Confirmation - Balance, Grounding, DI Box

Discussion in 'Microphones (live or studio)' started by Codemonkey, Sep 28, 2008.

  1. Codemonkey

    Codemonkey Well-Known Member

    Dec 11, 2007
    Scotland, UK
    tl;dr: DI fixes ground loop?

    I've had the Church on my back (one guy anyway) about the loop for deaf people. Now while I'll not go into the politics and just say I'd prefer people to use their hearing aids to amplify the ambient sound, not tune into a dodgy loop...
    ANYWAY I digress.

    Mixer uses effect send to feed loop amplifier at far end of building, people are complaining about all sorts, squealing, buzzing, too quiet etc.

    Today I got to (finally) hear the problem. Ground loop. Also it sounds like the gain structure is whacked, but the guys who installed the system have checked and said the effect send (feeds the loop) should be set as what I now have it set as. BUT I'm not convinced on their testing methods.

    The loop amplifier has a VU on it, but I can't read it and adjust volume knobs at the same time (20m apart). I can't move it. I also don't have a hearing aid to listen with. So I guess I'm a bit stuck on the volume front.

    As for the buzzing...
    Mixer output is a TRS UNbalanced jack (no idea why). Previously it was a TRS cable leading to the XLR which feeds the amp.
    *I'm not sure of the inputs on it.*

    I've changed it to use a short TS cable, plugged into a passive DI, which feeds the XLR. No idea if it works yet.

    If someone can confirm I've done the right thing by putting a DI in the path, great. I'll also find out the input type on the amp at some point.
    My thinking is, if the amp is unbalanced input then the signal will get cancelled (depending on what it does with the cold signal), but there'll be no buzz either. If the amp is balanced input, the benefits will be plentiful.

    Really I just need a hearing aid for 5 minutes and I'm hoping someone will have magical advice such as "open the XLR from the DI output and cut pin 3, guaranteed to work".
  2. IIRs

    IIRs Well-Known Member

    Oct 23, 2005
    If the mixer ouptut is a TRS jack that probably means it is balanced. You just need a balanced TRS jack to XLR cable: wire the Sleeve to pin 1, the Tip to pin 2 and the Ring to pin 3.
  3. Kev

    Kev Well-Known Member

    Nov 13, 2001
    does the amplifier have to be at the far end of the building ?

    it so then you need to know if it is balanced and at what level

    I suggest a transformer coupled input to the to the amp with enough grunt from the mixer to get the required level from the amp
    the transformer needs to be OK for these levels, so passive DI's may not be good as they are only OK for guitar levels and mic level output

    Is the mixer output balanced ?
    it probably doesn't matter
    as I suggest a transformer at both ends
    BUT at least one end
    and construct so you have an earth lift and a polarity flip (may not be needed but why not)

    the DI trick you are trying is fine
    IF the levels and impedances are in spec with the gear you have

    I think the DI should be at the amp
    so the common mode rejection can work on the noise induced down the long run

    before we go too far
    more info on the two units please
  4. Codemonkey

    Codemonkey Well-Known Member

    Dec 11, 2007
    Scotland, UK
    "If the mixer ouptut is a TRS jack that probably means it is balanced."
    Grabbed the manual, checked the spec, it says (and I quote) 1/4" TRS unbal. (unquote). I didn't believe it either.

    "does the amplifier have to be at the far end of the building ?"
    Yes. Unless I disconnect the induction loop wire from it, if that's even possible.

    "you need to know if it is balanced"
    I thought as much, I shall find out within 24 hours hopefully.

    "so you have an earth lift"
    Right, I'll poke that annoying ambiguous switch in.

    The only relevant info is the output is a TRS 1/4" unbalanced from an effect send, and it's at 150 ohm.
    As I said I'll find the info on the amplifier. It's about 5 years old and I have no idea of model number etc.
  5. GeckoMusic

    GeckoMusic Guest

    That wouldn't happen to be an AM transmitter would it? AM has quite a bit of competition these days. Some cell phone, blue tooth, and 802.11 signals get picked up by AM receivers. The carrier frequency for these protocols is much higher, but the data itself can look like an AM signal to some extent.

    As an experiment you might connect an mp3 player or lap top directly to the transmitter and see if the noise is still in the in-ear.
  6. Codemonkey

    Codemonkey Well-Known Member

    Dec 11, 2007
    Scotland, UK
    Gecko: :S no idea. An XLR comes out the back, into a box which splits into 2 brown wires that disappear down a hole.

    Anyway update.
    The input on the box is either some many-pin Aux line level thing that looks like a midi jack but I've never seen before, or a balanced mic input at 30-200 ohms.

    I'm feeding it the output from a DI box so I should be fine, if not for a slight level mismatch?

    I also screwed around a little while at Church tonight, and whoever said to put the volume knobs on the effect send at "3" was a buffoon. Going by the VU on the amp, I had a properly gain-staged mic up at 6 with no overload. So it looks like it's working in the sense that a signal is getting through.

    I just have no idea what signal...
  7. Kev

    Kev Well-Known Member

    Nov 13, 2001
    a passive VU meter is a very handy thing for the tool box
    2 is even better for send and return
    and for stereo
    see my web site
    there is a simple schematic there somewhere
    shows how it can be useful for seeing outputs and the effect of a load

    the the Rane and Jensen web sites for some good interfacing schematics
    it may help to see some options as a diagram
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