For a typical two wire lavaliere mic that specifies 5 Volts, the manufacturer is actually assuming that the transmitter has a 5 Volt supply in series with a bias resistor of 1k to 5k, depending on the brand of transmitter. The actual voltage at the mic will be 5 Volts minus the drop across the bias resistor. For example, a mic that is listed to draw 500 uAmp would produce a 2.5 Volt drop across a 5k bias resistor. The mic would only see 2.5 Volts (5V minus 2.5V drop). A different mic that pulled only 100 uA would see 4.5 Volts. So for most all transmitters, the voltage to the mic is all over the map. Generally the mics still work, because they actually can handle a wide range of voltages.
All the current Lectro transmitter models, such as the LMa, have a servo input that regulates the bias voltage to exactly 4 Volts under any condition of bias current. The voltage is set to 4 Volts by using the pin 2 to pin 4 wiring. This allows us to handle a wide range of microphones with any current draw with no concern about excessive voltage drop across the bias resistor and is unique to the Lectro transmitters. We chose 4 Volts because this was a typical design voltage and all the professional lavaliere mics we looked at worked very well at that voltage. The one exception is the tiny Countryman B6 and E6 models which require 2 Volts at high current. For the Countryman mics Pin 4 is NOT connected to Pin 2 and this sets the servo input to a regulated 2 Volts which is ideal for those lavaliere mics.