Weird possible grounding problem with a portable battery pack?

[robj144]

Seems like my stuff is just breaking down (see my keyboard thread in the Open Mic).

Anyhow, a couple of weeks ago, I got a Vox MV50 mini head and the tiny cab from a Sweetwater deal. I also have a portable battery pack with AC out to power my astrophotography gear. I had previously tried to use this battery pack with my Boss Kitana a few months ago just to see, and there was a horrible hum which seemed to be a grounding issue. Anyhow, it was not a big deal with the Kitana. But, with the portability of the Vox, I was hoping it would work. Hooked it up to the Vox and it was perfectly clean with zero hum. And, I mean absolutely no hum. The Vox has a transformer/converter and it's not just a single cable like the Kitana, so I figured that was why it was clean.

Used it like that for about two weeks being perfectly clean. The other day I turn it on with the battery pack and, all of a sudden, there's the same hum as the Kitana (I swear it was not there for the previous two weeks). First thought was a cable was going. Replaced every cable and nothing changed at all. Then, I thought it was the battery pack. Borrowed a friend's the other day and it's exactly the same hum with two different battery packs. Then, I thought it was the cable which plugs to the wall... changed it with another one I had laying around... nope.

This seriously makes zero sense. I don't know how it was perfectly clean at first and now hums. By the way, it does not hum at all connected to a wall outlet. It only hums running off the battery pack now.

Anyone have any ideas or experience???

[jonfields45]

I have found using battery powered gear when some of my signal chain is ungrounded AC powered, your typical stompbox power supply, is hit or miss for hum. Are you entirely off the grid when you are having a problem?