Originally Posted by RAFA
I think, the higher the slew rate, the faster or more detailed the sound is.
Why rail to rail?
Not so much about detail sound - you want it not to be slew rate limited, but excessively fast slew rate doesn't do you much good. That is, you need it to be fast enough for music. Faster doesn't make much sense since we don't need it to be fast enough for ultrasound range and slower is definitely not going to be good.
Rail-to-rail means the output of the opamp can swing very close to the supply voltage rail. Supply voltage in X3 is around +/-5V ~ +/-5.5V. So in theory, the stock AD8397's output can swing around +/- 5V (which is already more than X3's output volume in high gain) in full without clipping or inducing any noticeable noise. AD8620 on the other hand isn't rail-to-rail, so it will swing in much smaller range. If you have low sensitivity load that requires you to push for very high volume, AD8620 will begin to clip and distort. That's the downside of using AD8620 as it doesn't do rail-to-rail and doesn't have that much current output to deal with low sensitivity and low impedance load. But given those are more extreme cases, and usually not the kind of headphones I will use portably, AD8620 will still serve my purpose. Having a rail-to-rail opamp just gives you a bit more versatility when it comes to different headphone load. AD8397 in this case is actually technically a really good all-in-one solution. I would have kept it if not fro the obvious warmness. The way I see it is, if I really want to use a difficult to drive headphone, I'll just carry an amp with me. That's why AD8620 suits me just fine.