When I was a kid collecting broken radios to disassemble for parts, I encountered old single-loudspeaker radios that had four wires going to the loudspeaker.

So here's a question for the young people here: "Can you guess why it would be necessary for a single loudspeaker to be driven by two signals?"

Those speakers were known as dynamic speakers, they used the big coil on the back as apower suupply filter choke and to create the magnetic field for the voice coil.

Hi techcon.

You are correct. "Give that man a kewpie doll!"

Because it was not practical to achieve the necessary constant-level magnetic field strength using available permanent magnets back in the old days, they drove a DC signal through a speaker's "field coil." And that's the way they created their desired constant-level magnetic field.

By the way; I'm referring to those old floor-model radios in their truly gorgeous wooden cabinets.

techcon, I have another question for you. What problem was caused in those radios that DID use a speaker's field coil as an inductor to attenuate AC ripple in some DC power supply?

Only thing I can think of right off is some hum in the audio.

Hi techcon.

Again, you are correct. I see my questions are two easy for you.