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PostPosted: 09 Oct 2008, 17:06 
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Hi Bernardo,

So you are using the PA100, 23VAC and an 8ohm load?

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PostPosted: 09 Oct 2008, 17:31 
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No, right now Im using a PA100, 11VAC and 8R dummy load.

I'm using only 11VAC cause I thought I should try it first before delivering the full 23VAC. Im glad I did that.
There is a low pass (3.3nF and 680R) filter at the input, and 22pF in parallel to each feedback resistor. Each 3875 work fine alone, but when I tie them together, they instantly fry.

I tried a zobel network (2R7 and 100nF) and in just 1 second the 2R7 resistors fried (1/4w each). Not good.

Oh, and those 0.1R resistors get really hot instantly too after turning on the amp.

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PostPosted: 09 Oct 2008, 22:03 
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Hi Bernardo, I am a little confused. With the PA100 and an 8 ohm load, each chip will see a 16 ohm load. Thus for an 8 ohm speaker load, there is no advantage in using the PA100 configuration over a single chip. This is outlined in Table 1 of AN-1192. The PA100 should be used when you want to put a lot of power into a 2ohm or 4 ohm load.

If your speakers are 8ohms, and you want more power than what a single chip will deliver, use the BR100 configuration. However, you must use less than 28VDC. It seems your problem is too much voltage since a 16 ohm load worked fine with your BR100.

Cheers

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PostPosted: 10 Oct 2008, 09:26 
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Hi Gio!

OK, let me explain to you whats going on better.
At first, I did try the bridged config (BR100). That is what I first intended to build, as all my speakers are 8ohm and I needed more power. But, what I ended up running into many problems with this config:
1- Chips were getting too hot. After a couple seconds SPIKE was already kicking in.
2- A 8R load in series with my speakers fixed the problem, but that was just a temporary fix.
3- I tried 18VAC instead of 23VAC. That gave me about 25V DC at the 3875s. Still, I had the same crackling problem from my speakers.

After all this, my one and only conclusion was that my speakers impedance was going too low when playing and maybe that could explain why my 3875s couldnt handle pushing them.

So, just to see what would happen I decided to try the PA100. If my speakers were the problematic part I would know it. But, after trying that, all I could get was heat. Now I seem to be having some major oscillation problems, as the chips, when tied together in parallel fry instantly after turning on the amp.

Connecting each chip individually to the speaker terminals give me great results. But connecting both of their output together makes them oscillate violently at around 400kHz. No filter, decoupling cap or anyhting I tried could cure it.

Im alsmost giving up at ths point. Someythign is amish here but I canjt figure it out. BR100 gives me awfull crackling from speakers (SPIKE kicking in) and PA100 oscillates terribly. Maybe this amp does not like my speakers, but I need it to.
Those are the only ones I have for it.

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PostPosted: 10 Oct 2008, 09:54 
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I am on vacation til end of October. When I get back I can put together some LM3875 single chip amp PCBs for you and send them. I have a design already so I just have to print and dip some boards.
If thats fine with you I am pretty sure that this will work for you.
Ri

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PostPosted: 10 Oct 2008, 09:55 
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But make sure to send me a reminder PM at the end of October or for sure it will leave my mind.
Ri

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PostPosted: 10 Oct 2008, 11:03 
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Hi Ri!

That would be great. Thanks!
I'll remind you by the end of Oct. I hope I can fix this thing for good...
Once again, thanks for all the help, amd enjoy your vacation!

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PostPosted: 10 Oct 2008, 12:30 
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bsgd wrote:
OK, let me explain to you whats going on better.
At first, I did try the bridged config (BR100). That is what I first intended to build, as all my speakers are 8ohm and I needed more power. But, what I ended up running into many problems with this config:
1- Chips were getting too hot. After a couple seconds SPIKE was already kicking in.
2- A 8R load in series with my speakers fixed the problem, but that was just a temporary fix.
3- I tried 18VAC instead of 23VAC. That gave me about 25V DC at the 3875s. Still, I had the same crackling problem from my speakers.

Im alsmost giving up at ths point. Someythign is amish here but I canjt figure it out. BR100 gives me awfull crackling from speakers (SPIKE kicking in) and PA100 oscillates terribly. Maybe this amp does not like my speakers, but I need it to.
Those are the only ones I have for it.

Hi Bernardo, with those speakers, the PA100 is pretty much pointless. It is meant for low impedance speakers. Your BR100 worked with the 16 ohm load so the problem was the high voltage.

The BR100 reflects 1/2 of the impedance to each chip. You need to look at the impedance plot of your speakers. Sometimes what they call 8ohm speakers may actually have an impedance closer to 6 ohms in which case 25V would be too high. What speakers are you trying to drive?

Something you can easily try. Get a household fan, put in on high and right over the chip. If it works with the fan or runs for a bit longer before the SPiKe protection turns on, then you know you either need to increase the cooling or drop the voltage a little more.

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PostPosted: 10 Oct 2008, 13:24 
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Or, dont you have a Variac or some adjustable supply you can test with? Its okay to vary the voltage while its running, esepecially if you are testing with less and less voltage.
Ri

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PostPosted: 10 Oct 2008, 15:22 
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Gio, yes, I know I should use the BR100 insatead of the PA100. But I needed to know why wouldnt the BR100 work, cause maybe my speakers were to blame. But then, after vonverting to channel to parallel Im running into even more oscillation problems than before.
My speakers are a pair of B&W 601S3.

Yes, I did try the fan, but strange as it might seem it didnt help. The heatsinks were cool to the touch but crackling was still there. Turning off the amp, waiting about 60 secs and turning it on again would allow it to play fine for a few more secs before the crackling sets in again.

My left channel is still wired up as BR100, so I can test both PA100 and BR100 setups.


Ri, Unfortunately I dont have a VARIAC, but Im really considering getting one. They dont run cheap here, but I believe it might be useful to have one. Whats yours? I thought of getting a 1kW one, should be enough for most projects.

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