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Phantom Power Damaging Circuitry

Discussion in 'Amps and Cabs [BG]' started by rexspangle, Mar 24, 2001.

  1. hey

    I was reading through the manual a while back for the WM 2004 since I was considering to buy one (went 350 instead)
    and I noticed that they said this here about the XLR out:

    The Balanced XLR output is a true balanced output. No front panel controls affect its signal. The tone and output level are controlled only by the instrument plugged into one of the input jacks. To use this feature, run an XLR (Cannon) cable from the Balanced D.I. to the input of a tape machine, mixing console, etc. This output is also suitable for sending a signal to a house mixer in live situations.

    Wiring for the XLR connector is American standard (as follows):
    Pin 1 = ground, Pin 2 = +, Pin 3 = -


    It says that phantom power can destroy the circuitry - however my 350 manual says nothing about this (but still has a XLR out)

    What I am wondering is how to avoid phantom power especially if both of the systems I play through have one phantom power switch on the board - you CANNOT set it individually for each channel.

    who knows maybe i am looking at this wrong?

    any help?

  2. MikeyD


    Sep 9, 2000
    It's good to err on the side of caution. But it's hard to tell if phantom power could cause a problem unless the manual explicitly states so or you have a circuit diagram to look at.

    If there is a DC decoupling capacitor on each leg of the XLR, you'll probably be okay. One option is to hook up an ohmmeter set on high range. If the resistance climbs to infinity, it indicates a capacitor in series which would block the DC of the phantom power. Another option is to call the manufacturer's tech support people.

    - Mike
  3. thanx

    i checked the oneline manual and it says the same thing for the bass 350, for some reason my manual is missing pages???
  4. hello anybody?
  5. elros


    Apr 24, 2004
    Proprietor, Helland Musikk Teknologi
    On this topic:

    My church have a quite new (<6 months) Ampeg BA210 HP combo amp - good sounding thing indeed - but the DI output recently quit working.

    We run it directly to our Yamaha ProMix01 and it gets 48 volt phantom power. It has worked very well until now.
    Did the phantom power kill it?
    The thing is that I personally read the users manual thoroughly throu several times, looking for anything relating to phantom power on the DI. But it said nothing at all. So we hooked up, and it worked. Until now.


    But I suppose it is still under warranty, so I will probably call the supplier and ask what the heck the deal is.
  6. jja412

    jja412 Fine Gear Enthusiast Supporting Member

    Feb 2, 2004
    St. Louis
    I had a Super Redhead Combo and the DI went out for the same reason - really sucks if you use a Mackie board that has a single phantom power switch for all XLR inputs.
  7. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Most amp DIs are driven by an integrated circuit (cheaper than having a transformer). The IC can get toasted by phantrom power which is DC applied across the + and - leads of the XLR connection. SWR makes that disclaimer but it applies to most amps out there as well. Since most small PA systems apply the phantom power globally it can be a real hassle.

    It's not the phantom power itself that's the issue, it's faults in the wiring causing voltage surges that screw up the amp.

    General rules for using phantom power:

    1. NEVER turn on the phantom power until all the cables have been connected. Do not plug and unplug cables with the phantom power on.

    2. NEVER use low cost dynamic mikes with on-off switches that simply short the audio to ground (this shorts the phantom voltage to ground, too).

    3. You can use inline isolation transformers for amps, mikes, etc. that aren't happy with phantom power.

    4. Many amps with XLR DIs also have 1/4" DI outs (SWRs for instance) so carry a small 1/4"-XLR transformer (what I do) or a DI box.
  8. Big String

    Big String Supporting Member

    Apr 22, 2000
    Northwest Indiana
    I first ran into this problem when reseaching my Demeter Preamp. The H-Series had a similar warning about the Phantom Power. (Demeter since changed that and is not an issue anylonger) Anyway I ended up buying the 201s insteadh of the H series. We have a Mackie 1604VLZ mixer and have the same deal with our phantom which we use all the time for our horn section. Many mixers are not capable of "isolating" the xlr buss where the Phantom resides when engaged. I was alway worried about this when I had my Mesa 400+ and would not plug into that buss for fear of damaging my amp and many tubes. I could never get a straight answer from anyone on the effects of Phantom on paticular units. Some 'experts' would tell me if the mic, amp, unit, whatever.... didn't require the 48 volts, the current would just pass through. Others said if the unit didn't have the proper protection circuit via resistor etc., it could possibly be damaged.... sheeeze.....

