Balanced Line Technology

Balanced inputs and outputs have been used for many years in professional audio, but profound misconceptions about their operation and effectiveness still survive. Conventional wisdom about them is sometimes wrong. Balanced Line Technology web page gives you a good look at the balanced line technology in audio applications. The balanced line technology is also applicable to many other fields as well. Here is one piece of information picked from the Balanced Line Technology web page.

Ground voltages coupled in through the common ground impedance; often called “common-impedance coupling” in the literature. This is the root of most ground loop problems. the equipment safety grounds cause a loop ABCD; the mere existence of a loop in itself does no harm, but it is invariably immersed in a 50 Hz magnetic field that will induce mains-frequency current plus odd harmonics into it. This current produces a voltage drop down the non-negligible ground-wire resistance, and this once again effectively appears as a voltage source in each of the two signal lines. Since the CMRR is finite a proportion of this voltage will appear to be differential signal, and will be reproduced as such. The most common cause of ground-loop current is the connection of a system to two different “grounds” that are not actually at the same AC potential

23 Comments

  1. Twisted pair RCA cables again « Tomi Engdahl’s ePanorama blog says:

    [...] get rid of the noise that is coupled as common mode noise to the twisted pair. In systems that use Balanced Line Technology the twisted pair cable construction combined with differential receivers and signal sources make [...]

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  2. Value Trendsetter 30 says:

    Speaking of reviewing the content numerous will like this because it’s real and it’s good reading from an author that’s writing it for us to read

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  3. Tomi Engdahl says:

    Differential Signaling: Designing for Long, Fast, or Noisy Applications
    https://www.youtube.com/watch?v=DivVHJD_1Lg

    This video is your intro to Differential Signaling: Go faster, further.

    Bil Herd has covered single-ended topics like TTL, and CMOS, but when increase the speed or distance a signal needs to travel, Differential Signaling is key. This is why it is used in many modern standards like LVDS, CML, and LVPECL

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  4. Tomi Engdahl says:

    Cable shields
    http://www.edn.com/electronics-blogs/living-analog/4442166/Cable-shields?_mc=NL_EDN_EDT_EDN_analog_20160609&cid=NL_EDN_EDT_EDN_analog_20160609&elqTrackId=a7a526f434524aaebcb0179099f4f145&elq=67fb6fd9fb344b108c38beed0e9d229a&elqaid=32603&elqat=1&elqCampaignId=28476

    Of the different choices that are available for grounding a shield braid that encloses a differential pair of signal wires, please consider that the shield braid be grounded only at the signal source, at the input end, and not at the output end.

    As a first thought, and as something that is often advocated, grounding a shield at both ends may result in severe ground loop currents which could adversely impact EMI and isolation properties.

    As a second thought, with the shield grounded only at the output end
    the interground interference signal, Enoise, can induce a differential noise signal between the two outputs E1 and E2 that feed the differential amplifier

    As a third thought, grounding the shield only at the input end averts both the ground loop problem and the time constant mismatch problem.

    Enoise as a common mode signal so that no differential voltage is created between E1 and E2. The A2 differential amplifier is thereby protected from Enoise.

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  5. Tomi Engdahl says:

    The balanced line technology is also applicable to many other fields than audio as well:
    - Ethernet over twisted pair wiring uses balanced line technology
    - video over twisted pair technology convert video signals to balanced signal
    - wired telephones use long balanced lines
    - many interfaces (CAN, USB, RS-485, DMX-512 etc..) use balanced (differential) 2-wire interface
    - in electric power transmission, the three conductors used for three-phase power transmission are referred to as a balanced line since the instantaneous sum of the three line voltages is nominally zero

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  6. Tomi Engdahl says:

    https://ips.org.uk/encyclopedia/balanced-line/

    Balanced lines are typically – and quite incorrectly – explained as follows. A signal is split into two equal but antiphase parts in a balanced line driver. These signals are connected to each leg of a pair cable and eventually arrive at a balanced receiver. This device inverts one leg of the signal. In doing so it adds the antiphase signals together to reproduce the original one and also cancels out any in-phase interference signals that may have been induced in the cable en route. Almost all of this is wrong!

    The term phase should always be replaced by polarity, since phase implies shifting the signal in time rather than merely inverting it. But balanced lines are balanced irrespective of whether any signal is present and do not require symmetrical signals to operate correctly.

    IEC Standard 60268-3 explains the truth –

    “A balanced line is a two-conductor circuit in which both conductors … have the same impedance with respect to ground and to all other conductors”.

    If the impedance is the same for each leg then the voltage induced across it by interference will be the same, so there will be no difference in voltage between the two: there will be no “differential” interference voltage. Therefore balanced lines are distinguished by their ability to ignore interference and by nothing else.

    There is no requirement for each leg to be driven, let alone symmetrically, and polarity inversion is not always used in the receiver – a simple transformer, which is the classic balanced input, cannot invert one signal leg. What is universal in receivers is the ability to sense the line differentially (differential mode) – the difference in voltage between each leg – since this is the distinction between the wanted signal and most interference, which will be common (common mode) to each leg.

