Hi because i'm considering to buy an amp to replace my O2 (i'm tired to recharge it..) I was considering to buy the G103 s (budget reasons)
I would use it with my headphones : AKG K 702 - Beyerdynamic DT 990 PRO 250 oHm, Sennheiser HD 600
Plus I can easily get it in italy where I live.
But the big question ,is it good enough?
2nd I keep reading that It got 2 settings : hi - lo but I really cant see any switch on the amp
Can you help me please?
If you're interested in the G103S, then your budget is ~$290 USD / 275 euros. I haven't heard to the G103S, but I own (and really like) the G109-A, the now discontinued "Anniversary" edition of the G109-S (same power & basic design, but w/soft-start relay and a couple other tweaks that are welcome improvements).
Here's a review that compares both basic models. I always found this particular review educational re the benefit of stepping up from bottom model to next up:
http://www.innerfidelity.com/content/lake-people-g103p-and-g109p-headphone-amplifiers#RUhd4EV5L6w2Vsv8.97
Back to your real question: is the G103S good enough? (ie, sufficient for you; for your headphones, etc). The only person who could answer that is you, after hearing it. But just based on specs, I think the answer is yes. The G103S almost certainly has the power to drive the headphones you list.
The question then becomes: does it have the power to drive those headphones
very well, getting the most out of their particular designs? And the answer to that is probably, "Not quite." Particularly the HD600 is said to be, not exactly difficult to drive--but difficult to pair up w/a given amp for maximum sonic benefits. It's a high resolution headphone and different amps can bring out different things from it.
In my experience, amps are a little bit like cars: you can do well buying the cheapest, lowest power car (especially if it's well made & designed); but taking that car through the Alps might be a humbling experience...usually you benefit from having some "power in reserve." This quality of power in reserve proves extremely useful in headphone amps.
My guess is you would be happier w/the G109-S, based on my experience of the model: it sounds good on every headphone I've ever used with it. Other amps (far more powerful, more features) may sound subtly better in certain ways (particularly soundstaging and note separation), but literally ever sounds bad or even "just average" on the G109. And I'm including a power-hungry, high resolution planar, the ZMF Ori (it sounds terrific on the G109-A).
However, the retail cost of the G109 is probably over your budget (~$450 USD, after 15% winter discount). So my advice is to look for a used G109-S, which is probably easier to find used in Italy than it is here. It would probably be the equivalent of ~$350-$400 USD.