Pedal Power Squealing Noises (Troubleshooting Help Request)

[Big Monk]

I am having an issue that is likely very simply but i can't seem to figure out how to address it.

I have a pretty basic pedal wal-wart with a daisy chain connector. My son and I both practice through small Marshall MS-2 mini amps. I finished up a TB MkII build last night and when I plugged an MS-2 and the TB MkII together, I got no output but a high pitched squealing noise. With the MS-2 on battery and the TB MkII plugged in, no squeal. TB MkII on battery and MS-2 plugged in, no squeal.

Any ideas what is going on?

[duck_arse]

are you plugging a positive ground pedal into a daisy chain with a negative ground pedal, and shorting/shutting down the power supply via the audio cables, perhaps?

[Big Monk]

are you plugging a positive ground pedal into a daisy chain with a negative ground pedal, and shorting/shutting down the power supply via the audio cables, perhaps?

No. The MkII is negative ground. The Marshall MS-2 Mini-amp is also negative ground. They are not playing nice on the same supply. Major squealing.

As a reference point, I can use my Wah, Tremelo, etc. with the same supply and the Marshall and get no squealing at all.