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Old 23rd February 2007, 02:00 PM   #1
henkel is offline henkel  Malaysia
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Default LM3886 Heatsink size?

Hi!

Thanks to all the contributions in this forum, I finaly have enough courage to start my first GC.

I've just built an LM3886 amp and it sounds amazing! Just one problem though.

The heatsinks heat up really fast and the amp cuts off. Roughly about playing 3-4 songs only.

The various parameters involved:

Using 24V 4 ampere transformer to power the LM3886, used with 4ohm speakers. Current heatsink size is 6" x 2.5"

I intend to increase heatsink size to 8" x 3.5" as this is the current maximun allowable without relocating the amp's layout within casing.

My question is, would 8' x 3.5" enough to keep the LM3886 cool?

If not, then can you advise on heatsink size you think suitable from experience?

Heatsink dimensions are in inches.

Thanks.
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Old 23rd February 2007, 02:31 PM   #2
Fenris is offline Fenris  United States
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I've used 3" x 5" with no problems, I've also used a 2" x 6" copper bar, although it has a small fan blowing over it. I even have four mounted to a thin unfinned chassis. Of course, most everything I run is 8 ohm, so that probably helps.
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Old 23rd February 2007, 02:33 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
6inch by 2.5inch of what?
Aluminium cooking foil or a 10mm thick chunk of aluminium with 50mm deep fins at 8mm pitch with the fins aligned vertically so that cooling air can pass over them?

What happens if you disconnect the load? Goes the heatsink cool down or remain the same?

Going to 8inch by 3.5inch is not a particularly big increase.

What voltage are you supplying to the chipamp? 24Vac could be about +-36Vdc, which is at top limit for 8ohm speakers.
Did you follow the instructions in the data sheet for both voltage and heatsink?
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Old 23rd February 2007, 03:01 PM   #4
henkel is offline henkel  Malaysia
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Sorry for not being detailed enough.

The heatsinks currently used are made of aluminium with 1" fins which I bought off the shelf from local electronic stores.

The proposed 8" x 3.5" heatsinks are made from same aluminiun material and also have 1" fins.

Going thru the GC Gallery, I notice most pics have rather small heat sinks.

The heatsinks cool down within about 20 minutes or so when I switch amp off. When I restart the amp, it works fine.
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Old 23rd February 2007, 04:20 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
don't switch off.
How hot does it get when the load is disconnected?
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Old 23rd February 2007, 04:33 PM   #6
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I'm using a heatsink that looks like a CPU heatsink, probably no bigger than 3" x 3". The heatsink is inside my chassis, so there is NO airflow around them. I have one per LM3886 (it is a stereo amp). I picked them up for $1 at a local surplus store.

I think the load is roughly 6 ohms, and while I don't remember exactly what the DC voltage was, it was an 18V - 0 - 18V transformer.

It never overheated, and it has been run for at least a few hours straight, so by that point, it should've reached it's peak temp.

You don't need much heatsinking, just make sure it is a good design. Anything that looks like it could be used for a CPU should work. Since your average processor will dissipate magnitudes more power than the LM3886, most of those heatsinks WITHOUT the fan should still work for you.
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Old 23rd February 2007, 05:24 PM   #7
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I think Andrew is trying to determine if your amp is oscillating, or maybe shorting, which woud account for your heat problems. If your amp is on, but not connected to a load, and it still gets hot, heatsink size is not your problem.
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Old 23rd February 2007, 09:01 PM   #8
lgreen is offline lgreen  United States
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Quote:
Originally posted by BrianDonegan
I think Andrew is trying to determine if your amp is oscillating, or maybe shorting, which woud account for your heat problems. If your amp is on, but not connected to a load, and it still gets hot, heatsink size is not your problem.
Yeah, I think you may have an oscillation because many people have used heat sinks that size or smaller (including me) without having heat issues.
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Old 24th February 2007, 03:05 PM   #9
henkel is offline henkel  Malaysia
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Andrew,

The heatsinks runs cooler when the speaker is disconected. Just slightly warm like a normal amp should.
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Old 24th February 2007, 05:19 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
if it stays cool when no load is applied and quickly rises in temp to shutdown, then it seems like the speaker impedance is too low for the supply rail voltage.

Like I said earlier, 24Vac will give about +-36Vdc.
Have you measured that yet?
The manufacturer recommends maximum rail voltages of +-35Vdc for 8ohm and +-28Vdc for 4ohm. For these you must follow through the heatsink design process. It seems like you have omitted this design stage.
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