Class D

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

Because the filter is set at 50K does not mean it does not effect the sound.  It's intrinsic sound is still there.  No free lunch.  Everything makes a difference.  But you have to listen to know.  Years ago I worked for an audiophile company that made tube stuff.  They had a zobel network on the output of one of their amps that had a .1 microfarad cap. .  I told them to change their junk mylar cap to a polystyrene but the owner said it was operating at high frequencies out of the audio band so it should not matter.  He humored me and tried it.  Of course, it sounded much better.  There are people that sell external zobel networks and the best sounding ones use the most expensive parts.  Again, no free lunch.....everything makes a difference.