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PostPosted: 10 Oct 2008, 17:38 
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Sounds like the SPiKe protection kicks almost instantly so that is why the heatsinks don't even get hot. I could not find a spec for the B&W 601 S3, but the B&W 602 S3 specifies a nominal impedance of 8 ohms with a 3 ohm minimum. That does not leave much of a choice but to lower the voltage even more.

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PostPosted: 10 Oct 2008, 19:31 
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You really dont need a big variac since you will only use it to test and not to actually use it for a full time supply.
Not sure the brand of mine. I think its a few hundred VA and it has a 120V swing.
Ri

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PostPosted: 10 Oct 2008, 19:33 
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I still think you should hook up your speakers to another amp and see if they play well. You never know but its good to rule these things out.
Ri

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PostPosted: 11 Oct 2008, 08:12 
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Hi Ri, I did that!
They are fine on another amp. Its this chipamp that is giving me trouble, for sure.

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PostPosted: 11 Oct 2008, 08:15 
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gmilitano wrote:
Sounds like the SPiKe protection kicks almost instantly so that is why the heatsinks don't even get hot. I could not find a spec for the B&W 601 S3, but the B&W 602 S3 specifies a nominal impedance of 8 ohms with a 3 ohm minimum. That does not leave much of a choice but to lower the voltage even more.

Let me see if I can find the manual for them. I have it somewhere....

OK, found it. Minimum impedance is 3 Ohms, just like the 602.
Is this normal? I mean, does most 8Ohm speakers have that low minimum impedance?

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PostPosted: 11 Oct 2008, 08:21 
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Do you have any LM317. You could throw together a voltage regulator circuit with a pot for the resistor that determines the output voltage. With LM317 you get, I think, only 1 amp, but for your purposes it would be fine. This would allow you to confirm or eliminate a problem based on supply voltage.
Ri

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PostPosted: 11 Oct 2008, 08:37 
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I really dont think that a high voltage is the problem. In the LM3875 datasheet it says this in the design section:
"DESIGN A 40W/8Ω AUDIO AMPLIFIER
Given:
Power Output 40W
Load Impedance 8Ω
Input Level 1V(max)
Input Impedance 100 kΩ
Bandwidth 20 Hz–20 kHz ±0.25 dB
Equations (5), (6) give:
40W/8Ω Vopeak = 25.3V Iopeak = 3.16A
Therefore the supply required is: ±30.3V @3.16A
With 15% regulation and high line the final supply voltage is
±38.3V using Equation (7)."

But you might be getting spike protection anyway if you are getting those chips to hot. If they are attached to the heatsink but not touching the heatsink well enough or if ( I know we went over this but I forget your answer) if you are using the T package and not the TF package then you will have a problem if the heatsinks are touching or in anyway can share voltage. I think that the back of the chip is actually the - voltage. Not positive about this but am positive that it is either -,+, or GND and either way its bad for the two chips to be touching in anyway even through the heatsinks. If they touch you would be either sharing ground or you would go overvoltage.
Ri

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PostPosted: 11 Oct 2008, 08:45 
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But then again you said the amps work fine individually, so the stuff I just posted probably does not apply anyway. Its when you put them bridged or parallel that you are having issues. So I will leave the post up simply for the value of the infor from the datasheet, but it probably will not be helpful.
Ri

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PostPosted: 11 Oct 2008, 08:48 
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RiGuy wrote:
I really dont think that a high voltage is the problem. In the LM3875 datasheet it says this in the design section:
"DESIGN A 40W/8Ω AUDIO AMPLIFIER
Given:
Power Output 40W
Load Impedance 8Ω
Input Level 1V(max)
Input Impedance 100 kΩ
Bandwidth 20 Hz–20 kHz ±0.25 dB
Equations (5), (6) give:
40W/8Ω Vopeak = 25.3V Iopeak = 3.16A
Therefore the supply required is: ±30.3V @3.16A
With 15% regulation and high line the final supply voltage is
±38.3V using Equation (7)."

But you might be getting spike protection anyway if you are getting those chips to hot. If they are attached to the heatsink but not touching the heatsink well enough or if ( I know we went over this but I forget your answer) if you are using the T package and not the TF package then you will have a problem if the heatsinks are touching or in anyway can share voltage. I think that the back of the chip is actually the - voltage. Not positive about this but am positive that it is either -,+, or GND and either way its bad for the two chips to be touching in anyway even through the heatsinks. If they touch you would be either sharing ground or you would go overvoltage.

Well, one thing is for sure. Using a household fan made the chips run really cool. But I still got the crackling. But, anyway, I'm still suspicious that it might just be SPIKE protection because the amp playes fine for the first couple minutes. Then, it would start crackling now and then and keep that way until I turn it off and wait some time.

I'm using the TF package with Srtic Silver 5 thermal paste, so the chips are nicely heatsinked. Also, both heatsinks do not touch each other and the case is wood, so theres no way there could be a short there.

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PostPosted: 11 Oct 2008, 09:03 
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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.
Ri

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