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Old 19th July 2005, 06:46 PM   #1
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Default LM3886 BrianGT amp complete, slight hum

This is my 3-channel gainclone. I get a slight hum in all of the channels, but it's strongest in the left-most channel and weakest in the right most channel (left to right from the picture below). I'm guessing this is because of interference from either the AC mains lines or the transformer? Any way that I can lessen or eliminate this, or did my design simply place everything too close together?



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Old 19th July 2005, 06:52 PM   #2
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Although you have independent PSUs per channel, you are joining the grounds to earth, and with different lengths of cable.
If I'm looking correctly, the channel where you have more hum is exactly the one where you have the longest earth cable.
Make a test: disconnect the earth from all modules.
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Old 19th July 2005, 06:54 PM   #3
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I don't know if I'm understanding you correctly, I should try running the amp with none of the channels connected to earth (or "chassis") ground? Is this safe for the amp? If it is safe, why is there an earth ground connector on the PCBs?
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Old 19th July 2005, 06:56 PM   #4
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I think it's too close together. Are the boards touching? No chance a ground plain is shorting between the boards? They almost look stacked on top of each other.

Try mounting the trans on its side and away from the heatsinks.
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Old 19th July 2005, 06:58 PM   #5
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Quote:
Originally posted by vdi_nenna
I think it's too close together. Are the boards touching? No chance a ground plain is touching between the boards?

Try mounting the trans on its side and away from the heatsinks.

The boards are very close together, but they aren't electrically connected (well, other than all of the grounds go to the same place, and all of them get their AC from the same place). Moving the transformer at this point isn't an option for me, if thats the problem I'll just deal with it.
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Old 19th July 2005, 07:02 PM   #6
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Quote:
Originally posted by m0tion
I don't know if I'm understanding you correctly, I should try running the amp with none of the channels connected to earth (or "chassis") ground? Is this safe for the amp? If it is safe, why is there an earth ground connector on the PCBs?
Yes, it's safe.
That's the problem you have there.
Unscrew those wires, separate them, check out if they are not touching anything and power on the amp.
What do you hear now?
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Old 19th July 2005, 07:08 PM   #7
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Can you slide aluminum flashing around the edge of the transformer to shield it from the circuits? I can't tell if you have the space or not.

You can find some thin aluminum flashing at a hardware store.

If you do this, ground the shield to earth. Don't slice any wires!!
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Old 19th July 2005, 07:10 PM   #8
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And for more symmetrical looks, place the heatsink that differs from the others in the middle, lookes nicer that way

The hum in the left challen is most likely from the ac mains lines running right pass the left amplifier, try to twist the hot and neutral togerther. That will cancel out most of the fields radiated by the wires.

Very nice looks btw, well done!!
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Old 19th July 2005, 07:46 PM   #9
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Ok, tried a few things...

Carlos:
I don't know if this test is exactly the same, but it was a little easier and I'm lazy. I disconnected the ground wire that went to one of the channels and did a before/after test on that one particular channel. I didn't really notice any difference in the amplitude of the hum, sounded about the same to me.

vdi_nenna:
I was hoping that the barrier of heatsinks in between the transformer and the amp circuits would help shield them, do you really think a thin layer of flashing will help where the heatsinks do not?

Tekko:
Thanks for the compliment. I tried twisting those wires together and I think the amplitude of the hum may have decreased slightly, but it wasn't overwhelming.

The hum isn't really THAT loud to begin with, I have to put my ear about 6 inches away from the tweeter to hear it. It's probably even more noticable because this amp is being used in an active loudspeaker so the drivers are connected directly to the amp. I've decided to use the right-most channel for the tweeter and the left-most for the woofer because the hum is much more noticable on the tweeter. Thanks for all the suggestions so far and please keep them coming if you guys think of anything. At the bottom are two other pictures of the amp. I'm building another identical one for the other speaker in the active setup.


Pic 1

Pic 2
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Old 19th July 2005, 07:56 PM   #10
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You should always have a dc blocking cap between tweeter and amp.
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