How Do Speaker Field Coils Work?

What’s a field coil? How does a speaker field coil work? Why do old speakers have a field coil? What’s a pi filter? Learn about all this and more in this short video. We’re almost ready to discuss the receiving and amplifying section of our radio. But first, there’s one more piece of the power supply I haven’t mentioned. Interestingly, it involves the last element of our signal circuit: The speaker. On our schematic, the speaker is broken down into three sections, even though they’re all physically attached as one unit. This section is called the output transformer. This is the voice coil and cone. And this is the field coil. On the actual speaker, the cone, voice coil and output transformer are readily visible. But the field coil is hidden inside. Antique speakers work just like modern ones, but the output transformer is usually part of the amplifier circuit, and not attached to the speaker. I’ll discuss the output transformer and how a speaker works in more detail later. For now, all you need to know is that speakers require a magnet, and the field coil is an electromagnet. Nowadays speakers don’t use electromagnets because we have powerful permanent magnets that do the job without electricity. The electromagnet in our speaker is called a field coil, because it creates a magnetic field from a coiled wire. Another name for a coiled wire is an inductor. Inductors have the opposite property of capacitors. As current flows through the coil, the magnetic field that’s created reacts with the current and blocks AC, while passing DC. The field coil in our radio does two things. It serves as a magnet for the speaker and helps filter AC from our DC power supply. When an inductor is used as a filter, it’s called a choke. The field coil choke works with the two electrolytics to create a circuit called a pi filter. It’s called a pi filter, because it resembles the Greek letter pi. The pulsing DC from the rectifier is first filtered by C23 as we’ve discussed. It then runs through the field coil for further filtering. C24 finishes the job with a third level of filtering, providing a smooth DC signal to power the radio. Now that we’ve covered the power supply and filters, we can move on to the radio’s tuning and amplifying circuits. On the next video, we’ll learn about the very first stage in receiving a radio signal, the antenna. To stay updated, please subscribe and click the bell. And if you like this video, give it a thumbs up. See you soon. #radiorepair #radio #electronicsrepair #restoration #speaker #speakerrepair