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Noise on balanced line input

Discussion in 'Electronic Repair' started by klem kedidelhopper, Feb 16, 2013.

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  1. I have an interesting application which has a small problem. We
    installed a security system and a background music system in a
    building. The music is distributed via 70V lines at very low level
    throughout the building. The speakers are three inch 45 ohm units
    salvaged from an old apartment house intercom system. Each speaker is
    connected to the line with a small 1 watt 70V to 45 ohm transformer. I
    picked up a box of these transformers at an auction and the marriage
    of the speakers and transformers seemed a good fit. The music works
    very well but that's not the problem.

    The alarm system when activated on an audible zone applies 12VDC to a
    siren driver. The driver outputs a "whoop whoop" sound to a separate
    outdoor 8 ohm speaker. In addition to the connection to the speaker I
    also ran the output of the siren driver back to the background music
    amp. I connected the primary, (70V side) of a typical speaker line
    transformer to this siren output and the secondary, (speaker
    connection) to a low impedance balanced mic input on the amplifier. So
    when the alarm goes off, the sound of the siren is everywhere in the
    building. The transformer gives me isolation between the alarm system
    and the music system and as a bonus it just happens to step down the
    voltage the right amount to feed a balanced low level mic. input with
    the right amount of signal as well.

    This scenario also works well however with one slight problem. The
    cables for both systems are old CAT3's which were already in the
    building. All these cables are bundled together between two buildings,
    one where they're punched down onto 66blocks in one location to where
    the equipment is in the other. On the siren driver line, audible
    through the background music system speakers when the mic input is
    advanced to the proper level, there is a slight "fluttering" sound.

    I've heard this noise before when using my inductive probe to find a
    wire with tone on it and so I'm sure the sound is the data stream from
    the alarm system keypad getting into the siren driver circuit.I tried
    hanging different size caps across the mic input but none of them
    reduced the noise. I didn't have any chokes with me so I couldn't try
    anything in that regard, however I plan to. But I was curious if
    anyone else ever ran into a problem such as this and was ever able to
    remedy it. Or perhaps someone may have some suggestions I might try.
    Thanks for any assistance. Lenny.
  2. Guest

    Where does this transformer physically live? Close to the siren driver,
    close to the music amp, somewhere else? If you think you're picking up
    junk from the wires, it might help to move the transformer as close to
    the music amp as you can get it. I think (and I'm sure I'll be
    corrected if I'm wrong) that the mic inputs typically work with a lot
    lower signal levels than, say, a line or aux input (for a CD player or
    similar), so the mic input will be more sensitive to this kind of
    pickup. If the smaller signal only travels a couple of inches, it will
    be less likely to pick up junk.

    If you have a line or aux input on the amp, you may be able to fix it
    by moving the siren input to that. The suspected induced noise will
    still be there, but the lower sensitivity of the line/aux input will
    make it harder to hear.
    Are you sure? You might not be the first person to re-use existing
    wiring in that installation, especially if it runs between two buildings
    - grabbing a pair from the old telco wire is way easier than bringing a
    shovel or a ladder. :) I have seen old Bell 25-pair 1A2 PBX cables
    repurposed for *Ethernet*; it actually works sort of OK at 10 megabit,
    100 megabit is iffy. My point is, there may be something besides the
    alarm wiring that is radiating junk into the pair that you used for
    the siren tone.

    You might go to where the transformer is, disconnect it from the siren
    driver, short the two primary wires together, and see if you still
    hear the noise. If it goes away, it may be coming from the siren
    driver itself; changing inputs on the music amp might be the fastest
    way out of that situation. If it stays, then it's getting picked up on
    the wiring somewhere.

    If it's getting picked up on the wiring, an idea: if the alarm "brain"
    has enough 12 V drive to run two siren drivers, run the 12 volt siren
    driver supply to where the music amp is. Put another siren driver
    there, and put an 8 to 16 ohm power resistor (look at the siren driver
    spec) across its output. Then put the ends of a pot (1K maybe?) across
    that, and run the pot wiper and one end into the amp. That gives you
    a local and hopefully "clean" source of siren noise for the music amp.
    If the 12 V from the alarm is really too noisy, still run it to the
    music amp, but use it to switch a little 12 V relay. Use a local
    "clean" 12 V DC wall wart to power the local siren driver at the music
    If you have any ferrite beads (those lumps at the end of some computer
    cords), looping the cable through one a few times right before the mic
    input at the music amp might help a little. I think those tend to help
    with higher frequencies than what you probably have, though.

