AB168 PP light

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This topic contains 14 replies, has 5 voices, and was last updated by Profile photo of Mike C Mike C 3 years, 6 months ago.

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  • #69224
    Profile photo of debzdoodle
    debzdoodle
    Participant

    Had a weird thing occur on the AB168 this past weekend.
    Bass player has been using his EBS micro bass pre amp powered with 48v for a few months but on friday he decided to bring his TC amp DI to FOH instead – no 48v necessary.
    We were plugging in everything and noticed that the channel on the AB168 that the bass was connected to had the pp light illuminated BUT the 48v was NOT turned on at the mixer – I double checked. The channel was correctly assigned to D Snake.
    Not sure if this has happened before but we had never noticed it.
    Why might this happen??

    #69225
    Profile photo of GCumbee
    GCumbee
    Participant

    If this was an analog snake I could understand. Backfeeding voltage. But with a digital snake I’m not sure. Something weird. I really can’t see how that channel would be lit. Maybe voltage on the device was somehow triggering the logic circuit in the front end of the console that fooled the AB. Strange.

    #69229
    Profile photo of ahjeff
    ahjeff
    Moderator

    I think this is working as intended. From the user guide, “The Phantom Power indicator detects voltage at the socket whether supplied by the AB168 or received from an external source”.

    Hope this helps.

    – Jeff, A&H

    #69237
    Profile photo of GCumbee
    GCumbee
    Participant

    Well I guess I was on the right track. Thanks Jeff.

    #69258
    Profile photo of debzdoodle
    debzdoodle
    Participant

    Thanks guys …. “Received from an external source” – what source?

    #69259
    Profile photo of airickess
    airickess
    Participant

    The bass player’s DI.

    #69265
    Profile photo of debzdoodle
    debzdoodle
    Participant

    He wasn’t using a DI box or pedal. The point is he was using his amp head direct to the PA. This is my confusion.

    #69270
    Profile photo of Mike C
    Mike C
    Participant

    I take it unplugging from the amp and the light on the channel goes out.
    Is the output on the amp an XLR line out male or a 1/4 inch.
    Are you sure it’s a line output and not a speaker out.

    What model amp is it.

    With a volt meter set on DC volts measure the XLR output on the amp, measuring between pins 2 and 1 and then pins 3 and 1. If the amp is putting out DC voltage on an XLR line
    out jack I’m going to say that’s not a good thing!

    Do you have an isolation transformer to put inline between the amp and the AB168 input alos you could try a pin 1 lift on the XLR.

    #69273
    Profile photo of debzdoodle
    debzdoodle
    Participant

    The pp light only showed up when the amp was connected to it. DI output is XLR and was definitely the one used – speaker outs are SpeakOn.
    TC Helicon BH550 amp.
    There has to be some leakage somewhere that was detected by the AB168. A DI box is probably the answer – strange though.

    #69278
    Profile photo of Mike C
    Mike C
    Participant

    I would say the amp has an issue or a design flaw. Maybe TC was going for the purest signal path to the XLR line out and did not put any DC blocking caps in the circuit but failed to design a circuit that did not have any DC offset.

    If you get a change to measure the voltage on the XLR out I would be curious to know
    what the voltage is.

    I would put a 1 to 1 isolation transformer between the amp and the AB168, a pin 1 lift would break the circuit so the AB168 would not really see the voltage.

    #69391
    Profile photo of debzdoodle
    debzdoodle
    Participant

    Thanks Mike….Haven’t had a chance to measure yet but I got a 1:1 isolation transformer – a nice small inline one so we will check it out and report back.

    #69409
    Profile photo of Mike C
    Mike C
    Participant

    Thanks Mike….Haven’t had a chance to measure yet but I got a 1:1 isolation transformer – a nice small inline one so we will check it out and report back.

    Let us know what you find.

    #69416
    Profile photo of Mike C
    Mike C
    Participant

    What transformer did you get?

    #69421
    Profile photo of debzdoodle
    debzdoodle
    Participant
    #69427
    Profile photo of Mike C
    Mike C
    Participant

    That will work and it’s always handy to have one or two of those handy
    for last minute input output interface isolation.

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