Heat sinks and C/W ratings.

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I'm looking at making an amp using LM4782 3-channel chip from National Semiconductor. I'm planning on driving it with two 12-volt batteries connected in series giving + and - 12 volt rails. In looking at the datasheet I've calculated that the heat sink needs to be somewhere in the 8 C/W assuming ambient air and 8-ohm speakers.

Being new to all this, my question is as follows. Do these chips dissipate the full wattage at all times? Or does it depend totally on how high the inbound signal (i.e. volume) is driven? My gut tells me it's the later.

I'll probably try and get some stock Aavid brand extrusion and may add a fan to the amp enclosure just to keep the heatsink small.
 
Power dissipation in the amp depends on the output power. For a class B and most A/B amps (which the LM chips are), power is very small at idle, increasing to maximum at half output power, then decreasing to a low value again at maximum output power (for a sine wave).

Don't forget that the temperature inside your enclosure will rise above ambient (unless you have serious airflow in there or mount the heatsinks externally where there are completely unobstructed), which increases the heatsink requirements. Also, lower temperatures increase the lifespan of semiconductor devices, so it doesn't hurt to over-specify.
 
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