Fix DC interference with guitar pedals?

The DC adapter for my pedal board recently died on me for no apparent reason. I found that an adapter for an old BT home hub was an exact match except for the polarity so I cut the cable and connected them the other way round and it works fine, but when I use the pedals I get a relay high screeching sound. I know its the adapter because I tried with my friends BOSS PSA (All the pedals in my board are BOSS) adapter and got no interference. How could I fix this?