The Master module has 2 jobs, bringing power to the AE Modular, and being a Midi/CV converter. It is possible to have an AE modular without a Master, you just need a suitable 5V supply (the Master module converts the 9V supplied to 5V for internal use). There is a 1 unit module in the Tangible Waves shop for precisely this purpose.
There are various synths and keyboards/ sequencers available with Gate/CV outs but you will need to keep an eye on the maximum voltage they send; the AE modular will be damaged if provided with more than 5 volts, (or negative voltage). This problem can be avoided by use of the 4 I/O module, or a Midi synth/keyboard/ sequencer and the Master module.
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These merchants probably sell this module. Huh?