Film-Tech Forum ARCHIVE: Poping Noise In Sound

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Posted by Don Sneed (Member # 955) on 12-21-2002, 12:02 PM:

I need some ideas to get this noise to stop, I have a EPRAD Starlet processor/amp. that has a poping noise in the sound each time the Potts platter brain microswitch in engaged, all three platter brains are doing this, the automation is a Xetron Maxi-8, every one of the manual switches do the same poping noise in the sound, I found no ground wire is run to the projector, automation, sound, & electrical plugs, the ground is being supply by the conduct & raceway....I tried using the conduct for ground to the sound system & platter, but no work, poping noise is still there, I am going to try the .1 mfd @ 600v capacitor idea across the microswitches tonight & see if that will stop the noise from the platter brains....question is would this help, if so wher do I connect the capacitor, across the microswitch terminals, or to the platter & one contact on the switch ?? Would getting an electrican to run a ground wire to the automation, projector, platter & sound system would help & stop this...any ideas from you guys is much needed....thanks....

Posted by Mark Gulbrandsen (Member # 72) on 12-21-2002, 01:35 PM:

Don,
Are the processor and platter on the same phase, or the same circuit? I've had this happen before and moving the platter over to a different phase cured it. Also, if this is not the case look on the AC line(carefully) with your scope to see if the noise is comming back in that way.....or some other way. A line filter may also help on the input to both the platter and processor. You could also install the Strong Linear feed sensors on the payout heads, you'd only have to contend with the noise on startup then. The best thing is to get rid of that primative processor and install something for real! Yes, you would put the caps across the micros. And the noise filters on the AC lines going in to the platter and processor, locating them inside the piece of equipment if possible. Adding a dedicated grounding system is a good and safe thing to do but is no guarantee of eliminating the noise problem at hand. Also, you really have to ground all the way back to the buildings water main input to achieve a real good ground. I suspect the processor is at fault here.....do you have the popping noise at all with just the power amps turned on and the processor off? Is the wiring from the cells to the processor correct, and the wiring from the processor to the amps correct and all of high quality shielded wire? These types of problems are always a good challenge!
Also....does that processor even have a true balanced cell input....I don't remember that it does.....A balanced input is very important from a noise standpoint.
Mark @ CLACO

Posted by Gordon McLeod (Member # 33) on 12-21-2002, 05:50 PM:

The StarScope/Starlet used an unbalanced cell input Ie only a hot and ground from each half of the cell
Also it was a very high impedance input (4.7K) and a very sensitive high gain input amp LM382(no longer made device)
That is why the excitor lamps had to run at a low voltage on them
The cap(add a 100ohm resistor in serries with the cap) across the contacts will help but remeber some voltage potential will be across the motor conector if it is unplugged
Try using a 3 cond + shield cable for the cell Belden 8770 or the like and ensure the shield is floated.
To lower the sensivity of the preamp change R1-R4 from 4.7K to 500ohm and then you will need to boast the excitor to compensate and that may help raise the difference between signal and the popping noise (not the eq of the cell will change)

Posted by Ken Layton (Member # 133) on 12-21-2002, 08:00 PM:

We had the same problem at one of our theaters in one house only and we had a Dolby CP-55. We also had older Strong/Potts platters. We changed the payout control plates from microswitch units to phase control and problem solved!

Posted by Don Sneed (Member # 955) on 12-21-2002, 09:27 PM:

Thank you Mark, Gordon & Ken for the help, working in a small town (I can tell I'm not in Houston) I could not find a .1 cap anywhere...Mark, the platter & sound system ( I think eveything is all on the same phase, only two circuit was run to power everything [Eek!]

I will take this advise...

Gordon..thanks for the input info...& Ken I will have to checked into the phase control...by the way what is the phase control ??? that is the first time I heard of that...

Here is what I did to elimate the noise..not the pop but the buzz noise...I connected the shield & the neg. together at the processor end, & connected the shield to the soundhead frame...thety are using exciter lamps & stereo solar cell...the poping noise also went low, low enough for me to hear knowing what to listen for, but not enough to get anymore complaints from the patrons, I still want to get rid of totally, but for now it is OK, on my next visit I will elimate the poping noise..I had made notes on the input from you guys, thank you so very much for the help...have a great Christmas....Don

Posted by Gordon McLeod (Member # 33) on 12-21-2002, 09:29 PM:

The cell input is on a RCA pair of connectors the shield should connect to one of the shells
A 500:4K transformer on each of the inputs would help the most

Posted by Mark Gulbrandsen (Member # 72) on 12-21-2002, 10:21 PM:

If at all possible do the theatre ownwer a big favor while your there next time and pull out the Eprad!! That piece was beyond obsolete when it came out. A nice used CP-50 would be far better for them and is much more immune to noise. By the time you tie the shield to neg and frame, hang on a couple of input xformers, and add what ever other grounding it takes you will end up with a true kluge......The phase controls are not cheap but are very effective as long as its an Alpha platter system. Never put one on an older "Stupid Platter" and I doubt the decks would stay in time for very long.
Mark

Posted by Rick Long (Member # 213) on 12-21-2002, 11:33 PM:

How about a couple of .05's in parallel, (one may do it anyways), across the wires feeding the micro-switch? Worked for me.