This is topic Non Sync hum in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Antonio Marcheselli (Member # 373) on 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
Posted by Gordon McLeod (Member # 33) on 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
Posted by Josh Jones (Member # 447) on 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
Posted by Joe Beres (Member # 678) on 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.
Posted by Paul G. Thompson (Member # 655) on 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.
Posted by Antonio Marcheselli (Member # 373) on 04-27-2002, 04:04 AM:
Thanks for all suggestions, I'll try them today.Antonio
Posted by Matt Close (Member # 996) on 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 .....
Posted by Antonio Marcheselli (Member # 373) on 04-27-2002, 05:02 AM:
I've heard of isolation transformer many times. What are them?Antonio
Posted by John Anastasio (Member # 607) on 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.
Posted by Paul G. Thompson (Member # 655) on 04-27-2002, 12:59 PM:
You can get some el-cheapos from your electronics store.
Posted by Matt Close (Member # 996) on 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
Posted by Don Sneed (Member # 955) on 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...
Posted by Antonio Marcheselli (Member # 373) on 04-28-2002, 04:57 AM:
Thanks to all!Antonio
Posted by Phil Hill (Member # 371) on 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
Posted by Manny Knowles (Member # 1171) on 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.
Posted by Gordon McLeod (Member # 33) on 05-11-2002, 09:54 AM:
Ideally eah end is teminated int a transformer
If youa re a multiplex the odds are you have a distribution amplifier that sends a balanced source out to each screen so te problem is usually in the termination
Posted by Manny Knowles (Member # 1171) on 05-11-2002, 01:07 PM:
I have seen these for XLR and 1/4" Phono (TS/TRS) plugs, but I haven't yet seen them for stripped wires...Do they even exist in that variety?------------------
~Manny.
Now...where was I ?
Leonard Shelby, MEMENTO.
Posted by Gordon McLeod (Member # 33) on 05-11-2002, 02:06 PM:
Mouser electronics and Newark all sell transformers
The ones in XLR cases are usually mic balanceing and are 600:10K ohms not 10K:10K