How many 'c/W
 

For a LM3875 running at +/-25V into 4 ohms,is a 2 'c/W heatsink enough??

Depends.

What is the ambient air temperature?

Is that using the non-isolated T package or isolated TF package?

Is there a mica washer if using the T package?

What is the min and maximum supply voltage rails?

Or you can just download the Overture Design Guide from National, put all the info in yourseld and see what it tells you about thermals.

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

Middle column about 1/2 way down the page.

-SL

Preferably not more than 50'c.

T package.

Yes.

Stable 25V.

My Gainclone works fine on 24V rails an heatsinks about the size of my hand, but a bit shorter, about 3 inches... gets to about 50 degrees, but seems quite happy... Someday I will integrate everything into a new case and get larger sinks.... but noone died in the meantime.

What is the exact size of your heatsink???

speaking from memory.. was a while since I made the case.... about 8 x 5 x 1.5cm.... i.e a little small... idealy you need something just a bit bigger than this....

Could one get away with using a smaller than recommended heatsink if used with a small quiet fan? +/-18 with small 8ohm bookshelf speakers.

Fans make a big difirence to the efficiency of a heatsink.... unplug you PC's CPU fan to see what I mean (just jokeing - please don't)... I would give it a test but sit close and make sure those sinks are not getting too hot....

Fans introduce a host of issues of their own from the noise created by the motor to (on badly designed systems) microphonics due to induced vibration...