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Author Topic: Amp for driving 2 QSC DCS-SB-5218s? Jay Wyatt Film Handler Posts: 33 From: Los Angeles, CA Registered: Mar 2015 posted 01-06-2016 11:28 AM                    quote: Marco Giustini Auditorium is 60 feet large. It's 86 feet long. 6dB is always a good thing but maybe a little overkilled as a starting point? I was responding to the query on the smaller rooms (60ft deep). Overkill perhaps, but it can't hurt to over spec a bit. Found an mistake in previous post: the QSC recommends a 1600W amp for the SB-5218. This would allow you to use 2x DCA1622s, each in bridge mono. Link to Amp Specs  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-06-2016 12:16 PM                     6dB Headroom? For what? Just like to sell higher powered amps? Cinema sound is VERY well defined. It has a known reference, it has a known maximum level. A legitimate argument could be made that even 1dB of "headroom" is a complete waste. 6dB is quadrupling one's amplifier power. So you are preparing for what? Running the fader at "9" or so for a typical Hollywood movie? As for amp recommendations for a given speaker...again, it may make things easy for the uninformed to specify things that will work but what power is needed to hit the SPL levels desired is what you need...not some arbitrary number of a one size fits all.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-06-2016 12:24 PM                     Apologies I missed the previous message about 60ft rooms. Getting rid of the 3678 speakers and move the 4722 there sounds like a good idea to me. The 3635 sub in particular is barely ok for a small living room. 6dB are really not necessary but 3dB is not a bad idea to avoid running speakers at 100%,particularly low frequencies. This is assuming a film being run at 7 of course.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-06-2016 12:30 PM                     The specifications I use allow for the speaker to run 100% pink noise for 2-hours. So what is a few seconds or even a few minutes going to do to it? You need what power you need to fill the room you are working with. Again, things are very well defined in Cinema so over specifying (something almost never done anyway...as this thread is an example of how things are woefully under-specified demonstrates) is not necessary and does nothing more than make someone feel good, I guess. I'd rather that money go to something that actually will make a difference than into wasted capacity.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-06-2016 01:28 PM                     AES rating is good but i thought that figure meant "power you can feed to the speaker for two hours before it breaks". I thought it didn't say anything about distortion and power compression. If that's not the case then I'll have to have a look at the testing protocols. Otherwise I stand my ground that 3dB headroom would be a good idea to reduce/avoid power compression and limit distortion. Then I agree that the real world says otherwise but when someone asks help to do things right I start with what is right: we are always in time to compromise.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-06-2016 01:49 PM                     You are welcomed to do what you want, for sure...we all are. Again, pink noise would be a worst case signal and 2-hours continuous would also be over-the-top in terms of headroom. As for power compression...depends on the driver and how it dissipates heat. I like the JBL 2242 (used in the 4645C)...it is a real 800-watt driver and sounds wonderful too. I absolutely don't consider having a 3-dB headroom is a criteria for "right." It is superfluous in a cinema design. But okay...lets do that for the 86 foot room. Excuse me Mr. customer, you need 10,900 watts (and the speakers to handle that so please increase your two subs to 5 (double-18s) and buy 5 amplifiers to go with them so you can have 3dB of "headroom" that you'll never, ever use. You see, it just gets silly. I'm sure most shook their heads at my 5455-watt requirement! All of the sudden 4 subs and 2 amps don't look so bad and the amps aren't the big breaker busters either. Remember, on a 20A/120V circuit, you only have 2400VA available so if you think you are going to get more than that out of the amp (particularly on subs), you might want to consider 220V power (something our Euro friends get without anything special). You'll never lose a driver or amp due to under-specification if you at least cover the required amount. And people will still look at you sideways for having so much power/subs. Then they'll turn it down to 5.5 because the movies are too loud (particularly the previews) so you'll get your 5-6dB of headroom there.  |  IP: Logged Jay Wyatt Film Handler Posts: 33 From: Los Angeles, CA Registered: Mar 2015 posted 01-06-2016 02:31 PM                    quote: Steve Guttag The specifications I use allow for the speaker to run 100% pink noise for 2-hours. I ran by dB loss vs distance calculations with SPL Peak Max. If you're using the Continuous (IEC 2Hr) rating with a 115dB req. its the same as the 121 req. (6dB+115) using Peak values, since SPL Max Peak = ~6dB + SPL Max Continous Via ISO 2969:1987/SMPTE ST 202:2010 "When compared with a full-range screen channel, the subwoofer channel must be capable of producing +10 dB of in-band gain" So the 105dB requirement at the sweet spot for Center becomes 115 for subwoofers. Can we agree on that being the required spec or are you using something else?  