Can we fix the audio for A5/A6/A7?

[joerg]

Am Mi  2. April 2008 schrieben Sie, Andy Green:
> - gpg control packet
> Somebody in the thread at some point said:
> 
> Hi Joerg -
> 
> Thanks for finding this.
> 
> |> 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!!!!!!
> 
> They're ~32R apparently, with 33R in series (R4405 / R4407) so it is ~65R.
Yep, so *no* standard replacement headphone will fit. No Motorola, no Sony, 
just OM-headset (as long as at least these have 600~1000Ohm for f0=300~400Hz!)
No use to tell in wiki about the standard pinout of connector.

> Clearly a certain
> sized capacitor is needed to couple in low frequencies with-
> out severe attenuation. But in many cases the speakers  <============
> used in portable systems, whether internal or external, have
> little ability to reproduce signals below 150Hz. Thus, large
> value input and output capacitors may not increase system
> performance.''

That's just nonsense, it applies to speakers, not to headphones (see data for 
an arbitrary headphone anywhere in the web). The whole paragraph is on 
selecting right size of CI for bridged mode, and bridged mode is implicitly 
for speaker. The headphone stereo circuitry isn't really discussed therein. 
As you say, the application example shows 100uF.


> So the impact is we lost some bass end... maybe not a noticeable amount
> to what we would have with 100u due to the transducer performance.

We have a f0 of ~160Hz WITH OPEN CIRCUIT, just due to the 1k pulldowns!
With a 1000 Ohm headset, this will be 320Hz for -3dB!!!
That's noticeable for sure. It's plain crap.


> If 
> there's no room for adjusting the value of the caps on the A5 or A6 PCB
> then I would just accept this.
At very very least we can increase the thing to 4.7u, it fits on current PCB.
Think one of my first tasks in .tw will be retrofitting the 100uF and compare 
the sound. Maybe i can do a nice foto footage for the wiki, so anybody with 
good soldering skills can do the same, if they are interested in HiFi.

cheers
jOERG