Class D

It's often the case that extra filters are added to allow the device to pass FCC requirements and/or to improve stability into arbitrary loads.

If you are making modifications for your own use, you can get away with it, but if you are trying to make a commercial product, you will need to make sure the product continues to meet FCC requirements for emissions before offering the product for sale.

No one in their right mind would use a ferrite core coil in the midrange and tweeter xover of a serious speaker. Ferrite messes up the sound. So, why do we have ferrite coils on the output of all class D amps? It is because they are smaller, cost less, have less resistance, run cooler and have better contained fields so there would be less rf outputting the amp.

Less resistance is definitely important leading to high damping. They also have a closed magnetic path which means they are not effected by external steel which could even include component leads ... and they don't turn external steel (which could be anything) into a speaker either.


Given the very low distortion and IM, it is hard to argue that the design choice is poor. All depends on the ferrite material and where you operate it on the curve. Lots of well rated audio products have output transformers. Choose your poison.

You can theorize all you want. What I do is listen. Most EVERYONE who has listened to ferrite core coils in a xover.....knows they suck. I now know they also suck as "full range" coils on the output of a class D amp. You can only know how something "sounds" by listening. If you want to build just for numbers than a ferrite core is fine. I build for sound......the best sound wins. Of course, it is just my opinion. Until others listen to air core coils on the output of a class D amp it is just.....my opinion....based on what I hear. There are lots of opinions where they have used them in speaker xovers.....and the overwhelming opinion is that ferrite sucks. Look at all the pics of the xovers of super high end speakers.....do you see any ferrite core coils used in the mids and highs? Don’t think so. He he.....I am sure someone will find one and post.....just to be right....he he.

You would be hard pressed to find a ferrite core in a mid/tweeter for even a mid range speaker. Air core inductors are fairly cheap at the frequencies needed.  For woofers, there are trade-offs w.r.t. resistance / core impacts.


In speakers, the cross-over is in the audio band. In a Class-D amplifier it is not.  In a speaker you don't normally have steel nearby. In a Class-D amplifier you may.

Ferrite messes up the sound. So, why do we have ferrite coils on the output of all class D amps? It is because they are smaller, cost less, have less resistance, run cooler and have better contained fields so there would be less rf outputting the amp. It is not because they sound good. If you use a better sounding air core coil the board has to be bigger, the coil costs more, you have to use a large gauge coil to get the resistance down, and you might have to add more shielding in the amp to make it pass rf standards.

You got that right at least!! and it’s why ML in their megabuck class-D flop, use big "air cores" in their No.53 monoblock, and used 4 of them in series to get a far steeper slope to cut the switching frequency noise out, so it doesn’t affect the audio band phase integrity.
https://www.stereophile.com/images/styles/600_wide/public/011212ML53-600.jpg