LM3875 with snubberized PS on the same PCB ??? - diyAudio
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Old 13th December 2005, 06:12 AM   #1
methar is offline methar  Thailand
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Default LM3875 with snubberized PS on the same PCB ???

Hi,

I plan building a power amp using LM3875 for my OPA3124 pre (ESP88). I wonder if

1). Can the LM3875 and big PS (10,000+1500FC+0.1 per rail) be in the same board? ( One tranny 24-0-24 240VA per channel)

2). Will it hurt from the current ripple can cause hum or ground problem?

3). Will having 1 Ohm res between the main ground and the signal ground help?

4) Will having 0.1 Ohm 5 W in series between the 10,000 cap and 1,500 cap help?

5). Is the (10,000+1,500FC) overkilled, or (4,700 + 1,500FC) per rail is sufficient?

Thanks in advance!
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Old 13th December 2005, 02:55 PM   #2
nina is offline nina  United Kingdom
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1). Can the LM3875 and big PS (10,000+1500FC+0.1 per rail) be in the same board? ( One tranny 24-0-24 240VA per channel)

Nina>> That's fine. Dual mono config.

2). Will it hurt from the current ripple can cause hum or ground problem? 3). Will having 1 Ohm res between the main ground and the signal ground help?

Nina>> 1 Ohm is too small for that current. 100 Ohm 1W should be better, I think. ( I saw on the Leach amp, they use ~ 82 Ohm).

4) Will having 0.1 Ohm 5 W in series between the 10,000 cap and 1,500 cap help?

Nina>> Don't know. May be Carlos, Nuuk and people here can answer you this question.

5). Is the (10,000+1,500FC) overkilled, or (4,700 + 1,500FC) per rail is sufficient?

Nina>> According to Nelson Pass's article, The bigger the better. Big capacitors hold more charge. So, 10,000 is better if you already have/can affort it.


Carlos, in this case, do we really needs both RC ( the 0.1R one orginally at PS, and the 0.47R near the clip) ???

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Old 13th December 2005, 04:46 PM   #3
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Search for "Mr Ed" , Russ White did a PCB layout (and built it) with LM3886 and onboard PSU (10000uF+220uF i think) and he had a 1ohm resistor i think..

I'm gonna build one to like "Mr ED" but with MUR860 diodes instead, like this.

http://www.ettnet.se/~tobias/diy/lm3..._ed_copy_1.jpg
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Old 13th December 2005, 07:56 PM   #4
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Default Re: LM3875 with snubberized PS on the same PCB ???

Quote:
Originally posted by methar
4) Will having 0.1 Ohm 5 W in series between the 10,000 cap and 1,500 cap help?
No
You need high capacitance after the resistor. I suggest 20,000uF or more.
Check my schematic.
Everything is possible to make on a single board, but it may not be practical.
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Old 14th December 2005, 06:45 AM   #5
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Why " not practical ", Carlos ? Look at my PCB - any problems with wiring, no hum, easy assembling... Yes, no space for own experiments, it is " problem "...
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Old 14th December 2005, 04:38 PM   #6
methar is offline methar  Thailand
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Nina: Thanks for your comment. I would try the 4,700+1,500 per rail first, and keep four of my 10,000UF cap for other projects


tobias_svensk:

I did search and look for a member named Mr Ed but in vain. Could you hint me more?

BTW, thanks for the pic. THAT is what in my mind now ( except mine will be point-2-point on vero board, and the chip is in flat position, because I cannot make the PCB -___-" )


Carlos:

How about the RC(s)? Do we still need both of them even they are on the same board?


Upupa Epops :

I cannot find your PCB on the posts. Could you share us your PCB and experience? Thanks!
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Old 14th December 2005, 06:48 PM   #7
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methar, it was a member called Russ White and the PCB was called "Mr ED"

My NI chipamp Mark-II - Mr. Ed :)
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Old 14th December 2005, 08:12 PM   #8
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Quote:
Originally posted by Upupa Epops
Why " not practical ", Carlos ? Look at my PCB - any problems with wiring, no hum, easy assembling... Yes, no space for own experiments, it is " problem "...
It may not be practical in terms of size, just that.
If you put diodes, 4,700uF + 1R + 2~3x10,000uF caps on the amp board it will be a big PCB, in some cases not practical to assemble on a chassis/heatsink.
It can be made, of course.

Btw I have no hum too.
But I understand what you mean, two channels on one board is less likely to give hum problems if you don't know how to wire things properly.
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Old 14th December 2005, 08:14 PM   #9
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Quote:
Originally posted by methar
Carlos:

How about the RC(s)? Do we still need both of them even they are on the same board?
Yes, I would always use both.
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