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Author
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Topic: Amplifier power for JBL ASB7128
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Mark Strube
Master Film Handler
Posts: 322
From: Milwaukee, WI, United States
Registered: Feb 2007
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posted 12-19-2017 01:44 PM
I sent the following message to JBL and I'm rather suspicious of the accuracy of the response I received. First, my message:
quote:
We currently have TWO of the ASB7128's in our largest screen at one of our theaters. However, I suspect they are very under-powered, and I may be swapping out our amplifiers soon. What specific amp wattage would you recommend that I look for to make proper use of these monsters? Thanks!
The response of a technician named Arturo...
quote:
The ASB7128's are rated for
800W at 4-ohms each
The Crown XTi 2002 is a nice match
https://www.crownaudio.com/en-US/products/xti-2002
Hope this helps
I then replied...
quote:
Thanks for the reply. Are we talking about the same speaker? I’m referring to your high-powered dual 18” subwoofer, the ASB7128.
On the spec sheet I see it’s rated for 4000 watts continuous RMS, and 16,000 watts peak.
I also see a “long-term power rating” of 2400W, and 9600W peak.
Is an 800 watt amp really enough for this?
To which Arturo replied...
quote:
That is Correct
1600W is the peak, but
The continuous program is 800W at 4-ohms
The Xti2002 will do exactly that
I still feel like we're talking about two different speakers here. The JBL spec sheet can't be that far off, and I have a hard time believing that a 800 watt amp can correctly support a dual cabinet JBL speaker that is quoted as having 16,000 watt peak handling with a 3.5" excursion.
Any guidance would be appreciated, I'm trying to get our large house properly powered. (And I'll be leaning towards QSC amps, not Crown.) Thanks!
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-19-2017 04:12 PM
The 7128 uses two 2269 drivers. They can handle 2000-watts (each) continuous pink noise or 4000-watts for the cabinet. For subwoofer duty, that is the operative rating. The 2269 is a real Bad-Ass driver
That doesn't size your amp however, it merely tells you how big it CAN be before running your amp at full power you will cause speaker damage.
You need to know what the room length is (speaker to last seat) and then use the sensitivity (99dB 1W/m) to see how much power you NEED for your room.
Any reason you/someone chose the ASB line? Are they flown? The ASB are indeed a sturdy box with grill but they won't have the best bottom end. You can get the 4642A box with the 2269H drivers. It is the 5628. Quite possibly the scariest subwoofer made for the cinema market at the moment (because of those drivers). Your sensitivity drops to 95dB but your bottom end drops to 18Hz too. And the drivers have the ability to do it.
Remember too, when driving something like this at high power to think about 220V amplifiers. Subwoofers can have sustained passages (think the take-off sequence in Apollo 13...we reconed dozens of speakers due to that one). All of the "1/8" power ratings go out the window that you can normally use for stage speakers and surrounds. Subs have to be able to deliver continuous power for longer than the typical reserve in an amplifier's power supply. So, if you are on a 20A circuit, even with 100% efficiency, you top out at 2,400 watts. Run at 208V (2-legs in a 3-phase system), you drop your current demands almost in half.
And, if your not trying to play down to the low-end of the 2269, then I'd use the 2242 driver and the systems built around it. It will get you down to 22-25Hz in the right box (e.g. 4645C) and B-6 alignment. It is only an 800-watt driver (continuous) but you efficiency goes up accordingly too.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-20-2017 07:19 AM
Dave, you are wrong on clipping in subwoofers. Clipping the amp on a high-powered subwoofer application is absolutely a damaging thing. In addition to the forces it places on the driver components (coming to a dead halt in a near discontinuous manner) it also halts the cooling of the voice coil. Most drivers depend on the driver motion to force air across the voice coil. Naturally, it all depends on the severity of the clipping and the power in the system. Again, as someone that recones drivers, you an tell a driver was clipped just looking at the voice coil. It will have dark bands at the ends from where it cooked with no pole piece to help dissipate the heat. Typically, it is not uniform either and it can even appear as dark only on one end or the other (depends if the amp didn't clip uniformly as well as the mechanics of the particular driver).
In system with passive crossovers (full-range systems) then clipping and the resulting harmonics get "crossed over" so the poor HF systems get to be the recipients of the high level harmonics and indeed take them out (another reason to avoid passive crossovers in high-power systems).
But looking at the 2269 driver, the X-max is 19mm (just shy of 1-1/4 inches). In comparison, the 2242 is only 7.87mm, the 2241 is 7.62mm. Even the oldie 2245, with foam surrounds would jump the gap at 9.65mm.
Again, if you look at what the 2269 driver can do, it is a pretty scary thing. It isn't as efficient so you need more power, which it can drink, just fine and what you get in return (presuming you have the right box for it) is very loud and very deep bass.
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Mark Strube
Master Film Handler
Posts: 322
From: Milwaukee, WI, United States
Registered: Feb 2007
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posted 12-21-2017 04:43 PM
Thank you all for the information so far!
