I had the AHA120 (unit number 10 I think), it is good but there is a roll-off in the highs. Makes the treble peaks of the K70X disappear for example, not a problem for everyone I reckon. However, I wouldn't put it in the portable amp category, given the size.

That said, it's built like a tank, has a great battery, and is transparent. On top of that, Justin is a nice guy to deal with, and is knowledgeable too.

MP, make sure you don't confuse the AHA120 and the µHA120. The latter is smaller, but not as good as its big brother from what I heard (Class A vs Class AB).

I was under the impression that the O2 was really great bang for the buck (its about 150euro's here). So I ordered one from the UK (build time is +-3 weeks), hoping to use it as a transportable rig with my HD650. Now I'v read from multiple people that the O2 is not that good with the HD650  .

I can still cancel my order if I am in time, but ofcourse that leads to the follow-up question:

What is a better portable amp for the HD650's for +-150euro's?

I do not need a DAC, I just need an amp. It may be battery powered, but it is not necessary. Its primary use is for the HD650, classical music, but some pop/rock also.

Any help is greatly appreciated!!

-Victor 

I haven't heard any negative things about the O2 + HD650's synergy specifically. If I had to guess, I'd think the combo was decent enough since the O2's harshness would sort of balance out any perceived veil on the 650s.

In general the O2 is a good amp for what it is. It was designed to be linear and powerful, and in that sense it succeeds. However it's rather unrefined---particularly in the treble---and sounds pretty two-dimensional. It's also not the last word (even in its respective price range) on low level detail despite people trying to argue that the THD measurements show it is.

Really, it just depends on your priorities and what you want. At the price range you've indicated I think the O2 is pretty competitive. There's the upcoming C5 from nwavdeuche JDS Labs. Personally, I'd recommend the Leckerton UHA-6S:

http://www.leckertonaudio.com/products/uha-6s-mkii/

It has a DAC built in, but you can bypass it. At that price it's still a bargain for the amp itself IMHO. You can also roll opamps if you want to tweak the sound a bit. It'll be more portable than the O2 as well.

If you don't have to have a battery, you may want to consider the Schiit Magni.

Hmm, it seems the iPad works great with the CCK, and can indeed stream its data via USB to an outboard DAC. The iPhone's however cannot (some say the iPhone 4 + jailbreak can).

What do you guys use as a portable player? Any portable player's out there which you can connect using a USB connection to a DAC?

Makes me want to listen to my stuff, which I still haven't touched since October last year...

I've swapped the opamp in Leckerton to ADA4898-1, the successor of AD797. It's a great success, noticeably better than OPA209 that was in before - and most importantly, immune to GSM noise.
Available from most Element 14 stores, like Newark (only dual ADA4898-2 right now, will need a different adaptor) and Farnell for cheaps - $6 or so. Half as much as AD797ANZ or AD797ARZ.

The sound is as spacious as OPA209, if not slightly more so. Definitely not like the mid-biased AD8610, maybe close to AD797, but I haven't heard that in the UHA-6. Excellent microdetailing, slightly but noticeably better than OPA209. Blacker background. Balance is almost the same. Definitely sounds like BJT opamp, which it is.
I can definitely say "wow". UHA-6s mkII was already excellent, now it's better still.

Lower noise floor in high gain modes. Better 2nd and 3rd order harmonics by ~2 dB. (others aren't measurable by my old 107 dB SNR analyzer) Better crosstalk performance at all frequencies, by 5 dB at low end to 13 dB at 20k. Frequency response is more linear, with less highest end roll by 0.5 dB - not that it should matter. Likely due to slightly lower output impedance of the opamp and/or higher current capability.

I can't even utilize it to the fullest potential. I couldn't attach the pad below it that's supposed to be connected to Vss- (generic DIP adaptor doesn't have a pad) and the amp supplies only 6.3 V, which is low for this opamp (rated 5 to 16V). It also has a power down pin, which is not used by the amp - and could save battery when there's no input.

For the curious: http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/ada4898-1/products/product.html

I've also learned that one of my probes (for the scope) has almost 2.5 uF capacitance - which drove the amp unstable in low gain mode at around half volume - until I've swapped the probe to something reasonable with 30 nF - which is still huge, way more than any IEM or headphone should be bar silly crossovers or filters. Not even SE-5 has such capacitance.
This could be fully averted if there was a tiny (2-3 pF) capacitor in parallel with the opamp's feedback loop, but Leckerton UHA-6S mkII doesn't have one and I didn't have time to make a custom DIP adaptor, used a standard one.