Buzz/Hum Hm?

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Buzz/Hum Hm?

Post by awjoe »

I wonder if you could suggest ways for me to deal with this small problem.
(I call it a small problem because it's smaller than it used to be.)

Here's what I'm trying to do - I want to run both my KRKvxt8's and my Avantone mixcube through my new Mackie Big Knob Passive monitor controller and have minimal buzz/hum.

There's no problem with the KRK's through the Big Knob A Channel - I'm using TRS 1/4" jacks and it's quiet.

I then tried unbalanced cables with the Avantone, but the noise was terrible. So I hooked up the Avantone this way:

* XLR into the Avantone, which splits into two XLR female jacks.

* I attached two XLR male to 1/4" balanced jacks to the females, and the 1/4" balanced jacks into the Mackie Big Knob controller B Channel inputs.

I now get buzz/hum from the Avantone (but much, much less than when I used an unbalanced connector), but here's the kicker - I'm now getting a bit of hum/buzz from the KRK's as well, hum/buzz that wasn't there before. If I turn the Avantone off, the hum through the KRK's stops. Bad Avantone. Bad.

Any ideas about how to get my monitors as quiet as a quiet thing?
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Re: Buzz/Hum Hm?

Post by ef37a »

To be brutally honest Awjoe, I suggest you ditch the Mackie PBK in favour of a better controller (even a small mixer is better IMHO!)

The Passive BK suffers from two (three if you count that they have done the same Bad Thing twice!)

First off, the 'cold' side of the balanced (not really) input TRS has the low resistance of 1k to ground. This is low enough to cause distortion with some sources. Even if there is no distortion the input resistances are about an order apart and so all 'balance' is lost.
Secondly the 'hot' 'cold' output lines are again at very different resistances and therefore you are NOT getting anything like a balanced output.

My PBK is now gathering dust in a spare bedroom (free to any passing oik!) and I use a Behringer X 1202 to 'gather' signals from my TV (via an optical D/A) a CD players, feeds to and from my M4 and ultimately a DI feed from an HT-20. Sounds very good to me on my Tannoy 5As and noise levels are well below audibility.

Dave.
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Re: Buzz/Hum Hm?

Post by Hugh Robjohns »

You probably need to isolate the Avantone ground from that of the Passive controller to break the ground-loop which is causing the hums. So I'd suggest installing an ART DTI or Cleanbox2 between the controller and aventones.
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Re: Buzz/Hum Hm?

Post by ef37a »

Hugh Robjohns wrote: Sat May 21, 2022 11:31 am You probably need to isolate the Avantone ground from that of the Passive controller to break the ground-loop which is causing the hums. So I'd suggest installing an ART DTI or Cleanbox2 between the controller and aventones.

I am sure you are right Hugh in that Awjoe's problem is a ground loop I just wanted to make him aware of the problems with the BK which might make diagnosis more difficult? I.e. hum in a 'balanced' system that is not by any means balanced is not a good start!

My first fix was a pair of SparkFun U/B to balanced out modules to feed the Tannoys. I only needed the hot signal from the BK. Then I reasoned that the feed from the hi fi amp to the BK might not be so good so I put the Sparks in that line and now get a balanced speaker feed from the mixer.
One day I shall treat myself to a better, active controller.

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Re: Buzz/Hum Hm?

Post by awjoe »

Dave: I just got the Big Knob, so I don't want to ditch it right away lol. I like everything about it, other than the small hum issue when I add the Avantone to the system.

Hugh: I'll try the ART DTI - I've got an ART ZDirect for bass recording purposes, and I've liked it.

Thanks very much, you two. My tech chops don't extend beyond 'use balanced cables for this'.
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Re: Buzz/Hum Hm?

Post by awjoe »

In the end, satisfaction was had, not by the insertion of an isolator, but simply by plugging the second monitor (the mixcube) into a socket on a different circuit. Duh.
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Re: Buzz/Hum Hm?

Post by Martin Walker »

awjoe wrote: Mon May 23, 2022 12:35 am In the end, satisfaction was had, not by the insertion of an isolator, but simply by plugging the second monitor (the mixcube) into a socket on a different circuit. Duh.

I'm glad your hum has gone away, but it will be back another day ;)

Essentially by plugging in your monitor to a different mains socket you have changed the size of the ground loop, which (as luck would have it) has reduced the amount of hum. However, that ground loop is still there, and when you plug some other gear item in that hum could return with a vengeance. Just enjoy the relative silence for now.
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Re: Buzz/Hum Hm?

Post by awjoe »

Martin, I'm glad you educated me on that. Otherwise, when the hum returned, I'd have been suspecting the work of demons or something. :thumbup:
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Re: Buzz/Hum Hm?

Post by Hugh Robjohns »

awjoe wrote: Mon May 23, 2022 12:35 am In the end, satisfaction was had, not by the insertion of an isolator, but simply by plugging the second monitor (the mixcube) into a socket on a different circuit. Duh.

Ah yes. Rule 1: Always make sure all the gear is powered from the same wall socket, ideally with a star-distribution of plugboards. This minimises the size of the 'loop' and ensures everything is tied back to the same single safety earth reference.

Using different wall sockets almost guaranties a ground-loop with different safety earth reference voltages to different bits of gear, and hence audible ground-loop hums.

But as Martin says, the physical ground-loop is still present, it's just that both ends are at the same ground potential, so there is very little noise current flowing and hence inaudible hum. So adding more gear, and especially if that is powered from a different wall socket, is likely to alter the ground current and may thus bring back audible hum.

The isolation transformer suggested earlier inherently breaks the loop and provides a permanent fix.
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Re: Buzz/Hum Hm?

Post by awjoe »

FWIW, the problem cropped up when all devices were plugged into the same electrical source, the same plugboard. The problem ended (Martin says the fix is likely temporary) when I plugged one of the devices (the mixcube) into a different socket entirely. This doesn't follow the pattern you're describing, so it looks like it's a case of 'variables produce variability', and I expect the problem will re-appear at some point, and from what you've said and from what I've been able to understand from reading about it, the isolator looks to be the simplest solution.

I'll get this working though, because the advantages of having the contoller are numerous and powerful.
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Re: Buzz/Hum Hm?

Post by blinddrew »

I have a similar situation with a valve mic. If it's plugged into the same socket as everything else I get hiss and hum.
Stick it in a socket on the other side of the room and it behaves itself perfectly.
Ground loops hey! :roll::headbang:
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Re: Buzz/Hum Hm?

Post by Hugh Robjohns »

Oh... sorry. Confirmation bias: I read what I expected to be the case and not the actual case!

Ground-loops are little buggers! :think::ugeek:

There are a number of possible explanations for your apparent good fortune, but the scariest one is that the alternative wall socket is lacking a safety earth completely, or it has a high enough ground impedance to subdue the loop current to a negligible level.

Alternatively, it's possible that you have multiple loops and the current via one route ends up being in the opposite polarity to another, effectively cancelling each other out.
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