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Author Topic: Setting SPL Levels Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-08-2005 12:23 PM                     300mV would only be best on certain processors...not as a general statement. It all depends on where a particular piece of equipment's Signal-to-noise is maximized. This is not the same place for all processors nor even all Dolby processors. In general, the earlier you do amplification and the fewest places you do it, the better. Also, in general, running amps wide-open will not yeild the lowest noise.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-08-2005 12:42 PM                     Is there a way to determine the optimum signal to noise output setting for a particular processor?  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-11-2005 10:42 AM                        In one booth the amps are all Crowns. They originally were set up with the pots wide open and even marked with little stick on dots, not to mark levels but to insure that booth personnel could visually see that one pot was to be run at full and the other at zero because they were stereo amps that were used in mono mode -- only one pot controlled the level. Thing is, if you went up to the stage and listened to the wide opened amp, you did hear some noise component. I pulled the inputs on the amps and still there was noise -- not alot, mind you but it sure wasn't that dead silence that you should get from the amp. In my mind, the amp should never add any noise. I had someone in the booth turned down each amp's pot as I stayed on the stage listening. It only took a slight reduction on the pots -- maybe at most 1/5 of a turn to get them completely silent (to the ear at least). We remarked the pots and then made adjustments accordingly at the processor. As Steve says, running amps wide open may not give you the best SN.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 04-12-2005 06:42 PM                        Most amps have their level control "first thing" after the input. The noise level should not change when the input is unplugged and the pot is rotated from one end to the other. When you attach an input (processor) you are then attaching the output stage of the processor to the input of the amplifier. It is the output stage of the processor that is the noise generator. Some processors are worse than others. Of particular note is the CP-100. With this processor it is correct to run the trimpots up full and reduce the noise with the power amplifier input. In olden days, I made this modification at the Uptown in D.C. Other processors such as the CP-500/650 do not seem to have this problem. Remember there are only 2 calibrated locations on an amplifier pot: full on and full off.  |  IP: Logged Ken Jacquart Film Handler Posts: 82 From: San Francisco, CA, USA Registered: Mar 2000 posted 06-22-2005 12:18 AM                        Sam is correct. Most installations that i've seen have the amp gains turned maximum. Let's be real here... CP's of today and amplifiers of today are so darn quiet that I challenge anyone to hear the difference between methods. True enough... optimization to 300mV may techncially provide a better S/N, but if you can't hear the difference, should one play it safe and turn the amps to maximum? An amplifier at maximum can't be "tweaked" louder and if it is replace with an identical amplifier, can be set easily...  |  IP: Logged Ari Saarinen Film Handler Posts: 9 From: Vantaa / Finland Registered: May 2005 posted 06-22-2005 01:00 AM                        Steve is very right about this issue. Even that todays equimpnets are very high quality, but also same time amplifier powers have gone much higher (at least here in Europe). All most every new multiplex have had tri-amp stage channels allready last 5-6 years. It's allways best way to tune system with correct signal structure. About todays high quality products... even one three letter brand digital x-overs from California are sometimes more like noise generator than silent ones (SG-01)... but you can avoid it in most cases... if things are done correctly in signal structure to get best possible signal to noise ratio.  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 06-22-2005 02:55 AM                        Dunno which manufacturer you mean, but we just installed 14 systems with digital crossovers made in California, and the systems were very quiet with the amps turned all the way up. I have revised my earlier practice of setting the amps at approx. 2/3 and the processor channel output levels as close to bus level as possible. Man, you wouldn't believe how much booth personnel tinker with equipment here, and how many creative ways they find to break and blow stuff up.  |  IP: Logged Ari Saarinen Film Handler Posts: 9 From: Vantaa / Finland Registered: May 2005 posted 06-22-2005 03:07 AM                        I did mean older THX x-over version SG-01...  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 06-22-2005 06:37 AM                     The QSC amps have a 6-dB hit when you use the "data ports". As such, when you turn your amps wide open, you actually are still 6dB shy of wide open as compared to using the XLR or phoenix connectors. Since I have seen the DCM clip out many times before the amp...I would always turn the amps up full when working with that unit. As for calibrated pots...nope they are not calibrated at both ends, just the one (zero ohms)...at the other end they will vary a bit but hopefully within the tolerence set by the manufacturer of the pot. I set the gain structure for the lowest noise, unless I have exposed pots (not much anymore since the DCA amps by QSC have cover plates). The input attenuator should be thought of as a "sensitivity" control rather than a "gain" or "attenuator"...it should be set to yield the best S/N. I can certainly understand the desire to set to max since it is easy and repeatable and only gives the tinkerer the ability to lower levels.  |  IP: Logged

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