Modification done.
Modifications:
Changed opamp to opa2156
Changed 1k and 3.3k resistors at feedback network to 220ohm
Changed volume potentiometer to bourn 1kohm
Cosmetic change: the original volume pot used D shaft, not compatible. Adopted the original o2's pot cap.
Performance change: lower THD at high gain. Lower noise. (really no other performance change eg: loaded thd at low gain)
Subjective change (This is all in my head and not really happened!)
Original atom sounds energetic and pushed out. Both ends are not well extended or controlled. Can be a little bit veil and thick. Little bit congested in the mids. Soundstage is very wide so that's good.
After modification, the change is made to the whole spectrum. Bass is tighter fuller but more controlled, less thick and more forward. Imaging is better, clearer and more stable. Highs are a bit more cleaner and clearer. Not really more highs but you can hear more clearly. Transparency is better. Mids are more controlled but now has a bit of glossy type of sound, very smooth.
At the end: if you believed me, please dial the difference to the minimum you possibly can. The differences are subtle but consistent.
Thanks.
Edit: one this really annoyed me was that the volume pot is balanced only after 10 o clock. New pot balanced after 8 o'clock.
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