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PostPosted: 02 Oct 2008, 12:09 
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Good links Ri, thanks.

bsgd wrote:
Slide some gounded shielding where? Around the tranny?
Oh,forgot to mention, the tranny itself is connected to the bus ground.

Try disconnecting the ground to the transformer frame to see if that has any effect. For shielding, it can be around the transformer or you can just put some metal between the transformer and the signal and chips. You will have to ground this piece.

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PostPosted: 02 Oct 2008, 12:21 
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I know that the LM3886 made the LM3875 obsolete, but is the LM3875 to be treated exactly the same as the LM3886? Couldnt treating it as such result in wrong part values and therefore oscillations?
And I am sure that this was taken into consideration but the pinout is very different.
Ri

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PostPosted: 02 Oct 2008, 14:32 
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gmilitano wrote:
Try disconnecting the ground to the transformer frame to see if that has any effect. For shielding, it can be around the transformer or you can just put some metal between the transformer and the signal and chips. You will have to ground this piece.

I'll try that. Thanks Gio!

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PostPosted: 02 Oct 2008, 14:34 
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RiGuy wrote:
I know that the LM3886 made the LM3875 obsolete, but is the LM3875 to be treated exactly the same as the LM3886? Couldnt treating it as such result in wrong part values and therefore oscillations?
And I am sure that this was taken into consideration but the pinout is very different.

Well, I'm not sure, but I thought the AN-1192 paper could be used for both 3875 and 3886. And regarding the pinout I followed the 3875 datasheet. The circuit is fine, as the amp plays fine on low levels, but gives some awful crackling noises at higher levels.

If its not oscillation, what else could be causing the crackles?

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PostPosted: 02 Oct 2008, 18:17 
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Hello again!

OK, I have some news.

First, ungrounding the tranny did nothing... also, couldnt stop the crackles by isolating the chips from the trannies.
BUT... I decided to do a test using speakers. Found a pair of old 4 ohm speakers here and connect them in series with a 8R resistor (otherwise each cip would see only 2R, not good) to the amp.
The result was that the first time I played a problematic song, it played fine. The second time it was all crackling noises and almost no music.

So, my conclusion is that I have a heat problem. Apparently my 3875s are heating up too much and the protection circuit is deactivating and reactivating the chips, causing pops and crackles from the speaker.

I dont know what to do now, and I believe my only option would be to try a parallel circuit instead of bridged. I heard 3875s dont like 4 ohm loads.

What do you guys think?

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PostPosted: 02 Oct 2008, 18:32 
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Oh, btw, how will a parallel circuit with 3875s behave with 8Ohm loads? My main speakers are 8ohm...

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PostPosted: 02 Oct 2008, 18:40 
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AN1192 goes over that in the BPA200 I think. I will check it and see if I find the reference. But, I think it normals out then. Like if you have an 8Ohm it sees 8Ohm.
While you wait....
Could you get in a real quiet room with one of your original speakers and put your ear up to the cone and push the cone in and out a few times? If it sounds scratchy.. wow thats bad news. It worried me that when you tried the second pair that it got crackly after time but not right away. Does it always have to heat up or are both sets of speakers crackly instantly now? Maybe it is spike protection and I hope so, but it could also be burnt voice coils.
Ri

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PostPosted: 02 Oct 2008, 18:51 
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SO, I dont see mention of it specifically. But they refer time and again to using 8Ohm and 4Ohm speakers. If you do a Bridged and Parallel then I think you will be fine, theoretically :), with those speakers. God rest your pocketbook.
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PostPosted: 02 Oct 2008, 19:03 
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RiGuy wrote:
AN1192 goes over that in the BPA200 I think. I will check it and see if I find the reference. But, I think it normals out then. Like if you have an 8Ohm it sees 8Ohm.
While you wait....
Could you get in a real quiet room with one of your original speakers and put your ear up to the cone and push the cone in and out a few times? If it sounds scratchy.. wow thats bad news. It worried me that when you tried the second pair that it got crackly after time but not right away. Does it always have to heat up or are both sets of speakers crackly instantly now? Maybe it is spike protection and I hope so, but it could also be burnt voice coils.

Oh my god. Now you got me worried.
Ill try that now. One weird thing that does happen is I tried once pushing my main speakers cones and I heard crackles from the tweetwers!! Yes, I can induce crackles in that amp.... but I dont think thats waht you mean, you just think my coils are physically damaged right?? For that test the speaker itself does not need to be connected to anything right?

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PostPosted: 02 Oct 2008, 19:04 
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RiGuy wrote:
SO, I dont see mention of it specifically. But they refer time and again to using 8Ohm and 4Ohm speakers. If you do a Bridged and Parallel then I think you will be fine, theoretically :), with those speakers. God rest your pocketbook.

I was actually thinking of doing a parallel circuit, the BPA100, and not a bridged-parallel as the BPA200. What do you think?

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