LM3886 Heatsink

[star882]

Wire the fan to only turn on at high load.

If you really want quiet, water cooling is the way to go. Or just switch to a hybrid digital design.

(BTW, if you ask for quiet cooling for a high speed CPU, they'll likely agree with water cooling.)

[Redshift187]

Originally posted by dfdye
Will one of the heat sinks described in the original post effectively cool one chip of an amp?

That depends on your definition of "effectively". Do you mean just saving the chip from destruction, or do you mean allowing it to play without ever triggering SPIKE? Because you may not be able to hear SPIKE, but some people definitely can.

[gtforme00]

Originally posted by star882
Wire the fan to only turn on at high load.

If you really want quiet, water cooling is the way to go. Or just switch to a hybrid digital design.

(BTW, if you ask for quiet cooling for a high speed CPU, they'll likely agree with water cooling.)

Water cooling a chip amp? Go recompute, star. Water cooling can never be more quiet than passive.

This is why you don't listen to pure logic when designing anything practical. Computers (such as the quoted poster) have the logic thing down great, but can't come to reasonable suggestions.

But this is all an aside from the point.

I use a heatsink similar to David's (dfdye) for my stereo pair of LM3886TF's. It works fine with my +/-24 V supply, doesn't even get warm. I would not be able to use my heatsink with 4ohm loads at +/-28V though, due to the TF package.

For your application, I would use one slot A heatsink per chip. (with widely spaced fins, dfdyes are a good example).

-David

[pacificblue]

Originally posted by dfdye
Will one of the heat sinks described in the original post effectively cool one chip of an amp? Isn't that the question we are trying to answer?

Exactly. And we cannot answer that, because ormo does not specify the conditions exactly.

Your approach is to assume that ormo will use the amplifiers the same way you do, at civilized listening level and comfortable ambient temperatures below 80 °F, which is around 27 °C.

Andrew's approach to show up, how to calculate the heatsink for a given set of conditions is better, because it also works, if ormo uses the amplifier in one of numerous different ways.

Originally posted by star882
Wire the fan to only turn on at high load.

If you really want quiet, water cooling is the way to go.

When you use active cooling, don't forget the temperature limiter that switches the amp off at a certain point. Fans and water pumps can fail, and there have been several posts reporting that the SpiKe protection system is not always up to its task.

[star882]

It's actually possible to have water cooling without a water pump. The water will become less dense as it warms up, and that can be taken advantage of to make it flow. (It would take more work in design, but it could work in theory.) Water has a much higher heat capacity than air so it is more practical for natural convection. That will so help keep temperatures more stable.

If you use a liquid with a low boiling point (such as tetrafluoroethane), it works even better since the liquid boils into a vapor, which is much, much less dense. Indeed, such an approach is very common and is known as heatpipes. However, the tools required to make one of those cooling systems cost quite a bit and therefore is generally not cost effective for most DIYers. (I have some HVAC tools I used to repair an A/C at my old house so I do have all the tools required. But most DIYers do not.)

Or just go for a hybrid digital design. For high power levels (which was once 100w and above but is now more like 5w and above thanks to the decreasing cost of new technology and rising cost of certain raw materials), it costs less to initially build and less to operate. The extra cost of a hybrid digital design is more than made up for by reduced costs in heatsinking and power supplies. Indeed, a 60w hybrid (2x TPA3122D2) one of my friends built doesn't even have a heatsink for the chips and only two small heatsinks for the power supply (old 90w laptop supply). Even at high output, it barely runs warm.

[pacificblue]

Originally posted by star882
Or just go for a hybrid digital design.

Would somebody, who cannot select a heatsink on his own, be up to the task of designing and implementing a switching amplifier? Electro-magnetic compatibility is no trivial topic. Most Class D amplifiers need planes for shielding and sometimes for heatsinking as well. How many Class D amplifiers work with a single layer PCB?

Is a 10 W @ 10 % THD amplifier (TPA3122D2 acc. to datasheet) an adequate subsitute for a 68 W @ 0,1 % THD amplifier (LM3886 acc. to datasheet)?

[star882]

Originally posted by pacificblue
Would somebody, who cannot select a heatsink on his own, be up to the task of designing and implementing a switching amplifier? Electro-magnetic compatibility is no trivial topic. Most Class D amplifiers need planes for shielding and sometimes for heatsinking as well. How many Class D amplifiers work with a single layer PCB?

Is a 10 W @ 10 % THD amplifier (TPA3122D2 acc. to datasheet) an adequate subsitute for a 68 W @ 0,1 % THD amplifier (LM3886 acc. to datasheet)?

The TPA3122D2 would not be a good substitute for that application, but there are chips out there that would be good substitutes. There's a new hybrid by TI that is rated to 600w. And discrete very easily goes even higher. Remember, every modern sine wave inverter is essentially a very powerful digital amplifier! Some of them, such as the ones in enterprise grade UPSes, output extremely pure sine waves.

Of course, there's also modules that take most of the work out of building a hybrid or pure digital amplifier. They're very easy to use and are therefore highly recommended for beginners. Avoiding EMI can really be as simple as enclosing it in a metal box and using filters on inputs and outputs.

At high power levels (above about 5w at this point), standard analog really doesn't make sense as it would cost more due to heatsinking and power supply requirements. And the frequency range is actually improving to the extent that some engineers are actually working on digital DSL line drivers, which is a very demanding application (well into the low MHz).

[Samuel Jayaraj]

I have built 6 monoblocs using LM3886TF chips with supply of +-25V and transformer rating of 120VA. As most here have experienced, when you use them for normal domestic use, they barely get warm even at relatively loud levels. Ofcourse, it would depend a lot on speaker sensitivity.

However, when I used these to really push 12" PA speakers (2 way, 99db sensitivity) it didn't take long for the heatsinks to really heat up and it did take long for them to cool down. My heatsinks measured 200 x 88 x 33mm. The fins were 25mm and the heatsink base plate was 8 mm thick.

All of these amps have been installed in a friends place and I won't be able to re-test.

Andrew is right that one would require compliance with National's specs when you have reactive loads and high drive levels.