Heatsinking issues

pjpoes · 17th September 2005, 06:12 PM

http://www.mpja.com/productview.asp?product=14674+HS

The above link is to the heatsink I am using. Ok, so I am concerned with the temperature I am getting for my Chip amp. I was under the impression that the amps ran cool to the touch, or rather, just a little warm. This amp is running warmer than my large main amp. I dont have a thermometer to test with yet, but I can get back to you with that. The chip and mounting screw is now staying to about the same temperature as the rest of the heatsink, probably slightly higher, but not like before, however now the hole heatsink is quite warm. Not too warm to touch, but quite warm. Is the heatsink too small, did I introduce some other problem? Is that one not large enough to have two amp modults mounted to?

The only other thing I could think of is the power supply, running to high of rail voltages. I am currently running the 330VA 25-0-25v Avel transformer, are my rail voltages going to be too high, I was under the impression this was within the safe range, but at the higher end. My rail voltage appears to be about 36 volts. Is that high?

DC Dave · 17th September 2005, 07:02 PM

Need more info.

What IC are you using? Depending on what chip you are using and under what load that heatsink might not be big enough for two chips.

What is the impedence of your speakers (4ohm, 8ohm)? The LM3875 cannot drive 4ohm loads with the voltage levels you are using.

Have you checked the DC offset at the outputs? High DC offset can create added heat.

What about only using one chip on the heatsink. Does it still get hot?

pjpoes · 17th September 2005, 07:37 PM

I am using an LM3875 chip into an 8ohm nominal impedence. The Speakers are JmLabs Electra 905's with the crossovers remaid using better parts, but all values were basicly kept the same. I would expect Impedence to have not changed from that modification.

With just one amp it gets less hot, but still warm, two amps is definatly making it hotter, but that would only be expected, two heat sources instead of one. Again, it doesn't get so hot it I can't keep my hand on it, I just was under the impression that the amps didn't run hot at all.

As for the DC Offset, how exactly do I measure that, I asked this once before and really didn't get an answer. Do I simply hook a load, like an 8ohm resister, and plug the amp in, then measure the dc voltage across the resistor. DO I have to short the input? Let me know, as I was intrested to measure the dc offset.

DC Dave · 17th September 2005, 08:01 PM

To check DC offset you can connect a 10 ohm resistor to the outputs and measure the DC across the resistor. You do not short the input.

At +-36 volts the LM3875 will dissipate close to 35 watts of heat. Two of them on one heatsink would mean 70 watts to be dissipated. With out knowing how hot the heatsink is, it does not sound too abnormal what you are feeling.

I run my LM3875 amp from a +-25volt supply. With that voltage it barely gets warm with a smaller heatsink. The more voltage you apply for a given load, the more heat will be produced. You are suppling at the upper end of what the LM3875 can handle, therefore more heat.

peranders · 17th September 2005, 08:34 PM

What is hot really? It's a little vauge description about your heat problems.

Since the LM3875 is a class AB amp normal theories applies. I recommend that you read the following documents where heatsink issues are described.

http://www.national.com/ds/LM/LM3875.pdf
http://www.national.com/an/AN/AN-1192.pdf
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf

Nordic · 17th September 2005, 09:41 PM

Just a question, did you use any thermal paste?

jleaman · 17th September 2005, 09:49 PM

is there any short ? Does it get hot with no load and no input on the amp. ? just power ?

pjpoes · 18th September 2005, 02:27 AM

Yes I used thermal paste, though maybe not enough, but I dont see it causing this problem.

As for what is heat, I dont know, I need to measure, this may be normal. As for the remark about if it gets hot with no load, yes it does, but not as hot, or even what I would call hot hot. It gets warm to the touch, but easily something I can place my hand on comfortably. However, again, this is still quite a bit warmer than my very cool running Acurus amp. I dont see any shorts, I can't imagine I have any, but anything is possible, any thoughts on a place where a short would not strongly effect sound, but would cause such heat. One thing was my power wires, I didn't have wire of small enough guage without being way too small, so I simply trimmed the wire to fit into the hole, and so the edges were a bit scraggly, but I dont believe I had any loose ends that could have touched anything they shouldn't. I will look it over, or posisbly redo it with better wire.

As for the heatsink, it looks like it says its just bare aluminum. I think I read somewhere, maybe on the ESP sight, that anodized aluminum works much better. Something about black oil even better. Not sure what Black Oil is, but for the anodized, if it says bare aluminum, is it probably not anodized, should I consider doing that? Thanks.

pjpoes · 18th September 2005, 03:21 AM

I take back the comment about it getting hot with no load, it was hooked to the speakers when I said that, sorry. It does not with no load.

As for the DC offset, I measured it with a 8ohm resistor, its all I had, it was left over from a crossover. With that, I get 52mv, that seems high, but if you mostly measure with a 10ohm load, is that my problem. Well anyway, now how do I go about lowering my dc offset?

jaudio · 18th September 2005, 07:09 AM

From what I read on this forum, 52mv is a little high but should not be a problem. What schematic are you using?