    Matt Roberds
  3. I appreciate your ideas Matt. The transformer is mounted right on the
    PA amp with a 6 inch secondary wire to the XLR connector. I'm certain
    that the noise is being picked up by the cables. none are shielded and
    the best there is is the CAT3. You also may be right about the chokes.
    I've used them in the past for RF suppression, but this is at an
    audible rate.

    I made a service call once to a house very near an AM broadcast
    transmitter. The station was just about as loud on the telephone as a
    normal conversation would have been. I fed the CO line into two small
    chokes, (value unknown), and then hung a cap, (again I don't remember
    what I used. I wish I had written it down), across that. When I
    connected the premises phone line up to my filter the line was clean.

    I did try the other high level AUX input but couldn't get it to work
    with my 45 ohm transformer. I suspect the step down effect was
    working against me at that point. Eliminating the transformer and
    connecting the AUX input directly to the siren driver output didn't
    work at all either. I'm not really sure why though. I might need a
    transformer with a less aggressive turns ratio for that input to work.
    Maybe I could try to capacitivly couple to the aux input but that
    could be noisy too. Lenny
  4. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"

    ** If both the lines involved REALLY are *equal level & anti-phase
    balanced * AND twisted - there should be no such problem.

    So I bet that is not the real case.

    .... Phil
  5. Phil Allison

    Phil Allison Guest

    "Jeff Liebermann"
    klem kedidelhopper
    ** Absolute nonsense.

    70V is the maximum voltage output from the amplifier and there is nothing
    constant about it.
    ** Volume pots allow the level to be set at the operators convenience.

    It is very common to have a low, background music level and a much higher
    speech level for announcements.

    ** More nonsense.

    The real advantages include the use of long cable runs with low losses and
    having many distributed speakers - which do not need to be all of the same
    rating type or level.

    ..... Phil
  6. To further explain this, the amplifier's, (which is located in one
    building), output is taken off the 70 V tap, however as Phil alluded
    to the voltage on the line could be and in fact in this case is very
    low, ,just a few volts. This is all I need because, again this is VERY
    low background music and the level seems to be satisfactory throughout
    the building. As far as the lines being balanced, I'm assuming that
    they still are as I have done nothing anywhere that I can think of to
    cause an unbalance. As I mentioned before this balanced line output
    leaves the amp on two pairs of a CAT3 cable. I used two pairs because
    they are the main feed for all the speakers. This "main" cable runs
    from this building underground through PVC pipe to the 66 block in the
    other building. At that location all the other CAT3 wires also appear
    on that block as well. On each cable that was reused the brown pair is
    connected to a speaker. in a different room. These brown pairs are all
    placed in parallel and then connected to the 70V feed. So this is all
    well. It is not however the problem. The problem is occurring when I
    try to sample the siren driver output from the alarm and apply it to
    the low level input of the music amplifier. That's when I hear the
    data noise which I'm certain is being picked up by the "antenna" which
    is the sample wire and fed into the mic input of the amplifier.
  7. When the alarm is activated, 12VDC is applied to the "siren driver".
    The driver outputs a "whoop whoop" sound to an 8 ohm speaker. The
    speaker is located in a different building than the alarm system. The
    CAT3 cable to the speaker is punched down onto a 66 block near the
    alarm system control box. This cable serves two functions. It sends
    the siren driver output to the speaker, and also by cross connecting
    another of it's pairs on the 66 block also sends the siren driver
    output back to the PA amp where I connected it to the 70V transformer
    primary and then connected the 45 ohm secondary to the mic input.
    That's when I heard the "ticking" or "fluttering" noise. I really do
    think that the cables are physically in good shape. Lenny
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