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  1. #21
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    I was talking about the connector on the mixer, not what cable you're using. There are new DJMs with either 3 pin or 2 pin connections.

    Edit: Not the best picture, but you can see the ground pin on the DJM900:
    http://www.agiprodj.com/images/pione...ed_edition.jpg
    Last edited by makar1; 02-02-2014 at 09:00 AM.
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  2. #22
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    My question is - how do you know if you should intervene at the POWER source, or in the AUDIO SIGNAL CHAIN???
    I have a couple isolation transformers, but I rarely use them. I generally opt instead to address the issue by altering my power situation. I start by looking at how I have things plugged in, and see if I can avoid having multiple paths to earth in the first place. If this doesn't fix the issue, or is for some reason not possible, then I unplug mains power from my laptop (and run off battery for a moment). If the buzz stops, I use the HumX on the laptop mains plug and it's fixed. If the ground loop persists, then I try it on the mixer (follow all proper power on/off sequences for your venue). 95 times out of 100 one of these steps will fix your issue. If it doesn't, you need to start looking for racked gear or equipment with "noisy power" (anything with a motor for example). Prime culprits are fog machines, refrigerators, etc. Get these devices off the power circuits you've got your audio gear plugged into (borrow an extension cord). This brings us to something else...

    There is only one type of ground loop hum - 50/60Hz. The rest is just general interference.
    Ground loops can manifest themselves in many ways - not just an easy to pinpoint 50/60 cycle hum. I've had computers that only made noise when the mouse was being used - or the hard drive was being accessed (for example). I've had ground loops only manifest themselves when a door to an amp rack was fully closed. That's because the conditions that are causing the ground differential that causes a loop CAN MODULATE. When that happens, it can also modulate the sound of the buzz in a way that makes it sound more like RF chatter - or make the sound only appear at certain times. Another thing to watch out for is noise being dumped to ground by a piece of gear (especially "chassis ground" in a rack).

    A good rule of thumb for chasing down ground problems is this. Treat your computer / audio interface first. If that doesn't fix it, look for things that connect multiple devices (like a mixer, or an equipment rack). Start troubleshooting from your computer down the signal chain.

  3. #23
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    Cheers for this. Have bookmarked it and will check it out!
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  4. #24
    DJTT Moderator bloke Karlos Santos's Avatar
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    Quote Originally Posted by makar1 View Post
    I was talking about the connector on the mixer, not what cable you're using. There are new DJMs with either 3 pin or 2 pin connections.
    Got ya.

    Quote Originally Posted by nem0nic View Post
    Get these devices off the power circuits you've got your audio gear plugged into (borrow an extension cord). This brings us to something else...
    If only it were that easy in some of the crappy bars in the UK. The fridges are the prime cause of the hum and the electrics in old pubs and bars are very rarely designed in a way that you can avoid them or something similar.

    I did what you suggested at a pub once were we used a lengthy extension cable to plug into the living quarters above the pub. That worked but we couldn't do it in the winter (or most of the year in UK) as we had to go out of a window, up an outside wall and back in through a window into the landlords living room

    I would be better off patching into a street light at some venues

  5. #25
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    I used to have ground loop issues quite a bit when I first started mobile djing. Most of the time I could prevent it by plugging everything into one outlet and using a ground loop isolater between my mixer and unbalanced DSP. I also used an adapter on my laptop power cable. This worked most of the time.

    I have since switched to a balanced DSP and now everything from my mixer to my speakers/amps uses balanced xlr connections. I still use the plug adaptor on my laptop, but have not had a ground loop problem since, even when plugging into different outlets.

    My advice is to use balanced connections wherever possible, as it is the only way to guaranty a hum free system.
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  6. #26
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    My advice is to use balanced connections wherever possible, as it is the only way to guaranty a hum free system.
    Balanced connections do not in ANY WAY guarantee a hum-free connection. Ground loops manifest themselves in balanced audio signal paths all the time. Unbalanced connections are MORE susceptible to ground loops, but by no means are ground loops exclusive to unbalanced connections.

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