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How do I cure ground loop hum?

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How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 12:43 am
by musicbox
I have a hum which I really would like to remove. In order to identify the source I have unplugged every thing from the mains. The power supply is a clean, dedicated seperate supply to the studio in the form of three spurs. The hum appears as soon as I plug in the Mackie 32.8 power supply and does not change at all as all the other equipment is plugged in. I have therefore concluded that the power supply is the source of the problem. The question is how do I get rid of it? and am I right in my diagnosis?

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 9:15 am
by Stickybud
O that's Martin Walker, the PC guru, in his signature it says: You hum it, and i'll trace the ground loop. He's the man to ask if any. I think a power conditioner is a good investment if you have a lot of audio gear. Good luck finding it.

:angel: :headbang:

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 10:31 am
by musicbox
I think you are right! Any advice Martin?

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 12:33 pm
by mattneighbour
Is it an acoustic hum coming from the psu, or is it a ground loop hum caused when the Mackie is plugged in to other equipment? Or is it mains hum that gets from the Mackie to the monitors?

If you could plug the Mackie only into your monitoring system and report, it would help us diagnose the problem. Also try just plugging in the Mackie and listening on headphones - any hum?

Matt

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 3:37 pm
by musicbox
It isn't an acoustic hum. I am told it is mains hum. I am not at the desk at the moment but I will check with headphones and report back either later this afternoon or early evening. i have tried different monitors ( my active Mackies and passive Yamahas and the hum is still there. Nothing else is connected to the mains but the desk and speakers. However, in my tests so far I have left all audio connections in situ and only disconnected the power. I am grateful for your input.

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 4:08 pm
by Martin Walker
Here I am ;)

Matt's highlighted an important point - you first need to know for sure that the Mackie mixer is humming before being plugged into active speakers or amp and passive speakers. if you can hear that hum through headphones with just the mixer plugged in, it's a mixer fault (possibly from a failing electrolytic smoothing capacitor in the PSU, or similar inside the mixer, depending on its design - all capacitors tend to degrade over time, which is why so many old cassette decks exhibit alarming hum levels).

Given that it's an external PSU, make sure also that it isn't situated near your audio cables either (leaving it on the floor with loads of other cables draped nearby is a recipe for disaster).

If your hum only appears when you plug in the amp/speakers its a ground loop caused by both the mixer and amp being earthed. This can be cured by balanced cables between mixer and amp, or (in the case of an unbalanced mixer output and balanced active speaker input) pseudo-balanced cables.


Martin

Re: How do I cure ground loop hum?

PostPosted: Wed Mar 23, 2005 11:42 pm
by musicbox
I have disconnected the speakers and can still hear the hum through headphones. Am i right in thinking that this means the PSU? I don't think that it is the desk it's self as I have had the problem since the studio was built but, because of ribbon cable problems, Mackie have replaced the desk TWICE so I think it unlikely that three desks have been at fault but I have never returned the PSU. What do you think, a trip to Mackie with the PSU?

Re: How do I cure ground loop hum?

PostPosted: Thu Mar 24, 2005 12:50 pm
by mattneighbour
It's worth making sure the PSU is as far from any audio cables as possible first - I've had problems with an old Quad amp radiating hum from its transformer on to any nearby cables.

Anyway, it seems that we can rule out ground loops.

(Wow, that's a refreshing change!)

Matt

Re: How do I cure ground loop hum?

PostPosted: Thu Mar 24, 2005 2:39 pm
by Martin Walker
musicbox - I did note that you said previously "However, in my tests so far I have left all audio connections in situ and only disconnected the power." You have to physically unplug the other cables from the mixer if you're to rule out ground loops - even if the other gear is switched off you can get ground loops if its cables are still connected to the mixer

As a double check, I'd remove your Mackie desk from its current resting place and take to another room, plug it in, and listen on headphones again. As I mentioned, and Matt has repeated, the PSU can induce hum in nearby audio cables, so this should make sure that none of your other audio gear or cables are causing the problem. Just don't place the mixer on the floor, as this might just put it into close proximity to some mains cables and result in further hum!


Martin

Re: How do I cure ground loop hum?

PostPosted: Fri Mar 25, 2005 6:31 pm
by musicbox
Hello again
I think I have found the offending unit. Having telephoned Mackie they suggested that I unplug everything, remove the PSU from the rack and listen again through headphones, which I did and.....No Hum. I then started to plug in the equipment to the mains until the hum returned, which it did when I plugged in my Digitech vocalist. Without this connected the system is quiet. So, I know what it is, now what do I do about it? The vocalist is connected to the desk using balanced cables. Mackie guessed that the PSU in contact with the metal rack strip was a problem. Apparently I can buy isolation strips to cure that?? So what do I do with the digitech? Incidentley the PSU is not near any audio cables.