17th September 2005, 06:12 PM	#1
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	Heatsinking issues http://www.mpja.com/productview.asp?product=14674+HS The above link is to the heatsink I am using. Ok, so I am concerned with the temperature I am getting for my Chip amp. I was under the impression that the amps ran cool to the touch, or rather, just a little warm. This amp is running warmer than my large main amp. I dont have a thermometer to test with yet, but I can get back to you with that. The chip and mounting screw is now staying to about the same temperature as the rest of the heatsink, probably slightly higher, but not like before, however now the hole heatsink is quite warm. Not too warm to touch, but quite warm. Is the heatsink too small, did I introduce some other problem? Is that one not large enough to have two amp modults mounted to? The only other thing I could think of is the power supply, running to high of rail voltages. I am currently running the 330VA 25-0-25v Avel transformer, are my rail voltages going to be too high, I was under the impression this was within the safe range, but at the higher end. My rail voltage appears to be about 36 volts. Is that high?

17th September 2005, 08:34 PM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	What is hot really? It's a little vauge description about your heat problems. Since the LM3875 is a class AB amp normal theories applies. I recommend that you read the following documents where heatsink issues are described. http://www.national.com/ds/LM/LM3875.pdf http://www.national.com/an/AN/AN-1192.pdf http://www.national.com/appinfo/audi...gn_Guide13.xls http://www.national.com/appinfo/audi...sign_Guide.pdf __________________ /Per-Anders (my first name) or P-A as my friends call me

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17th September 2005, 07:02 PM	#2
DC Dave diyAudio Member Join Date: Feb 2004 Location: Washington DC	Need more info. What IC are you using? Depending on what chip you are using and under what load that heatsink might not be big enough for two chips. What is the impedence of your speakers (4ohm, 8ohm)? The LM3875 cannot drive 4ohm loads with the voltage levels you are using. Have you checked the DC offset at the outputs? High DC offset can create added heat. What about only using one chip on the heatsink. Does it still get hot?

17th September 2005, 07:37 PM	#3
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	I am using an LM3875 chip into an 8ohm nominal impedence. The Speakers are JmLabs Electra 905's with the crossovers remaid using better parts, but all values were basicly kept the same. I would expect Impedence to have not changed from that modification. With just one amp it gets less hot, but still warm, two amps is definatly making it hotter, but that would only be expected, two heat sources instead of one. Again, it doesn't get so hot it I can't keep my hand on it, I just was under the impression that the amps didn't run hot at all. As for the DC Offset, how exactly do I measure that, I asked this once before and really didn't get an answer. Do I simply hook a load, like an 8ohm resister, and plug the amp in, then measure the dc voltage across the resistor. DO I have to short the input? Let me know, as I was intrested to measure the dc offset.

17th September 2005, 08:01 PM	#4
DC Dave diyAudio Member Join Date: Feb 2004 Location: Washington DC	To check DC offset you can connect a 10 ohm resistor to the outputs and measure the DC across the resistor. You do not short the input. At +-36 volts the LM3875 will dissipate close to 35 watts of heat. Two of them on one heatsink would mean 70 watts to be dissipated. With out knowing how hot the heatsink is, it does not sound too abnormal what you are feeling. I run my LM3875 amp from a +-25volt supply. With that voltage it barely gets warm with a smaller heatsink. The more voltage you apply for a given load, the more heat will be produced. You are suppling at the upper end of what the LM3875 can handle, therefore more heat.

17th September 2005, 09:41 PM	#6
Nordic diyAudio Member Join Date: Sep 2005	Just a question, did you use any thermal paste?

17th September 2005, 09:49 PM	#7
jleaman Banned Join Date: Apr 2002	is there any short ? Does it get hot with no load and no input on the amp. ? just power ?

18th September 2005, 02:27 AM	#8
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	Yes I used thermal paste, though maybe not enough, but I dont see it causing this problem. As for what is heat, I dont know, I need to measure, this may be normal. As for the remark about if it gets hot with no load, yes it does, but not as hot, or even what I would call hot hot. It gets warm to the touch, but easily something I can place my hand on comfortably. However, again, this is still quite a bit warmer than my very cool running Acurus amp. I dont see any shorts, I can't imagine I have any, but anything is possible, any thoughts on a place where a short would not strongly effect sound, but would cause such heat. One thing was my power wires, I didn't have wire of small enough guage without being way too small, so I simply trimmed the wire to fit into the hole, and so the edges were a bit scraggly, but I dont believe I had any loose ends that could have touched anything they shouldn't. I will look it over, or posisbly redo it with better wire. As for the heatsink, it looks like it says its just bare aluminum. I think I read somewhere, maybe on the ESP sight, that anodized aluminum works much better. Something about black oil even better. Not sure what Black Oil is, but for the anodized, if it says bare aluminum, is it probably not anodized, should I consider doing that? Thanks.

18th September 2005, 03:21 AM	#9
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	I take back the comment about it getting hot with no load, it was hooked to the speakers when I said that, sorry. It does not with no load. As for the DC offset, I measured it with a 8ohm resistor, its all I had, it was left over from a crossover. With that, I get 52mv, that seems high, but if you mostly measure with a 10ohm load, is that my problem. Well anyway, now how do I go about lowering my dc offset?

18th September 2005, 07:09 AM	#10
jaudio diyAudio Member Join Date: Oct 2004	From what I read on this forum, 52mv is a little high but should not be a problem. What schematic are you using?

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