Tony Allgood answers questions regarding chip selection in the MIDIdac:

User ?? U3 - OP77F @ $4.72/OP177G @ $1.56

TA:
I have found that the OP77 is getting hard to get.
Which is fine since the I also use the OP177... >

User ?? U13 - LF412C @ $0.77/LT1057C @ $4.50

TA:
Probably not worth paying the extra I would think.
I happily use LF412.
Somewhat better than the TL074 used in the Kenton Pro-2.

User ?? U11,14 - LT1013C @ $1.90/OPA2277PA @ $3.29

TA:
In the UK, its the other way around (prices).
I can't get the LT part without paying loads.
Both are suitable in this location, and both are excellent DC parts.