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Thread: Ground Loops 101

  1. #1
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Ground Loops 101

    This article is to help show what causes those pesky ground loops many of us have had to deal with at one time or another. Essentially what happens is that a signal is created between two ground points in the system. The gain from source to speaker can be very high and amplifies the “ground” signal, leading to that familiar raspy buzzing sound.

    Consider the system shown below. Please note this is not meant to be technically rigorous but to illustrate how ground loops can form. Red lines are positive signal wires, black lines are negative signal wires, and dashed green lines show the current flow. The source (CD player, BD player, turntable, cable box, etc.) is connected to earth ground but the preamp and power amp “float”. This shows a system where only the source has a three-prong plug. The signal current can only flow one way and terminates at the single (source) ground terminal. Ground noise affects only the source, but since all components reference the source, the noise is suppressed. Any ground voltage at the source moves its (+) and (-) outputs equally. With no other ground connections the preamp and power amp follow. No ground loop, no nasty ground loop hum.

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    Now consider the case when all components have three-way plugs or are otherwise grounded. Now there are three ground loops formed: source to preamp, preamp to amp, and source to amp. Any ground leakage or noise current in any component can circulate among these loops. The currents through the resistors cause voltage fluctuations that affect each component. Since some of the signal current now goes into the ground loops, instead of the straight signal path shown above, the voltage induced is seen as signal by the components. They cannot tell if it is “true” signal voltage or voltage caused by ground currents. Since the ground current is often 60 Hz leakage from transformers or unbalanced loads that is the source of the 60 Hz (and harmonic) buzz commonly associated with ground loops. Note that any ground noise can cause this effect, whether 60 Hz power line signals or EMI/RFI.

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    A simple simulation can illustrate the severity of the problem. Vsrc may be a turntable, CD player, cable box, etc. It is not used for these simulations thus its amplitude is zero. Notice the source ground, Gsrc, is connected to earth ground by another signal VGsrc. This represents the induced ground signal and is coupled to the source ground pin Gsrc through resistor R3. VGsrc is a 1 uV, 60 Hz signal representing induced ground noise. The preamp and amplifier (represented by ideal gain stages Epre and Eamp) are also connected to earth ground through resistors R4 and R5. Finally, signal grounds are connected through R1 (source to preamp) and R2 (preamp to power amplifier). The preamp has 60 dB of gain, the amplifier has 30 dB, and the 8-ohm load R6 represents a loudspeaker.

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    This example shows R1, R2, and R3 small (1 u-ohm), shorting the source, preamp, and amplifier grounds to the induced signal source VGsrc. They represent ideal cables and ideal source chassis ground. R4 and R5 are large (10^6 ohms) to float the preamp and power amp (e.g. two-prong power plugs). Because there is no low-impedance path to ground from the preamp and power amp, no ground loop is formed, as shown earlier. You can see this is the plot below; the signals are near 0 V at all points (f = femtovolt = 10^-15 V; u = microvolt = 10^-6 V). The top plot shows the source (blue), preamp (red), and amplifier (green) output signals. The bottom plot shows the ground signals at the source (blue), preamp (red), and amplifier (green). Even with only 1 micro-ohm there is some signal generated by the ground signal but it is very tiny (< 100 fV, or 0.000000000000001 V)

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    Now observe the results when R4 and R5 are also 1 micro-ohm, essentially shorting signal and earth grounds through tiny resistors. This represents the case when all components have three-prong plugs. The resistors act as voltage dividers to couple the signal grounds at each point in the chain. The 1 uVpk ground signal has been amplified to about 10 mVpk (about 6 uW) at the speaker. That would be about 36 dB SPL from a pair of 90 dB/W/m speakers 6’ (2 m) away. Still small, but what if the ground signal was 100 uV? That does not seem like much, but then the voltage across the speaker would be about 1 Vpk or about 62.5 mW. With a pair of 90 dB/W/m speakers the volume (SPL) six feet away would be about 75 dB, definitely audible (I would say loud!), and that is with no room gain. We have a ground loop; the signal ground goes one way, and chassis ground another, providing two places for the signal to flow. Now ground is not really ground so any ground signal is amplified, to our detriment.

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    Clearly small leakage or noise currents in the ground loops can cause very audible problems (as we know). Note cables typically have resistance of milli-ohms, not micro-ohms, and components may be plugged into different circuits so the ground path can be very long. Larger (parasitic) resistor values and additional ground signal sources will generate larger and potentially more complex noise as the signals interact among the components and with the primary signal from the source. Even low-level noise can often be noticed by its absence when the system is modified to eliminate ground loops.

