Sorry - can't comment on the original X3 as it is one I never had a chance to hear.  Pretty sure it isn't supported by the K5 though.
 
I'll try the line-in and line-out later tonight when I get home, but I'd imagine that it should work the way you've suggested.  What plays through the dock will be dependent on the switch (line-in or dock).  But again, I'll try and test later in the evening.

Thanks. Regarding the X3 (1st Gen). I know its not supported on the K5. I just meant that I could use the X3 as-is without the dock and still have X5ii as my portable... So the ability to "swap easily for portable" isn't really one of the selling points for the K5.

I'm a little worried, in general about the micro usb interconnections FiiO uses. On my X3II the socket feels poorly made and likely to cause problems sooner or later.
 
I'd feel happier if when I push a lead into it, it gave me a satisfying and reassuring 0% failure rate at first push. It doesn't get anywhere near that.
 
Perhaps it's just the micro usb standard that's poor. The mini usb standard wasn't as worrying. It felt built to last, shame it was over run by 'progress'.

Yes - agree to a point.  The X3ii can be a little tricky to dock at times.  The X7, E17K and X5ii are easy. Not sure if it is because of the X3ii's smaller size ....

Can you use this straight up as a headphone amp+dac connected to pc via usb Without docking any dacs?
Just the k5 and the PC with Sennheiser HD 650 will it work?

Not via USB - that would be a digital transmission, and the K5 has no DAC.  You could run line-in (into the rear input RCAs) though, and simply use the K5 as an amp. It wouldn't make a lot of sense buying the K5 then though - as the dock is actually one of it's main features.

Something's not right. Fiio states a distortion at 1khz of being less than .002%, not .02%.

Thanks for picking that up - it just means its measuring my sound card's limits rather than FiiOs K5.  I need to get a better external soundcard if I'm going to carry on measuring amps etc.  I'll alter the review.

Your review was great, much learned. Above someone asked the same question as I will ask.
I am looking for an amplifier for my Dt990s (I will use it as a desktop amp) and as I live in Sweden I don't have much choice other than certain Fiio products and not much more I'm afraid. So some guy on this forum recommended the K5 as I can get my hands on it here. What do you think? And I know the main feature is the DAC..
I did find a used O2 but all the ports in the front would be a bother as well as the built-in battery would start to leak someday if I just left it plugged in.

Actually the K5 doesn't have a DAC - so you'd need to dock a FiiO product in there to take care of the DAC part.  The E17K is a pretty nice option and combines well (in case you don't have one).
 
As far as the amp goes - it should drive your DT990 with no issues.  It handled my T1 (600 ohm) pretty well.  

Hi Broko, i just got am5 amp for my x7, i am using it with beyerdynamic dt 990 pro, sound is superb, but some of flac files are to silent, its also because i am using eq (V-shape), but the question is ... is the k5 dock better than am5 module in sound quality direction? Or just in volume amount? Thanks

The K5 has 5 times the output power of the AM5 into a 300 ohm load - so to drive the 990 (if you are finding it too low volume), then you need to look at a more powerful amp.  if you are using EQ, it will drop the volume level further

First, sorry my english is so bad, hope u can understand it..
Hi Broko, I have Fiio X3ii and I want to buy HD650. Can E17K handle it at amp power section? Or K5 is better?
 
Or I need more power to get best result of HD650 like Chord Mojo, iDSD or CyperLabs Piccolo? or any others idea..
Regards

Hi Avnel - your English is fine.  The E17K should handle volume OK, the K5 is a little better for a desktop setting simply because it will have higher voltage and current output. From my experience with the HD600 & HD800S - I would definitely recommend the iFi Micro iDSD if your budget stretches that far.  Very good DAC, great amp section.  It is my day to day rig.

what happened to the enterprise? I can't seem to find it online

I'll ask Lee.

Hi, great review!
 
I have the X3ii and the K5 and I'm trying to power my HE-400i headphones, but even with Spotify, Windows, and the X3ii (in docked DAC mode) all maxed out, I still need to go to about 12 o'clock on high gain before it gets loud. I don't have any way of measuring the dB or anything, but I was expecting a lot more volume from this amp.
 
Could the X3ii be throttling the max volume of the K5? Any reason why it should be quieter than the E17k in DAC mode? I think it's weird that the volume on the X3ii still works when the player is in DAC mode, and docked in the K5.

Go into play settings on the X3ii. Change the line-out mode to fixed level. The reason the volume is low is that you have the X3ii line-out set to volume adjustable, and volume isn't fully enabled.

I have a question, it may sound foolish but I felt so confused.
How K5 has the same output power (150mW@300 ohm) with E12 and A5 where it doubles them @32 ohm? It's also 20 Vp-p where those two amps are something like 15 Vp-p. 

You'll find its in implementation of the various OP amps.  The A5 is the more powerful overall - tells in the extra peak to peak voltage.  All the specs mean is that it has the ability to put more power into a lower impedance load, whilst still maintaining the high power output into higher impedance loads.  The reality si that for most headphones - even the 150 mW into 300 ohms is overkill