Phantom power XLR jack wiring help please

Discussion in 'Pickups & Electronics [BG]' started by Ted88, Dec 28, 2019.

  1. Ted88


    Dec 28, 2019
    I have an active bass which features both an XLR jack and an ordinary 1/4" jack. After a bit of research I've found that the XLR would have originally been able to supply phantom power to the preamp, but it's since been been disconnected. I'd like to make it original again and wire it back up... I'm just a bit lost on one of the connections as I've never had a bass with an XLR/phantom power jack before and can't find any diagrams relating to a bass.

    It's a 3 pin XLR and the bass also takes a 9v battery. One connection on the XLR is still in place and this goes to the positive + wire from the preamp (which also goes to the 9V battery clip). The other pin is ground, so presume I solder that to the ground on the 1/4" jack. The third pin (minus I think?) is the one I'm lost on - I can't see anywhere it came from!

    I was leaning towards thinking the third wire should be going to output from the volume pot or to the tip on the 1/4" jack, but I'd be guessing and wanted to be sure before testing anything. So any help would be gratefully received! Thank you.
  2. For true balanced operation, you would need an audio transformer or an onboard preamp that has a balanced line output. Since bass amplifiers having a balanced line input are rare or non-existent, I doubt it was ever configured for true balanced operation, much less using phantom power, too.

    More likely, to take advantage of the robust, locking XLR connector, it was connected as an unbalanced output adapted to a 1/4" cable on the other end, or a slimmer possibility is that it connected to a power supply / audio breakout box that supplied power to the onboard preamp, thereby eliminating the onboard battery.
  3. Ted88


    Dec 28, 2019
    Thanks for the reply. The jack is definitely for Phantom power, I found a few different mentions of it being for this. One of them in an old magazine article featuring a review of the bass. The other was a post by someone who had talked to the luthier himself (no longer in business).

    What I hadn’t considered was that it might be just for power...So maybe it plugs into a power supply like you suggested, then all the sound signal goes through the 1/4” jack. Meaning the bass would have to have both plugged in.

    How would I got about wiring the jack for this? Would it be to the ring on the 1/4” jack (where the battery minus goes)?
  4. No, the 1/4" jack wouldn't be used; pin 1 on the XLR would be signal and power supply ground, Pin 2 would be your unbalanced audio signal, and pin 3 would be B+ power for the preamp.

    Pins 2 and 3 function could be reversed; convention is to put the power and ground pins as far from each other as possible, making pin 2 for power, and pin 3 for audio.

    Note: By definition this isn't really phantom power. True phantom power is superimposed and supplied on the same conductors as used for the balanced audio signal. The scheme I described isn't balanced, and separate, individual conductors are used for power and audio signal.
    You can't plug this into a conventional balanced microphone preamp, even though they use the same type connectors. Because of this possibility , if I was to use these type of XLR connectors, I would make them 4 pin so they can't be erroneously connected to anything else.

    What make is your bass? I would be interested in reading what the intention was to fit it with an XLR connector.
    Last edited: Dec 29, 2019
    Crater likes this.
  5. Ted88


    Dec 28, 2019
    Thank you, appreciate the help. My unfamiliarity with XLR jacks and very little info on these basses has made it difficult! Its one of these Ashley Pangborn Warlord 'Mark King' (IM Feb 85). It mentions the phantom power in that article.

    Obviously heavily Alembic influenced, which I believe often also have an XLR jack which supplies power, but they normally use more pins.

    So what could it be plugged into then with this setup? I’ve seen mentions of similar setups where the XLR wouldn’t function unless the 1/4” jack was also plugged in...if that changes anything?
  6. If the 1/4" has to be used in conjunction with the xlr, the xlr is most likely used to supply power, albeit a bulky, clunky way of powering it. Had they eliminated the fancy power LED, they would have increased the battery life considerably. In this scenario, a 3 pin xlr would allow for split power supply operation. Such a powering scheme would increase the potential for ground loops.

    The whole XLR thing appears it was a period fad, especially with the British makers. Personally, I can't see much use for it; an external DI box offers more utility function with ground lift, level pads, etc.
  7. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yep, that was my rationale for using a four pin one: