What is this in a Class D amplifier?

[sagarverma]

Hello,
i got my hands on a class d amplifier schematic with pcb and all.
but the documentation is all in German.i translated that to English using machine translation(pain in ***,took a good four hours),but still documentation is difficult to interpret.
one component in the pcb cant be identified properly,i am attaching the image,can anyone tell me what is it?(air core inductor????)

[Tekko]

Can you take a pic of it ? I´d say its the footprint for the amps output filter inductors, but i could be wrong.

[Danko]

Somewhere on my computer I have that .pdf, and as far as I remember,
that's an inductor.

[sagarverma]

hi Tekko,
>>>> Can you take a pic of it ? <<<<<,,i didnt get u on this count.

the amplifier pcb is double sided, it is the o/p inductor but what baffles me is the total number of green dots. I am attaching the image.

Hi danko
it is the inductor but what r the various green for.An inductor has two terminals,but here ,,going above my head
BTW,do have its translated version so life can be simple for me,

[Danko]

Thoose green "dots" are for mechanically hording the inductor.

[Genomerics]

Indeed.

It is probably an RM core. The two sets of four pads are for the pins on the bobbin which the wire is wound on and the other two pads are for the metal clips that hold the two core halves together.

http://www.ferroxcube.com/prod/assets/rm_frnt.pdf

[sagarverma]

Genomerics
thanks for the pic,now i got that.
can this inductor be replaced by torroid inductor.

[Genomerics]

I don't see why not..... however,

Once gapped the ferrites used in things like RM and POT cores are extremely linear up to saturation.

The materials used in toroids are usually an 'exotic' mix of nickel manganese and iron powder used to achieve a high saturation flux density with reasonable losses.

They are really targeted at low cost SMPS output filter applications where the ripple current, and resulting core losses, is substantially smaller than the DC current. A class D amplifier might have significant ripple current in the output filter inductor.

One of the things they do is 'soft saturate'. It means that you might design for a particular zero DC bias value but you have to accept that the operating inductance with a DC current will fall, perhaps up to 50%, 40uH might go down to 20uH.

You might account for that in your design and in an SMPS it's not necessarily a major concern. However for a class D amplifier that sort of behaviour might be unacceptable.

I wouldn't like to comment on the effects on percieved sound quality.

DNA

[sagarverma]

DNA,
i dont have any problem in using the said inductor but its subject to its availability at my local supplier.
the schematic shows the use of four such inductors,the values are 11.4845uh and 16.241uh.Can u suggest the which one to use Rm5,Rm4,Rm8 or what?what should be number of turns of what thickness wire to achieve these values of inductances?
i will go to market soon to get these if they are available.

[Genomerics]

Ahh.... two inductors, this is the fourth order one....

The values you give are a little bit too precise but I wouldn't be overly concerned. Just don't expect to do better than 11u5 and 16u or there about.

It's a little bit messy but have a look at....

http://www.genomerics.org/inductors/inductors.html

That might give you some idea about how to get an answer. Unfortunately it's unfinished and does not strictly apply to Class D
amplifiers.

If you give me the values of the capacitors used in the filter, your topology (half bridge, full bridge), supply voltage and target output power or load impedance I can have a fiddle about and suggest a couple of designs...

It would be good to have a complete picture of the output filter including any damping or zobel networks. The first inductor is going to have the highest ripple and will therefore suffer the greatest losses.

DNA