Film-Tech Forum ARCHIVE: Harmonic Buzzing Noise in Analog

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Posted by Danial Simmonds (Member # 4638) on 06-25-2009, 03:36 AM:

Hello again,

I have came across a very weird problem.
I guarantee it is nothing to do with A-Chain.

From the front channel speakers, subwoofers and surround there is this buzzing noise that is like the electrical harmonic noise coming from them.

I have literally tried almost everything. I have done the A-Chain and B-Chain alignment, swapped all the cards of the CP500 and even swapped the QSC Amps...

This problem only occurred after a huge renovation was made opening an IT center above the cinema.

If you guys got any ideas please let me know.

Thanks alot...

Regards
Danial

Posted by Gordon McLeod (Member # 33) on 06-25-2009, 03:08 PM:

Disconnect all sources that fed the cp and see if it is still there if not then add one source at a time till it reappears and then start looking at it.
If it is still there with the cp with no sources connected then try disconnecting the cp output and see if the noise is there

Posted by Louis Bornwasser (Member # 3063) on 06-25-2009, 08:09 PM:

CP-500: there is supposed to be a small jumper from one of the LOW on the output plug to the chassis. (I use the sub woofer LOW.)

This is not in the older manuals and is quite often missing.

Hope it helps. Louis

Posted by Eric Robinson (Member # 2966) on 06-29-2009, 10:07 PM:

Yes...follow Gordons advice.
Also, you can hook an oscilloscope up to the output of one of your amps far a visual of the buzz. This way you can see when you have eliminated it.

Posted by Danial Simmonds (Member # 4638) on 06-30-2009, 01:02 AM:

Hey guys,

Thanks, i found that the problem is definitely from electricity. The noise will only go away after i disconnect the power for the analog pre-amp card located at the projector or disconnect the optical input behind the processor...

The projector is V5 Cinemeccanica... I got a power converter 240VAC to 24VDC and took a different power source and the noise disappear.

The noise only happens in Analog. Digital and other sources is okay. However this problem only started after a new floor (Computer shops and mobiles) was opened above the cineplex.

What you guys think?

Thanks
Danial

Posted by Hugh McCullough (Member # 1542) on 06-30-2009, 05:07 AM:

Danial.

Had a similar problem in the 1970s.
We suddenly started to pick up a low buzzing on both optical, and magnetic sound. No digital in those days.
We eventually traced the problem to the Underground station next door.
They had had the station refurbished, and a new escalator installed.
It was the motor of this escalator that we were picking up.
We contacted the underground people, and within about two days they had cured the problem at their end.
Has this new floor above the cinema got an escalator, or some other motorised machinery that could be causing the trouble?

Posted by Danial Simmonds (Member # 4638) on 06-30-2009, 09:48 AM:

Well ya, they had placed a total of 2 new escalator's right in front of the cinema, and also they tapped in the electricity just right before the cinema termination... But the shopping mall doesn't want to do anything.. would there be a solution around this?

Posted by Jack Ondracek (Member # 1466) on 06-30-2009, 12:55 PM:

Years ago, I had an intermittent whining sound in an audio studio I ran. I eventually traced it to a building next door. The owners had upgraded their elevator, including a new hoist with an electronic power supply.

The problem was similar to the interference you can get when cheap light dimmers are hooked up nearby. In this case, the SCRs in the motor's variable power supply were not filtered. It only took some capacitors across the input power to fix the problem. Something along those lines might be what's happening to you, and filtering at the offending equipment could take care of the problem.

Posted by Danial Simmonds (Member # 4638) on 06-30-2009, 09:37 PM:

Hi Jack,

Thanks for the reply...

So i should try to put capacitors at the power input to the analog pre-amp?

Thanks
Dan.

Posted by Jack Ondracek (Member # 1466) on 06-30-2009, 10:24 PM:

If your situation is similar to mine, the best place to install filters is where the power goes into the offending equipment. That prevents the interference from going into the power system in the first place.

I wasn't able to eliminate the problem until I did that.

Jack

Posted by Mark Gulbrandsen (Member # 72) on 06-30-2009, 10:33 PM:

I had a similar problem when I installed a screening room in Houston. There was a factory about 100 yards from the office park and the company made railroad tank cars. You can imagine the types of welding equipment they had over there... the stuff threw back so much garbage onto the power line that you could see it in the projected light on the screen. The only cure there that worked was to install switching power supplies for the lamphouses.

Mark

Posted by Justin Gorka (Member # 3727) on 07-03-2009, 05:59 PM:

Deaf loop? I had it a few years back. Do you have deaf loop? If so check all the cables in the trunking. One minor short will do the job(Coke normally is good at it!)

Posted by Danial Simmonds (Member # 4638) on 07-03-2009, 09:47 PM:

Now the offending supply to the projector is a 3 phase power input, were using cinemeccanica machine's here and the power for the analog is taken from one of the 3phase lines...

However what i did was this;
I took the power source "single phase" from another power point in the projection room, hooked it up to a 230VAC Step down 24VDC for the analog pre-amp power and it did got rid of the noise for 2 halls but the rest it only reduced it...

Any suggestions on where and what filters to install?

Thank you all for your kind advice :-)

Regards
Dan.