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 01-06-2016 02:45 PM                        At what point do the room's acoustical characteristics enter into this discussion? Or do they? No doubt, a "live" room would make it easier to achieve 115dBC in that mythical perfect seat (2/3 back, slightly off-center) than a heavily soundproofed room. And aren't speaker sensitivity ratings determined in an anechoic chamber?  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-06-2016 03:18 PM                     I politely disagree, Steve. Yes, you'll never break anything by not specifying any headroom. Yes, the sound system will sound good without specifying any headroom. And yes, nobody will care and they'll end up installing a massively underspec'd system. But in my opinion the starting point when considering what is 'right' and what is 'compromised' is to allow some headroom, even 1dB is better than nothing. Also as Scott points out you may be unlucky and you may end up with a not ideal acoustic and suddenly your 0dB headroom becomes -3dB (yes, it could also be otherwise). The best sounding rooms are not designed to run 0dBFS at 100% of their capacity. That is my benchmark. Then we can talk and we can compromise to meet your budget but at least we know where we are. The 2242 is indeed a great speaker but if memory serves it shows a 3dB power compression at rated power - that is, the speaker will soon drop its output level by 3dB when run at 800W. Hence the plan is to allow 3dB headroom so the sub is only run at 400W, reducing/eliminating power compression and reducing distortion. Yes, very few people will follow these guidelines but - again - at least we know where we are. Don't get me wrong Steve, your advice is always sound and I truly respect your knowledge and your experience. Jay Yes, I agree  |  IP: Logged Jay Wyatt Film Handler Posts: 33 From: Los Angeles, CA Registered: Mar 2015 posted 01-06-2016 03:30 PM                    quote: Scott Norwood At what point do the room's acoustical characteristics enter into this discussion? Or do they? They certainly make a difference, problem is you don't normally receive RT-60 times or absorptive coefficients when asked to spec a room. One would hope that the room is relatively dead (0.8-1.2 Rt60) and quiet (NC-30 or NC-35) but who knows. That's why 'rule-of-thumb' or 'ballpark' figures normally get used.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 01-06-2016 03:32 PM                       Acoustical considerations like room gain are not a factor as we are after not blowing up the speaker by overdriving the speaker on soundtrack peaks. Steve perfectly bridges the gap between theoretical and reality. When SR.D came out Lonny Jennings and I taught the Dolby classes and spent a lot of time doing the math for our students explaining how many subs and how much amp power was really needed. We were met by a lot of skeptiscim and some followed our advice but most only got close. Cinemas were bigger back then so the requirements were pretty daunting. Mostly they got away with under specing because no one would play movies at the required 7.0 on the fader.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-06-2016 04:14 PM                     quote: Sam D. Chavez Acoustical considerations like room gain are not a factor as we are after not blowing up the speaker by overdriving the speaker on soundtrack peaks. Why not, Sam? If you end up with some acoustic issue and you have to boost the channel - or maybe some frequencies - a few dB's, there it goes your headroom!  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-06-2016 04:40 PM                     Whereas I've had a hand in SMPTE 202...yes 115dB is sort of the right level. Why "sort of." Because the 105dB is 105dBc and presumes that the stage change has the response defined by the ISO 2969 curve. C weighting is flat from 20-20KHz. So the typical 1/3-octave band frequency is only running at about 72dB and when they all sum up, you arrive at 85dBc. For the subwoofer channel, we don't have anything that approaches 20Hz - 20KHz which is why it is best set not with an SPL meter but a suitable RTA such that the in-band level is 10dB hotter than the in-band level of the stage channel. This is not to say that it measures 115dBc though. It has been my experience, with subwoofers, one is more likely to get a peak if there is any build up rather than a notch...hence most cinema processors only provide cut on the subwoofer channel. Again, you are welcome to disagree with me on any or all points, have at it. The only thing you can't disagree on are where things are based on facts.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-06-2016 05:06 PM                     I believe I elaborated my points enough to corroborate my thinking. You make me sound like my points are based on incorrect facts, and I feel that that is a little unfair. I feel I am just being a little more cautious and more conservative on my *benchmark*. While I'm happy to accept some degree of disagreement can we agree that we are mostly on the same page on the matter?  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 01-06-2016 08:34 PM                       quote: Why not, Sam? If you end up with some acoustic issue and you have to boost the channel - or maybe some frequencies - a few dB's, there it goes your headroom! You have a point but first you need to have enough power to hit the desired peak SPLs not to clip the amp or overdrive the speaker. Good EQ practice is to cut first and then make small boosts where needed. Often deep holes in response are room nodes that cannot be fixed. Say you boost 160 hz 6 dB and it only comes up a couple of dB on the display. You leave it is what I say.  |  IP: Logged

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