Phantom Power DI Problem

Discussion in 'Microphones (live or studio)' started by JimboJ, May 12, 2006.

  1. JimboJ

    JimboJ Active Member

    May 1, 2005
    Recording bass guitar at a concert the other day, the bass player’s amplifier gave off a suspicious whine when the bass was plugged into my Whirlwind IMP-2 passive direct box. The ground lift switch on the DI did not get rid of the noise. I suspect that my DI is not blocking the phantom power that my PreSonus Firepod is sending down the line. I can’t switch off the phantom power on that channel because the Firepod has on/off phantom power switches for 4 channels at a time and I have condenser microphones on every other channel

    Something similar happened a week before when I split the signal from the microphone used by a Fender Passport PA system; the PA whined until I unplugged the cable connected to my Firepod, which was sending phantom power down the cable. I have used the Firepod subsequently with another PA – splitting the signal from the dynamic PA microphone – and haven’t had a problem.

    I did a little surfing last night and found the Rapco ISOBLOX. The ad copy for the device says: The ISOBLOX is another tool in the war against unwanted noise. It will eliminate the dreaded 60 cycle hum and outside noise that can be caused by older electrical systems. The best reason to have this is that if you have an isolated mic signal and you play an electric instrument... you won't get shocked in the chops. ISOBLOX is an In-line XLR(M) to XLR(F)that provides isolation to microphone level lines. - 1 to 1 Isolation 600 Ohm.

    Any opinions on whether that will take care of the problem or am I misdiagnosing the problem? Both times this has happened, I’ve been in older venues with questionable wiring, so the bass amp and PA may not have had proper grounding.


    -- James
  2. Nirvalica

    Nirvalica Guest

    nah, won't work. i dunno what to tell you.
  3. Boswell

    Boswell Moderator Distinguished Member

    Apr 19, 2006
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    You could try turning off the phantom power for that group of channels to see if the whine disappears. The other mics would lose power, but at least it would tell you something. My memory of the IMP-2 is that it's an isolated transformer-type passive DI box, so no d.c. can get back from output to input unless there's a fault. However, if there's a.c. ripple on the phantom power line, the transformer type of box will work backwards and inject that into the amplifier. I would check the FirePod mic inputs with phantom power enabled with a scope (on AC coupled) to see if the rail is clean.

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