Film-Tech Forum ARCHIVE: Amp for driving 2 QSC DCS-SB-5218s?

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Posted by Andrew Thomas (Member # 7000) on 01-04-2016, 09:46 PM:

When Christie originally installed our system, they used a single Crown XLS-1500 to drive these subs. Not very long after opening, the first amp died. We RMA'd the thing 4 times and it always goes out after a few weeks, so I think the unit must not be able to keep up with these two beasts.

Could you guys recommend a specific make and model of amp for driving two of these simultaneously?

Posted by David Buckley (Member # 2600) on 01-04-2016, 10:20 PM:

OK, a QSC DCS-SB-5218 is a 4 ohm speaker with a recommended amplifier power of 1600W RMS. You have two of them, so they are either being driven by one channel each of the amplifier, or paralleled to 2 ohms, and being driven by the amp in bridge mode.

The amp into 4 ohms can deliver 525 watts, rather than the 1600W recommended. Thus seriously underpowered. There is no 2 ohm bridged output quoted for the amp.

In bridge mode, the amp can deliver 1500W into 4 ohms, which is a good match for one of your subs. So an easy upgrade path would be another amp. This also gives a degree of redundancy, in that if you lose an amp or a speaker, you still have a surviving amp and speaker so the show can go on, albeit with reduced LFE, as long as you don't turn up the level to make up for the missing sub...

Posted by Mark Gulbrandsen (Member # 72) on 01-04-2016, 10:47 PM:

For sure go with the larger QSC DCS series amplifiers. They sound great and are uber reliable.... The kind you put in and forget about.

Mark

Posted by Tony Bandiera Jr (Member # 2365) on 01-04-2016, 10:56 PM:

Specifically, a pair of DCA 2422's or better yet, 3022's each in bridge mono mode or a single 3422 in stereo mode, each sub on its own channel (But I feel that a single 3422 will still be a bit underpowered, but at least the amp will hold up.)

And if possible, seriously consider getting rid of ALL Crown amps in the system...they are crap amplifiers for the cost. (They are also noisier and a lot less reliable that DCA's.)

Posted by Andrew Thomas (Member # 7000) on 01-04-2016, 10:58 PM:

quote: Tony Bandiera Jr
And if possible, seriously consider getting rid of ALL Crown amps in the system...they are crap amplifiers for the cost. (They are also noisier and a lot less reliable that DCA's.)

I should have noted that the plan is indeed to replace all of the Crown amps over time. So thanks for the recommendations.

Posted by Marco Giustini (Member # 4544) on 01-05-2016, 01:46 PM:

If the two subs are paralleled into a 2ohm unit and the amp is bridged, then we know why this is happening.

Otherwise the 1500 is a fairly small amp for two beats like that but in fairness it depends on the auditorium. If the subs are very overspec'd for the room and you're not going to feed them any more that a handful of watts, then a 1500 could be ok.

Could you tell us more about the room (length) and the wiring configuration?

That said, QSC are indeed "wire and forget". But I guess a qsc would fail as well if drove outside of its specs!

Posted by Andrew Thomas (Member # 7000) on 01-05-2016, 02:24 PM:

The room (including space behind the screen) is 86 feet long, 60 feet wide, and ceilings range for 18 to 24 feet high. Sloped floor.

The subs are currently wired by two separate runs from the same amp, which looking at the user manual on QSC site, looks like the wrong way to wire them up. Am I reading it correct, that if driven from the same monoblock/bridged amp, you should run one wire to the first sub on the terminal #1, then run a wire from terminal #2 on the first sub to terminal #1 on the second sub?

Posted by Steve Guttag (Member # 268) on 01-05-2016, 02:56 PM:

Andrew, here is the no BS answer. Given your room dimensions, and those two subwoofers, you need (and are you ready for this?) 5,450 watts or thereabouts. You only have 2000-watts of power handling capacity in the subs so if you had the power, you'd blow em apart.

So lets presume it is one sub on each side of the amp, you have 1050 watts. Not quite 5450 is it?

So the next question is...was your room tuned to "spec"? That is, using a -20dB pink noise signal, are your subs, in the pass band 10dB hotter than your stage channels with the same level of pink noise? If the answer is YES...then the next question is ... where do you run your fader? Is it around 7 (or 0dB for those that think they are in a recording studio or something).

