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Strange hum

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Redflag
Strange hum
on Sep 21, 2006 at 11:08:12 am

Hi there

Today in a studio the sound guy was getting a pretty load hum off the mixer when he physically touched the faders or the jack where the head phones go into the mixer.When he didnt touch anything it was fine.The mixer is totally pro one that cost over 3 grand.

One thing, it was literally first time used back from full service? anyway he couldnt account for this noise,wasnt from cable,any idea,s?
Iam presuming it wasnt on the tape either.No time to check.

Thanks



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Matte
Re: Strange hum
on Sep 21, 2006 at 11:35:24 am

Thanks for fill us in!


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Redflag
Re: Strange hum
on Sep 21, 2006 at 12:43:54 pm

eh! whats the problem Matt?



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Matte
Re: Strange hum
on Sep 21, 2006 at 7:27:41 pm

I thought you must have been just telling us a story.

If you had an actual question, I figured you would have mentioned things like:
brand name, and model number of the console and the type and amount of gear plugged into it, the age of the console, the number of inputs, European or USA power supply, Line inputs open, Miic inputs open, Settings of the sub-masters and masters, patch-panels, Audio DA, monitor amps, the reason it went into "full service" in the first place, and why you don't ask this question of the person you paid to service the console...
at least a FEW things of interest like that.

Without any kind of information whatever, how can we help just sitting out here in the vacuum of cyberspace?

Hence, my cryptic and incomplete first response.


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Redflag
Re: Strange hum
on Sep 21, 2006 at 9:22:56 pm

haha yes now I see it!
The mixer is Japanese,sold in the US as Kame 302.The service was routine,not specific.There was only one 416 shotgun mic plugged in,at normal volume ie an interview in a studio.
Iam not the soundguy,but as i said,he had no answer to this hum which happened only when he touched the faders or headphone plug,where it joins the mixer.
So sorry not high on tech input,except the mixer is a good one,it was just serviced,one 416 going into it.
and this mystry hum thing.
Just wondered if any of the experts would have any idea what it might have caused it ..thats all.

Thanks again for your time



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Matte
Re: Strange hum
on Sep 21, 2006 at 11:08:33 pm

Could be that when the board was serviced, the tech failed to reconnect an internal grounding strap/cable.
The board likely has a "floating" ground internally (as opposed to all of the grounds/shields being connected to the "earth-ground" on the power plug.)

You need a qualified tech to check the board for open grounds/shields at all of the exposed metal at points where the operator is in close proximity. I'd go FIRST to the outfit that recently serviced the board... they really "owe you" a properly operating device.

Also, if the board's power plug is a 3-prong grounded type:

A) If there is no ground-lift AC adapter connected at the wall socket, add one.

B) If there IS a ground-lift AC adapter connected at the wall socket, remove it.

Do this just to "test" if it makes a difference, as operating full-time "ungrounded" has a potential for hazardous electric shock.

Did you check to make sure you had a good cable (shield attached at both ends) coming if from the MIC?

Another possibility is that one or more of the internal boards is not seated correctly after being serviced.

Still another is lack of proper grounding/shielding on the output cable(s), monitor and otherwise, but a LINE-levels, that's usually not a major issue.


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Ray Palmer
Re: Strange hum
on Sep 22, 2006 at 1:32:07 am

Matt has a great point. First rule of trouble shooting.... reduce the number of variables.
I would first remove all inputs and outputs and monitor the mixer on a set of headphones. If the hum is still there, then it is indeed either the mixer or headphones.
If the hum is still there and lifting or adding the AC ground doesn't fix the hum, I would open the mixer, reseat the boards and look for grounds not attached.


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Redflag
Re: Strange hum
on Sep 22, 2006 at 11:36:22 am

Hi there

wow great thanks for advise.Today I had time to quickly set up the mixer with the same cable,headphone and mic.The cable has been used for a while and I believe is properly shielded,and in good condition.
In a car park I didnt hear any hum off the faders or head phone cable all was totaly fine.

The hum occurred in a studio that had alot of other cables,lighting boards etc around,I guess this might have had sometimd to do with the hum? would this also imply something is not grounded in the mixer?

Iam sorry not very tech again.Another problem is Iam in Japan and comunication is a problem,as otherwise ofcourse I would have just called the guys who did the service,which is the company that makes the mixer(Sigma) over here.So I would expect they know what they dont...but you never know I guess !!

Thanks again for both your time



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Redflag
Re: Strange hum
on Sep 22, 2006 at 11:39:31 am

Sorry I didnt make this clear,although we were in a studio,the mixer is smallish 3 channel field mixer,not a big desk.It was a sound stage not a recording studio.Sorry stupid, I just realized the mistake!!



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Ray Palmer
Re: Strange hum
on Sep 22, 2006 at 2:56:23 pm

If it was a field mixer, can you run this off of batteries as a test? Eleminate the AC power factor all together!
If you have hum while on batteries, and no mics plugged in, you really have a problem with the mixer.



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Redflag
Re: Strange hum
on Sep 22, 2006 at 10:52:19 pm

Yes the mixer is only ran on batteries,so not AC problem.Plugged it up with same cable and mic just in a car park....and no hum at all?

Must have something to do with the studio environment,but why? it s on line level out to the camera,and phantom +48v to the shotgun mic..Sen 416



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