Lm4651

[soundNERD]

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

 

[JoeBob]

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.

 

[soundNERD]

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

 

[soundNERD]

Well...do I or don't i need the inductor?? Or does nobody know?

 

[tlmadsen]

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.

 

[soundNERD]

can i use a different inductor, possibly a small one?

 

[tlmadsen]

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

 

[audioPT]

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

 

[soundNERD]

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

 

[tlmadsen]

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

 

[soundNERD]

Thomas,
My question was really referrring to the type of parts, not the brand. For example, when it says to use mylar, do I really have to use a mylar capacitor?

-Mike

 

[JoeBob]

No, you don't need to use mylar, but where they say mylar, just use any film cap you want. The best at a reasonable price would probably be a polypropelene type.