I recently bought the Sound Devices MixPre-D mic pre and like it a lot for most of its functionality. The main issue for me is running out of power. Because of the 48V phantom need, I'm only getting about 1.5 hours of time from the internal two AA batteries. Does anyone have a serious solution to this via a tidy rechargeable, cost-effective, beefy battery pack? I need at least 4-5 hours of record time. Thanks