    What I finally did. I aquired a cable with the normal XLR (3 pin) balance connector on one end (connected to amp) and a 1/4" TRS balanced connector on the other end which went into the my channel receptacle on the mixer, only the 1/4" accomplishing the same thing. I don't think it is truly balanced when going over 18 feet with that type of cable though. I am always close to the mixer and it never was an issue. Now, all my equipment have Jensen Transfomers and are protected from the Phantom.
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Phantom power is not applied across the + and - legs of the XLR input. What you describe would be T power, which is rare except in location recording. For example, many Nagra recorders have T power on the inputs.

    Phantom power is a positive voltage applied through equal-value resistors to the + and - legs on the XLR input, with the ground as the negative. The reason it's called phantom power is that it doesn't show up as a DC voltage across the signal legs. Dynamic mics with on/off switches and balanced outputs generally work by shorting the + and - signal legs when switched off and opening the connection when switched on. This can be done safely precisely because there is no DC voltage across the switch.
  10. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Bob, you are of course correct about it not being across the + and -. My bad. By the way "T-power" as you called it is common on MiniDisc recorders to power external condenser mikes.

    In any event, the issue is the same, if something disrupts the phantom voltage which is supposed to be applied equally between + and - bad things can happen. Causes of this can be a bad connector or cable or even plugging or unplugging the cable.

    I once bought a cheap Peavey dynamic mike which came with a warning in the manual about using with phantom power; for some reason rather than shorting + and 1 with the on-off switch, they shorted the signal to ground :eek: I would hope this is pretty unusual, but such mikes do apparently exist.
  11. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Phantom power is supposed to be +48 V applied through a pair of 6.8k ohm resistors, one to each side of the mic line. Some units use less than 48V.

    The BA210 should not have been damaged by normal phantom power, because it is capacitor isolated. But it is always possible that something happened. An extra high phantom voltage, transient, etc.

    SVT series units generally have a transformer and would not be affected.

    Other manufacturers might not use isolating capacitors etc, and it is possible that a unit could be damaged. But the maximum current available is fairly low, typically a little under 15 milliamps (0.015 A) when shorted to ground, about 10 ma into a typical IC with 15V supply voltages. That could damage an unprotected IC, but isn't likely to burn up anything.

    I would always follow the recomendations of the manufacturer. If they say phantom power may cause damage, stay away from it.

    It is generally possible to use a DI or the like to avoid problems.
  12. Big String

    Big String Supporting Member

    Apr 22, 2000
    Northwest Indiana
    Hey Bob or Brain

    Indulge Me. From what I stated in my post on this subject, did I get a bunch of smoke blown up me were the Sun don't shine ?
    Like I said, I never really got a straight answer from some of the folks I called about it.

    Also, will the xlr/trs 1/4 be the same as using the xlr/xlr as far as being balanced.
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Having transformers to isolate your rig circuitry from phantom power is a good thing, if it wasn't already designed to be impervious to it.

    I once plugged the balanced output of my BBE preamp into a directly into a mixer input that had phantom power. The output section of the pre just wouldn't put out anything, even out of the unbalanced output, which was going to my power amp. Fortunately, nothing burned up. From then on, I've only used my direct box, with its nice Triad transformer, driven from the unbalanced output of the preamp (the balanced out is going to the amp now) to drive the direct box.

    If the cable has two independent signal conductors plus the ground/shield, and two signal connections plus ground/shield at each end, then it's balanced, regardless of whether the connectors are XLR or TRS.
  14. Yes.


    Balanced is in the circuity, not in the cables. That said, you have to have two-conductor plus shield to carry balanced lines. It makes no difference if TRS or XLR is used, as both carry 2 conductors plus shield.

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