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Old 24th May 2007, 02:47 AM   #1
ronjodu is offline ronjodu  United States
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Default Safe operating temperature.

I built a BrianGT Dual Mono with 2 28/28 torroids. This is for my son's MP3 player. It works awsome.

Because I know he and his dorm room buddies will crank this full 24/7 I decided to do a little "destructive test" with it. I super glued a LM34 temperature sensor to one of the LM3886's, fired everything up and listened to some of my 90's dance music at full volume for about 2 hours watching the temperatures fluctuate between a cool 150 F to 210 F. The temperature drops rapidly between songs, sometimes 10 to 15 degrees. At what I would call an acceptable listening volume the chip temperature hovers around 110 to 120 degrees F.

The amp outputs are fed into a 4 ohm subwoofer rated 50 watt nominal and 100 watt max. It then splits to two 8 ohm desk speakers(rated 20 watt nominal 40 watt max).

My question is do these temps sound reasonable or should I consider more heatsink, given the use I think this will get?
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Old 24th May 2007, 03:08 AM   #2
sam9 is offline sam9  United States
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Generally cooler will live longer, however, I think the built in thermal shut down is 180C, so you certainly have room to spare in that regard.
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Old 24th May 2007, 09:29 PM   #3
ronjodu is offline ronjodu  United States
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Thanks for the reply.

FWIW the chip temp with no input to it is a constant 90 degrees F. Room ambient is 72.

I need to find some other things to test or I'll have to give the amp over to my son.
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Old 24th May 2007, 10:07 PM   #4
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Simple rule of thumb, (pun intended).

If you can keep a finger on the chip for ten seconds or so whilst running at full, you'll probably be okay. If you can't you need bigger heatsinks.
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Old 24th May 2007, 11:03 PM   #5
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I agree with pinkmouse- if you can't leave your fingers on it for some seconds, it's too hot, probably over 160F. Remember too that you tested at normal room temperature. If the room doesn't have air conditioning and hits 90, the heat sink will rise by 30 degrees too. The real temperature of interest is the die/junction temperature, but calculating back to that can be a bit uncertain, thus I go with the "gotta be able to touch it" rule.
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Old 24th May 2007, 11:37 PM   #6
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Cooler will mean longer life but the life might still be 10 years so does it matter if longer? 210F is 100C and that is still within the operating. Remember that either thermal shutdown (about 155C for the record) or the Spike protection will kick in if the chip gets really hot. So if neither of those are kicking in then things are fine under the conditions you tested. If the room is hotter then one of the protections will kick in to keep the part from blowing up.

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Old 25th May 2007, 10:05 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
surface of the chip at 210degF sounds too hot for comfort.
I would expect the amp to run much cooler using 8ohm speakers.
It may run a little quieter using the high impedance and save them getting their equipment confiscated.

If you decide to keep the 4ohm speakers, then I would recommend a sink at least twice as big as present, or even better three to four times bigger.

An alternative is to fit a thermal switch to cut the mains power when the sink temp reaches 140degF (60degC) or 158degF (70degC). He might get the message when it switches off for a couple of minutes every five minutes. If he's real attentive to his lessons he'll soon learn how to bypass the thermal switch.
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Old 31st May 2007, 12:11 AM   #8
! is offline !  United States
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In the present test scenario, the chip is staying cool enough and you would hear SPIKE cutouts if it weren't. The voltage from 28-0-28 transformer is higher than optimal for this configuration though, by not having a substantially larger heatsink you are likely limiting the max output possible BUT it only needs be as loud as you deem sufficient.

Further if the tested speakers are the ones intended for use with the amp they are already a limitation. If they are not the intended speakers you should consider impedance and efficiency vs desired output level with of the intended speakers. Also, we have no idea if your idea of loud is same as theirs and dance music not necessarily same level of bass unless some modern techno type.

I also assume you are testing with the MP3 player as source instead of something with a potentially higher output level. Further there is the assumption your son is aware of basic amplifier placement, need to keep things away from chassis vents (on top?) and not under other components in a stack since it is a less than (probably) optimal environment you were testing in, but we don't know the exact test environment such as whether all your tests were with the amp case closed up or open.

If all test parameters are equivalent to the real operating conditions, it is cool "enough". If you wanted to build in more margin, only you can decide to what lengths vs benefit it will have. I'm inclined to suggest that if the amp is finished, otherwise, leave it as-is but if you were to build the next one it might use lower voltage and then consider larger heatsink if practical.
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