ipod video with DC Coupled headphone output

[strider]

I read the manual WM's, and I
 found an interesting idea.
The idea to remake out of my Ipod.
this steps:

1) remake wm8758 driver
and recompile firmware

2) remove lineout capacitors  and connect to headphones GND
Warning!
 New headphones GND should`not be connected to GND of mainboard, as previously.

3) remove out1L/R capacitors and connect to headphones L/R

and I got excellent sound)))
but now can not use the original firmware.(if I use original firmware then i can damage the out1 and out2)

[Chronon]

I'm not really sure of the purpose of this.  Is there some special purpose that requires DC signals?

[strider]

only manual build and hands

[gomem]

I'm not really sure of the purpose of this.  Is there some special purpose that requires DC signals?

I think the idea of the DC coupling is to try and improve SQ, especially on the bass. AC coupling with a low impedance load like headphones can degrade bass response, if the capacitors not up to par. Some people reckoned the original iPod Shuffle had better SQ than the other iPods, because it was DC coupled. Expensive HiFi amplifiers tend to use DC coupling for the same reasons, rather than putting the signal through electrolytic capacitors which could degrade the SQ.

[Chronon]

That makes a certain amount of sense, though you can push the corner frequency of the high-pass filter down by using higher impedance phones.  My phones present a 64 ohm load.

Alternatively, if you're cracking the case you could replace (or simply augment) the capacitors.

[soap]

Alternatively, if you're cracking the case you could replace (or simply augment) the capacitors.

Not enough room in any of the cases for any of the capacitors I've seen proposed for fixing the bass roll-off @ 16 ohms.

1) remake wm8758 driver
and recompile firmware

Please explain this part to the noob here (me!), what needs changed in the audio driver?

[Multiplex]

Please explain this part to the noob here (me!), what needs changed in the audio driver?

There was this post a little while ago http://forums.rockbox.org/index.php?topic=23221.0

[Chronon]

Not enough room in any of the cases for any of the capacitors I've seen proposed for fixing the bass roll-off @ 16 ohms.

I see.  I haven't checked to see how much extra capacitance is needed and how much space this would typically require. 

[NiHaoMike]

Just add about 100 ohms of resistance in series with each channel to push down the RC corner frequency. I did that by modifying my headphones, although it can also be done by modifying the player or by making an adapter. In my case, the JVC HA-FX34 has a stock impedance of about 16 ohms, so the mod puts them up to 116 ohms. Some volume is lost but the iPod can still play much louder than I want.

Another benefit is that variations in headphone impedance over frequency will be reduced, smoothing your frequency response.