A good quality magnetic microphone is more better then a carbon microphone. The circuit below is a very small good designed and using few parts to convert a carbon microphone. The circuit contains only one transistor one capacitor and three resistors. The circuit takes a carbon mic
input and changes it in to the magnetic type. In this circuit you don't have to use ground connection even if you build this circuit in a metal box, magnetic element mic is a substitute for the carbon element. The circuit use minimum components so the entire converter amplifier can be mounted in original microphone unit to save Q1 from destruction the unit must connected properly for the first time. The Positive "+" lead which is going in Transistor Q1 collector should be connected to the microphone input that delivering a positive volt.