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Old 30th September 2003, 02:20 AM   #1
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Default Lm4651

I would like to build the LM4651/52 subwoofer amp, but have searched and seen many posts about the inductor. Do I need that inductor or can I use an active crossover? Also, if anybody has built this amp, how is the sound, reliability, and simplicity? Thanks, Mike
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Old 30th September 2003, 03:34 AM   #2
JoeBob is offline JoeBob  Canada
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I do believe the inductor acts as a filter to remove any high-frequency from the switching on the OUTPUT (so no active xover here) and by doing so reconstructs the sine-wave. Not a wizz by any means on class-d amps, but I think you're going to need that inductor on the output.
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Old 2nd October 2003, 01:41 AM   #3
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So, I can't do away with the Inductor to save some money and space? It won't work without it? Also, my other question is how good is the sound and heat output?

Thanks, Mike
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Old 5th October 2003, 01:10 PM   #4
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Well...do I or don't i need the inductor?? Or does nobody know?
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Old 9th October 2003, 09:50 PM   #5
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YES, you do need the inductor for sure.

The output of a class-D amplifier is basically a square-wave. The LC-circuit on the output lowpass filter this into the "original" input signal, just amplified.
The output inductor on a audio calss-D amplifier is one of the more tricky parts to get right.
The inductor and capacitor togher with the load makes a lowpass filter, but since the load can change, the x-over freq. for your LC-filter also change. This means that you have to take different loads into the senario when you select the size of your inductor !!

Be also aware of the "RF-noise" a class-D amplifier can give. You should put the amplifier in some kind of metal-case.

Have fun

THomas

P.S. Check that everything works (squarewaves) before you mount the LM4652. I didn't first time around and it took me quite some time to get it going.
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Old 11th October 2003, 03:03 AM   #6
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can i use a different inductor, possibly a small one?
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Old 20th October 2003, 06:46 PM   #7
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Yes, but it depends very much on the freq. range you want to cover and your load.
They are not that expensive; 4-5 $, so if you are locky you will only save a dollar or two.
There are some recommendations in the datasheet for the LM4651/52 somewhere.

By "size" you should not only talk about the induction of your inductor, but also of the amp-rating.

Thomas
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Old 20th October 2003, 07:03 PM   #8
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soundNERD

Always use double faced PCB.

My first attempt was on a breadboard and... my hi-fi started making some 'music'...
After that, in a pre-made PCB (those with holes and pads), the music was another: hot ans... puff!! Some smoke...

So, I decided to make the double faced PCB that National has in the datasheet and... still work!

The high frequencies of the PWM are realy HIGH to use a single sided PCB.
If you check the DS (datasheet), the fsw goes up to 200khz.

Don't forget the output inductors!

You really need them!

Regards,

Pedro Martins
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Old 21st October 2003, 01:33 AM   #9
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One more question:
On the datasheet, the schematic says the type of caps and resistors used. can I just use any resistor and cap type?

Thanks again, Mike
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Old 21st October 2003, 07:32 AM   #10
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Hi Mike

Yes you can use any kind of resistor or capacitor as long as you use the same type as speceified. I.e. if they say 50 ohm/3 W you have to use at least a 3 W resistor.
It don't have to be a specific brand.

Thomas
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