If the answer is YES to both questions...KABOOM! It is a system incapable of delivering the demands of any movie that chooses to use the full dynamic range of the DCinema system.

You could double your subs and go to an XLS 2500 and STILL not have enough for that sized room. However, if you use 4 subs and four XLS 1000s (one bridged into each sub), then you'd finally have enough sub and power to match the room, tuned to industry standard levels and the fader at "7"

Now if you run your fader lower (doesn't have to be much) and the power required change dramatically...just lowering to say "5.5" would drop the power requirements by 5dB. Doesn't sound like a lot but 5dB to your power requirements drop to a mere 1725-watts...within the power of your subs but still beyond your amp.

I think it is a dumb way to design a sound system but I guess one sells more mis-designed systems than to tell one what they really need and scare them to the guy that designs cheap ones.

Your best bet, really is to add two more subs and put in a pair of QSC DCA2422 amps (drive each sub off of one channel of each amplifier). It will last forever, never blow apart and be able to play whatever Hollywood can throw at it.

Posted by George Germisashvili (Member # 6597) on 01-05-2016, 03:11 PM:

You can use 1pc QSC DCA3422 for SB5218, regarding wiring http://appguide.qsccinema.com/Downloads/LC80_System6_Diagram.pdf

Also you can use online application from QSC http://appguide.qsccinema.com/_layouts/15/qsc/welcome.aspx

Posted by Marco Giustini (Member # 4544) on 01-05-2016, 03:13 PM:

What Steve said.

However, it sounds to me that you're running BOTH subs with a BRIDGED XLS 1500. As said before, that amp does not allow a 2 Ohms load when bridged and this is probably the main cause of the failure.

Anyway specifying an XLS1500 for a 26m long room is just nonsense.

Posted by Andrew Thomas (Member # 7000) on 01-05-2016, 04:58 PM:

Steve, very helpful info. The short and sweet answer is that when Christie came out and did the install (we re-opened a long closed 5 plex), neither the guy who originally did the site survey nor the guys who did the install actually knew a thing about audio.

Which is why I'm trying to learn more myself, trying to get the right equipment, and eventually having the right person come out and tune everything.

Amusingly, I don't think a single patron actually cares. But I want to do my best, even if it isn't readily apparent that there are deficiencies. Most of our customers are just happy it isn't the disgusting place it used to be, but I want more than that.

If you guys don't mind, can I tell you the rest of the system and maybe get your recommendations on what else I should look to add?

The way it is currently set up is this

L/C/R stage speakers: JBL-4722, each with a separate XLS1000
Subs: QSC SB5218, single XLS1500
L/R Surrounds: 16 JBL 8320s (8 left, 8 right) on a single XLS1000.

As I said, I want to replace all the Crown's with QSC amps, but should I also be looking to add more surround speakers? I have a USL JSD-60 processor, which I think can handle 7.1. There are actually speakers mounted on the back wall (2 each for left and right), so I think I should be able to get them on another amp and feed the additional signal in.

Thanks for taking the time to respond guys, I appreciate your valuable knowledge!

Posted by Steve Guttag (Member # 268) on 01-05-2016, 06:15 PM:

Andrew,

The problem with your system really is that it wasn't designed, it was cut and pasted using equipment that isn't necessarily junk but isn't necessarily adequate or proper for the room you describe. The JBL 4722 is a good BUDGET speaker but I wouldn't use it in an 86 foot long room.

The JBL 8320 isn't the worst surround speaker in the world but is vastly inferior to the 8340A and the 8350 (The 8350 response is easily the best of the three in that line and the 8320 is very much the worst).

The 5218 isn't a bad sub but 7218s should have been used, if using QSC double 18s. Why? Power handling and sensitivity. When you are talking about subwoofer where power demands start to get crazy and a mere 3dB of savings via sensitivity cuts your power requirements in HALF...you need to look at that sort of stuff.

I'm firmly in the QSC camp for amplifiers. The cost/feature/benefits are just all there. I'm partial to the DCA line too.

