Before permanent magnets were invented, speakers used an electromagnet and I ran across some photos of these relics.

Some of these beasts were heavy duty units and they had a hum bucking coil to cancel any variation in the DC supply.

 

Sure, saw them all the time in old tube radios and amps.

 

Back in 1942 a 15 in speaker gave enough wire to set up a 2 party telephone system about 900 ft. +- apart.

 

Before permanent magnets were invented, speakers used an electromagnet and I ran across some photos of these relics.

Before permanent magnets were invented?

Really?

Let's see...

The electron was discovered about 120 years ago.

The electromagnet was invented just under 200 years ago.

Permanent magnets have been made and used for about 2500 years.

The first loudspeaker, in 1921, used a permanent magnet (though it was mounted on the cone instead of the speaker coil).

 

Years ago a good number of the older tube type radios used an electro-magnet for the speaker. These same radios also had a power supply which provided filament voltage for the tubes and a B+ high voltage for the tube plate supply. The HV came off a transformer and was rectified by a rectifier tube (the 5Y3 comes to mind) and filtered by a cap. passed on to a choke and passed to another cap in a Pi filter. The choke was often the speakers electro magnet.

Here is a simple example:


While I would not say "before permanent magnets were invented" electro magnets were used in plenty of early radio speakers as well as just audio amplifiers. It was actually, at the time, a pretty slick design idea.

<EDIT> I had forgotten about it. In the guest bedroom I have an old Atwater Kent Radio originally battery powered. Beside it I have an old speaker which while not for that radio looks cool beside it. I just took a good look at the thing and sure enough there are 4 wires in the original cloth insulated cord which is about 5 maybe 6 feet long. Nice looking and maybe 1920s vintage. I think as a kid I dragged it out of my grandparents house. </EDIT>

Ron

 

Before ceramic magnets came to the market in the late 1960s, Alnico (and a few other alloys) were the norm, but they were also very pricey. Smaller speakers for consumer products had an Alnico cylinder, but the cost would be prohibitive for the larger home speakers.

I had a project to build a seismometer which used a moving coil transducer and it required a large cylindrical magnet about 3" outside diameter. I recall reading through a catalog from a magnet supplier and the prices for Alnico cylinders in a large diameter (and weight) were sky high. So I came up with a field coil arrangement like the ones shown in the photos. However, when ceramic magnets came into mass production, they were the best thing since sliced bread.

 

As a schoolboy in the fifties I was "roadie" to a 5th form skiffle band. I fixed up a guitar pickup with an ex-gov carbon throat mike and a speaker from an old radio with the fretworked speaker grill in the design of a sun with rays. The speaker inside had an electro-magnet, which I believe doubled up as a smoothing choke for the HT..

 

Hi,

Ceramic magnets didnt appear until the 1950's i think, even though natural magnets were around long before that.

Yes i had the opportunity to use a speaker with no permanent magnet many many years ago. I pulled it out of some tube thing but i cant remember what it was inside of now.

What puzzled me was that it had four wires, not two. It was obvious that the wires were two pairs not just four individual wires, so what i did was i used one pair and tried to get some sound out of it with an amplifier and i think the sound came out but it was very low amplitude. It took me a while to figure out that it had no magnet and that must mean that it required a current to create a pseudo magnet from the two other wires. After that i got it to work. It was a really nice speaker too and a decent size. Long gone now though as it was discarded as i got new speakers of the more modern type.

I used speakers as big as 12 to 18 inches in the past but i assume they all had ceramic magnets. I have not kept up on speaker designs so i dont know how they have changed over the past say 40 years, and i think they should have with the introduction of the rare earth magnets maybe in the 1970's. I would think that would make them better just like it did with motors. Dont know for sure though, but i would think the stronger magnets vs volume (size) would make the speaker more sensitive to electrical current and thus lower the winding turns count, etc.

Out of all the threads i have read and replied to in the past maybe 30 years this is only about the second thread that i have ever seen anyone talk about speakers that dont have permanent magnets but have a second coil instead. Most other subjects come up more often. Technology changes drastically over the years.