Film-Tech Forum ARCHIVE: Buzzing

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Posted by Kevin Wheat (Member # 81) on 03-23-2000, 10:24 AM:

Several of our auditoriums have developed a buzzing that I can't seem to locate. It is not in the amps. We recently made the conversion to Jax lights,which were wire wrong originally, but they were to have been fixed by our tech. The buzzing is constant through the movie and nonsync. Any suggestions would help.

Posted by John Pytlak (Member # 331) on 03-23-2000, 10:44 AM:

Start with the speakers that have the buzzing and work your way back:

Are all channels affected?
Is the buzz 60hz?
Does it vary in loudness as you change the fader?
Is is a smooth hum (sine wave) or raspy (clipped or distorted)?
When exactly did the buzzing start? (e.g., immediately after the Jaxlight wiring problem?) Any chance the Jaxlight power supplies were damaged during installation, putting some AC ripple on the line?

My guess is that you have a ground loop somewhere, perhaps when someone changed the wiring during the Jaxlight installation. Since the buzz is also in nonsync, it's probably somewhere in the processor, or between the processor and the power amps.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Professional Motion Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by Gordon McLeod (Member # 33) on 03-23-2000, 11:16 AM:

The big question list
What type of processors, amps, monitors do you have

When trouble shooting it is a good idea to break the problem in the middle so listen to the monitor and
a determin which channel (s) the buzz is in and also switch between amp and processor out and look for a difference
If it is only on amp out look at the amp connections
Most likely it is on both since my gut reaction is it is the shield on the cell cable
The Jax lite has a little preamp that mounts in the soundhead and that is where the fun starts
1q try unhooking the cell input from the processor and see if it goes away
yes it is in the jax hookup if no try the nonsync input

If in the jax check that the shielded cable is not terminated at both ends. It should only be terminated at the processor!
Also try reversing the input power connection to the little preamp and also isolate it from the soundhead casting
If you have a Mod2/3/4/5 processor from Smart UltraStereo has a post on there web site as does smart regarding special hookup instructions
goodluck

Posted by Joe Redifer (Member # 3) on 03-23-2000, 03:27 PM:

Just an odd question: How can a Jaxlite be wired wrong? From the ads I gathered that you just plop 'em in like an exciter bulb and do an A-chain.

Posted by Richard Hamilton (Member # 321) on 03-23-2000, 10:16 PM:

I found one problem with the Jaxlight is the fact that the polarity is not marked on the input from the existing exciter supply to the preamp power supply. I have always wondered why the power input on the Jaxlight preamp doesnt matter what the polarity is. The first one that I installed, I had a terrible buzzing noise and the only fix was to switch the exciter power supply from DC to AC.

Rick

Posted by David Johnson (Member # 173) on 03-24-2000, 02:48 AM:

With an open sound head such as a Kinoton, AC light from the room or light reflected from the porthole can give rise to quite a loud hum. The problem is even worse if a "Jax lite" is fitted as the light output from the LED is much lower than the old exciter lamp hence the need for the small preamp. This system will increase the signal to noise ratio.
David

Posted by John Pytlak (Member # 331) on 03-24-2000, 05:32 AM:

David ---

The possibility of picking up room light (especially flourescents or lights on an SCR dimmer) by the solar cell is worth checking. I forgot the need for additional amplification with the JaxLight, which would make it more sensitive to stray light. The quick check to see if this is the part of the problem would be to turn off the room lights. But it doesn't explain why the buzz also occurs in nonsync too. I still suspect a ground loop caused when the wiring was changed to install the JaxLight.

Posted by Gordon McLeod (Member # 33) on 03-24-2000, 10:39 AM:

The preamp that comes with the jax lite has its own bridge rectifier in it so that it will work on ac or dc supplies
The usual culprit is iether the shild terminated at both ends or the ground of the excitor lamp powersupply versus the input dc to the preamp as that can also create a loop

Posted by Jesse Skeen (Member # 586) on 09-14-2000, 02:39 AM:

The hearing-impaired systems can cause noise in the sound system too; this was a common problem at the theater I used to work at. If your theater has these, try re-aiming the antenna or turning the system off and see if the noise goes away. I found a buzzing problem at the theater I just started working at; turning off the hearing-impaired transmitter helped but the noise did not go away completely, I'll have to play with it some more next time I'm there.

