LM3875 rail voltage question

Greg3333 · 25th April 2006, 07:41 PM

First of all, sorry if this has been covered before. I've done a few searches and can't find a definitive answer.

I've started putting together my first GC monoblocks based on Peter Daniel's LM3875 boards but I've run across a problem with my power supply. I was planning on using Noratel 120VA 2x25V toroids (230V AC) from http://www.farnell.co.uk (part no 9532358). The problem is that I've found I'm getting around 29V AC from the toroid (with the outlet power measured at about 243V AC) which is giving me around ±40V DC at the rails after rectification (again, using Peter Daniel boards).

Is this too much for the LM3875 or does this depend on other factors?

For cooling I'm planning on using an open-ended aluminum enclosure, with the chip mounted to a piece of alloy bar attached directly to the wall of the enclosure.

Any help is much appreciated.

Thanks

Greg

peranders · 25th April 2006, 09:13 PM

2 x 29 VAC unloaded is OK.

Greg3333 · 25th April 2006, 09:34 PM

Thanks Per-Anders. Is it likely to drop when the amp's loaded?

In theory UK mains power should be 230VAC. My main concern is that if it gets much higher I may go over ±42VDC which is (as far as I know) the absolute maximum the LM3875 can handle.

25th April 2006, 07:41 PM	#1
Greg3333 diyAudio Member Join Date: May 2005 Location: Yorkshire, England	LM3875 rail voltage question First of all, sorry if this has been covered before. I've done a few searches and can't find a definitive answer. I've started putting together my first GC monoblocks based on Peter Daniel's LM3875 boards but I've run across a problem with my power supply. I was planning on using Noratel 120VA 2x25V toroids (230V AC) from http://www.farnell.co.uk (part no 9532358). The problem is that I've found I'm getting around 29V AC from the toroid (with the outlet power measured at about 243V AC) which is giving me around ±40V DC at the rails after rectification (again, using Peter Daniel boards). Is this too much for the LM3875 or does this depend on other factors? For cooling I'm planning on using an open-ended aluminum enclosure, with the chip mounted to a piece of alloy bar attached directly to the wall of the enclosure. Any help is much appreciated. Thanks Greg

25th April 2006, 09:13 PM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	2 x 29 VAC unloaded is OK. __________________ /Per-Anders (my first name) or P-A as my friends call me

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25th April 2006, 09:34 PM	#3
Greg3333 diyAudio Member Join Date: May 2005 Location: Yorkshire, England	Thanks Per-Anders. Is it likely to drop when the amp's loaded? In theory UK mains power should be 230VAC. My main concern is that if it gets much higher I may go over ±42VDC which is (as far as I know) the absolute maximum the LM3875 can handle.

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