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Old 4th July 2005, 11:30 AM   #1
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Default LM4780 and noise (question)

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?
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Old 4th July 2005, 11:36 AM   #2
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Default Re: LM4780 and noise (question)

Quote:
Originally posted by 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?

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.
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Old 5th July 2005, 04:07 PM   #3
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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?
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Old 5th July 2005, 10:25 PM   #4
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some background noise is common with grounding problems. lots of problems and lots of solutions for that, so check that area.
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Old 6th July 2005, 06:28 AM   #5
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What I search for?

The keyword is "Grounding" in the search section? But for my application must I choose acurate solution?
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Old 6th July 2005, 01:51 PM   #6
Leolabs is offline Leolabs  Malaysia
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Check your PCB layout and grounding.
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Old 6th July 2005, 04:25 PM   #7
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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
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Old 7th July 2005, 06:25 AM   #8
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What value of the resistor must I choose ?
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Old 7th July 2005, 12:02 PM   #9
SRMcGee is offline SRMcGee  United States
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I've used 10 ohm 0.25 watt resistors, which worked very well.
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Old 7th July 2005, 01:06 PM   #10
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Default Re: LM4780 and noise (question)

Quote:
Originally posted by 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?
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.
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