LM3875 Transformer Question

tomman888 · 30th January 2009, 01:34 AM

I was looking to build an LM3875 classic kit from audiosector.com and am assuming that the kit does not include the power transformer. I need a 25+25v toroidal @ 160+VA for this chip, correct?

pacificblue · 30th January 2009, 07:07 AM

Depends on your speakers. For an 8 Ohm speaker 2 x 25 V is okay. With 4 Ohm speakers you should not go above 2 x 18 V. Maybe 2 x 22 V with the unisolated package and a very big heatsink.

160 VA is liberal for one channel, and just enough for two channels at 2 x 25 V.

bill6300gp · 1st February 2009, 08:07 AM

My LM3875 Amplifier works on TB4 Speakers with 24-0-24V EI-transformer.
The transformers are usually hotter than heatsink.

So I think that the voltage is OK for LM3875.

Luck · 1st February 2009, 09:05 AM

Why not use a higher voltage and a bigger heatsink?yes,it works,but still not in the best way!

pacificblue · 1st February 2009, 05:33 PM

Quote:

Originally posted by Luck
Why not use a higher voltage and a bigger heatsink?

Because the amount of heat the IC can dissipate is limited. From a certain point on a bigger heatsink does not improve the dissipation significantly anymore. See AN-1192 for that.

See also How To Heatsink.

30th January 2009, 01:34 AM	#1
tomman888 diyAudio Member Join Date: Nov 2006	LM3875 Transformer Question I was looking to build an LM3875 classic kit from audiosector.com and am assuming that the kit does not include the power transformer. I need a 25+25v toroidal @ 160+VA for this chip, correct?

30th January 2009, 07:07 AM	#2
pacificblue diyAudio Member Join Date: May 2008	Depends on your speakers. For an 8 Ohm speaker 2 x 25 V is okay. With 4 Ohm speakers you should not go above 2 x 18 V. Maybe 2 x 22 V with the unisolated package and a very big heatsink. 160 VA is liberal for one channel, and just enough for two channels at 2 x 25 V. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

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1st February 2009, 08:07 AM	#3
bill6300gp diyAudio Member Join Date: Oct 2008	My LM3875 Amplifier works on TB4 Speakers with 24-0-24V EI-transformer. The transformers are usually hotter than heatsink. So I think that the voltage is OK for LM3875.

1st February 2009, 09:05 AM	#4
Luck diyAudio Member Join Date: Jul 2006	Why not use a higher voltage and a bigger heatsink?yes,it works,but still not in the best way!

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