[CSMR]

TIC,
Headphone and subwoofer are handled through a dac i believe for xr25/45/50/70(found if you search through one of the old xr10 posts on DIYaudio), xr10 uses straight from front channels for headphone, and not sure on sub-out. Only difference for 25/45 to 50/70 is that for the front two channels there are different ti power chips used, with the 50/70 having the better ti chips. From what I've read on avsforum and audiocircle, headphone out on the 45/25 and 50/70 are not good. I have the xr10 and I don't think it sounds bad at all, better than my Rio Karma (which sounds similar to an Ipod because they use the same DAC/headphone amp chip). JRC makes the headphone op-amp on the xr10, not sure what is used on the other units. When you plug in headohones on the xr10, main speakers are shut-off, confirming that it does use the front channels.

Don't quite see what you mean tschanrm. Are you saying the speaker amps power the headphones? But then you wouldn't have an op-amp. And if they don't then if you have a digital input there has to be a DAC somewhere.

[tomek]

I wonder if the a/d converters are on at all times and causing the noise in the xr10.  I tested this on my xr-10, if I switch to a digital input, I hear a slight hiss when volume reaches -23, but I have to put my head right next to the speaker.  But, if I switch from analog to digital and don't play anything, I get no hiss at -23 until I play something then pause it.  

yes, this is exactly what i noticed.  it has no noise until i actually play soemthing and then pause it, then there is a continuous hiss.  

really, this hiss is very, very small.  my speakers are 97db efficient, and i still need to put my head within a foot of the tweeters to hear it.  but nonetheless, on the xr50, there was NOTHING.  dead quiet!

i wonder how important this is though.  i still think this amp sounds great, but i don't have the xr50 around to compare it to anymore.

[tschanrm]

CSMR,

Sorry I should have clarified better.  No the headphone out does not get amplified by the mosfets.  Instead, when headphones are plugged in, the tas5012 for the front channels sends the signal to an opamp to convert it to line level, then to the headphone amp, at least thats what it shows in the schematic I'm looking at. Looking at the schematic and physically inside the xr10, there are no DAC's inside it.  One other thing that I noticed is that according to the schematic, all the opamps are biased into class A, which is nice.

i wonder how important this is though. i still think this amp sounds great, but i don't have the xr50 around to compare it to anymore.

I wonder too, tonight I'm gonna try and just unplug the analog connectors inside the unit to see if that does anything, they are labeled CP1004 and CP1001 in case you want to try.  Although, I don't think this will do anything, because the a/d converters are located right next to the dsp chip, which means they must be powered and in the circuit at all times, unless I solder them out.

edit:  Just as I thought, unplugging the cables did nothing, and I can't unplug the power from the analog in board because the digital inputs are on the same ribbon cable, so no way to isolate the buffer opamps fo noise either.   But opening this up again did give me a chance to look at the dsp circuit closer.  After looking up specs for all the parts, it seems this unit should can accept and produce 192khz even though the manual states it only supports 96).  Digital transceiver is akm4114 (supports 192khz), AAC/DTS/DD chip is Crystal CS49329, A/D converters are TI's pcm1804 (supprt 192khz), and the TAS5012 supports 192khz.

The pcm1804 is used in the Audigy 4, EMU 1820 & 0404, and Denon's new AVR-2805 to the top end AVR5805, so it is not low end, but there are better ones out there (AK5394).  However, considering that this amp has been out since 2002/2003 and that newly produced products are using it still tells me that Panasonic really put some quality parts in this reciever.

So for now I'm leaving the adc's in, as they are good and probably have a minimum effect on the sound.

edit 2:  I also found out the ti chips the xr25/45 use, its the tas5182 and the tas5036.

 And also, looking in my XR10,  does anybody know what Panasonic PF's are?  They are all black with silver vertical strip for the negative marking.  All that reads on them, in this order is :  M (the m is in a little square box, I think for matsushita), CE, PF.  They are 63v 800uf and 50v 1000uf.