[2008-02-22]

Here is a nice simple way to add phantom power to a tube mic preamp. It uses a stack of 2 24V zener diodes to shunt-regulate the voltage. It also has a nice big resistor and a big capacitor after the switch that will give you a slow-rise effect so that you don't slam the mic with a 48V spike when you flip the switch. The bleeder resistor will also let the 48V bleed off when you turn the phantom power switch off, and don't have a mic connected.

This circuit could also be used with different zeners to make a 100V or other supply for tube mics. Heck, if you have a 100V tube mic, you could use a gas regulator tube instead of the zeners. That would be sweet. You could even make the phantom power voltage selectable between multiple zener stacks.