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Old 23rd July 2009, 01:54 PM   #1
ormo is offline ormo  United Kingdom
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Default LM3886 Heatsink

Hi,

I will soon have a stereo amp running with one LM3886 on each channel. They are powering one 4ohm speaker each and being ran from a +-28v supply.

The chips will likely be bolted to a thin sheet of aluminium (less than 1mm think) which will be bolted to the base of this heatsink:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...:X:RTQ:GB:1123

My questions: is this heatsink going to be suitable, if not where is goot to look within the UK and how much with thermal transfer compound actually help?

Thanks
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Old 23rd July 2009, 02:56 PM   #2
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You have about 40 W power dissipation per chip. Thermal resistance for 25 C is 1.9 C/W. I wonder the CPU heatsink is sufficient. You can visit http://www.wakefield.com/ to get an idea for some CPU heatsink performance.

How many CPU heatsink do you use? How far does the heatsink from LM3886? Can you put 3886 on the heatsink surface? The aluminium sheet is too thin to be a good heat conductor.

Always use thermal junction compounds (thermal grease) even you have a huge heatsink.
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Old 23rd July 2009, 06:22 PM   #3
ormo is offline ormo  United Kingdom
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Right, so basically I need to:

* Always use thermal grease
* Find a bigger heatsink
* Cut out a few chunks of the aluminium and attach the chips direct to the heatsink

Would something like this be more suitable? It's 80mm x 80mm, not sure of the thermal resistance

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=290333103902
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Old 23rd July 2009, 06:22 PM   #4
ormo is offline ormo  United Kingdom
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Another note: (I'm not allowed to edit posts)

The above heatsink is quoted to be suitable for processors up to 89W. Would it therefore be ok for two chip amps going up to roughly 80W at max?
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Old 23rd July 2009, 07:29 PM   #5
Atilla is offline Atilla  Norway
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The above heat-sink was made to cool this wattage of CPU thermal dissipation, but there is a catch. It was made to do so with forced cooling, provided by the fan attached to it. Most CPU coolers are pretty damn good, but they are rated to do their job with a fan attached.

Do not try to run 2 chips attached to one of those heat-sinks. It will not work. You can probably get away with it, using one per chip, without forced cooling. If there is reasonable airflow in the case (or outside of it, depends on how you're mounting them) it should be working reasonably fine.

As for attaching them to the heat-sink, with an intermediate step, it's probably worse, no matter what you do.

If CPU heat-sinks are getting your attention look for the following or similar:

- "Silent" heatsinks - Zalman, thermaltake, Akasa and the likes.
- Intel Xeon copper heatsinks
- Heatsinks where the fins have good gaps between them and can be mounted with the fins vertically ( similar to the second one you posted)
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Old 23rd July 2009, 09:24 PM   #6
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Quote:
Originally posted by ormo
Would something like this be more suitable? It's 80mm x 80mm, not sure of the thermal resistance
If you are working with the T package, it will be too small. Consider Atilla's advice to add a fan.
Even the optimistic Overture Design Guide shows a possible design problem for the TF package due to its limited dissipation capability, and that is for a single channel.

What ambient temperature do you expect around the heatsink? Will the heatsink be mounted inside the case or with the fins sticking out?

Quote:
Originally posted by ormo
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Old 23rd July 2009, 11:34 PM   #7
star882 is offline star882  United States
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The CPU heatsink will work fine. Just remember that good ventilation is required for optimum performance.

How much power are you looking at? If it's a lot, a hybrid or pure digital design would make more sense.
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Old 23rd July 2009, 11:39 PM   #8
dfdye is offline dfdye  United States
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I am currently using an old slot-A heat sink to cool two lm3886's with absolutely no problems. I do not run hot, and I don't press the power limits of the chips, but I have never had heat issues, even with a chassis that is not the best in terms of air flow.

I definitely second the motion that you should avoid tightly spaced heat sink fins for CPU coolers unless you are going with forced air cooling. Just glancing at the heat sinks you mentioned in your first post, for the cost, I would go with one sink per chip and you should be fine if you follow the mounting instructions everyone keeps repeating.

Good luck!
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Old 24th July 2009, 03:38 AM   #9
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You can also mount the amp chips to an aluminum plate and then attach four or more CPU heatsinks to the plate.

The thermal conductivity losses with the added "spreader plate" should be more than offset by considerably more fin area.

Same construction rules apply: Thermal paste, clean mounting surfaces, use the correct screw torque when mounting the chips, and mount the CPU heatsink fins vertically for improved convection cooling.

Forced air (fans) will improve cooling efficiency, but can be noisy and require an additional +V supply.
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Old 24th July 2009, 05:44 AM   #10
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Quote:
Originally posted by dfdye
I am currently using an old slot-A heat sink to cool two lm3886's with absolutely no problems.
Do you use the T or the TF package?
Do you use 28 V rails in combination with 4 Ohm speakers?
What is the ambient temperature around the heatsinks?

Quote:
Originally posted by DCPreamp
The thermal conductivity losses with the added "spreader plate" should be more than offset by considerably more fin area.
It does not work that way. If the spreader plate cannot transport the heat away from the IC fast enough, the additional fins make no difference.

With the heatsink right on top of the IC on the spreader plate, there is little loss. In that case the spreader plate will rather improve the thermal resistance due to its additional dissipation. If the heatsinks are in different locations, the spreader plate must be thick enough to carry the heat away from the IC.
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