"Unbalanced-Balanced" to "Balanced"

Discussion in 'General Electronics Chat' started by Precious Molina, Sep 24, 2015.

  1. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    lets say, im in quiet small recording studio, i have a dynamic microphone unbalanced to 5meters "short" cable to unbalanced preamp.
    outout signal of preamp is unbalanced, my question is
    Do i need to balanced it first before plugin it in balanced xlr?
    were short distance like 5meters, im afraid if the signal going to degrade

    Bruno putzeys said:
    "Differential transmission of audio doesn’t mean
    you need to make a symmetrical voltage.
    An input that expects a symmetrical signal is not
    differential because it’s trying to involve a third
    node into the equation whereas voltages are only
    measured between two nodes."

    i dont know if i understand that in a wrong way (english is not my native language)
    balanced sometimes changing signal
     
    Last edited: Sep 24, 2015
  2. crutschow

    Expert

    Mar 14, 2008
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    Not sure I understand the question.
    Is everything unbalanced except the xlr connector?
    Can you post a diagram of what you intend to do?
     
  3. peter taylor

    Member

    Apr 1, 2013
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    A balanced three wire cable is normally used for a condenser microphone because it picks up less noise.
    An unbalanced 2 wire cable picks up more noise.

    A balanced cable can be used for greater lengths, and is always preferable for dynamic (coil) or condenser (capacitor) microphones.
     
  4. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    Do i need to do this (image below) before plugin it in balanced input?
    [​IMG]

    i thinks it still acting like a balanced since the input is balanced, i think its still canceling the noise
     
    Last edited: Sep 24, 2015
  5. peter taylor

    Member

    Apr 1, 2013
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    Yes. Or you will lose half your signal.
     
  6. peter taylor

    Member

    Apr 1, 2013
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    Sorry, I did not read your post properly.

    By the time the unbalanced signal reaches your circuit, it has already picked up noise.

    You have to send the signal down a balanced cable in the first place.

    Your circuit will not improve the signal.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Read this paper. There is a lot more excellent information on balanced/unbalanced audio systems on their website. Especially read section, 2.4, "A simple Alternative".
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What is the gain of the unbalanced-to-unbalanced preamp? Also, what is its output impedance? And of course, can you post a schematic?

    If the preamp has 20-40 dB of gain and a reasonably low output impedance, like 600 ohms or fewer, then I don't think noise pickup will be a problem with quality shielded cable. Figure 2.1 in Les's paper should do it.

    ak
     
  9. peter taylor

    Member

    Apr 1, 2013
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    If this is the case, then why balanced ?
     
  10. peter taylor

    Member

    Apr 1, 2013
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    Dynamic microphones have low impedance, low voltage and high current (compared to condenser microphones), and are less prone Electromagnetic Interference (EMI, or 'hiss'). These typically use unbalanced cable with 6.5 mm phone jacks.

    Condenser microphones have high inpedance, low current and high voltage, and are more prone to 'hiss'. These have three wire balanced cable and XLR jacks, because balanced cable reduces interference.

    If dynamic microphone cables are run more than 5 meters, I would choose balanced cable.

    Wouldn't you ?
     
  11. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    yes both cold and hot, and in the differential they gonna cancel it out right?
    i found it this is where i read it, but i dont know i understand it well i think
    http://www.hypex.nl/docs/papers/The G Word.pdf

    thanks this help

    20db and bufered ouput opamp so i think its less than 1k ohm

    im just thinking is it worth the effort of making "20db 1kohms unbalanced signal to balanced in a single box, since im gonna insert it in balance soundcard, cause the job of the balance is to eliminate the "same signal"?
     
  12. bertus

    Administrator

    Apr 5, 2008
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  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I don't think so. The vast majority of dynamic microphones in recording studios, on stages, and in other professional environments operate as a balanced source because that's what they are at heart, with either an XLR connector (usually built into the microphone itself) or a 1/4" TRS plug. You can tie one end to ground in a TR plug, and that is common in vocal mics for bands because that is what PA amps take because PA amps are cheap, but that is a subset of the world of dynamic mics. And (referring to a previous post), tying one end to ground to unbalance it does not eliminate half the signal. OTOH, an electret element is by definition an asymmetrical source. Wiring tricks can provide phantom power, make it appear as a balanced source to a balanced input, etc., but the majority of electret mics are wired unbalanced. Note that electret mics in personal electronic devices, headsets, computers, etc. number in the billions, and far outnumber the high end balanced devices.

    ak
     
  14. peter taylor

    Member

    Apr 1, 2013
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    Microphones have weak signals. When this signal has to travel along a cable before it is amplified, it will pick up noise.
    Noise is also weak, but can be as strong as your microphone signal. They both enter your amplifier, and you end up with both amplified vocals, and amplified noise. The noise can sometimes drown out your microphone signal.
    To stop this, the cable will be shielded. The center signal wire is surrounded by a copper braid or silver foil , so external noise is grounded or reflected, before it reaches the center signal wire.
    If the signal is very weak, as is the case with good quality condenser microphones, a balanced cable is used. This also has a shield which is grounded, but there are two signal wires that are twisted together.
    Your microphone signal is sent down one wire, and an inverted version is sent down the other wire.
    At the amplifier end, a circuit will recombine the complimentary signals to a singe signal, by inverting one wire, and then amplify it.
    What happens is, external noise will have the same polarity on both wires, and when one wire is inverted and combined with the other wire, the noise will cancel itself out.
    But since the microphone signal is already inverted on one wire, inverting it and combining it with the other wire gives you the original microphone signal, minus the noise.
    I think (and correct me if I'm wrong), that the hot wire on a three pin XLR plug, is simply the un-inverted signal, so that it can be joined directly to the center pin of an unbalanced 6.5 mm phone jack, along with the ground. The cold pin is the inverted signal, and could also be connected in an unbalanced configuration.
    The reason they are distinguished is that, ultimately, running a loud speaker from a non-inverted signal, and driving another load speaker from the same inverted signal, means that the sound vibrations in air will want to cancel.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Disclaimer - my pro radio and TV days are way behind me. Some information may have shifted from true to historically accurate.

    3 pin XLR wiring.
    Normal: 1 = shield, 2 = in-phase, 3 = out-of-phase.
    CBS: 1 = shield, 2 = out-of-phase, 3 = in-phase (hottest is the highest).

    The in-phase signal is a positive-going voltage when air pressure on the diaphragm is increasing. A true dynamic microphone is a completely floating, medium-impedance current source. Because it is floating, either end can be grounded to create an unbalanced source. The standard is to ground the out of phase leg, and use the in phase leg as the audio signal. Note that the out of phase leg *must* be connected to the preamp input signal return (GND). The shield connection is optional, and should never play a part in the audio signal current path. Also, because a dynamic mic is an inductor, the preamp input impedance has a direct effect on the tonal characteristics (brightness, warmness, etc.) and transient response of the microphone.

    An electret element needs a DC excitation potential, and the return path for that current *is* the audio signal current return path. Besides connecting one to a balanced input using methods in the papers linked above, many electret studio mics have other electronics inside to create in phase and out of phase signals that are independent of the bias voltage ground, some have transformers for that "transformer sound" and some have built-in vacuum tube preamps for that "warm, tube sound".

    All of this makes discussing the "right" way to interface an electret microphone *very* dependent on the exact microphone being used.

    ak
     
  16. peter taylor

    Member

    Apr 1, 2013
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    An introverted engineer will look at his shoes when talking to you. :mad:

    An extroverted engineer will look at your shoes when talking to you. :confused:
     
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