Edited in response to ThreePhaseEel's note below.
Can I simply try to find a way to wall mount one of these female adapters into the wall, and then run 18 gauge wire from the back of this device to the back of the amp? Not sure how I would wall mount this female jack, and I presume I need to do it in a box, but all that is doable if I can find the right mounting system.
I have wired speakers in my wall. I bought a small amplifier that can be installed in the wall in a 1-gang box. I posted a link below. The unit is 12v DC and can be powered in either of two ways -- there is a jack in the front for a plug in adapter that comes with the unit, or it can be hard wired with DC 12v power in the back. I'd like not to have a cord running down my wall, so the question is, how to power it from the back?
I was thinking there are a couple of options. First, I don't mind having a 12v plug down lower in the wall behind a tv stand. So, I was wondering if there's a way to wall mount a DC power input jack next off the floor near an existing plug, plug an adapter in there, and then just fish 12v wire behind the wall up to the back of the amplifier. But I can't seem to find anything like a 12v wall mounted input jack. It looks as though there's no code problem running 12v wire behind drywall.
Option 2 would be to figure out a clean way to install a 120 to 12 power transformer and then run DC wires from the transformer through the wall to the back of the amp. But I can't figure out the right way to do that either. There are a couple of nearby outlets to take power from if I could figure out a clean solution for my transformer, but I'm not seeing anything. There's one company that sells a small transformer that can supposedly fit in the back of a 1 gang box, but I'm skeptical that it's ok to put a transformer in the wall, even in an accessible junction box, and it does not look like the device is UL listed. I do not have an attic or basement. Have to do everything inside the walls. Is there a transformer that will fit inside an old work 1 gang box? I'm ok having a blank cover on the box to make it accessible. Anything else I could do?