Quoted: <IMO, JRC4558 can be replaced, to NE5532 if you want go for budget one. That cheapo op-amp are some better than JRC4558, for better ones try go for LME49720 or LME49860 depends on the power supply voltage. Other Burr Brown op-amp such as OPA2604 or OPA2134 can try out also but i personally not like it for swapping purpose, coz it does not sound like the way it like.>

Thanks for your suggestions! What do you think of S5532(1986/87/89), OP275 and AD827SQ/883B(quite expensive...)? Between S5532 and OP275, which one is better? One more thing, ACS 45.1 runs at a single 12V power supply(correct?).

The limitation of the source? Is it the TDA7370B, or from the sound card, the computer? So, for fun purpose as well as improvement, what else cound be done on it at this moment?

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Added on May 5, 2011, 4:19 pm

Yes, S5532 is NE5532. It has a big "S" before the printed model number, and they were manufactured in the older days, someone said those older-manufactured stocks is good at sound. By the way, I have no idea of it.