Posted by Greg Pauley (Member # 503) on 09-14-2000, 10:09 AM:

9 times out of 10 this is a ground loop problem. Check your grounds first.

Posted by Scott Norwood (Member # 30) on 09-14-2000, 10:20 AM:

I don't know about Jax lights, but with regular exciter lamps it's worth checking to make sure that someone didn't accidentally flip the exciter supply switch to "AC/Emergency"; this would cause a 60Hz hum...not sure if this is a problem with Jax lights.

Posted by Pat Moore (Member # 380) on 09-14-2000, 10:22 AM:

Assuming this is not a new problem that suddenly started last week, this kind of troubleshooting is a pain and can be tedious. All the above are good suggestions. Some others:

Since the buzzing is present at all times (both film and non-sync), it's probably not anything in the projector A-Chain. Something is getting into the preamp or power amp input. Check other sound formats -- is it the same "buzz" in analog and digital? Any more prevalent in stage channels than surrounds? This will give you some paths to follow. Problems in one or two channels might indicate a wiring and/or ground and/or component problem. If it's "everywhere" there's an external noise source that needs to be isolated.

With everything powered up, start turning things off to isolate the source. Dimmers are a real obvious source -- be sure to de-power them, not just dim them down -- same for aisle lights and such if their transformers are in the booth. Are there neon signs? Is air conditioning running full time? Booth monitors occasionally cause some problems, as well as non-sync sources, etc.

You need to be a bit of a detective and have a little patience. I know of one instance where noise was finally traced back to speed controls for overhead fans -- NOT an AMC Theatre, by the way. Good luck.

Posted by John Walsh (Member # 168) on 09-14-2000, 11:11 AM:

This is probably not it, but we had a big problem with the filaments buzzing in the light fixtures. Also, at another place, no one noticed the electrictian ran the theater lights and speaker wires in the same conduit.

Posted by John Pytlak (Member # 331) on 09-14-2000, 12:37 PM:

SCR dimmers are notorious for generating "hash", as they essentially work by "chopping" the sinusoidal AC power to the lights being controlled. The harmonics can extend into the RF spectrum (e.g., the buzz picked up by an AM radio in a location with SCR dimmers). Extraneous light shining on the solar cell pick-up, or just the "hash" picked up by low level audio cabling and being amplified, can result in hum or buzz in the sound.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by Aaron Sisemore (Member # 145) on 09-14-2000, 03:12 PM:

Slide projectors are another culprit for nasty untraceable hummmmmmms... Hell, I can hear the slide projectors in the TELEPHONES at one of our theaters

Aaron

Posted by Randy Stankey (Member # 64) on 09-14-2000, 03:14 PM:

With Jax-Lites, you have to install a preamp to get enough signal to the processor, don't you? If that's the case, I'd be looking in that area. Don't you have to have shielded wires in there?

With all that amplifiaction going on BEFORE you even get to the regular preamp ANY little bit of interference would be amplified too.

Posted by Marc Jones (Member # 29) on 09-14-2000, 08:06 PM:

I agree with Pat
Is your booth monitor wired into the outputs of the processor or from the outputs of the amplifiers.
If wired into the processor do you here hum in the monitor? this might help you find your problem.
You also might check the Output terminals from the amps that all connections are good and tight.
Good Luck

Posted by Mike Spaeth (Member # 524) on 09-15-2000, 03:13 AM:

not my Jax-lights. The original install was done using non-shielded wire covered by tin-foil. Luckily, this was recently corrected.

Posted by Mike Jones (Member # 522) on 09-19-2000, 04:45 PM:

"any suggestions?"

-yeah! don't use Jax lights!

Posted by Richard Hamilton (Member # 321) on 09-19-2000, 07:25 PM:

since this is occuring in a few auditoriums, I would suspect possibly a power problem. Do these auditoriums have a common breaker box? Was there any recent electrical work done? Any work on a dimmer, where they might now share a nuetral? How about any neon added to the theatre?

Rick

Posted by John Walsh (Member # 168) on 09-20-2000, 11:03 AM:

I think we should make it a new rule: If you post a problem for others to answer, you must then tell us the outcome.

Otherwise, we glue your zipper shutter closed!