GTA02A5: Headset Amp circuitry, max output power to headset

[joerg]

Urgent Suggestion:
make C4110, C4111 100uF or at least as high as whatever my fit on PCB 
(currently it is 1uF! in circuit diagram A5_1220)!!
make R4405, R4407 1R or 0R (is 33R now)

reason:
Avoid max volume of headset being too low. 
Allow for usage of low impedance headsets. 
Have better sound by not cutting bass with typical headset.
For multimedia usage, a good and loud sound is desirable.
Don't waste power.
Better adjust max volume by use of Wolfson mixer (+driver) than by series R. 
(this is about FREEdom to kill your ears. If there are legal issues, fulfill 
them by sw, not by hw design)

details:
Application info and datasheet do not demand any R, but clearly say that C has 
to be a certain minimum size.
with VDD=3,3V, the LM4853 has a typ. Po of 40mW to 33R in SE mode 
(http://cache.national.com/ds/LM/LM4853.pdf page 4). However due to R4405 / 
R4407 we need highZ-headphones (>>33R) thus further reducing max Po. 
Even with Z of earpiece = 32R there were only 20mW of 40 left to each, other 
half is heating device via  R4405, R4407 (energy loss!)
LowZ-headsets won't work at all, 8R will get as less as 8mW with no bass.
With 1uF we might get weak bass anyway, even with highZ-headphones.
From datasheet, p5:
> Output coupling capacitor which blocks the DC voltage at the amplifier’s
> output. Forms a high pass filter(!!!) with the single-ended load RL at fO =
> 1/(2π RLCO).
With e.g. *typical* (->ds, p.12) 32R-headphones, RL = 65 Ohm! Could anybody 
check the Z for OM-headset? 
Please calculate f0 for this value!!!!!! 

From application notes of LM4853, there's no indication for the defice being 
sensible to output shortcircuit, so any series R in output is useless.

This change should not affect any aspect of tested device stability, and 
hopefully does not need a layout change (depending on size of C)
I really hope this will make it into MP!

cheers
jOERG