    Balanced lines are, in effect, extended Wheatstone bridges and the driver, cable and receiver all have a role in maintaining the balance condition.

    Poor tolerance resistors or bad contacts at any one point can have severe consequences in damaging the ability of the entire circuit to reject common-mode interference even though this may not be immediately obvious in the handling of the wanted, differential signal. Under some broadcast conditions a 1 ohm common-mode impedance difference can reduce interference rejection by 15-20 dB.

    As with so many audio matters balance will vary across the frequency range. Good transformers can maintain their common-mode rejection across a very wide bandwidth but many electronic input circuits lose this ability at higher frequencies and can be very poor at rejecting RF interference.

    Most (though not all) electronic inputs cannot “float” – the maximum common-mode voltage is limited to the voltage swing of the line receiver – and large levels of interference may restrict the undistorted dynamic range of programme. Transformer-connected systems can float

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  7. Tomi Engdahl says:

    SparkFun THAT 1206 InGenius Breakout
    https://www.sparkfun.com/products/14002

    The SparkFun THAT 1206 InGenius Breakout Board offers an easy solution to adding a balanced audio input to your circuits. The THAT InGenius technology has been designed for high-grade analog line receiving and offers a low distortion and high common mode rejection in real-world audio applications. Each breakout board combines the THAT 1206 IC, its supporting components, and a ¼” TRS (Tip Ring Sleeve) socket. With these powers combined, you will find it very easy to use the input drivers on breadboards and in projects!

    SparkFun THAT 1646 OutSmarts Breakout
    https://www.sparkfun.com/products/14003

    The THAT 1646 OutSmarts Breakout Board offers an easy solution to adding a balanced audio output to your circuits. The THAT OutSmarts technology has been designed as a high-grade analog line driver and offers a low distortion and high common mode rejection in real-world audio applications. Each breakout board combines the THAT 1646 IC, its supporting components and a ¼” TRS (Tip Ring Sleeve) socket. With these powers combined, you will find it very easy to use the output drivers on breadboards and in projects!

    The THAT 1646 OutSmarts Breakout and its sibling, the THAT 1206 InGenius Breakout, perform mirror-image signal conversion.

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  8. Tomi Engdahl says:

    With balanced signals with decent equipment 500 ft distance is no big issue. I have used over 100 meters long cable runs with balanced analog audio.

    When you have audio cable between two different buildings, I would recommend using an audio isolation transfomer on that run, because different building can have somewhat different ground potentials. Worst case without isolation there is potential danger of huge ground loop noise, electrocution and even fried audio cable / equipment.

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  9. Tomi Engdahl says:

    Discussion from https://www.facebook.com/groups/517919938413134/permalink/1654766964728420/

    Some studio guys prefer unbalanced. They claim the differential circuit alters the sound. Use quality cables and you should be fine as people have said.

    That’s a new one, I’ve never heard anyone say they prefer the tone of unbalanced.
    The only downside of balanced inputs are that the Johnson Noise tends to be very slightly higher than an equivalent unbalanced front end. But when you consider the advantages, like immunity to ground loops, I’d prefer the balanced interface any day of the week

    The extra amp stage required for balancing is almost always audible. Many mastering engineers use unbalanced gear for this reason.

    I didn’t say I prefer unbalanced to balanced. However, I did read an article once on a mastering engineer who did.

    This audiophile article states the same.

    Just reporting….

    https://audiophilereview.com/audiophile-news/balanced-or-unbalanced-which-is-better/

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  10. Tomi Engdahl says:

    Understanding Common-Mode Signals
    https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/2045.html

    Abstract: To understand how common-mode signals are created and then suppressed, you should first understand the interaction of shields and grounds in common cable configurations. The following discussion defines a common-mode signal, reviews the common cable configurations, considers shielded vs. unshielded cables, and describes typical grounding practices. The article discusses methods whereby common-mode signals are created and rejected.

    The primary focus of this discussion is on RS-485/RS-422 cables and signals, but the discussion also applies to telephone, audio, video, and computer-network signals.

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  11. Tomi Engdahl says:

    Common Mode Analysis of Ethernet Transformers
    https://www.researchgate.net/publication/245513121_Common_Mode_Analysis_of_Ethernet_Transformers

    In this letter, a distributed model for Ethernet transformers subject to common mode signals is developed. Explicit formulas for transfer functions are derived and illustrated by numerical examples.

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  12. Tomi Engdahl says:

    Balanced vs. Unbalanced audio
    https://www.youtube.com/watch?v=PzEmKPTb18g

    Have you ever wondered what the differences are between the RCA single-ended connections and the XLR balanced connections in audio? PS Audio CEO and founder Paul McGowan shares with us the what’s, whys and wherefores of this subject.

    Why don’t more preamps have XLR?
    https://www.youtube.com/watch?v=XjiWv0ROjz0

    Tons of source equipment have both XLR and RCA outputs but very few preamps have more than one XLR input. Why is that? It just doesn’t seem to make sense.

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