Phantom Power Conundrum???

Discussion in 'Recording/Live Sound' started by jokerjkny, Jun 23, 2004.


  1. jokerjkny

    jokerjkny Member

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    hey all,

    so i've got a set of Audix ADX-90 mini-condensor mics for the toms on my kit, which need phantom power. but i also have an acoustic guitar with an onboard preamp, and a bass preamp with a DI, all going into the same snake/mixer.

    can i still use the phantom power, and not damage the other instruments?
     
  2. GaryNattrass

    GaryNattrass Member

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    So long as the other instuments are connected via a balanced DI it will be OK.

    Phantom power is a 12-52 volt signal fed down balanced lines with an earth a positive and a negative connection hence the 3 pin XLR which is the most common connector.

    You can also get 3 pin DIN connectors and 3 pin Tip Ring Sleeve jacks.

    The phantom power is fed as an earth (pin1) plus+48volts (pin3) and minus-48volts(pin2).
    Pins 2 and 3 also have a 4.7k resistor on each leg to maintain true phase and an equal +ve and -ve voltage.

    A source needing phantom power will utilise the voltage to power the mic or DI box but if you plug a balanced dynamic mic or balanced passive DI box across the three pins then the two 48voltages across the pins 2 and 3 will phase cancel each other out and therefore give a zero charge with no damage.

    Where it all gets messy is if you plug an unbalanced source such as an output from an accoustic guitar pre amp across a balanced 3-pin input which has phantom power applied. You could have a 48 volt supply going into your unbalanced pre-amp and it could damage it.

    An unbalanced jack should always be fed into a balanced mixing desk with phantom power present via a balanced or active DI box.

    Hope this helps.
     
  3. MichaelK

    MichaelK Member

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    This I never knew.

    Thank you, gary.
     
  4. GaryNattrass

    GaryNattrass Member

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    Also beware when re-plugging or re-patching anything that has a phantom power switched on and feeding it.

    As you re-plug you can potentially get 48 volts going across your desk and if you have a 10k PA system on the end you may have to sweep up the HF drivers off the floor!

    Your studio monitors could be damaged too if you re-patch a mic input with 48volts across it.


    Also if you ever get a frying type noise whilst using a condensor mic with phantom it is because it is not receiving both +ve and -ve voltages equally in a balanced way.

    A bit of switch cleaner on the connectors will help but remember the bit above and mute your monitors before re-plugging any phantom power fed source.
     
  5. jokerjkny

    jokerjkny Member

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    thx gary, great info!

    but one more thing, i've got a guy using a Taylor with one of those new fangled "Expression" systems. it outputs a signal that's "balanced", and uses a 1/4" to XLR plug that goes right into the snake.

    no probs, right?
     
  6. GaryNattrass

    GaryNattrass Member

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    Thats right he will have no problems because the jack from the Taylor expression system is a Balanced TRS jack to a balanced XLR.

    Therefore the phantom power will be distributed across the tip and ring as +48 and -48 volts. The sleeve acts as the earth for the system.

    As mentioned you are OK with phanton so long as any connector is balanced.

    It only gets risky if you are going balanced to unbalanced without buffering the signal through a DI box.
     

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