Film-Tech Forum ARCHIVE: Buzzing in sound

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Posted by Tom Wienholt (Member # 1510) on 07-31-2003, 09:17 PM:

I have recently noticed a buzzing sound in one of my auditoriums. The buzzing is in all channels and is always there, whether in non-sync or stereo mode. The sound system is a CP50 with smart amps. I thought the amps were the problem so I swithched them out and the problem continued. The buzzing is always at a constant volume, even if the main fader is turned up or down. Also, if the processor is turned off the buzzing continues but is much louder. I tried turning of circuit breakers off one-by-one in the breaker box to see if the buzzing was from another circuit in the booth but the buzzing was constant. Also, i checked the wiring between the processor and amps as well as the cell and processor but all was correct. What could this be?

Posted by Gordon McLeod (Member # 33) on 07-31-2003, 09:45 PM:

Sounds like a ground issue

Posted by Scott Manley (Member # 867) on 07-31-2003, 10:00 PM:

Recently we found one house had a hum.....has a Ultra Strereo...cant think of the amps.....ended up that the power for the rack wasnt grounded properly...ran a ground wire from the rack (screw on a junction box) to part of the gutter. Hum went away.

Posted by Josh Jones (Member # 447) on 07-31-2003, 11:37 PM:

I also agree with the grounding issue theory. Find a good long piece of say 12 gauge wire and connect it to the frame of the rack. Then connect it to the gutter or breaker panel closest to this theater in the booth. Does the buzz go away? Also look into the usual suspect items like lighting dimmers, flourescent lamps, large transformer powered equipment and the like. Is the buzz still present with the amps off? Also, if that doesnt help, try running the dimmer up and down, listening for a change in the buzz's sound. A choke in the dimmer could be going bad in this case.

Good luck in your quest towords perfection [Wink]

Josh

P.S. Something else I noticed, be sure that all the signal cables have their shield grounded on ONE END ONLY, usually the amplifying or receiving end!! this can save you a lot of grief.

Posted by Darryl Spicer (Member # 711) on 08-01-2003, 02:36 AM:

God it's been a long time since I screwed around with a cp50. when you say the CP50 is off do you mean it is off at the power source breaker or on the power supply. I can't remember but didn't the CP50 go into bypass when you turned it off.

Do you hear the buzzing down stairs when the breakers to all booth equipment is off?

I had one of our theaters call me once about hearing buzzing coming from behind the screen. They did not hear it in the monitor but could hear it down stairs. I told the to look behind the screen to see if there was a neon power controle box behind it. Turns out there was and it was putting out a buzzing noise.

Posted by Gordon McLeod (Member # 33) on 08-01-2003, 11:14 AM:

I always pulled a #6 insulated ground wire directly to each rack from the main distribution panel and it was bonded to the isolated ground recepticals individually

Posted by Alan Haigh (Member # 1781) on 08-01-2003, 06:09 PM:

Could be power supply in booth monitor system "influencing" all the main amp outputs

Posted by Don Sneed (Member # 955) on 08-01-2003, 06:19 PM:

I would try to locate the noise source, try removing any non-sync wires to see if the music system could be ground looping...if noise still there, remove the solar cell inputs, keep removing wires (mark them of course) until noise stops, them connect them back as came off...non-sync, input, ect..ect...this would be the starting point for me after grounding & un-grounding didn't work....one thought, many as us use to put the non-sync on proj. # 2 preamp & padded down the signal to mic. level....check to see if the music source is connected to the proj.# 2 (if so...thats my first place to look)...good Luck !!!

Posted by Don Sneed (Member # 955) on 08-03-2003, 02:28 PM:

Tom, did you get a chance to see where the problem is & corrected it ???

Posted by Tom Wienholt (Member # 1510) on 09-08-2003, 08:35 PM:

Don
I did solve the problem. I discovered that the jumper on the back of the CP50 that connects the signal ground to the chassis was removed. I put a jumper between the two pins and the problem was solved. Thanks

Posted by Paul G. Thompson (Member # 655) on 12-07-2003, 05:08 PM:

After a couple of days of trying to solve an elusive ground loop hum in one of our theaters, the source was identified. I pass this information along to the rest of the technicians who encounter elusive ground loop hums in the equipment at hand.

First, the preliminaries:

1. A hum was reported to be present in the left channel of the Ultra-Stereo JS-100. The hum was present in non-sync as well as SVA format. There is no automation format selections interfaced with the Ultra Stereo. Non-Sync formats and SVA with surrounds are all active in this setup.
2. Disconnecting all inputs from the reverse scan reader silenced the hum. The hum reappeared when reconnected. Disconnecting the left channel lead to the Ultra Stereo optical pre-amp silenced the hum. Reconnecting the left channel and punching the system up in Mono, the hum disappeared. Returning to SVA with surrounds, the hum came back.
3. The hum goes away when the LED power supply for the reverse scan is physically unplugged. After plugging it back in, the light tube of the reverse scan was physically blocked off with my finger to keep ambient light from entering. The hum remained.
4. Inspection of all the cables for the optical sound revealed no abnormalities.
5. Scott Hicks (Thanks, Scott) from ACE sent me an entire reverse scan assembly (including the power supply) for troubleshooting purposes.
6. Substitution of the power supply and reverse scan pickup (which took a total of 5 minutes) ruled out the reverse scan components.
7. Suspecting a grounding issue with the sound equipment rack, investigation revealed it was improperly grounded. Grounding of the equipment rack made the hum increase substantially.
8. Plugging all of the equipment from the sound rack into a power strip with a long extension cord was then tried. No matter what receptacle it was plugged into, the system really hummed.
9. Disconnected the Non-Sync input removed the hum. Obviously, heavy current was flowing through the non-sync shield.

All of the non-sync apparatus primary power is derived from booth No. 3, which was an add-on years ago. There appears to be a difference of potential between the neutral and ground connections between the add-on and the original booth. The power feeds come from a different transformer of the power vault in the underneath the street in front of the theater.

Installing an isolation transformer in the non-sync feed temporarily corrected the problem until I can get an electrical contractor to take a look at the building's primary power.

The thing that really threw me off is:

1. Why just the left channel.
2. What did the LED power supply have to do with the non-sync?

I was taught properly this statement from the United States Navy:

Never make a flat statement when it comes to electrical and/or electronic theory.

Posted by Jean-Michel Grin (Member # 1360) on 12-08-2003, 07:41 AM:

Hi all,
I remember me the day we replace a CP200 with a CP 650, and improved the direct scan sound reader with an reverse scan model on a Cinemeccanica V8, we experienced a lot of hum and buzz. By boss told me to measure with a voltmeter the difference of grounds between the projector chassis and the sound rack chassis.
More than 80 volts difference ! [Eek!]

We discovered that the technician who did the installation before us had lifted all grounds on the sound racks and the ground of the solar cell cable was attached to the projector base [Eek!]

All the grounding of the sound equipement passed trought this screening.

Bye,