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PostPosted: 02 Oct 2008, 19:09 
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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?[/quote]

Well, dont worry til its fried :) Just might want to check.
Will check into your other question.
Ri

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PostPosted: 02 Oct 2008, 19:23 
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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.

OK, just tried it and... I dont know!!!
My left speaker gives no scratchy sound (actually it gives no sound no matter how hard I push the cone). But my right one kinda does. I dont know if I would call that scratchy, but its not silent too. Any other way of testing it? I cant connect it to another (good) amp now cause its too late.
Maybe I'll test it tomorrow....

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PostPosted: 02 Oct 2008, 19:31 
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Well, the way I found my speaker was fried was that it was scratchy when I moved the cone. Find out if there is DC coming across the amp outputs. This is what fries a speaker.
Can be frustrating when the amp misbehaves huh? Trust me I know.
http://www.shine7.com/audio/pa100.htm
The guy, over and over, builds beautiful amps. You can use your 4 ohm speakers here.
Ri

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PostPosted: 02 Oct 2008, 21:19 
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Hi Bernardo, perhaps you could try a lower voltage to make sure it is not the protection circuitry kicking in.

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PostPosted: 03 Oct 2008, 08:20 
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RiGuy wrote:
Well, the way I found my speaker was fried was that it was scratchy when I moved the cone. Find out if there is DC coming across the amp outputs. This is what fries a speaker.
Can be frustrating when the amp misbehaves huh? Trust me I know.
http://www.shine7.com/audio/pa100.htm
The guy, over and over, builds beautiful amps. You can use your 4 ohm speakers here.

So, is it just me or 3886s are more common in bridged circuits??
Ive seen that page before, really beautiful amps.

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PostPosted: 03 Oct 2008, 08:22 
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gmilitano wrote:
Hi Bernardo, perhaps you could try a lower voltage to make sure it is not the protection circuitry kicking in.

Well, thats is a nice idea... but how could I lower the voltage? I dont have a variac :| Also, what voltage should I use? Right now I get about 29V under load an 30.5 idling.

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PostPosted: 03 Oct 2008, 08:34 
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Bernardo. I really think you should check for a DC voltage across the amps outputs. While you have a speaker hooked up. BUT FIRST you might try hooking those crackly speakers up to an amp that you know works well and turn it up and listen for crackling. Then you will know if you have issues with your speakers and we would pretty much guarantee a bunch of DC coming out of the amp at that point. But still please test for DC across the outputs while it has a load. Also check the inputs.
RI

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PostPosted: 03 Oct 2008, 12:30 
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Hi Bernardo. Try another transformer. Say a small 15V CT. Don't worry about the size of the transformer. You just want to check that it is not the SPiKe protection that is being activated. If you don't have a lower voltage transformer, try using a 16 ohm load on the output (two 8 ohm speakers or add a power resistor in series to get 16 ohms.) No that I type it out, it makes more sense to use a 16 ohm load.

Ok, here is the note from AN-1192
AN-1192 wrote:
4.6 THERMAL CONCLUSION
Because of National’s portfolio of products and the capabilities of the bridged/parallel circuit, the bridged solution is applicable for a power output window between 80W and 120W. Trying to exceed this power level without a rigorous thermal design will be difficult to achieve. More caution needs to be applied along with better thermal management for bridged circuit designs. The proposed bridged/parallel solution is a more robust design than the bridged circuit, allowing higher output power levels to be obtained by paralleling the two bridged sets of ICs. Table 1 below summarizes the maximum supply voltages for each type of configuration and load impedance while keeping Pdmax per LM3886 IC to less than 40W.
Code:
TABLE 1 Maximum Power Supply Voltages
      2Ω   4Ω   6Ω   8Ω  16Ω
BR100 NR* ±20V ±24V ±28V ±37V

So you can see for an 8 ohm load, you want to be less than 28V. If you are using 23VAC, that is about 32VDC so there is a very good chance you are activating the SPiKe protection. Try a 16 ohm load or lower voltage and see what happens.

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PostPosted: 08 Oct 2008, 14:01 
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Hello all!

Im back, had some busy days but now Im back.
OK, so I checked my speakers and they are fine, no harm done. Also, there is no DC at the outputs of the amp.

That 16ohm idea is great Gio, and I'll try that right now and post back the results.

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PostPosted: 08 Oct 2008, 14:33 
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Hi Gio!

Connecting a 16Ohm load apparently did it!!!!
Crackling is gone, although power is greatly reduced (I know, that was expected).

So, the big question now is what to do? Rewire the amp to use parallel chips (PA100) or replace the tranny with a lower voltage one?

Rewiring would make each chip see an 8Ohm load, but I'm not sure if that's the best thing to do.

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