For your big rooms with that long throw...you really should consider something like the SC 423 (or SC424 if you don't mind more amp channels and more wiring...the SC424 is one of the most pleasing sounding cinema speakers out there. I'm using the 424 as the "base" model with 2 woofers, depending on the particulars the 434 or 444 may make more sense (the quantity of woofers can affect power handling or sensitivity).

In the small to medium rooms, the 4722 is just fine though again, not the best speaker in the world is FAR from the worst too. It has a real good bang for the buck.

When it comes to sound, you have three powers in play at all times

1) Subjectivity (what YOU think sounds best)
2) The physics of how sound works and what is required for it to work properly.
3) Budget. What you can afford versus what you may want.

If you do end up going with the QSC 4-way stage speakers, I DO recommend "quad-amping" them. The VHF diaphragm is much easier to tune/control if by a separate amp rather than EQ.

And if the 4-ways are not in the cards, the SC-423 is awesome too.

Posted by Andrew Thomas (Member # 7000) on 01-05-2016, 06:37 PM:

Thanks again. I'm thinking then that it may be a good idea to move a lot of this equipment from our large auditorium to one of our smaller rooms.

We have three smaller rooms that are roughly 30Wx60Dx20H. In those auditoriums we have

L/C/R Stage Speakers: JBL 3678
Sub: JBL 3635
Surrounds: JBL 8320

What would you suggest we do there?

Honestly, we just went with what was recommended to us and assumed that Christie knew what was up. But we are 2 years into this thing, making great money, and I just want to keep pumping cash back in to make this the best theater I can...even though we charge $5 for first run product and most of our customers think our A/V is amazing [Razz]

Posted by Jay Wyatt (Member # 8768) on 01-05-2016, 07:51 PM:

60ft (18.3m) long room
Goal is to be capable of 121dB peak levels @ halfway point of room. (115dB required for digital soundtracks + 6dB headroom)

Inverse Square law for 121dB @ 30ft/9.1m gives us a 140dB SPL peak requirement at 1 meter.

Looking at sub datasheets (assuming mutually coupled):
2x JBL 4642As yield 142 dB
or
2x QSC SB-2180 would give ~142dB
or
2x QSC SB-5218 ~141

4642A are rated for 1200W continuous and the SB-2180 is 1100W continuousand SB-5218 is 1000W so we want ~2400W per amp channel @4ohm.

A DCA 2422 can deliver 2400W in bridge mono mode to 4ohm. So 2 of these Amplifiers - each in bridge mono - will do the trick.

Posted by Marco Giustini (Member # 4544) on 01-06-2016, 02:29 AM:

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?

Posted by Jay Wyatt (Member # 8768) on 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

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marco Giustini (Member # 4544) on 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.

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marco Giustini (Member # 4544) on 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.

Posted by Steve Guttag (Member # 268) on 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.

Posted by Jay Wyatt (Member # 8768) on 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?

Posted by Scott Norwood (Member # 30) on 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?

Posted by Marco Giustini (Member # 4544) on 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 [Smile]

Posted by Jay Wyatt (Member # 8768) on 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.

Posted by Sam D. Chavez (Member # 1841) on 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.

Posted by Marco Giustini (Member # 4544) on 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!

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marco Giustini (Member # 4544) on 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?

Posted by Sam D. Chavez (Member # 1841) on 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.

Posted by George Germisashvili (Member # 6597) on 01-07-2016, 12:26 AM:

https://bgaudioclub.org/uploads/docs/Yamaha_Sound_Reinforcement_Handbook_2nd_Edition_Gary_Davis_Ralph_Jones.pdf

Super book ,everybody can get answers for sound questions.

Posted by Marco Giustini (Member # 4544) on 01-07-2016, 02:15 AM:

I agree with you Sam.
It's just that sometimes you'll end up in one of those rooms where a little boost of something is needed which will be eating up your power.

Also, say you need to boost some frequencies: you can cut all the others but in the end you'll then need to turn up the whole channel and the outcome is the same.

Posted by Sam D. Chavez (Member # 1841) on 01-07-2016, 10:01 AM:

Marco

I understand your point for sure. My overall point is not to over EQ as it eats up headroom.

Posted by Marco Giustini (Member # 4544) on 01-07-2016, 02:17 PM:

Absolutely, Sam.