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I've set up stage monitors that only took 1/4" cables as input, and I took it for granted that they just worked until I realized they did not plug in to the stage's power source. Eventually I learned that they ran on this scary-sounding thing called phantom power, but I still don't really know how it works.

What are the mechanics behind phantom power? What equipment most commonly uses phantom power? What are its limitations?

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This kind of question would really be better off on electronics or Sound Design. Anyway I don't think you're actually talking about phantom power: as you see on Wiki or Kevin's answer, that's mostly used to power microphones but definitely not sufficient for stage monitors. Also, phantom power almost always uses XLR connectors, not 1/4". –  leftaroundabout Jul 24 at 22:50

2 Answers 2

Stage monitors generally come in two varieties- powered and unpowered. The first need AC power (220v in U.K.) to run, just as an ordinary guitar amp., p.a. etc. would.The other is passive, in that it is a speaker cab.As such, it will need an amp. externally to run it, as well as being connected, often by jack plug, to the sound out.

Phantom power is DC, and 48v. Way too small to run a monitor. It is used to power condenser mics, as are more used in recording situations. The plugs on these are usually XLR ( sometimes called cannon), usually with 3 pins, for a balanced connection. Someone has fed you misinformation.

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Just wanted to add: Be careful not to connect a powered (amplifier output) lead to an already-powered monitor : the input of powered monitors is expecting tiny voltages like 0.5 volts - the output from an amp, which might use the same kind of 1/4" cable, is loads more, maybe hundreds of volts and will most likely blow the input of the powered amp. Best to be sure what is powered and what isn't prior to plugging stuff in. You can tell a powered monitor in that it'll also have a power lead and normally some controls. An unbpowered monitor normally just has a speaker input - no other controls. –  user2808054 Jul 25 at 10:09
    
Thanks - fair point. A speaker output from an amp will power a speaker. A line out from an amp will be good for the input of a powered monitor. And the two plugs could look identical. –  Tim Jul 25 at 10:24
    
I see. Yeah the monitors were connected to a return from the mixer. Still, the monitors were definitely only connected by 1/4" cables, so what causes the sound in that case? –  pugles Jul 25 at 17:51

Phantom power is just a way to transmit DC power over microphone cables. It's most used for condenser mics and direct boxes (DIs).

Phantom powering consists of a phantom circuit where direct current is applied equally through the two signal lines of a balanced audio connector (in modern equipment, usually an XLR connector). The supply voltage is referenced to the ground pin of the connector (pin 1 of an XLR), which normally is connected to the cable shield or a ground wire in the cable or both. When phantom powering was introduced, one of its advantages was that the same type of balanced, shielded microphone cable that studios were already using for dynamic microphones could be used for condenser microphones.

A microphone or other device can obtain DC power from either signal line to ground terminal, and two capacitors block this DC from appearing at the output. R1 and R2 should be 6.8k ohms for 48 volt phantom, and R3 should not be used.

With phantom power, the supply voltage is effectively invisible to balanced microphones that do not use it, which includes most dynamic microphones. A balanced signal consists only of the differences in voltage between two signal lines; phantom powering places the same DC voltage on both signal lines of a balanced connection.

(Source: http://en.wikipedia.org/wiki/Phantom_power)

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