Introduction
Phantom power, a staple of the audio industry, is an essential element in the operation of many professional-grade microphones and other studio peripherals. Almost universally supplied at a voltage of 48V, it can also be found at other voltages depending on the device’s needs. This article will delve into the intricacies of phantom power systems and peripherals, exploring the mechanisms of how DC power is applied safely, electrical protection provisions, limitations, and more.
Understanding the Flows
Phantom power is designed such that the audio and DC powering signals are basically orthogonal: the two paths should have minimal interaction if managed correctly. The audio signals (sent from the remote source) are AC coupled and are driven in a push-pull fashion down the differential twisted pair in the cable; the DC positive phantom voltage – 48V – is split and “resistively” fed equally down the twisted pair cable (pins 2, 3), but its return current (back to the source) is via the pin ground connection (pin 1). To make this all work takes careful design and specification of the mic amp circuitry, as well as the remote source or microphone.
The Electrical Connection
Electrically on the sending side, phantom power is usually delivered by two 6.81kΩ resistors, each connected to one leg of the balanced audio line (pin 2 and pin 3 on the XLR connector) and the other ends to a low noise, stable +48V power source. This forms a resistive network that supplies current evenly down into the two signal lines.
On the receiving (but really the audio source!) end, usually a microphone, the phantom current is used to power the internal preamplifier circuit. The DC power is picked off by the microphones internal power management circuitry, with power consumption set such that enough current is taken to run the preamplifier, but not too much current is used, which would limit the differential audio output swing.
Current Delivery in Phantom Power
The amount of current a phantom power supply can deliver depends on the resistive load presented by the microphone. However, most condenser microphones only require a few milliamps of current. In the extreme, if both pin 2, pin 3 of the XLR is shorted to GND (pin 1), the maximum current flowing as a result of this would be of the order of: (48 / (6810/2)) amperes; or around 14mA. The resultant voltage at the pins will be very near GND potential, so phantom presents little in the way of safety risk to humans, tending to fail “safe” even if abused.
Electrical Provisions in Phantom Power
Audio equipment that provides phantom power should be designed to handle scenarios where connected devices do not require – or may be damaged by – inadvertent application of the bias. For safety, such equipment often includes input blocking capacitors on the mic / line input sections, which prevent DC from affecting the audio processing. With the current limiting series output resistors, there is an inherent current limiting if phantom is inadvertently applied, preventing damage to most well designed output stages of prosumer equipment.
Withstanding Phantom Power in Non-Phantom Powered Devices
Many devices connected via XLR cables do not require phantom power. For example, dynamic microphones do not require an external power source, but most will operate with or without phantom power enabled. Other equipment that could be plugged in via XLR – say a remote, self-powered mic amp or a piece of Audio Outboard rack gear – may well have an electronic balanced output stage that does not need phantom power to operate, but is now exposed to an operator incorrectly applying it. In these instances, it is essential that such remote devices incorporate output stage circuitry to cope with the possibility of accidental exposure to phantom power. Traditionally handled by designing in output transformers, this is often achieved in modern equipment through (suitably rated) DC blocking capacitors on each output, which blocks the DC phantom power voltage, while allowing the low impedance AC audio output signal to pass.
Upcoming Universality for Phantom Power
Phantom power is a robust, simple and well-designed technology that has seen both much success and little variation over many decades within the Professional audio industry. By understanding how the DC voltage is applied to the XLR, the current delivery, and safety measures associated with phantom power, audio professionals – as well as Prosumer enthusiasts – can maximize their sound quality while ensuring their equipment is protected.
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