LM3875 Fuse Query

john blackburn · 25th March 2007, 02:19 AM

I've got a recently completed Chipamp.com LM3875 using a 2 amp slow blow fuse in the power inlet from a Vikash "Amp plus" pack. It has a single 2x22v 300va transformer also from Vikash and seems a bit harsh on fuses! If the amp is turned on with no input playing it's fine and will then play for hours, but if it's switched on with the cd playing it pops the fuse. This is at lowish volumes using a pair of "sacrificial" 8ohm speakers. I have a pair of FR125s on the shelf awaiting boxes making but its one thing at a time......
The DC offsets with the inputs shorted are -17.3mv on the left and +6.5mv on the right.
Would it be ok to up it to a 3 or 4 amp and try it, or is that asking for trouble? I would hate to fry it at this stage of the game.
I've read of the inrush being high with toroids and assume the fuse can cope with that but gets stressed when it has to dive straight into amplifying as well as the inrush.
The heatsinks ( also Amp plus pack) are barely lukewarm after several hours at modest volumes and the sound is better than my current shop bought amp even on modest test speakers.

chipco3434 · 25th March 2007, 02:40 AM

Then the answer is 3A.

It will be perfectly safe with a 3.

john blackburn · 25th March 2007, 05:39 PM

Cheers for the advice. I was unsure about it and didn't want to damage anything by stupidity!
It has taken a lot of reading and learning to get it to this stage but the results are looking like they will be well worth it.
Many thanks
JB

25th March 2007, 02:19 AM	#1
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	LM3875 Fuse Query I've got a recently completed Chipamp.com LM3875 using a 2 amp slow blow fuse in the power inlet from a Vikash "Amp plus" pack. It has a single 2x22v 300va transformer also from Vikash and seems a bit harsh on fuses! If the amp is turned on with no input playing it's fine and will then play for hours, but if it's switched on with the cd playing it pops the fuse. This is at lowish volumes using a pair of "sacrificial" 8ohm speakers. I have a pair of FR125s on the shelf awaiting boxes making but its one thing at a time...... The DC offsets with the inputs shorted are -17.3mv on the left and +6.5mv on the right. Would it be ok to up it to a 3 or 4 amp and try it, or is that asking for trouble? I would hate to fry it at this stage of the game. I've read of the inrush being high with toroids and assume the fuse can cope with that but gets stressed when it has to dive straight into amplifying as well as the inrush. The heatsinks ( also Amp plus pack) are barely lukewarm after several hours at modest volumes and the sound is better than my current shop bought amp even on modest test speakers.

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25th March 2007, 02:40 AM	#2
chipco3434 diyAudio Member Join Date: Feb 2004 Location: Racine, Wisconsin	Then the answer is 3A. It will be perfectly safe with a 3.

25th March 2007, 05:39 PM	#3
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	Cheers for the advice. I was unsure about it and didn't want to damage anything by stupidity! It has taken a lot of reading and learning to get it to this stage but the results are looking like they will be well worth it. Many thanks JB

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