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Old 20th November 2006, 02:19 PM   #1
henkel is offline henkel  Malaysia
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Default LM3886GC, Does it prefer 4ohms or 8ohms?

Hi!

Planning to build my LM3886 soon, also planing to build a pair of speakers for it. Does the LM 3886 prefer 4ohms or 8ohms drivers?

Thanks.
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Old 20th November 2006, 02:35 PM   #2
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8 Ohm definately.

I recently completed 4 channels and made the mistake of getting the plastic parts for convenience. Also, I'm running at 35-0-35 when you should run at 28-0-28 if you use 4 ohm. I'm running it on my Jamo's which are 4ohm and these chips are getting HOT! I have a very big heatsink on them, but the losses on the plastic package is too high, so it cannot transfer the heat quick enough. They sound quite good though, for incase you didn't know
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Old 20th November 2006, 04:07 PM   #3
Triode is offline Triode  Netherlands
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4 0hms is ok for the LM3886. See http://www.national.com/pf/LM/LM3886.html
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Old 20th November 2006, 11:17 PM   #4
pwan is offline pwan  United Kingdom
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depends on the supply.

For 4 Ohm speaker, LM3875 run fine at +/- 24V while the LM3886 can run with +/- 28V. If you have good & big heatsink, I run LM3886 with +/-35V on 4Ohm speaker happily.
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Old 21st November 2006, 12:48 AM   #5
Leolabs is offline Leolabs  Malaysia
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It's all depends on your power supply and heatsink.
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Old 21st November 2006, 01:27 AM   #6
CarlosT is offline CarlosT  United States
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I guess I don't understand the power supply voltage issue. I heard a few times now that 4 ohm loads work better with lower voltages. Is this because the load is drawing more power and a lower voltage power supply keeps things under control? Even if you have a high voltage power supply, couldn't one compensate by reducing the gain...playing with the feedback resistor? Excuse my ignorance...I'm very new at this
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Old 21st November 2006, 01:59 AM   #7
henkel is offline henkel  Malaysia
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Thanks for the replies,

reason for me asking is that I have a lot of 4 ohms drivers lying about from the car audio days and if the LM3886 GC can run them, I'd be saving some speaker building $$$.
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Old 21st November 2006, 04:08 AM   #8
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Again, I'm speaking from the experience I had and from what I read.

Apparently you keep the voltage lower into 4 ohms to prevent heat spots building up on the chip. There is not a hell of a lot of dissipation area on the chip, so there is not a lot of contact surface area to dissipate heat. You can have as big a heatsink as you want, the thermal footprint of the chip is still small!

I think my heatsinks are big enough
Click the image to open in full size.

By using 28-0-28V supplies, you limit the dissipation in the chip if you want to run 4 ohms. When I first read it I didn't take it too seriously, but now that I've run it for a while I realised they were quite serious about it! Anyway, mine won't be used on 4 ohms permanently, they will run on my studio monitors once I'm done building all my speakers.

The point I was trying to make (like an idiot, I must say!) is that if you were to run 4 ohms, you need to have your PSU at 28-0-28V AND you shouldn't use the plastic package. Use the metal tab version instead.

I hope I haven't confused the issue any more
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Old 21st November 2006, 04:57 AM   #9
Nordic is offline Nordic  South Africa
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Think of the load at the end of your amp as being powered....
remember all the power doesn't go to the load, but it DOES go to the chip, so if you hit the chip's maximum level, you will likely overheat it it and burn it out.

The minimum rateing for lm3886 is about 7 AMP (you can look it up in datasheet) - this means with variations between chips, they should not blow up till they reach at least 7 ampere - in fact it shows a possible maximum up to 11A depending on the chip in your hand.

now take your load (speaker/driver) and divide it into the supply you got hooked up.
40V (almost max for lm3875 level) would on lm3875 run into 8 ohms. 40/8 = 5amps at 40V

for the lm3886 you can see that 4ohms x 7ampere = 28V. (as it is the minimum figure - remember some could make it to 11A) so it could in theory run onslightly higher rails....

But it should make the whole thing transparent to you for future use.
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Old 21st November 2006, 05:32 AM   #10
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by Nordic
...now take your load (speaker/driver) and divide it into the supply you got hooked up.
40V (almost max for lm3875 level) would on lm3875 run into 8 ohms. 40/8 = 5amps at 40V...
So running 40V with an LM3875 might get you in trouble going to a 4 ohm speaker?
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