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ground loop issue with iMac, hum eliminator not helping

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ground loop issue with iMac, hum eliminator not helping

Postby donnie darko » Fri Apr 03, 2020 11:47 am

Hi there,

I have a simple set up, 2017 iMac into an M-Audio M-Track audio interface, then out to my monitors. I am having electrical interference which my Morgan acoustics balanced hum eliminator is not fixing. It worked on my old Mac G5, but is not working on my current, nor my previous iMac.

I output the interface using a balanced cable, into the hum eliminator then another balanced cable into the monitors, the interference does disappear, but at the same time the volume level dips, the lower frequencies are suppressed and there is a slight amount of distortion, does anyone know what is causing this?

I don't hear any inference when listening through my headphones plugged into the interface.

Thanks folks,

Don
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Re: ground loop issue with iMac, hum eliminator not helping

Postby blinddrew » Fri Apr 03, 2020 12:42 pm

donnie darko wrote:but at the same time the volume level dips, the lower frequencies are suppressed and there is a slight amount of distortion, does anyone know what is causing this?
That sounds like you've got a balanced to unbalanced connection going on somewhere. Or one channel is getting polarity inverted somewhere.
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Re: ground loop issue with iMac, hum eliminator not helping

Postby Hugh Robjohns » Fri Apr 03, 2020 12:49 pm

donnie darko wrote:I output the interface using a balanced cable, into the hum eliminator then another balanced cable into the monitors, the interference does disappear...

So the hum eliminator is solving the ground-loop problem, then!

[quote]...but at the same time the volume level dips, the lower frequencies are suppressed and there is a slight amount of distortion, does anyone know what is causing this?-/quote]

All artefacts that are typical of a poor quality or inappropriate transformer, I'm afraid... Basically, the transformers in that Morgan box are probably being saturated by the relatively high level audio signal you're routing through it.

You have three choices:

1. If possible, reduce the output level from your interface and turn up the sensitivity of the speakers. That will reduce the signal level through the transformers and might be sufficient to take it out of saturation to reduce the loss of level and LF, as well as lower the distortion... but this solution might not be fully effective or practical.

2. Invest in a better quality line isolator box. I routinely recommend the ART DTI box which is twice the price of the Morgan and it's pretty good... Most people seem happy with its qualiuty, anyway (including me for most things!) It's still not perfect, though and a really top-quality transformer isolation box with really good Jensen, Lundahl, or Sowter transformers would set you back about £250.

3. Ditch the transformer box completely and perform some minor surgery to the connecting balanced cables. At the speaker ends of both balanced cables, open the connectors and snip out the cable screen connections (to the sleeve or pin 1). You'll probably need to use some insulating tape to make sure there isn't a stray connection when you reassemble the connector. It's also a good idea to mark the cables in some way to remind you that they've had surgery!

That should break the ground loops and avoid introducing the level dip, distortion and loss of low-end that comes with underspecified transformers.

H
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Re: ground loop issue with iMac, hum eliminator not helping

Postby donnie darko » Fri Apr 03, 2020 4:11 pm

Thank you both for your replies!

Yes I suppose the hum killer did do it's job haha, but is not quite good enough, was only £25 when I bought it several years ago.

I think I'll give the ground snip a go. I always thought balanced cables were supposed to eliminate this kind of noise interference anyway, if removing the ground wire will eliminate the noise, why don't they sell the cables without it? just curious.

Thanks guys

Don
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Re: ground loop issue with iMac, hum eliminator not helping

Postby Hugh Robjohns » Fri Apr 03, 2020 4:38 pm

donnie darko wrote:Yes I suppose the hum killer did do it's job haha, but is not quite good enough, was only £25 when I bought it several years ago.

You do generally get what you pay for... Good audio transformers that can handle high levels of audio without distortions are most definitely not cheap!

I always thought balanced cables were supposed to eliminate this kind of noise interference anyway

The problem is not interference in the usual sense -- there is no EM or RF interference getting into the cable. Instead, the problem here is that one of the devices -- the interface or the speakers (or both) is susceptible to circulating ground currents.

In well-designed gear, circulating ground currents will naturally flow around the Faraday screening of a system (equipment cases & cable shields) and they should be sent directly into the mains safety earthing. Unfortunately, in some equipment it is allowed to pollute the audio circuitry's ground reference through bad design, and that's how it becomes audible.

...if removing the ground wire will eliminate the noise, why don't they sell the cables without it? just curious.
[/quote]

The assumption is that the equipment is properly engineered, and the requirement is not to stop circulating ground currents, but rather it is to minimisme external EM and RF interference. So almost all commercial cables are provided with a screen which is connected to the ground terminal/chassis at both ends.

I say 'almost' because it used to be common practice to make PO316 style (post-office) patchbay cables with only the hot and cold wires connected, and no cable screen at all.

The presumption was that interference wouldn't be likely in a well-engineered studio, and omitting the screen avoided any potential ground-loop issues and made the cables more flexible.

In my BBC days patch cables came in lots of colours: red, green, black, and none had screens... but the yellow patch cables were always wired polarity-reversed, and only the blue-coloured patch cables could be relied upon to have a full screen connection!

Seriously!

Let us know how you get on with the cable surgery.
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Re: ground loop issue with iMac, hum eliminator not helping

Postby donnie darko » Fri Apr 03, 2020 6:53 pm

Well, I don't know what to say other thank you! that's done the trick.

I got slightly worried when I snipped the first one, connected to the speaker I got a really loud hum lol, but once I turned the audio interface back on it stopped. Would that hum be caused by RF & EM?

Don
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Re: ground loop issue with iMac, hum eliminator not helping

Postby Hugh Robjohns » Fri Apr 03, 2020 7:45 pm

Glad you're sorted. The loud hum is probably a down to a combination of things, but basically interference getting in to the cables now the screening is reduced.

However, as long as you turn the speakers on last, and off first, it shouldn't be a problem.
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Re: ground loop issue with iMac, hum eliminator not helping

Postby donnie darko » Sat Apr 04, 2020 11:56 am

Thanks for your help Hugh, I'd been stuck with this for 8 years :roll:

Don
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Re: ground loop issue with iMac, hum eliminator not helping

Postby Hugh Robjohns » Sat Apr 04, 2020 12:32 pm

:D :thumbup:
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