Mike,

If it's a Vocalist VR (you didn't mention the model), then the power unit is a Harman Pro Group PS750 9.75V AC, 820mA power adapter.

Also, it should list on the back of the unit next to the power input "9.75 - 820 mA".

You find the user manual (PDF) here : http://www.digitech.com/discontinued.htm

Gr.
John

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Have a nice day