Heat Sink for 8 channel lm3886 GC

srfranci · 16th January 2005, 03:16 AM

Anyone got some recomendations? Each channel is wired P2P. The sink is going into a wood case, so I was thinking of some heatsink with the fins mounted through the bottom of the case. How big does it have to be?

thomas997 · 17th January 2005, 05:06 AM

What voltage are you running the chips at?

srfranci · 17th January 2005, 05:16 AM

+-30V Its going to be used to power my orions when i finish them.

Ori · 17th January 2005, 05:05 PM

No bottom of the case mounting for these power levels. You need airflow!
You have to figure out the worst-case instantaneous power in each channel. Let's assume it's 40 watts (from your -+30V) for the 2 front channels, 40W for the 2 sub (I assume you bridge) and 20W for the other 4 channels. Total of 240W (your supply must deliver double that power!).
Back to the analysis. It's very unlikely that this power level is sustained for a few minutes. Let's assume about 1/3 of it average, or about 80W.
The efficiency of the class AB chips is about 50% worst-case, so your heatsink dissipates 80W. It has to limit temperature rise from ambient of 50 degrees celsius to about 80 degrees C The thermal resistance is then: (80-50) degC / 80W = 0.375 degC/W.
This is quite a large heat sink, but that's OK, because it has to support 8 chips mounted on it. You could also split them into 2 groups and use two 0.75 degC/W heatsinks on both sides of the cabinet. There is an advantage to that, because the
two smaller heatsinks are more efficient than one large piece and the temperature spread (hot spots) becomes more even.
Look in the AAVID online catalog for examples. Pick a profile with somewhat chunky metal, which increases the thermal capacity of the heat sink and limits instantaneous temperature rise.

srfranci · 17th January 2005, 05:22 PM

Sorry I should have made it clearer. These are for orions, they are stereo speakers, each with 4 active channels. Anyway Im sure the power dissapation is about the same. Thanks for your help. Ill check out that place.

BlackDog · 17th January 2005, 07:13 PM

I would think that you could probably get away with something similar to my 6 channel LM3875 GC. The heatsinking I used turned out to be overkill, but might be more appropriate for 8 channels of a more powerful chip. My power supply is +-25V DC.

Bear in mind that the amp driving your tweeter needs very little power relative to the bass units...

See the link to my pictures below...

Steve

16th January 2005, 03:16 AM	#1
srfranci diyAudio Member Join Date: Oct 2004	Heat Sink for 8 channel lm3886 GC Anyone got some recomendations? Each channel is wired P2P. The sink is going into a wood case, so I was thinking of some heatsink with the fins mounted through the bottom of the case. How big does it have to be?

17th January 2005, 05:05 PM	#4
Ori diyAudio Member Join Date: Oct 2004 Location: San Jose	Calculating your heat sink(s) No bottom of the case mounting for these power levels. You need airflow! You have to figure out the worst-case instantaneous power in each channel. Let's assume it's 40 watts (from your -+30V) for the 2 front channels, 40W for the 2 sub (I assume you bridge) and 20W for the other 4 channels. Total of 240W (your supply must deliver double that power!). Back to the analysis. It's very unlikely that this power level is sustained for a few minutes. Let's assume about 1/3 of it average, or about 80W. The efficiency of the class AB chips is about 50% worst-case, so your heatsink dissipates 80W. It has to limit temperature rise from ambient of 50 degrees celsius to about 80 degrees C The thermal resistance is then: (80-50) degC / 80W = 0.375 degC/W. This is quite a large heat sink, but that's OK, because it has to support 8 chips mounted on it. You could also split them into 2 groups and use two 0.75 degC/W heatsinks on both sides of the cabinet. There is an advantage to that, because the two smaller heatsinks are more efficient than one large piece and the temperature spread (hot spots) becomes more even. Look in the AAVID online catalog for examples. Pick a profile with somewhat chunky metal, which increases the thermal capacity of the heat sink and limits instantaneous temperature rise.

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17th January 2005, 05:06 AM	#2
thomas997 diyAudio Member Join Date: Feb 2004 Location: Vancouver	What voltage are you running the chips at?

17th January 2005, 05:16 AM	#3
srfranci diyAudio Member Join Date: Oct 2004	+-30V Its going to be used to power my orions when i finish them.

17th January 2005, 05:22 PM	#5
srfranci diyAudio Member Join Date: Oct 2004	Sorry I should have made it clearer. These are for orions, they are stereo speakers, each with 4 active channels. Anyway Im sure the power dissapation is about the same. Thanks for your help. Ill check out that place.

17th January 2005, 07:13 PM	#6
BlackDog diyAudio Member Join Date: Feb 2004 Location: Wellington, New Zealand	I would think that you could probably get away with something similar to my 6 channel LM3875 GC. The heatsinking I used turned out to be overkill, but might be more appropriate for 8 channels of a more powerful chip. My power supply is +-25V DC. Bear in mind that the amp driving your tweeter needs very little power relative to the bass units... See the link to my pictures below... Steve

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