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Old 14th January 2017, 01:27 PM   #1
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Default Is 27v + 27v AC too high for a LM3886?

Hi, after the Peeceebee I want to build a Gainclone.
I have a toroidal 27+27V, is too high for a LM3886?
I think to use the schematic by Carlos.
Thanks
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Old 14th January 2017, 01:43 PM   #2
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Maybe just READ the data sheet?
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Old 14th January 2017, 01:55 PM   #3
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Quote:
Originally Posted by mondogenerator View Post
Maybe just READ the data sheet?





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Old 14th January 2017, 01:58 PM   #4
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Originally Posted by franziscko View Post
I have a toroidal 27+27V, is too high for a LM3886?
Short answer: Yes, it is too high.

Long answer: 2*27VAC*sqrt(2)(AC peak)*1.1(mains tolerance)*1.15(typical transformer regulation) = 96.32VDC - this is 2 volts above the absolute maximum supply voltage for the LM3886 without the signal applied.
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Old 14th January 2017, 02:57 PM   #5
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mmmhhh.... my formulas are different....
(27*1.41)-1.4(diode voltage drop) = 36.67 VDC.
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Old 14th January 2017, 03:06 PM   #6
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Its 27V under load or not? Besides, this is given at 230VAC, if you experience a power grid surge and voltage is for example 245VAC, would you take the risk? 35V is perhaps the max if you run 8ohm speakers, if they`re 4ohm or similar, it would be too much.
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Old 14th January 2017, 03:11 PM   #7
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If you ask people who looks at datasheets or just people who believe what other tells them. buahahah... you'll get a 'yes' answer. None of them ever walk over the borders. The only way to know the answer is walk over there yourself.

I have been using these little LM3886T at 96VDC for a long time. The datasheet stated that it has all the internal protections, Over-under voltage, SPIKE, temperature...etc. So I ran them on 2ohm without a fan, and a normal size heat sink, to see how good the protections are. They didn't give up after the tests. And so, I left them wired up with 96vdc. One thing for sure, I cant go back to the lower voltages, the musics in the 80s dont sound good with lower voltages.

BTW.. 27+27vac is not enough to get beyond 80vdc. Check the math, and again try it to see. I know, I have a 30vac which barely gives me 82. So, I have to use a 36+36vac to get me to 96vdc
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Old 14th January 2017, 04:59 PM   #8
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Quote:
Originally Posted by franziscko View Post
mmmhhh.... my formulas are different....
(27*1.41)-1.4(diode voltage drop) = 36.67 VDC.
First of all if the print on an industrial grade transformer states 27VAC output, it typically means that you would measure such voltage under full rated load and at nominal input voltage which is also stated on the transformer. The problem is that under no load transformer would output slightly higher voltage typically in the range of 10-20% extra. This is called transformer regulation and is also part of the transformer specification. Next there is no guaranty that your mains voltage would not rise by 10% above nominal value since it is allowed by the standard in EU. This would result in extra 10% rise of the output voltage. All these increase are multiplicative hence my calculation.

Now to the more interesting part - thermal consideration of the LM3886 itself. TI has a lot of info regarding the operation conditions in the datasheet, so here I second post #2. Also Tomchr of neurochrome.com has done all the math for the interested reader. He also gives recommendations for the supply voltages etc. Just read! And don't listen to those who suggest to explore beyond borders until you gain enough knowledge to make an informed decision about the dangers it brings.
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Old 14th January 2017, 05:52 PM   #9
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There you have it.. Should not do what people tell you. If you want to know the truth, the only way is to do it yourself. As for me, I walked the talk.

Everything has a risk. Just thinking of doing it yourself is dangerous. There are lots of customer products that use LM3886 like B&K CT600, don't have to do it yourself. Keep away from doing stuff yourself if are fear of danger.

DIY = Danger In You..

hahaha.. trying to recover from the bad math. Well, one thing for sure, bad math is totally dangerous.
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Old 15th January 2017, 12:08 AM   #10
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Your lucky day.
you can go with a regulated powersupply, and its fine as it is.
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