As far as I can tell, many digital-to-analog converters do not include operational amplifiers on output lines. If you want them, you need to use this mod. Keeping with our no-limits approach, I’ve decided to add them to all our input and output lines. On the output side, the AD797 seems to be the default option with the ES9018, but I’m wondering if an OPA627 or OPA637 could be more suited.
In our initial design, the operational amplifiers will be used by default. But in future revisions, we will allow them to be by-passed, most likely by using some relays. This will be especially useful when connecting microphones to the analog audio inputs.