Film-Tech Forum ARCHIVE: CP55 non-sync buzz

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Posted by Tom Wienholt (Member # 1510) on 06-06-2009, 09:28 PM:

Here is the situation:
-CP55 has a buzz ONLY when in the non-sync format, but only when a signal is connected to the non-sync inputs
-Signal is coming from a distribution amp
-Identical processor, amps, distribution amp signal, etc. are being used in a second auditorium with no buzz
-The only difference between the two systems is that the one with the buzz is 50 feet from the sound rack containing the distribution amp so there is a 50 foot run of shielded cable between the racks for the non-sync signal
-Checked all wiring and grounds and there are no apparent problems with the wiring

Not sure what else to check. Could there be a problem with one of the cards in the CP55? Perhaps there is another explanation. Any help would be appreciated. Thanks.

Posted by Elia Orselli (Member # 4673) on 06-07-2009, 02:20 AM:

I have the same problem with aux input, but I'm sure that in my cinema the reason is that the cable has some shielding problem and it run from a side of the booth to the other where there's the audio rack. Because this cable is the link between a dvd reader and a surround processor, I'm going to change the processor with a digital one and the shielded cable with a toslink one.

Bye
Elia Orselli

Posted by Demetris Thoupis (Member # 833) on 06-07-2009, 03:51 AM:

Try hooking a direct source near the CP to see if it goes away. Then work your way backwards

Posted by Louis Bornwasser (Member # 3063) on 06-07-2009, 07:00 AM:

Any cable can pick up hum or buzz depending upon what it is near.

I would get another (much longer) cable and run it on the floor to see if it also has hum. If not, then you need to reroute the original cable.

btw: most cinemas use the incorrect cable. If you are using a Smart distribution amp, a 2 conductor shielded cable is NOT correct; you are mixing l/r together under the shield. You either need 2 of these cables or its equivalent. Shield connected at one end only. Louis

Posted by Scott Christopher (Member # 834) on 06-07-2009, 08:26 PM:

I had this problem with the CP500 & CP650. It turns out it was a ground loop which was fixed by installing an ground loop isolation transformer. You can buy them already with male/female RCA's attached.

This also fixed a problem of the play button on a DVD player not responding all the time on a CP500.

Posted by Frank Angel (Member # 248) on 06-08-2009, 05:49 AM:

Of course it's always best to try to discover where wiring it faulty and what's creating ground loop hum in the first place, but that can be tricky in large, complicated systems. Barring that, try this in a pinch: Ground Loop Isolator - RCA Unbalanced from MCM Electronics.

It's cheap enough to keep in your tool box so that you always have one handy to stick into an unbalanced circuit to see if a ground loop is the culprit. If it is and this box eliminates the hum, then you can try to find the loop and eliminate it; or, just leave the unit in place and buy another one for your tool box. [thumbsup]

Posted by Gordon McLeod (Member # 33) on 06-08-2009, 01:22 PM:

What isn't mentioned is what brand of distribution amp is used
If it is a smart swap the auditorium feeds and see if the problem moves that would determine if it is the card
If it doesn't move then I would suspect the wireing

Posted by Tom Wienholt (Member # 1510) on 06-08-2009, 07:25 PM:

So I decided to purchase a $16 ground loop isolator from radio shack just to see if it would solve the problem. It did and I'm satisfied just leaving the isolator hooked up permanently. Thanks for all of the help.

I'm still confused as to where the buzz was coming from. I have a second auditorium and lobby music all coming from the same distribution amp with no buzz or hum at all. I even swapped the outputs to rule out any problems with the output channels. But, like I said, I'm fine with having the ground loop isolator hooked up to eliminate the buzz. Thanks again.

Posted by Scott Christopher (Member # 834) on 06-08-2009, 09:52 PM:

We had an isolation transformer at every amp rack for the NS1 feed to eliminate possible loops between equipment - It works well.

Posted by Eric Robinson (Member # 2966) on 06-09-2009, 01:10 AM:

Sometimes when I have buzzing problems I connect a heavy gauge wire to the rear metal frames of all equipment involved. This usually takes care of the problem.

This may not be good advice though. If it works; however, someone should be able to explain to you why this technique is working.