. On a lark i put it up for auction and it almost paid for my cost of the entire hifi.

dave

 

[454Casull]

There is some more info on the virtues of Nd-Magnets in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=5857

Found an old book published around 1990 in the library (G. Schwamkrug, Lautsprecher - Dichtung und Wahrheit).

According to the chapter on the magnetic system, the problem isn't so much the modulation of the magnet itself by the VC, but more so modulation of the magnetization of the iron system.

One way to avoid this is to run the iron in saturation (I have to look up the figure again but I believe it was on the order of 1.3 T). If the iron is well in saturation, some modulation of the external field by the VC will not have any effect on the magnetization of the iron. Moreover, the iron has no chance to pass the magnetization on into the magnet.

Apparently, it is absolutely impossible to saturate iron with a ferrite magnet because the flux density of ferrite is too low. A properly designed system with an Alnico or Nd-Magnet, on the other hand, will do that automatically.

The same chapter claims that a properly designed copper ring will both decrease inductance as well as reduce distortion by up to 20 dB by keeping the VC field from entering into the iron. I guess I will have to look up conductivities typical iron and steel materials...

An improperly designed copper ring is said to make disto worse...

Eric

How do copper rings keep the field of the VC out of the iron circuit?

 

[Ron E]

With conventional electric motors if you weaken the field magnets (either permanent or electromagnets) the motor goes faster. So would it follw that a weakened speaker magnet would make the speaker more sensitive, or at least lowers the impedance because it now needs a greater cone excursion to generate the same back emf as before, that stops it from drawing infinite current? (ignoring dc coil resistance)

This "speeding up" of the VC with reduced magnetic field happens in practice with speakers because of the reduction of Bl with excursion, and especially because of assymetry in the BL field. It is not desirable because it is accompanied by DC offset - also called oilcanning or dynamic offset. It is usually compensated for by making a progressive spider.

 

[johnnyx]

I used to have a valve (tube) radio which had a loudspeaker that had had it's magnet energised from the HT, it was made by Rola, who were bought out by Celestion. The company was called Rola - Celestion for a while. Pity I didn't keep it.

Here is a link explaining Barkhausen Noise