High Performance, High Fidelity, High Current Audio Buffer

Here is a new National chip that might be useful for headphone amplifiers.

National LME49600

See the data sheet at:

http://tinyurl.com/6jrlsb

ZAP

Probably a useful replacement for some older hybrids. I guess this is out of that group at National that's gotten some publicity lately for focusing on analog audio, which I suppose is a positive development.

Note that the blurb bullets at the beginning associate with the use as a buffer in a closed-loop with a fancy opamp. And what the devil does "Pure fidelity. Pure performance" mean?

I do find some troubling things in the application part of the datasheet. One in particular is this penchant once again to thinking that the use of servos to eliminate conventional coupling capacitors means there are no capacitors in the signal path. Rubbish.

Another quibble: Figure 4 shows what is implied to be a superior topology with d.c. coupling at the input, including to the volume control. Using the opamp shown with its 10nA bias current is going to be a recipe for some scratchy nasties when the control is adjusted, and the ~1 sec servo tau won't keep up. Bad idea.

Another concern is the peaking shown in the graphs on page 5, and worse, the absence of a most important datum, the source impedance that was used to generate the curves. This should be fixed.

But still in all, probably a part worth playing with. Thanks for posting it.

> a new National chip

Direct link: http://cache.national.com/ds/LM/LME49600.pdf

Over at Headwize they say it is the old BUF634, which draws mixed opinions as an audio chip.

Are National and TI sleeping together? Or just horse-trading?

> some troubling things in the application part of the datasheet

It has been a looong time since Widlar, Jung, or Pease wrote sales-sheets (oops... spec sheets). It has always been a chore for mid-level engineers: the guys (and gals now I suppose) who can't yet actually design anything, but are close enough to fake it on paper.

Anyway of the three, only Jung knows the intracacies of audio. Pease is brilliant and says things which need to be said, and would understand the issues, but does not know audio in depth. Widlar has left the field and this mortal coil. Lots of bright guys in the semi industry, but audio is not their main market. Even when the sales-sheet tries to pretend it is.

I saw some recent and perhaps New! "audio opamp" from National where all the application circuits were reprinted from some 1980 chip (and a few of those were trailing-edge at the time).

Here is one related thread:

http://www.groupdiy.com/index.php?topic=27449

Looks very similar, but contains a be - protection from reverse voltage and thermal threshold sensor, also I don't understand how to make complementary devices with common base, but separate collectors and emitters...