AIR-Core Vs Ferrite-Core Inductor in Class-D amplifiers - diyAudio
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Old 21st January 2004, 06:55 AM   #1
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Arrow AIR-Core Vs Ferrite-Core Inductor in Class-D amplifiers

Recently i look around several class-d amps which used air-core as well as ferrite core inductors at their outputs to suppress switching frequency component.

AS I am working on class-D amp project , Kindly Help me which filter should i use for suppressing switching frequency components and best performance in terms of sonic quality.
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Old 21st January 2004, 07:51 AM   #2
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Why don't you use something inbetween ? A ferrite-core with an air-gap.

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Charles
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Old 21st January 2004, 08:13 AM   #3
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Hello Charles, I dont able to understand what is meant by A ferrite-core with an air-gap.
PLZ Clarify.
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amp_man_1
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Old 21st January 2004, 09:32 AM   #4
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Have a look at this one:

http://www.lcaudio.dk/com/zpn10.jpg

(Hi Lars ! )

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.

Regards

Charles
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Old 21st January 2004, 10:48 AM   #5
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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.
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Old 21st January 2004, 11:03 AM   #6
ukram is offline ukram  Finland
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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?
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Old 21st January 2004, 11:17 AM   #7
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Quote:
I guess using as big coil as possible makes current changes small?
Are you talking about the OUTPUT-FILTER inductor ?

If so, the peak current in the inductor at idle is:

carrier_peak_voltage / (2 * carrier_frequency * inductance)


Please keep in mind that this current is reactive and is only causing some conduction losses in the output stages, while too large a coil would lead to a low filter cutoff frequency.


Regards

Charles
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Old 21st January 2004, 05:14 PM   #8
subwo1 is offline subwo1  United States
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Default Hi Charles

Quote:
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.
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Old 21st January 2004, 09:01 PM   #9
ukram is offline ukram  Finland
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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?
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Old 22nd January 2004, 06:14 AM   #10
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Quote:
probably meaning yet another inductor after output filter to isolate speaker from feedback loop. Is this really necessary?
That depends on your feedback topology, i.e. whether you are going to take-off feedback before or after the output filter AND wheter you are using a self-oscillating topology.

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Charles
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