Ralph, I was reading about Classic Audio Speakers the other day. They also sell their components separately to DIY guys. How do the field coils work? I am not familiar with this design and have never gotten into the specifics of horn design. Probably should study this a bit. Those Hartsfield speakers are real lookers. I remember when I was a kid listening to JBLs with that same horn lens that were mighty impressive back then.
One way to describe how field coils work is ’pretty good’ :)
Field coils preceded permanent magnets. Permanent magnets are easier/cheaper to make so when the industry sorted out how to do it field coils went away. Field coils are simply a large coil through which a DC current is sent. This causes it to have a magnetic field which is focused in the voice coil gap of the loudspeaker. What makes them special, like ESLs, is that the magnetic field does not sag like it does with any permanent magnet speaker motor when current is applied to the voice coil. So they respond faster and with greater linearity. Of course its important to have a good supply for the field coil just like it is for the static field of an ESL.
In the old days the field coil was used as a choke in the power supply of the radio or whatever with which it was used, serving double duty to smooth the power supply while also being thus energized. These days the power to run them comes from a separate power supply. Of course, audiophiles got involved with that, so if you read about the power supplies for field coils sooner or later you’ll read about Tungar bulbs, which are low voltage high current gas-tube rectifiers, which audiophiles claim sound better :) Since some of the modern field coils are quite large, that this is so is highly dubious since regulating the supply is pretty important. The regulator would of course negate any benefit that a Tungar bulb might bring.
Put another way, field coils are the only dynamic speaker tech that keeps up with the speed of ESLs. Combine that with a horn and the driver really doesn’t have to move all that far, improving linearity. In this way you can make a dynamic driver that has similar low distortion as you see in ESLs.