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rhys h  
#1 Posted : Thursday, March 26, 2009 1:19:53 AM(UTC)
rhys h

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Hi,

Can someone tell me what heatsink rating i need, i will be running 30v +/- in the standard configuration, with one chip per heatsink. I have posted this question over at DIYAUDIO and i have been told it is impossible to get over 50W output in bridged config without overheating? d'oh!
francolargo  
#2 Posted : Monday, April 27, 2009 4:33:40 PM(UTC)
francolargo

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If you check the data sheet, you'll see that 15 watts of output into a 6 ohm load causes 60 watts of dissipated power (+/- 30 V). Thus, you have to dissipate heat very efficiently because even at moderate outputs the chips will dissipate at their maximum. It is beneficial if your heat sinks can radiate (flat black finish preferred) in addition to working by free convection. The built-in thermal protection doesn't trigger until 150C, but I have heard that the 3886 chips don't sound as good once the protective system has been triggered. I would make the heat sink as large as your case solution will allow and use a premium heat transfer paste, such as those designed for computer CPUs. http://www.national.com/ds/LM/LM4780.pdf
zonneveld  
#3 Posted : Tuesday, May 26, 2009 12:52:15 AM(UTC)
zonneveld

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Although Russ and Brian spoke comforting words about the capability of driving 4 Ohm loads with the Sympatico, i still struggle with this. If you use the nationals design tool;

http://www.national.com/analog/audio/

The design tool gives a warning with 4 Ohm loads and the voltages reccomended by TP

heatsink ratings:
28 V rails/4 Ohm load= 0.72 C/W/chip (20-0-20 transf.)
30 V rails/4 Ohm load= 0.53 C/W/chip (22-0-22 transf.)
33 V rails/4 Ohm load= 0.30 C/W/chip (24-0-24 transf.)

with 8 Ohm loads everthing looks fine:

28 V rails/8 Ohm load= 2.13 C/W/chip (20-0-20 transf.)
30 V rails/8 Ohm load= 1.76 C/W/chip (22-0-22 transf.)
33 V rails/8 Ohm load= 1.33 C/W/chip (24-0-24 transf.)

Does this has to do with the fact that the design tool is for single ended set up and that balanced set ups are a whole different story when it comes to power dissipation?

Could anyone explain?


Gerard

Huge heatsink when using 33V rails (heatsink would measure 150x250x80)

Edited by user Tuesday, May 26, 2009 1:45:07 AM(UTC)  | Reason: Not specified

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Russ White  
#4 Posted : Tuesday, May 26, 2009 6:06:19 AM(UTC)
Russ White

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What it comes down to is how much power your plan to constantly push through the amplifier.

I usually go with a little bigger HS then I think I need, but you don't need to go crazy.

The HS the Brian recommends works great as long as you don't overwork the amp. I have used them myself and found you could run the amp very hard without any worry.

Remember that National's guidelines are for constant load. A constant load is not a normal circumstance.

Cheers!
Russ
zonneveld  
#5 Posted : Tuesday, May 26, 2009 6:40:15 AM(UTC)
zonneveld

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Yes that must be it. I Just had some problems with overheating in the past that made me a little paranoid. I will be using 8 Ohm speakers anyway, but just wanted to make sure i could use 4 Ohm in the future as well. I'll go for the heatsink rating Brian suggest and rest at peace with it.

Thanks,

Gerard
schnak  
#6 Posted : Tuesday, June 8, 2010 9:20:58 AM(UTC)
schnak

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Location: seattle, wa

Couple of questions. Is this amp common ground with respect to speaker output?
I've looked and cant find the recommended size of heatsink. Can anyone point me in the right direction? If I order the kit the only other stuff needed would be a transformer, case, and heat sinks and connectors, correct? Thanks.
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