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Old 16th February 2009, 04:32 AM   #1
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Default Class D - output LPF

Hey all,

I am new to audio and am following a reference guide from IR for class D audio.

Attached is an image detailing my inquiry.

My two questions are:

1. Why bother with the inductive LPF at the output, if the speaker itself represents essentially an inductance and resistance in series?

2. Assuming 1. is debunked, what type of inductor should I choose? I usually notice people use large circular airgap inductors in such power applications.

Thanks in advance
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Old 16th February 2009, 08:25 AM   #2
markhu is offline markhu  China
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Actually, that is the demodulation filters.all class D need this .
That used be retuned the Analogue signal.
The inductors need enough current.
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Old 16th February 2009, 08:51 AM   #3
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If you send the unsmoothed switching waveform straight into the speaker leads you will probably knock out every radio and TV for quite a distance.
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Old 16th February 2009, 09:27 AM   #4
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There are several class D chip amplifiers out there which are intended to operate directly into speakers, e.g. MAX9703/4.

The principal critical factors in the feasability of such designs are power, frequency, spread spectrum clock (if any), and lead length from amp to speaker. As already pointed out, you can create a load of EMI if you don't know what you're doing. In the worst case this is a criminal offence in most countries.

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Old 19th February 2009, 01:40 PM   #5
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Fair enough, I realize the EMI could be an issue but I am more just thinking theoretically.

Furthermore, sizing your LR low pass requires some knowledge of the speaker, as it provides not only your R, but adds to your L. So the question becomes, how does one adequately set the parameters of the LR filter on the output stage? Do I require intricate knowledge of the speaker?
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Old 19th February 2009, 01:40 PM   #6
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Also, once we reach the conclusion the output inductor is necessary as I'm certain is the case, what type is appropriate?

I know of current carrying capacity, but aren't there lots of different types of inductors just like there are capacitors. So air core etc.. Normally I see those big hoopy ones on high power output stages, is there anything special about them, or is it just the only option when looking into large currents flowing?
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Old 19th February 2009, 06:01 PM   #7
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No, in fact an (additional) inductor is required neither in theory nor in practice.

Yes, there are many types of inductors.

The subject of your enquiry is beyond the scope of a correspondence on a forum. If you are interested you could do worse than read the datasheets for the chips I have mentioned, or do a search for similar amplifier chips or modules, or simply google 'inductor', 'capacitor', 'filter', etc.

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Old 19th February 2009, 06:17 PM   #8
kubeek is offline kubeek  Czech Republic
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I have another question if you donīt mind, I once saw a schematic where there were two output chokes in series between output transistors, and the output and feddback were taken from the midle of them.

What are the andvantages and disadvantages of this compared to standard toplogy?
I think there was mentioned no shoot-through in the output transistors which seems like an advantage, so why isnīt it used more often?
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Old 20th February 2009, 01:40 AM   #9
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Ok my final question. I have sized my L at the output stage based on the resistance of the speaker. Is this inappropriate? Should I be sizing the L simple as a supressor and leave the speaker out?
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Old 20th March 2009, 02:09 AM   #10
audioC is offline audioC  United States
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You have figured correctly.
The L (coil) is based on the speaker impedance as part of the output filter network.
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