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Low Output impedance while powerful enough to drive demanding headphones (HD6xx & K712) to average level. Portable design do not harm the performance. In addition, replaceable OP-AMP let me have more fun on it.
Currently using Muse02 and found that the sound can be much more Vocal oriented, and one of my friend comment is "have the feel of listening to tube amp more than some tube amp" . Highly Recommend owner of O2 who want would like to look for new feel of the O2.
Edit: In addition, also tried O2+Muse02 to drive Massdrop Plus and IE800, which are both absolutely fine.
Is it easy to replace the OP-AMP?
sort of yes, just un-screw that 4 screws from the volume knob side and you can pull the whole board out.
Just to beware a few things:
(not sure how you familiar with op-amp, for safety please bear with me as once I almost burnt my Sennheiser IE800, or simply jump to point 3 and refer to the picture):
1. you will need a 2ch op-amp (e.g: Muse02, OPA2228)
2. beware the gain ratio
3. physically the op-amp is between the caps, so my solution is simply add one more socket (on the left there is a muse02 and a bundled OP-AMP socket, get one if yours OP-AMP didn't bundle) and stacked them all up, so you can easily change it in future.
Even stacked, the chassis is still high enough to not touching the op-amp.
The red arrow is the op-amp.
Thanks for the explanation! However what do you mean by 'beware the gain ratio'? I have basic knowledge in Electrical Engineering so you can use some technical terms here.
Also I think there are more than 1 OP-AMPs in the circuit. Did you replace all of them or you only replace the one with the red arrow?
Thanks! i was about to swwap some opamp but realise there are 4 and not sure which one to swap. thanks for point it out!
Btw what are the other 3 for?
I am not totally understand neither.
However, my friend who solder amplifier himself did saw the schematic and told me, among the other 3, 2 are in the same direction, which is controlling the output power.
The one on the rear most is for battery management, never touch it as it is not an opamp.
Forgot to mention, I did tried to swap all 3 opamp to Muse02, and found that the sound is so "thick"/"sticky" (sorry i don't know how to describe that feel, but it is no way i call it good after such change.)
Muse02 is not cheap even when I get it in Japan last year(2017 Mar), so better save money on just getting 1x Muse02.
Cost of additional 2x Muse02 can buy you a lot of other 2channel opamp to try out, which I think you will have more fun and allow you to change opamp to co-operate with different headphone.
Notes for who wanted to run Muse02 on battery: Muse02 use more battery than the stock one (i can't remember, should be NE5532), for me it is still acceptable, charging every 1.5weeks with average 1hour listen daily.
just found the link of the designer comment about differet opamp, most are very low cost.