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Ground loop noise, why it is and what you might do about it

Posted 19th January 2016 at 12:37 AM by bwaslo

I recently was motivated to figure out numerically what might be happening when you get hum out of your power amplifier only when connecting other cables to it. And why a fix I sometimes use works so well.

Before you go blowing your budget on expensive audio isolation transformers, please take a look!


Hope someone finds this useful.

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  1. Old Comment
    The use of a 10R resistance in parallel with a diode bridge to shunt fault voltage/current directly is pretty commonly used as part of the amplifier audio ground to earth connection. This is not the same as putting resistance in series with the shield, but is in the ground loop path.

    The nature of the problem is this: when you have TWO paths (cable shield AND earth ground) you form a loop that is available for inductive coupling to AC fields, e.g. from the mains, that are all around us. This is shown by the dashed line in Figure 1 at the first link below, and the text in the section "What Causes Earth (Ground) Loops?" further down that page.

    Another great way to form ground loops is when you have MULTIPLE shielded cables running between two pieces of equipment (even if they are NOT earthed!). If you separate the cables, you again are forming a loop with their shields and hum pickup ensues. I learned this from my own experiences. I could pick up and move the cables farther apart or closer together, and this would change the amount of hum present in the system. In that case adding small (e.g 4.7R) resistors to each shield was effective.

    You need to break at least all but one of the paths (e.g. N-1 of the N paths) to avoid forming the loop.

    For more info, see:
    Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

    See the PS schematic on p11 of the pdf at link, below. An NTC thermistor is used instead of a resistor, but the concept is the same.
    Posted 19th January 2016 at 05:25 PM by CharlieLaub CharlieLaub is offline
    Updated 21st January 2016 at 04:21 AM by CharlieLaub
  2. Old Comment
    Triodethom's Avatar
    I find the NTC thermistor for me the most elegant for this use. One that is about 10 ohms cold and very low hot . This works as a high enough impedance for low current and low impedance when current is high if there is a safety fault. Un-balanced cable is the design of a frugal unbalanced mind . Using the chassis for more than safety is cost effective however not the most elegant way to do it.
    Posted 18th February 2016 at 01:55 PM by Triodethom Triodethom is offline

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