Radio Shack SSM-50 Stereo Sound Mixer AC adapter?

I have a Radio Shack SSM-50 Stereo Sound Mixer that I got about 13 years ago. I put it away when I changed over to PC, but I recently decided to try to integrate it. The original adapter is missing and my local store says they'll need voltage, amps, and polarity before they can recommend a replacement.

It says on the back that it's 18V but it says nothing about amps or polarity. Can anyone who has one of these units please tell me what it says on the adapter?
 
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