Summary: ...and that's how they get you. The Sony brand adapter is listed at $39. You can buy an universal adapter for under $20(the sony is rated at 9v and 800mA), but there is a catch. The universal adapter's plug is too long! It won't make the proper contact with these speakers. But there is a way to make them work. Find the right adapter plug and put a little piece of tinfoil in the hole of the adapter---don't put it in the speaker hole.
Summary: These speakers have been trying my patience from the start. Direct mode (no power source) is useless unless the speakers are next to your head. The moment I added a power source (either with batteries or the Sony AC-E90HG adapter), the buzzing noise was present and would not stop. It seemed to be related to the non-removeable stereo cable. The buzzing was problematic enough to send these speakers packing after only a day.