Film-Tech Forum ARCHIVE: Thump when turning on LED reader

This is topic Thump when turning on LED reader in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Tom Wienholt (Member # 1510) on 05-01-2004, 01:59 AM:

I recently installed a Kelmar reverse scan reader. When the LED is turned on, a thump can be heard in the auditorium. How can this be stopped?

Posted by Mike Olpin (Member # 1115) on 05-01-2004, 02:12 AM:

Put the processor in non-sync or mute before turning on the reader. If your theatre has automation, it can be set up to do this automatically when you start the show. For instance, our system turns on the LED reader as soon as start is pressed, but it does not toggle the processor over to SR until the dowser opens.

Posted by Tom Wienholt (Member # 1510) on 05-01-2004, 12:36 PM:

Yes, that would definately work. But, is there a way to make the LED turn on gradually instead of all at once to avoid a thump?

Posted by Gordon McLeod (Member # 33) on 05-01-2004, 01:01 PM:

The thump is more likely from the powering up of the preamp than the led
I would sujest wireing the led powersupply with the projector motor so it is on just before the changeover switching

Posted by Mark Gulbrandsen (Member # 72) on 05-01-2004, 08:29 PM:

These days I almost always hook the reverse scam supply across the motor feed terminals as many do not feature an on-off terminal as C.E. does. BACP reccomends theirs always be left on, but on the Kelmar there is no way other than to power it up and down and yes, the thump can actually be from both the plus and minus supplies comming up and the bias that the LED turning on causes on the cell itself. You basically have to do it as Mike suggests......most automation used today switches the processor between non-sync and SR, or digital with sr as the analog fallback. There is no feature for bringing the LED voltage up gradually and being able to do this would not completely eliminate the thump anyway.

Mark

Posted by Tom Wienholt (Member # 1510) on 05-01-2004, 08:59 PM:

Thanks for the help.

Posted by Scott Norwood (Member # 30) on 05-01-2004, 09:02 PM:

So what's the solution for installations which use an exciter-lamp changeover system? Does it have to be reworked with a processor that can do the sound changeover electronically?

I am sure that someone here will point out that exciter-lamp changeover systems are cheesy and I agree completely, but I've had to deal with many such setups, as they are fairly common.

Posted by Gordon McLeod (Member # 33) on 05-01-2004, 09:58 PM:

THe CE is the only one that would allow the led only to be off but you couldn't wire a changeover control to it identically as the old excitor one

Posted by Steve Guttag (Member # 268) on 05-02-2004, 01:20 AM:

CE does offer, upon request, their LS-40 power supply with the logic such that you can replace an exciter lamp changeover system (or an exciter lamp switched system). Just ask for the "Cardinal Mod" You will then need to connect the terminals in the rear to turn the LED on (Factory default is the reverse logic where the LED comes on unless you connect the terminals to turn it off).

Steve