LM4780 and noise (question)

[john_lenfr]

My problem is:

I can hear a very low noise (into speakers) when I sleep in my room (so it's very low but when I turn at max volume, it increases).

How can I remove this thing?

 

[Frank Berry]

My problem is:

I can hear a very low noise (into speakers) when I sleep in my room (so it's very low but when I turn at max volume, it increases).

How can I remove this thing?

Are you using a preamp?
If the noise increases when you increase the volume, the noise may be coming from your preamp or from your source (CD player, etc.).

We really need more information before we can offer too many suggestions.

 

[john_lenfr]

Of course!

So, I plug my mp3 player on it.

When I turn off my mp3 player, I can hear the noise.
I want to use my mp3 player+amplifier as an alarm clock.(I plug it with a RCA to Jack cable, 1.5 meter)

=>So I must turn on my amplifier when I sleep.

How can I remove this?

 

[asgorath]

some background noise is common with grounding problems. lots of problems and lots of solutions for that, so check that area.

 

[john_lenfr]

What I search for?

The keyword is "Grounding" in the search section? But for my application must I choose acurate solution?

 

[Leolabs]

Check your PCB layout and grounding.

 

[asgorath]

few tips....


keep signal ground (0V) seperate from power ground. Watch AC sources as they'll add noise to everything. Isolate and shield as much as possible. Connect signal ground to power ground with a resistor

 

[john_lenfr]

What value of the resistor must I choose ?

 

[SRMcGee]

I've used 10 ohm 0.25 watt resistors, which worked very well.

 

[jackinnj]

My problem is:

I can hear a very low noise (into speakers) when I sleep in my room (so it's very low but when I turn at max volume, it increases).

How can I remove this thing?

Is it a "random" noise (like the inter-station FM noise), or an "oscillatory" noise like motorboating? IF you havent' used the supply bypass caps which national semiconductor recommends -- THEN use the supply bypass caps which national recommends. (100nF in parallel with 10uF).

also check the DC offset of the amplifier -- the noise on the supply lines is amplified through the ampchip.

 

[john_lenfr]

[IMGDEAD]http://johnlenfr.free.fr/docs/pcblayout.JPG[/IMGDEAD]


I did this, exactely the same. (but cs7 & cs8 are not on the pcb because of the size)


So:

@ jackinnj :

You think of cs1 & cs2. So no need to add.


@ SRMcGee:

You think of Rm. So no need to add.


The sound is like a motorboard. But for accuracy, I tried to record it.I was at max level when I record with my mp3 player and a mic. I was at 3cm of the speaker:

NOISE (.mp3)

 

[jackinnj]

It sounds more like "chirping" than "motorboating" -- je ne sais pas ce qu'est le mot correct francais -- gazouillement -- comme un oiseau?

 

[john_lenfr]

I think this is both.

Motorbording+"gazouillement -- comme un oiseau"(chirping)