As a youngster, I saw some really old radios; the kind that occupied highly-polished wooden cabinets the size of a refrigerator. When those old radios were made, permanent magnets were not all that powerful, at least by today’s standards, and they were quite costly. Ergo, the loudspeakers in those radios derived their magnetic fields for their voice coils from electromagnets, which were called “field coils.”
The DC excitation of a field coil would come from using that coil as an inductor in an LC filter from which power supply voltage was derived. A super simplified arrangement is shown in Figure 1.
|Figure 1.||Here’s a simplified field coil speaker arrangement.|
Since the field coil doubled in service as a power supply filter choke, current passing through that coil carried a ripple current, 120 Hz in the case of a 60 Hz power line, and that ripple current would produce a low but discernible level of hum in the audio output.
|Figure 2.||This vintage field coil speaker photo was posted on eBay.|
The hum was low enough to be pretty much acceptable to most listeners but as permanent magnet technology matured so that such magnets became stronger and less costly, field coils were pretty much done away with in consumer products in favor of using permanent magnets.
The forgoing has for all these years been part of my personal gospel and I started writing this essay with the idea of describing a pretty clever but quite obsolete technology. I had envisioned field coil speakers as belonging with cats whiskers, coherers, and wax cylinder recordings.
Imagine my surprise when I discovered that field coil speakers are still being made and that there are aficionados with much to say regarding their virtues. I had no idea. Some even proclaim field coils are the future.
A quick Google search offers a whole lot more pictures both old and new if you’re interested.