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Listent to the XioSynth on these :
Novation XioSynth Description
Compact MIDI controller that also packs an 8-voice polyphonic synthesizer with arpeggiator, and an audio interface. Interestingly, it can operate as both a synthesizer and controller at the same time.
In 25 or 49-key format, it powers off batteries (6xAA ), USB or mains. Costs peanuts! Nice all-in-one laptop accessory for the mobile musician.
Size - 25-key: 468x68x190(mm) - 49-key: 798x68x190(mm), weight: 1.4 and 1.6kg respectively