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48V phantom power through dynamic or other non 48V mics- concerns?

Discussion in 'Pro Audio Equipment' started by jmm22, Nov 7, 2010.

  1. jmm22

    jmm22 Guest

    I have a Saffire USB 6 that has two inputs, but only one 48V switch for both ins. If only one of my mics is say an LDC, is there cause for concern with using non 48V mics on the other input, given that there is 48V at this second connection? I have tried it successfully already (in the sense that nothing blew up) with an EV676 on the other in while having an LDC connected, but I would like a better understanding. It's funny that the manual for the audio interface makes no mention of this aspect.
     
  2. TheJackAttack

    TheJackAttack Distinguished Member

    Provided your cables are wired correctly you should not have issues with modern dynamic microphones and P48. This is a fairly common question though.
     
  3. Boswell

    Boswell Moderator Distinguished Member

    The +48V phantom power is supplied as a common-mode voltage on the two internal conductors of a microphone cable referred to the outer screen (0V). Provided a dynamic microphone has its coil floating with respect to its case, it should not be worried by the coil being up at 48V, and the differential voltage developed by the coil across the two internal cable conductors due to varying sound pressure at the diaphragm will be passed on to the pre-amplifier as expected.

    Only if the microphone is not wired in this way, or if there is a fault in the cable or connectors will the coil see any of the 48V phantom power. The phantom-power short-circuit fault current is limited to about 14mA so as not to burn out a coil, but it can do irreparable damage to the suspension of a ribbon microphone, so the general advice is not to use ribbon microphones in a phantom-power environment in case a fault is present or develops. This is a general rule, but there are exceptions for certain makes of ribbon microphones.
     
  4. jmm22

    jmm22 Guest

    O.K. thanks.
     
  5. TheJackAttack

    TheJackAttack Distinguished Member

    Boswell,

    Do you have a recommendation for any sort of a filter one could place between a ribbon and a "regular" preamp to prevent hitting a ribbon with P48? Other than purchasing an active ribbon mic :) I often worry about "helpful" people or kids playing with the gear while I'm on stage performing and having to make David Royer's next boat payment repairing an SF12 or something. Right now, I often put the front case cover back on which does limit air flow/cooling. I could buy a ribbon preamp but True doesn't make a rack mount and don't own/afford any Grace at the moment.
     
  6. Boswell

    Boswell Moderator Distinguished Member

    I don't know of anything that is just a PP guard device. There are several different x10 (+20dB) XLR in-line signal boosters available that require phantom power to operate and not pass it on to the input connector. These are designed for ribbons and work acceptably, but I have no infomation about how they perform under fault conditions.

    A couple of years ago, frustrated that these commercial units were mono only, and suspicious of their fault condition behaviour, I designed a low-noise +20dB stereo unit that had clamp devices in the input circuit with a time-delay. Only when phantom power had been received correctly on both signal output pins of both channels and remained in that state would the input devices unclamp to allow the signal from the ribbon through. They re-clamped in a fraction of a millisecond on detection of a fault. The unit worked well normally and safely under all the fault conditions I could devise, but the manufacturer I had the initial interest from pulled out of a production deal citing the extra manufacturing cost of the clamp components as making the unit uneconomic.
     

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