Microphone Mythbusters, Vol. 9 – Do Wireless Bodypacks Have Phantom Power?
By Gino Sigismondi
In Microphone Mythbusters, Vol. 9, Gino Sigismondi answers this questions, “Do Wireless Bodypacks Have Phantom Power?”
The short answer to this question is “no.” The long answer is “not exactly.”
It’s a fairly common misconception that a bodypack wireless transmitter must have phantom power present at its microphone input, otherwise, how could condenser lavalier or headworn microphones work with them? It works because most condenser microphones that are meant to plug directly into wireless bodypacks do not utilize the preamp that is required when they are used in a wired application. The preamp is the portion of the complete condenser microphone that requires phantom power. Without the preamp, only a bias voltage is necessary for the microphone to work, and this is typically supplied by the bodypack transmitter. Let’s take a look at the difference between phantom power and bias voltage.
Phantom power is a dc voltage (11 – 52 volts), which powers the preamplifier of a condenser microphone. Phantom power is normally supplied by the microphone mixer, but may also be supplied by a separate phantom power supply. Phantom requires a balanced circuit in which XLR pins 2 and 3 carry the same dc voltage relative to pin 1. So if a mixer supplies 48 volts of phantom, XLR pins 2 and 3 of the microphone cable each carry 48 volts dc relative to pin 1. Of course, the mic cable carries the audio signal as well as the phantom voltage.
Mixers that supply phantom power contain current limiting resistors that act as control valves. If the microphone or cable is improperly wired, these resistors limit the flow of current to the microphone and thereby prevent damage to the phantom supply circuit.
A balanced dynamic microphone is not affected by phantom power. However, an unbalanced dynamic microphone will be affected. Although the microphone will probably not be damaged, it will not work properly.
Bias is a dc voltage (1.5 – 9 volts typically) that is provided on a single conductor. Unlike phantom power, bias does not require a balanced circuit. Bias supplies power to a Junction Field Effect Transistor (JFET) connected to the output of an electret condenser mic element. The JFET acts as an impedance converter that is a necessity in any microphone design that uses a condenser element. A condenser element has a high output impedance (>1,000,000Ω). The JFET input loads the output of the condenser element with an even higher impedance (>10,000,000Ω) to minimize loss of signal level. Also, the JFET output provides low source impedance (<1,000Ω) to feed the microphone preamplifier or bodypack transmitter.
Practically, this means that handheld, gooseneck, or studio microphones that have integrated preamps cannot be connected directly to a wireless bodypack. An external, battery-powered phantom power supply would need to be connected inline, before the bodypack, to properly power the microphone. However, most condenser microphones with detachable preamps will work just fine when connected directly to the bodypack, without the preamp. Note that when using a microphone from one manufacturer with a transmitter from another manufacturer, the connector and/or the wiring configuration will likely be different. Always check with the respective manufacturers for assistance. Note that in some condenser microphones, the bias voltage must be supplied on the same conductor as the audio. Condenser elements with a built in JFET use this configuration and employ a single conductor, shielded cable. Other condenser microphones utilize separate conductors for bias and for audio.
A dynamic microphone can be connected to a wireless bodypack without issue, provided the bodypack utilizes separate conductors for audio and bias. A dynamic microphone should not be connected to an input that supplies bias voltage (such as a wireless transmitter) because the audio and the bias voltage will travel down the same conductor. If this occurs, the frequency response of the microphone may be altered or the audio signal distorted. If a dynamic microphone must to be connected to input with bias voltage, a blocking capacitor must be used.
Remember, in a typical electret condenser microphone, it is the JFET that requires unbalanced bias and the preamplifier that requires balanced phantom power.