Dynamic mics are self-powered because they have a wire coil attached to the diaphragm and that assembly is next to a permanent magnet. So when the diaphragm is displaced by sound waves, that moves the coil in the presence of the magnetic field from the magnet and that induces an EMF (electromagnetic field) in the coil. When the mic is connected to a circuit like a recorder input, the EMF becomes an electric current (because now the electrons have some place to go

) and that current is converted into a voltage by resistance in the circuit.
The coil and diaphragm need to be a certain minimum size to generate a decent amount of current to get a good signal-to-noise ratio, so that's why dynamic mics are not small. One exception is bass drum mics because there is plenty of volume to drive the mic

.
If all or most of your older mics are electret condenser mics (you can usually tell by their smaller size compared to dynamic mics), then yes, they would need plug-in or phantom power to work.
I think you may need a bigger postage stamp now but that is A Good Thing.
