Hi burdie,
You do realize that you're actually rolling +/-22V OPAMPs with +/-17V OPAMPs (LME49720)? HPA-3B has
NJM7818/
NJM7918 (JRC actually) power regulators installed and these two are regulating a power voltage of +/-18V, so the two audio dual-OPAMPs from inside HPA-3B and also all output transistors are actually powered by +/-18V; this is why manufacturer choose
LME49860, because there are not many OPAMPs out there designed to work 24/7 at +/-18V (36V) voltage (Violectric/Lake People choose LME49860 from the same reason, but also because the sound is very good when used as voltage gain OPAMP). OPA134/2134 might handle +/-18V as well, but this is max. recommended voltage for them, like LME49990.
Feel free to use
LME49990 if you like them, but at least place a decent heatsink on top (glued with Arctic Alumina or with a good heat transfer tape). Their max. recommended voltage by manufacturer would be +/-18V.
As for the
LME49720HA, I would strongly recommend you to take them out from HPA-3B, because you'll most likely kill'em slowly...sorry.
http://www.ti.com/lit/ds/symlink/lme49720.pdf - their max. recommended voltage by manufacturer would be +/-17V, despite their
absolute max. voltage of +/-18V. Also, "(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur", so keeping LME49720 for long time at +/-18V would really not be a very good idea.
Thank you,
Raul.
P.S.: I like the sound of this headamp: it's strong and powerful and can drive all phones around here, balanced and unbalanced as well. Quite decent construction I'd say and good sounding amp, strongly recomended for its price.
L.E.: Adding a couple of pictures with the 2 regulators:
