Can you run a disc man off of an AC DC adapter that puts out more power than is needed?
IF the voltage rating is the same, the output in current (most likely in milliamps or ma) of the adapter is relatively inconsequential as long as it's at least what the Discman originally required, or a bit higher.
For instance, if the Discman requires 6 volts at 250 milliamps (250 ma) and you have a 6 volt adapter that has 475 ma. available, you should be able to run your Discman off of it with very little problem.
Also be sure that the different adapter has the same polarity at the plug as the original though, or it could be catastrophic.
Hope this helps,