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Old 16th December 2001, 05:47 AM   #1
JoeBob is offline JoeBob  Canada
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Default LM3875/LM3876 load question.

I've decided to start small and try making a little LM375 or LM3876 based amp. Now, national says these work with both 8ohm as well as 4ohm loads, but will they handle a 2ohm load? That's really all I need, thanks.
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Old 16th December 2001, 05:55 AM   #2
paulb is offline paulb  Canada
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Default Check this out

http://www.national.com/appinfo/audi...ation_Note.pdf
Gives insight into power capability of parallel ICs.
(fixed; the copy/paste made the ... real)
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Old 16th December 2001, 06:16 AM   #3
JoeBob is offline JoeBob  Canada
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You're link doesn't seem to work, probably the "..." in it...
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Old 16th December 2001, 07:18 AM   #4
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It just so happens that i am currently designing a very small and extremely cheap integrated amp based around the LM3875, LM1894 and PGA2310.

The LM3875 should run into a 2ohm load but output power will be down to around 25W and to avoid the protection circuit activating you will have to decrease this further leaving you with a usable power output of 15W rms with rather large heatsinks ..... you could run multiple LM3875's in parallel with small value (0.01ohms) resistors at the output of each chip.

Just keep the supply rails low say +/- 20V and the voltage gain of the amp very low and make sure the input signal is fairly small (it may need a set amount of attenuation on top of any variable attenuation used as a volume control).
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Old 16th December 2001, 09:03 AM   #5
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LM3886 has greater power handling, but will still activate its protection at very low voltage swings with this sort of load, unless you come up with an ingenious method of getting rid of the heat.

My woofers present a 2.5R load (2 parallel 5R units on each side), but I decided to bi-amp, so each driver gets its own LM3886.

If this isn't an option, parallel operation helps, but doesn't double the benefit. Though the current is shared, unfortunately the relationship between current and temperature rise isn't linear.

Also this design won't do 2R, the approach may help...

http://www.
national.com/appinfo/audio/files/BPA-200_Application_Note.pdf
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Old 16th December 2001, 12:39 PM   #6
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If the current sharing is equal or close enough to it and you use a heat sink with half the thermal resistance or give each chip its own heatsink with a low enough thermal resistance then you will get ~90% increase in current handling.

Also rest assured that the LM3875, LM3876 and LM3886 will work into a 2 ohm load just with very much reduced power output ... but as stated above they really are useless into a 1ohm load unless you parallel 4 or more of them. Therefore, using the bridged / bridged-parallel designs are not appropriate for a 2 ohm load as due to the nature of bridging each chip will see 1 or 2 ohms respectivly.

Jamie F,
As for the ingenious way of getting the heat out of the chip ..... they're called peltier (or thermoelectric) coolers.
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Old 16th December 2001, 01:39 PM   #7
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Default look at this link

http://www.dckits.com/advise.htm

Maybe it can help you.

else you can find a lots of info at HTTP://epanorama.net

jc dk.
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Old 16th December 2001, 02:52 PM   #8
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Peltiers are just good at lowering chip temperatures, e.g removing heat, but they do not do much in reducing overall heat production. I think a previous thread had something about it... overclocking circles also have alot of info on the subject
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Old 16th December 2001, 07:01 PM   #9
JoeBob is offline JoeBob  Canada
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I just saw the LM3886 datasheet, hadn't noticed it before...

I wanted to know if it could handle 2ohm load, because I wanted to bridge two of them to drive a 4ohm load for more power output, but if it's power output at 2ohms is rather low (since it would have to be) I guess this idea just plain sucks. Unless there's another way to use multiple LM3876's or LM3886's for greater power output for driving a 4ohm driver... (I'm currently reading the link partyjups gave me, maybe those will shed some light on the subject).
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Old 16th December 2001, 07:23 PM   #10
JoeBob is offline JoeBob  Canada
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Well. The LM3886 has a power output of 68w into 4ohms, I was wondering, do I really need more then that?

I mean, some people build 20w amps, and then I see 500w amps and higher (why such a big difference?). I'm going to use it for home listening, and while I like loud, I don't want something that makes me have to wear earplugs...

And after reading partyjups link on bridging and using LM3886's in parallel I think I better not try that on a first attempt, just more chance of me doing something wrong...
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