    HTH - Don
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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  2. #2
    Site Founder And Administrator Steve Williams's Avatar
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    Wow Don

    That more than helps. I even understood it

    Thanks for doing that
    Steve Williams
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    There's ALWAYS another Steve Williams BUT there's only "oneobgyn"
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  3. #3
    Member Sponsor [Technical Expert] DonH50's Avatar
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    One additional figure will help show why balanced connections are helpful. As you can see, with differential (balanced) circuits, the signal (+) and (-) lines and current flow are independent of the chassis or shield ground. Ground loops can still form but do not impact the signal. However, it is possible for ground currents to couple internal to the component through various “sneak” paths in the circuits. A commonly used technique is to lift (disconnect) the shield at one end of each interconnect, breaking the ground loops, but maintaining the desirable shield to block RFI/EMI.

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    All for now - Don

    p.s. Thanks Steve! I thought about another figure showing a larger signal but the plots look the same, just scaled up.
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
    Don's Technical Articles on WBF

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    Addicted to Best! Rodney Gold's Avatar
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    Thanks don, Exceptional article and for me was apropriate
    you won't believe it but I encountered a ground loop issue just 1/2 an hour ago
    I daisy chained an extra sub off an existing one with good cable , and one sub was humming.
    Both using 3 prong plugs , I think they may be on separate circuits.. not too sure
    Try powering one of the subs with a 2 prong power chord?
    Roon/Tidal ..Squeezebox touch . DIRAC...Twin Devialet D Premier amps , Vivid Audio Giya G1 Spirits .. fully treated room

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    [Industry Expert] Addicted to Best! DaveC's Avatar
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    Quote Originally Posted by Rodney Gold View Post
    Thanks don, Exceptional article and for me was apropriate
    you won't believe it but I encountered a ground loop issue just 1/2 an hour ago
    I daisy chained an extra sub off an existing one with good cable , and one sub was humming.
    Both using 3 prong plugs , I think they may be on separate circuits.. not too sure
    Try powering one of the subs with a 2 prong power chord?
    Put them all on the same circuit.
    Industry Affiliation:
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  6. #6
    Member Sponsor [Technical Expert] DonH50's Avatar
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    @Rodney Gold: Thanks! And what DaveC said. A cheater plug I will use to isolate the problem but not as a permanent fix. It can be dangerous if you aren't sure what you are doing and/or have a real ground problem (rare, but rare diseases still kill). If you can't do that, and do not have the ability to use XLR, then there are numerous isolation devices for either the power line ground or the signal ground available from places like Amazon, Parts Express, Radio Shack, etc. These things range from <$50 to upwards of $1000 for a high-end enclosed Jensen transformer or active unit. Amazon link below. I have used the Hum-X, not any of the others. When I picked up another pair of subs for the rear of the room, on a different circuit, I ordered them with XLR inputs and have no hum.

    HTH - Don

    http://www.amazon.com/s/ref=nb_sb_no...round+isolator

    p.s. One problem with transformers is that some do not have very extended LF response. Try to find one with a spec that is lower than your sub. Or try the Hum-X isolator for the power line instead.
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
    Don's Technical Articles on WBF

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    VIP/Donor [VIP/Donor] Folsom's Avatar
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    Schurter DENO. You need someone competent to put it in. I suppose you could put one in a small box, too, that had input and output for AC cords.

  8. #8
    Addicted to Best! Rodney Gold's Avatar
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    Yeh .. bity about the balanced thing , one set has , the other doesnt.
    I'll try the cheater plug first
    The whole room is actually driven off a 5kw pure sine wave inverter that uses 6 x 200ah batteries .. I will have to check the grounding arrangement
    Other subs I have used have groundlift , which normally cures the issue
    I can also not daisy chain , but use 4 discrete outputs of my DSP box to each of the 4 subs I have , thus one sub is not connected to another via a rca cable
    Roon/Tidal ..Squeezebox touch . DIRAC...Twin Devialet D Premier amps , Vivid Audio Giya G1 Spirits .. fully treated room

  9. #9
    Addicted to Best! Rodney Gold's Avatar
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    I tried a cheater plug and better cables .. reduced but did not eliminate hum .. my next step is to try daisychain using a balanced cable from the main sub and then use a XLR to rca converter
    Roon/Tidal ..Squeezebox touch . DIRAC...Twin Devialet D Premier amps , Vivid Audio Giya G1 Spirits .. fully treated room

  10. #10
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    Much of the ground-loop hum problem is caused by poorly designed audio components. If you have hum, you need to build a John Windt 'Hummer Tester'.

    That is a quick summary of the 1995 paper.
    Kevin

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