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Author Topic: Setting SPL Levels Pete Naples Phenomenal Film Handler Posts: 1565 From: Dunfermline, Scotland Registered: Feb 2001 posted 04-07-2005 03:49 AM                     Tom, in a word no! Whilst you will end up with the correct SPL levels, the S/N ratio and headroom might be awful. I've cured quite a few noisy systems by 'gain staging' this way, and it sounds audibly better to my ears. Even if you don't set the CP output to 300mv, you could try putting the CP outputs full up and pulling the amp gains down to get the correct SPL, I often turn the CP full up and then count down, say 3 turns on the pots, not very accurate but better than nothing.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-07-2005 10:01 AM                     The thing that confuses me is that for CP50's, there is an output adjustment on the equalization card for L,C,and R channels plus on the output card itself. How do I properly set the output to 300mV. Do I use the pink noise card or test film to do this. Any help would be great. Thanks.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-07-2005 11:08 AM                       I did not get in on the beginning of this topic so forgive me if I repeat something already stated. Regarding the CP50, there were any number of revisions to the manual as to this procedure having to do with the fact that the matrix card continued to evolve. The best way to set the Cat. 64 pots is inputs all the way down and outputs pots set (to around 200 mVolts)with a generator after b-chain EQ and before final SPL setting. This method is much simpler, takes wayward a-chain adjustments and the test film out of the loop, pun intended. You can then set the CP 50 output trim pots to suit your situation. I taught this method for many years at Dolby Labs  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-07-2005 05:34 PM                     So you are saying that the output levels should be adjusted by the output card and the amps can be turned all the way up? What about for a CP55 with Cat64B's that don't have an output adjustment on them? Or a CP50 with Cat64B's? Should the amps be turned all the way up and levels adjusted with the output card? Thanks for the help.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-07-2005 05:42 PM                     You can do what ever you want but ideally a soundsystems gain structure is optimal if the SPL is set on the power amps and the processors output trimmed to the processors buss level  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-07-2005 11:04 PM                     So basically the optimal processor output level is about 300mV per channel? So I should set the outputs to 300mV using a pink noise generator as the signal?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-07-2005 11:46 PM                     You have asked this question in various forms several times so why not just have "your qualified Dolby service engineer" come out and set it up right  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-08-2005 12:07 AM                     I know I have. It just seems like different people give different advice. I have set up many sound systems by simply turning the amp gains all the way up and adjusting the levels with the trimpots. This method has worked great but now that I have read these discussions I am considering "re-setting" the levels with the amp gains since I am being told that it will improve S/N ratios and improve sound quality. Just trying to be very clear on the method to use. Thats all.  |  IP: Logged Tracy Bellar Film Handler Posts: 72 From: Sciotoville, Oh. Registered: Mar 2005 posted 04-08-2005 12:11 AM                        In my theater we keep the amp gains up and set the processor. I know it isn't the best situation. However, in a theater where many employees other than myself have access to the booth and sometimes go in and bump a knob. They should keep thier hands off I know but it happens. We get complaints of the sound being too loud then too quiet. I wish the policy would be noone in the booth but trained people. If this would become a standard then I could see setting the gains down to where they should be. It's a quick fix to stopping someone from changing an amp thinking it was master vol. Amps should have a glass enclosed locked shield to keep hands that shouldn't touch.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-08-2005 12:26 AM                     Im not worried about people messing with the amp gains, they are on the back of my amps. I want to have the best sound quality possible with the equipment I have.  |  IP: Logged Tracy Bellar Film Handler Posts: 72 From: Sciotoville, Oh. Registered: Mar 2005 posted 04-08-2005 12:30 AM                        ours are on the front. Too many young kids working there that thinks if they have a nice stereo at home that they are qualified enough to change levels.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-08-2005 08:52 AM                     The best headroom and lowest noise will be found by useing the 300mv method  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 04-08-2005 09:57 AM                     I thought it was a good idea when the QSC USA's had the gain pot on the back ... accessable, but not too easily. Maybe they should include a feature on their next generation of amps where even the gain controls are only accessable with an RS232 port.. !  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 04-08-2005 10:07 AM                        I don't remember seeing the 300mV information in any of the Dolby manuals. Did I just miss it, or is it not there? If it's not there, then why isn't it? It sounds like it should be in the manual (if it isn't already). Most installations that I have seen use the "set the pots on the amps to maximum" approach. I can only guess that this is done to make it simple to replace an amp if it ever fails.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-08-2005 11:19 AM                       The main reason techs run the amps wide open is to keep the staff from turning up the surrounds in particular.  |  IP: Logged

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