AIR-Core Vs Ferrite-Core Inductor in Class-D amplifiers

[ukram]

I'm planning to take feedback after output filter, and the topology will be self-oscillating (+ maybe some periods of doing nothing in case of small signal, this can be changed in the programmable logic chip). Still that leaves the question open about how much does the speaker and speaker cable capacitance affect self-oscillation? I read somewhere that cable capacitance can be around 1 nF or less. Assuming i use 100 uH and 100 nF in LC filter, it would seem that cable is not a problem, adding only 1nF to the C, but i'm not sure about speakers.

[subwo1]

I can see usefulness in making the choke .1mH and the capacitor .1uF. The resulting resonant frequency may blend in fairly well in a self-oscillating amp.

[Rhesus]

There are HUGE emc-problems associated with air core inductors in power switching. So much so that there is no point in trying, if the amplifier has to comply with IEC standards or anything similar.

DIYers can choose to ignore it.

Gapped cores also have some of these properties. To minimize stray magnetic fields in these cores, keep the gap inside the coil, like if you use a double E-core, only gap the center leg.

There are many advantages to using cores, and there are good materials out there to. If you are going for a toroid design, then MPP cores should be high on the list. These are expensive though. The cool-mu cores are a less expensive, and an ok alternative.

Gapped 3F3 or 3C90 ferrite cores are good if you use E-I og E-E combinations.

[phase_accurate]

There are HUGE emc-problems associated with air core inductors in power switching. So much so that there is no point in trying, if the amplifier has to comply with IEC standards or anything similar.

Since it has been done it must be feasible, althoug not easily I have to admit.
Since we are DIYers and don't want to sell our stuff we don't care that much about complying with any standards.
As long as we don't disturb our neighbour's TV's and hearing-aids etc and we don't take any planes down with our equipment, it is O.K for us. We don't have any chances to test for compliance anyway.

Gapping only the center leg can be quite a task. For us DIYers it is much easier making the gap by inserting some insulating material. The EMC problems we had with a class-d amplifier were definitely not caused by the air gap.

Before I try to get some toroid made out of near unobtainium I get a standard RM, EFT or whatever core and gap it by myself.

Regards

Charles