GTA02A5: Headset Amp circuitry, max output power to headset

joerg joerg.twinklephone at
Sat Mar 29 22:33:03 CET 2008

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)

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)

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 
( 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!


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