Impedance Matching Problem?

Discussion in 'Pro Audio' started by levi, Apr 17, 2006.

  1. levi

    levi Guest

    Confessing ignorance here...(so what else is new?)

    I'd like to run an input into the 2nd channel of a (vintage)
    Fender Deluxe Reverb amp, the signal source coming from
    a small mixer (which serves to mix three or four microphones).

    Specs for the mixer lists an output impedance of 120 ohms.
    The signal works fine plugged into the line/instrument jack of
    another amp which has an advertised input impedance
    of 36.4k ohms.

    Specs on the Fender offers us input impedence choices of either
    1M ohms or 136k ohms (-6dB), the alternate choice is described
    as for use with "high output" or "preamped" sources. Neither of
    those seem to work, yet a single microphone will.

    Insight anyone? Thanks!
    levi, Apr 17, 2006
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  2. levi

    Phil Allison Guest


    ** No problem.

    You must a have a faulty lead.

    ........ Phil
    Phil Allison, Apr 17, 2006
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  3. levi

    Scott Dorsey Guest

    What happens when you try them? Does the whole thing become very clipped?
    Scott Dorsey, Apr 17, 2006
  4. levi

    levi Guest

    I meant "No audible sound from the amp" when I tried this
    setup (as I noted, a microphone - "straight in", except for an
    XLR -> TRS adapter, to the 1M ohm jack this channel works
    fine). The volume of the channel was set at "7". Neither jack
    seemed to work with the mixer.
    The first, obvious, test was to plug the cable into the other
    amp, but in retrospect maybe I didn't try that (but I thought I
    did) - testing instead that the mixer channel was working, by
    merely shifting the other cable over to this channel.
    It's a simple 10ft., 1/4" TS plug, cable.

    As Phil suggests, though, I will look/test closely to see if
    there is a faulty lead, and then maybe some sort of problem
    with the jack itself (it's an old amp, after all).
    The mixer (Behringer 802) provides a Main Mix level control,
    (which was set at the midpoint "0 dbU"). I am trying to use
    it's two Main Mix output channels (L & R), with one channel
    going to one amp, and the 2nd going to another. As to what
    electrical levels that may mean, I do not know nor do I find any
    documentation other than the specs I posted.

    Sorry. Ignorance = frustration, eh?!.
    I understand. Here are links to the specs I'm looking at:

    Related question:

    Should the output from the mixer go to the 1M ohm jack or the 136k ohm

    Thanks, Phil, Richard, and Scott.
    levi, Apr 17, 2006
  5. levi

    Scott Dorsey Guest

    Guess: the output of the console is impedance-balanced and only
    drives one leg of the line. The cable you have connects the
    tip to the XLR leg that isn't being driven.

    This is the unfortunate result that you will often run into when
    dealing with impedance-balanced lines in the same world with
    unbalanced stuff. Your local audio dealer will have a "phase reverse"
    barrel connector that swaps pins 2 and 3 on the XLR. I recommend
    trying one of these.

    Incidentally, a recent issue of Recording magazine had a DIY project
    for a reamp box which is intended for this application. It has an
    internal balancing transformer so it doesn't matter if you send it
    impedance balanced or differentially-driven input, and it has some
    gadgetry in there to model pickups which you can bypass if you don't
    want it.
    The best troubleshooting aid you can have is a pair of cheap headphones
    from the dollar store. Cut off the 1/8" plug, strip and tin the leads.
    Now you can touch one lead to ground and one lead to a signal pin and
    actually tell if there is signal on a line or not, and which pins are
    Scott Dorsey, Apr 17, 2006
  6. Scott: your article must be a huge success: Digikey is sold out of the
    transformer right now.
    darrelldklein, Apr 18, 2006
  7. levi

    Scott Dorsey Guest

    Try the MT-4151 instead, then. Same thing except the center tap is
    split. The CUI-branded 237-1155-ND will work also but is a couple bucks
    Scott Dorsey, Apr 18, 2006
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