The powered speakers need a power supply for the amp. Based on what I see for external speakers that is typically in the 5 to 9VDC range. If you have a headphone jack you may be able to drive a 600 ohm speaker directly from that, although how much volume you will get may be an issue. If you post some details about the tablet computer you are building you may get more help. The voltage of the tablet battery would be good to know since using that to power the speakers saves having another battery in the case.
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The powered speakers need a power supply for the amp. Based on what I see for external speakers that is typically in the 5 to 9VDC range. If you have a headphone jack you may be able to drive a 600 ohm speaker directly from that, although how much volume you will get may be an issue. If you post some details about the tablet computer you are building you may get more help. The voltage of the tablet battery would be good to know since using that to power the speakers saves having another battery in the case.