Scratchy sound coming from tweeter. LM3886 - diyAudio
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Old 19th July 2012, 02:23 AM   #1
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Default Scratchy sound coming from tweeter. LM3886

Hey guys,

My amp and speakers have been working pretty well. They are hooked up to my computer and it is verified as a good source. No noise.

The sound is hard to describe. It's hardly noticeable, but unfortunatley, after I first heard it, my ears pick it up every time. It comes out of the tweeter in one channel. It sounds almost like the tweeter is vibrating or buzzing, but it's securely locked down. I switched my speaker wires, and the same noise appears on the other speaker, so it is NOT the speaker. It is not the source either.

So it's the amp. I don't know exactly what would cause this sound. Only ideas I would have is intermittent connections? I kind of was running short on wire when I wired it all up so I had to use some cheap wire and perhaps the connections aren't good? How about a faulty potentiometer? could that make a scratchy, buzzing sound?

The buzzing sound is not constant. It goes with the music. So when the music hits, so does the buzz.

Just wondering.

Thanks.

Last edited by FenderBender11; 19th July 2012 at 02:29 AM.
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Old 19th July 2012, 03:12 AM   #2
Einric is offline Einric  United States
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Is this a new build?
Is the chip overheating and having SPIKE protection kick in maybe.
I have a similar problem with my LM4780 (2 3886's in a single package) and I haven't quite got my finger on the issue.
Mine might actually be the SPIKE protection kicking in.
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Old 19th July 2012, 03:18 AM   #3
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Quote:
Originally Posted by Einric View Post
Is this a new build?
Is the chip overheating and having SPIKE protection kick in maybe.
I have a similar problem with my LM4780 (2 3886's in a single package) and I haven't quite got my finger on the issue.
Mine might actually be the SPIKE protection kicking in.
I think it's definitely not overheating. This sound is evident at very low volume levels also. I don't actually think its LM3886 related actually. I mean it could be but I doubt it.

It's not a new build. Well I mean I don't think I noticed it before. I finished building it about 4 months ago. I think something must have come loose...

Thanks.

Last edited by FenderBender11; 19th July 2012 at 03:26 AM.
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Old 19th July 2012, 03:55 AM   #4
Einric is offline Einric  United States
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I would look at the I/O solder joints.
I know that one of my speakers will crackle if I wiggle one of the solder joints on my amp.
I have just not gotten around to re-soldering it.
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Old 19th July 2012, 03:00 PM   #5
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You did mention the computer to be a trusted source.
that's all fine.
But just for the sake of experiment, could You try the amp with a battery powered mp3 player of some sorth?
(ya, has to be battery powered.)
does this sound come when You short the inputs ?
does it come with open input?
Can You post a pick of the build, specialy I would mostly be intrested in grounding schematic and the area around the input caps.
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Old 19th July 2012, 03:07 PM   #6
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It's mostly just a kit build.

I'll do all those tests in a second. The sound as I mentioned, only comes when music is playing. It goes along with the music. So if there is a guitar playing, you'll hear the buzzing in the same rhythm as the guitar. But if nothing is playing, there is no sound.

Click the image to open in full size.
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Old 19th July 2012, 03:42 PM   #7
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Perhaps the amp is conditionally stable, and oscillates when audio input, or definitively oscillating.
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Old 19th July 2012, 06:37 PM   #8
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But the other two channels work marvelously. Hmmph. I don't know. When I say its barely audible, I mean its barely audible. Just sounds like the tweeter dome is buzzing a little bit.
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Old 19th July 2012, 07:09 PM   #9
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Quote:
Originally Posted by FenderBender11 View Post
But the other two channels work marvelously. Hmmph. I don't know. When I say its barely audible, I mean its barely audible. Just sounds like the tweeter dome is buzzing a little bit.
Ok, the other channels work fine, OK, but may be a fake cap in the circuit (For example, the power decoupling, the cap in zobel network, or some RC compensation network) that makes THIS amplifier to oscillate / become unstable, and don't affect the other(s).
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Old 19th July 2012, 08:55 PM   #10
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next test should be turning the volume controlls down to 10% on the computer and cranking up the volume on the amp.
then checking if the noise gets louder compared to the music.
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