An extract on #phantom
A phantom group is composed of three circuits that are derived from two single-channel circuits to form a phantom circuit. Here the phantom circuit is a third circuit derived from two suitably arranged pairs of wires, called side circuits, with each pair of wires being a circuit in itself and at the same time acting as one conductor of the third circuit. The "side circuits" within phantom circuits can be coupled to their respective voltage drops by center-tapped transformers, usually called "repeating coils". The center taps are on the line side of the side circuits. Current from the phantom circuit is split evenly by the center taps. This cancels crosstalk from the phantom circuit to the side circuits.
Phantom working increased the number of circuits on long distance routes in the early 20th century without putting up more wires. Phantoming declined with the adoption of carrier systems.
It is theoretically possible to create a phantom circuit from two other phantom circuits and so on up in a pyramid with a maximum 2n-1 circuits being derived from n original circuits. However, more than one level of phantoming is usually impractical. Isolation between the phantom circuit and the side circuits relies on accurate balance of the line and transformers. Imperfect balance results in crosstalk between the phantom and side circuits and this effect accumulates as each level of phantoms is added. Even small levels of crosstalk are unacceptable on analogue telecommunications circuits since speech crosstalk is still intelligible down to quite low levels.
Condenser microphones have impedance converter (current amplifier) circuitry that requires powering; in addition, the capsule of any non-electret, non-RF condenser microphone requires a polarizing voltage to be applied. Since the mid- to late 1960s most balanced, professional condenser microphones for recording and broadcast have used phantom powering. It can be provided by outboard AC or battery supplies, but nowadays is most often built into the mixing console, recorder or microphone preamplifier to which the microphones are connected.
By far the most common circuit uses +48 VDC fed through a matched pair of 6.8 kOhm resistors for each input channel. This arrangement has been standardized by the IEC and ISO, along with a less-commonly-used arrangement with +12 VDC and 680 Ohm feed resistors.
As a practical matter, phantom powering allows the same two-conductor shielded cables to be used for both dynamic microphones and condenser microphones, while being harmless to balanced microphones that aren't designed to consume it, since the circuit balance prevents any substantial DC from flowing through the output circuit of those microphones.