Those GC phantom power adapters appear to be a real problem. One of my collaborators bought one to power his mics, so that he could record with his 788. GC sells them without the wall-wart power supply as a matter of course: that's an added-cost *option*, according to their web site, for another 10 bucks. He brought it into my room, and I loaned him one of my 12V wall-warts, without knowing what it was for. Unfortunately, the adapter was set up for the wrong polarity (positive on the center pin), so
the phantom box *instantly* fried.
He took it back and exchanged it for another one. Tried another 12V adapter when he got it home, which was _also_ set up for the wrong polarity: smoke 2.
He took the carcass back in and after much argument, exchanged it for a Behringer 602, which is good enough for his purposes.
Here's what I know about the GC power adapter, which is actually a Rolls PB23: A) they appear to be sold incomplete as a matter of course, and B) I've never yet heard of one actually working, as a result. According to the Rolls web site, it is supposed to come with its own wallwart. The schematic is on their site. It's just a very simple voltage quadrupler.
Regrettably, they also designed it to use a pretty nonstandard polarity. It wants the center pin to be *negative*, and the outer ring to be positive- be forewarned, and forget using most common adapters. They didn't even design in protection diodes to make the thing reverse polarity proof. It just has a simple crowbar diode, so the question is whether the board or the wallwart will smoke first! So you have a greater-than-50% chance of destroying the damned thing the first time you use it, unless you use great care in picking your wallwart. Problem is, most normal human beings don't take the time to read the schematic first before trying to plug and play...
In my humble opinion, it is junk. You would do better to get a small mixer, or a standalone preamp with phantom built in- like the Midiman Audio Buddy. The only problem with the Audio Buddy is it only puts out 24V phantom- so it may not properly run many mics...