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Topic: Non Sync hum

Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 04-26-2002 04:49 PM

Hi everyone.

We have the audio out of a sound card's PC connected to Non Sync of our CP500. In the theater a "hum" can be heard. I'm not confident with line audio connection. What should I do to remove the hum?

Bye
Antonio

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 04-26-2002 04:51 PM

Possibly you have a ground loop
The best solution is to use two 10K to 10K audio isolation transformers to isolate the ground of the two systems

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Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 04-26-2002 10:36 PM

Or if you can't do the above mentioned, get a length of wire (12 guage or about 3-4mm) and run one from the computer's case to a known good ground. Then see if it goes away. 7 out of ten times it does.

Good Luck

Josh

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Joe Beres
Jedi Master Film Handler

Posts: 606
From: Minneapolis, MN, USA
Registered: Nov 2000

posted 04-26-2002 10:42 PM

Please forgive the somewhat electr[on]ically impaired, but how does one determine whether a ground is good or not? I have had problems similar to Antonio's in the past, but wasn't sure how to properly solve them.

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 04-27-2002 01:30 AM

Actually, Gordon has the best idea. Many theaters use a single source for non-sync, and the amplifier equipment racks may be on a difference phase of electrical power.

Also, if your building has a shitty electrical system, stand by for all kinds of hums in the non-sync if you use a common non-sync source.
Another alternative that may work very nicely is to use balanced cable and ground only one end of the shield. The hum is caused by a small amount of ac current flowing through the shield. By leaving one end ungrounded, the hum might go away. That is, of course, presuming your non-sync inputs are balanced input, which they are probably are not.

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 04-27-2002 04:04 AM

Thanks for all suggestions, I'll try them today.

Antonio

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Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 04-27-2002 04:57 AM

Always the isolation transformer method!! .... It has always been the only sure-fire way I have found to fix problems like this .....

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 04-27-2002 05:02 AM

I've heard of isolation transformer many times. What are them?

Antonio

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John Anastasio
Master Film Handler

Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000

posted 04-27-2002 07:30 AM

They're transformers which aren't designed to change the voltage or current, just couple the input and output magnetically instead of electrically. The primary and secondary windings are identical. The audio signal goes in, it's coupled through the transformer's core to the secondary, it comes out....it eliminates a direct connection between two pieces of equipment, so the grounds are no longer connected to each other either. Use a two-conductor shielded cable for the output and only ground the amplifier end of the shield.

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 04-27-2002 12:59 PM

You can get some el-cheapos from your electronics store.

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Matt Close
Expert Film Handler

Posts: 226
From: Hervey Bay, QLD, Australia
Registered: Sep 2001

posted 04-27-2002 06:07 PM

Make sure you buy a 10k - 10k one like Gordon said though..... Electonics stores will have a variety that all 'look the same' ... Using another type will give an impedence mismatch / level problems

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Don Sneed
Master Film Handler

Posts: 451
From: Texas City, TX, USA
Registered: Aug 2001

posted 04-28-2002 03:20 AM

The solution Gordon said is the way to go, we install in mulitplex theatres with two or more screens in the Ultra-stereo MDS-1400 & purchase the number of cards as needed for the screens, 2-screen per card....Smart also makes a great MDS System...if only one screen having problems, use the 10K/10K transformer....a 600/600 ohm will work if you cannot get a 10K/10K....but use the 10K if possible...

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 04-28-2002 04:57 AM

Thanks to all!

Antonio

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Phil Hill
I love my cootie bug

Posts: 7595
From: Hollywood, CA USA
Registered: Mar 2000

posted 04-28-2002 07:49 AM

Gordon is 100% correct. Use ONLY the 10K to 10K (or 15K to 15K) matching transformer. And Do NOT use the el-cheapos...they do not have the low-frequency response that the higher-priced units have so they do not sound very good.

The term "isolation transformer" usually applies to ac line voltage transformers that have a 1:1 ratio. Their purpose is to prevent damaging test equipment, or you from getting electocuted, when working on line-common equipment.... ALWAYS use proper grounding techniques.

I would forget about using some type of heavy grounding wire that was mentioned as this almost never works because of what was pointed out about the equipment being fed from different AC circuits. It also may produce unwanted ground loops and currents of it's own.

Gordon mentioned you need two transformers, this is correct as you need one for each stereo channel.

The best solution is the single 10k to 10k matching transformer for each channel. The 10k refers to the impedance of the circuit and means a high-impedance of 10-thousand ohms which is typical of single-ended audio circuits.

>>> Phil

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Manny Knowles
"What are these things and WHY are they BLUE???"

Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002

posted 05-11-2002 01:10 AM

I'm having this problem, too.

I've read (above) that a transformer is needed for each channel (left/right), but I'm wondering...

Do these transformers go on BOTH ends (out of the distributor AND into the cinema processor) ?

Is it a total of FOUR or just TWO per screen ?

------------------

~Manny.

Now...where was I ?
Leonard Shelby, MEMENTO.

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