I would likevto know if the K1803 preamp can be used with a electret microphone? I need to amplify it’s output to drive a audio amplifier. Will this unit do that?

Yes, normally this should work. Ofcourse you need to supply a certain voltage to your electret microphone if it hasn’t its own battery.

Thanks, but now another question - how do I supply voltage to the mike?

That depends on your microphone. Normally it is provided through a resistor connected to the power supply. Value: depends on microphone element but is rather high: 100k…1M.