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PostPosted: 11 Oct 2008, 09:28 
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RiGuy wrote:
TF package means no problems with voltage sharing. Man this is befuddling. I just reread AN1192 and it gives no new insight. I have seen an old design note on using LM3875 in bridge and parallel that included pcb designs but have not been able to find it since you posted your problem. THere was a link in DIYAudio that I followed to get it one day but didnt save it and am unsure how to find it again.
Anyway.
Wish I had more ideas at this time.

Hi Ri!

I've run out of ideas too. I believe I tried all that crossed my mind (crazy ideas...) and I'll I've read on the AN1192 note and still nothing.
To me, bridged and parallel chip circuits are just the worst thing ever.... :worried:

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PostPosted: 11 Oct 2008, 09:44 
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GREAT NEWS!!

I finnaly got somewhere. I just decided to try some snap-on ferrite cores everywhere inside the amp. I used 18 of them (I told you I decided to try them everywhere...) and my right channel (PA100) is working! No more heat problem.

I didnt test for crackling yet, but the heat is gone and the amp plays fine on low volumes. More to follow soon.
Looks like I had some really big HF noise there and the ferrites killed it.

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PostPosted: 11 Oct 2008, 10:23 
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That is good news. What do you figure is the source of the HF?

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PostPosted: 11 Oct 2008, 10:59 
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Hi Gio, I have absolutely no idea at all. There shouldn't be any HF noise inside there, as I have a pair of 70kHz cutoff freq low pass filters (before the buffer and right at each chip input). Also, one chip by itself does not oscillate. The real problem is connecting both of them together, that's when the problem begins. I'll start removing the chokes one by one and see which one causes the chips to heat up again.

Also Gio, do you think I should use the PA100 or BR100 in the end? If my speakers can go down to 3Ohm, each chip in bridged configuration will be driving only 1.5Ohms. Maybe that's when I get the crackling sound.
I believe I'll stick to the PA100 configure.

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PostPosted: 11 Oct 2008, 11:08 
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OK, so, after removing all ferrite chokes but one the heat problem is gone. Removing this one obviously brings the problem back.

This choke is placed exactly at the output of one of the paralleled 3875 (going to the speaker). Placing this choke a little farther (on the same wire) makes the heat problem come back.

I'll try rewiring the output from both 3875s with coaxial wire and see what happens.

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PostPosted: 11 Oct 2008, 11:26 
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OK, rewired everything. I even used coax cable for the speaker connection. No go. I still need to keep that choke there to avoit heat problems.

So, can this choke be a permanent fix? Or can these chips start oscillating again for some reason and fry anytime?

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PostPosted: 11 Oct 2008, 11:58 
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UPDATE: Putting one snap on choke next to any of the 3875s cure the problem completely. Moving it a bit farther from the chip makes the problem come back.
Also, it doesnt matter if the choke is next to chip1 or chip2. As long as its in the output wire (that goes to the speaker) rtight next to the chip the heat problem is gone.

Also, the sec I turn on the amp the snap on choke gives a loud buzz. Its a really quick buzz that just goes away in the blink of an eye.
So, this indicates that when I turn on the amp a sudden HF oscillation is present on that wire, and the snap on quickly absorbs it and then the 3875s stay cool....

But where is this coming from? No idea....

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PostPosted: 12 Oct 2008, 08:03 
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Thats insane and really cool. So, you put it on the output. I wonder if you could use a filter at output instead of the snap on. Not that the snap on is a bad idea to have there. Just wonder if a filter would do the same thing.
So how did you come up with this idea? Just desperation?
Is the output line just straight through the ferrite or did you wind it?
Anyway, really interesting and makes me want to try it on mine just to see if there is any difference. I have cleared up all problems on mine except a humm from my preamp and some HF from the power supply. I think its the fans on the supply but would be neat to try ferrites as I bet they dont mess with the sound as much as caps.
Ri

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PostPosted: 12 Oct 2008, 10:17 
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Hi Bernardo, I wonder if the problem is your speakers / speaker cable. Are the cables high capacitance? Perhaps the speakers are a very difficult load to drive. Perhaps you can try the amp with another pair of speakers.

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PostPosted: 12 Oct 2008, 12:28 
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I can't help but think you have a problem with earth loops. :|

Kind regards, John.


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