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Old 30th November 2006, 01:17 AM   #11
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Quote:
Originally posted by elementx

Where are you getting this info from?
http://www.national.com/ds/LM/LM3875.pdf says the chip can handle upto 84VDC,

50W into 8ohm @ +/-35vdc (70vdc)
68W into 4ohm @ +/-28vdc (56vdc)

I was going by the graph in the datasheet shown below. Where are you looking?
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Old 30th November 2006, 01:20 AM   #12
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i also wouldn't want to go much above 50W for this reason:
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Old 30th November 2006, 01:20 AM   #13
Minion is offline Minion  Canada
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To find out what value resistor to use for running the LED go here and use the LED resistor calculator....


http://led.linear1.org/1led.wiz
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Old 30th November 2006, 02:16 AM   #14
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Default Re: My first LM3875: check my parts please?

Quote:
Originally posted by meaghers
[B
Chassis:
3 x Hammond-56034 1441-16 Steel chassis 10"L x 6" W x 2"H

?? Is Steel or Aluminum better? They are the same price.


fuses:
what amperage, and fast or slo blow?
[/B]

Go with aluminum. It is much easier to drill and has better heat disapation.

2 amp slow blow should do the trick.

Good luck.
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Old 30th November 2006, 04:49 AM   #15
Nordic is offline Nordic  South Africa
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Sorry for the confusion, I understood the transformer rateing to be per secondary.

For 8 ohm loads you want to keep the transformer somewhere betweeen 24-0-24 and 28-0-28 to get closer to the 35V spec after diode and fwd voltage drop in chip.
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Old 30th November 2006, 07:29 AM   #16
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Quote:
Originally posted by Kooka


No, mine is the dual mono version kit, and it has 2 PSU boards. In this one there is not the R5 place and the led place...
Thanks anyway.

If its anything like the ChipAmp.com power boards then it is a 10k resistor for a bright LED or a 22k for a slightly less bright LED
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Old 30th November 2006, 07:41 AM   #17
Kooka is offline Kooka  Italy
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Quote:
Originally posted by ianpengelly



If its anything like the ChipAmp.com power boards then it is a 10k resistor for a bright LED or a 22k for a slightly less bright LED
I'll go for the poorly bright one! Thanks.
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Old 30th November 2006, 05:15 PM   #18
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Default Re: Re: My first LM3875: check my parts please?

Quote:
Originally posted by GG



Go with aluminum. It is much easier to drill and has better heat disapation.

2 amp slow blow should do the trick.

Good luck.
thanks! I was wondering about that. Aluminum it is.

What about a fuse before my speaker to protect it from accidents until I'm sure i've got it right?
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Old 1st December 2006, 04:16 AM   #19
gootee is offline gootee  United States
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Default Re: Re: Re: My first LM3875: check my parts please?

Quote:
Originally posted by meaghers
What about a fuse before my speaker to protect it from accidents until I'm sure i've got it right?
You could. But I wouldn't leave it in there permanently. It's possible that at low frequencies (or maybe also with low and high frequencies intermodulating), the fuse's resistance could change with the signal, causing distortion.

For anyone who's interested in checking it out: I haven't seen it, but there's an old JAES paper by Greiner of the University of Wisconson that charted nonlinear distortion due to fuse thermal effects.

- Tom Gootee

http://www.fullnet.com/u/tomg
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Old 1st December 2006, 07:39 AM   #20
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It's disadvantages are also well covered In Self's book. If you're worried about construction problems wire a lightbulb in series with the mains and use a dummy load or old speaker for testing.
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