Thanks for that Stew. I get hiss to my 8-Ohm speakers with the MG3 using 24V power from either the AC adapter -OR- from my 25.2V LiPo battery pack. And given that 12V power causes no hiss from either a battery on the external power jack or from internal AA batteries, I believe the problem is tied to how much voltage I'm using, independent of where it's coming from. (So I don't think it's a grounding issue...)
I like brunk's suggestion, above, that we ask Jan if it's possible to adjust the MG3 gain, but again, I think we can trim that just by adjusting the voltage. I will test with 16.8V four-cell LiPo batteries this evening, but I don't have any five-cell LiPo packs that could get me to 21.0V. I'm resisting the urge to buy an variable-voltage DC power supply, because I would only use it for testing. I'd much rather come up with a battery solution (at a voltage that doesn't induce hiss with my speakers). For testing purposes, I can discharge the six cell pack no further than 18.0V, but the hiss might disappear somewhere along the way, between 25.2V and 18.0V. I can also try my XP8000 with the inline voltage regulators (switch-mode) that allow me to select 15V, 16V, or 19V. Test, test, test... And then I'll have to find the ideal voltage all over again, for using the MG3 with the LCD-2 via the impedance match...
Edited by zilch0md - 8/23/13 at 4:06pm