The result is less core-saturation but also a lower AL value than the original core-set. But you still need less turns than a similarily sized air-core coil would need. Another advantage is that there is less EMC problems than using air-core coils.
I once thought about using a toroid-shaped "air" core. But that one would need much more windings than a cylindrical coil.
Someone else (on this forum, but I don't remember who it was)proposed two coils beneath each other connected in a way that they are cancelling out the stray field (in fact a cylindrical coil that is bent at half it's length). This could be a good compromise either.

SIR I DONT have access to ferrite cores right now, can u PLZ tell me that what are the disadvantages air core inductors and how can i calculate the number of turns for a given inductance and area of a core, can u plz tell me some formula to calculate it.

I'm going to use air core inductors in my amp, since it seems to be really easy and cheap to make them. How to measure or calculate the EMC? For example, how much di/dt is ok? I guess using as big coil as possible makes current changes small?

Someone else (on this forum, but I don't remember who it was)proposed two coils beneath each other connected in a way that they are cancelling out the stray field (in fact a cylindrical coil that is bent at half it's length). This could be a good compromise either.

Hi Charles, thanks for remembering this idea. Sometimes I rather stumble onto different things.

Amp_man, what I do is just try differing numbers of turns until the performance is optimized. But a ball park figure may be about 20uH for 30 turns on a 1" form for an air core choke. Using that as a reference, the inductance is proprtional to the square of the number of turns.

Yes, meant output filter. Another thing to worry is the speaker capacitance, how much can it be? I think someone mentioned "isolation", probably meaning yet another inductor after output filter to isolate speaker from feedback loop. Is this really necessary?