Is 27v + 27v AC too high for a LM3886? - diyAudio
 Is 27v + 27v AC too high for a LM3886?
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 14th January 2017, 02:27 PM #1 diyAudio Member   Join Date: Oct 2004 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
 14th January 2017, 02:43 PM #2 diyAudio Member     Join Date: Sep 2009 Location: City Of Villans Blog Entries: 1 Maybe just READ the data sheet? __________________ Balancing the things I must do, with the things I'd like to do...This is a skill (or a luxury) amongst many that I do not possess.
diyAudio Member

Join Date: Sep 2007
Location: Henderson, NV
Quote:
 Originally Posted by mondogenerator Maybe just READ the data sheet?

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diyAudio Member

Join Date: Dec 2010
Location: Germany
Quote:
 Originally Posted by franziscko 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.

 14th January 2017, 03:57 PM #5 diyAudio Member   Join Date: Oct 2004 mmmhhh.... my formulas are different.... (27*1.41)-1.4(diode voltage drop) = 36.67 VDC.
 14th January 2017, 04:06 PM #6 diyAudio Member   Join Date: Aug 2007 Location: Sofia 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.
 14th January 2017, 04:11 PM #7 diyAudio Member   Join Date: Aug 2015 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
diyAudio Member

Join Date: Dec 2010
Location: Germany
Quote:
 Originally Posted by franziscko 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.

 14th January 2017, 06:52 PM #9 diyAudio Member   Join Date: Aug 2015 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.
 15th January 2017, 01:08 AM #10 diyAudio Member   Join Date: Feb 2011 Your lucky day. you can go with a regulated powersupply, and its fine as it is. __________________ V*A=VA V*A*PowerFactor = WATT

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