I want to build an oscilloscope preamp and wonder what sort of op amp should I use? What's the maximum input level a smartphone can handle and should I bias the signal (for instance -5V/+5v to something like 0-1V)? What means of circuit protection should I use? Can I use diodes?
The microphone input will be sensitive to signals of millivolts, and definitely should be limited to 1V or less. It will probably be AC coupled internally.
See http://www.instructables.com/id/A-Preamplifier-for-Smartphone-Oscilloscopes/ and http://hackaday.com/2012/07/14/android-oscilloscope-built-from-parts-just-laying-around/ ; the latter of those is just an attenuator, relying on the built-in preamp. The former adds a unity gain buffer as an impedance converter, and will probably produce better results. In both cases it's only suitable for audio frequency ranges.