Airplane Seat Power Ground Loop?

I’ve experienced the following on several flights, 787’s mainly:

I plug my laptop into the seat power outlet.

I plug in my Audioengine D1 dac.

I plug headphones into the dac.

If I am HOLDING (touching) the dac in my hand, it sounds great.

If I do not hold/touch the dac, there is a hum/noise.

The hum disappears if I touch the dac.

The hum disappears if I unplug from seat power.

What is going on and how can I stop this?

My computer battery cannot hold up enough for long-haul flights and it's impractical to hold it all the time.

Thanks.

Thanks!