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Originally Posted by iceyfw
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That looks suspicious--HiFiMAN products always seem to be focused more on the boutique audiophile folks, and they end up having bad design and lots of measurable problems. Also, that DAC has an NJM4556 to start with, and you replaced it with an op-amp that's designed for extremely high-frequency tasks. Yeah, you could use an AD797 for a preamp stage in a speaker rig, but even Analog Devices claims it's been mostly used in non-audio applications.
People like to replace the NJM4556 because it costs 60 cents and they think that means it's crappy, but it's dumb. That opamp is specifically designed to drive headphones--it's one of the few opamps that measures well into loads below 150 ohms, and it's extremely high-current (70mA). The AD797 manages 50mA but not into headphone loads I'm sure.
These days, I pretty much don't trust a DAC or amp to be decent unless it's made by me... or Violectric, Benchmark or Centrance. Working on putting together an Objective2 and ODAC combo.