I'm trying to hook a cheap lavalier microphone to a Behringer HA400 micro preamp. The input line does not provide any power to the microphone. The microphone works fine when powered by a sound card with 3V in a PC. How do I add 3V to the tip of a 3.5mm jack? I don't need a filter, amp (that's what I'm hooking it to...) or anything else. Just plug in power.
The standard electret bias network is simply a 1.5V - 3V battery with a 2k2 or so resistor in series.
Connect the sleeve connection to ground and connect the free end of the resistor to the plug tip connconnection. The tip connection also feeds the preamp input.
You may need a DC blocking capacitor in series with the preamp input. Most preamp inputs contain their own DC block capacitors internally but some don't. A capacitor value anywhere from 1uF or so is good: (+) side of the cap goes to the mic / bias resistor; the (-) side of the cap goes to the preamp input.