3875 regulated operating points and lm338 heatsinking

[pjanda1]

Yup, it is right there in the data sheet. And, like I said, it looks to me like I shouldn't go higher than +/- 25V for a 4 ohm load (power seems to plummet after 27V or so and a 4 ohm load). But, many folks seem to do so. And many folks are smarter than me. Thats exactly why I'm asking!

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[AndrewT]

look at p10, that's where you saw the power plummet >56W (it's not the voltage that causes the power drop off, it's the limiting inside the chip that tries to prevent the user blowing up the chip).

My interpretation of the 4r0 graphs is that +-25V is OK, but needs a 2.8C/W (Ta<=25degC) sink to stay within National's absolute max ratings. I would use 1.4C/W
+-30V is also OK, but needs an even bigger heatsink. 0.8C/W for each chip is what I recommend for Ta<=25degC.

If you intend placing the heatsink inside a ventilated case then using Ta=40degC might be more appropriate.

Note also the a typical maximum output current is 6Apk and the minimum spec for maximum current is just 4Apk.
A 4r0 load driven to 56W requires output voltage = 21.2Vpk and output current = 5.3Apk
A 4ohm rated speaker driven to 56W of short term transient peak will demand at least 10Apk and in extreme cases could demand >15Apk.
This chip is not designed to drive 4ohm speaker.

[pacificblue]

Do you expect a lot of content in the range between 20 and 40 Hz?
The lowest notes on current drum sets and electric bass guitars is 40 Hz, so for pop music there is little need to build speakers and amps for that range.
The upright bass can go down to 30-something Hz, so if you like classic or certain types of Jazz music it can make sense. But then again to achive useful SPLs at 30 Hz you won't get far with a chip amp.
Then there is organ music, where 16 Hz is the lowest note for most big organs, but you don't listen to that do you? Otherwise you would not stop short at 20 Hz. The same goes for home theater, where even 10 Hz tones are present during spectacular effects.