This was 3 years ago, but if I remember correctly, the ASB line was recommended to us as an essentially bullet proof high-power solution to our big room. It’s unfortunate to find out the extended low-end response that I like to hear may be tough to achieve. We have QSC SB-7218’s in our other rooms and I love the sound and low-end extension of those.
The 2 cabinets are next to each other (coupled), they are not flown -- they are underneath the screen channels (between center and right). They are a bit higher up than the floor of the auditorium, however. I'm thinking we should also look into moving them to a lower level? The distance from speaker to last seat is 80 feet. This theater also has a high domed ceiling, if that makes any difference.
Currently, they're both connected to a single DCA 2422 in parallel mode. We're using a USL JSD-100 for the processor, tri-amp for the screen channels with crossover happening in the processor. (This processor's capabilities pretty much negates the need for a low-end boosting filter on the amp, right?)
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-22-2017 06:45 AM
The flaw in Scott's math (or the usage of the SA page) are:
Each DRIVER has 2000W capacity and there are 4 drivers in the system or 8000W capacity (for 2 straight hours). There are two boxes next to each other (mutually coupled) so the effective sensitivity of the system is 102dB 1W@1m and the "SPL" level has to be compensated for since you are not playing a wide-band signal. That is, the 115dBc presumes a 20-20KHz signal to be valid (at least, 60Hz to 4KHz...where I think the filter comes into play). You don't have that in a subwoofer, you have barely 1/3 of that range. When one sets a subwoofer level, they don't use an SPL meter (if they're smart) but uses an RTA and sets the in-band level 10dB above the in-band level of the Center speaker. If the subwoofer can play 25-120Hz, that allows for a relaxation of about 3dB of the single-number SPL. The wider its frequency response the higher the single-level should measure.
So the power requirement is about 2,650 watts. Split between the two cabinets, 1,325-watts.
There are several ways to accomplish this. Using my preferred amps, the DCA line from QSC, that would be either TWO DCA 3422, wired parallel and each driver gets its own amplifier channel or (how I would actually do it) TWO DCA 2422 configured Bridged Mono into each cabinet. You could probably do it with just a pair of DCA 1622s but with those drivers, I'd rather have more power than less. Put an SF-3 module on the back of the amp and you could probably use the "B6" filter to extend its response. It will put a boost with a Q of 2 on the bottom end around 20-25Hz that might mate up well with the natural response of the cabinet. The High-pass should be somewhere in the 20-25Hz range. You don't want to apply a boost if the box has unloaded the driver (there is a point where the driver will "feel" like it is in open air due to the box being too small for the reproduced frequency).
So, using the Crown amplifiers, two DSi 6000 if wiring "parallel" and using a separate run to each driver, or, at least a pair od DSi 2000 wired bridged into each cabinet. It is harder to pinpoint Crown's real power because they only want to publish their 1KHz spec...which has no meaning in a subwoofer application. You want a 20-20KHz spec. If you are nervous and have the money, go up one size the DSi 4000. Remember, they only have to be "cheating" the specs by a mere 3dB and you've cut the power capability in half! That big 3000W amplifier turns into a 1500W amplifier that you can actually use. The DSi amplifiers have a baby DSP onboard so you can apply a complementary filter right there on the amplifier via a parametric EQ and probably extend the bass down to what the system can really do. I haven't checked Crown's System IQ to see if the ASB7128 is on their list of pre-configured systems, if it is, it should be a good starting point.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-23-2017 06:16 PM
I can't and won't answer what what someone else says (I have enough trouble explaining myself!).
The sensitivity has to be supplied by the manufacturer. Like with all manufacturer specifications, look it over for any disqualifying information...like a sensitivity based on say 250Hz-1KHz...not meaningful for a subwoofer). Ideally, the sensitivity is over the range that you plan to play it.
As for the SPL level, that is defined by the format (105dBc for the stage channels, stupidly, 102dBc for each of the surrounds and +10dB referenced to Center for the subwoofer/LFE channel).
If you wanted to do the mathematics of it, you'd have to know the frequency range based on the weighting (and whatever the weighting is, in our case "C") "C" weighting is essentially 40Hz-8KHz (3dB down points). If you are doing 1/3-octave measurements (typical for cinema), you can use and RTA to measure each 3rd octave band for its SPL, apply the C weighting and then do a logarithmic summation to arrive at the C-weighted SPL. Since a subwoofer isn't going to play above 250Hz (and for most don't play above 120Hz), all of those bands above 120Hz aren't going to add into the summation and will lower the C-weighted SPL number. Heck, C-weighting already de-emphasizes the subwoofer frequencies. Typical "SPL" numbers that are +10dB for in-band level are something like 89-91 dBc (depending on how wide a response the sub has). It isn't 95dBc (when Center is 85dBc). This is why an SPL meter is worthless for setting subwoofer level (plus most people's SPL meter is of unknown response in the subwoofer frequencies). An RTA will allow one to set the level correctly, regardless of the sub's frequency response.
But using the information that the subwoofer is not a full-range channel and what portion of range it encompances, one can pretty safely lower the SPL requirement to 112dB from 115 and still leave a bit of headroom. Again, if you wanted to be precise, you'd have to know the exact frequency range of your sub in the space it is going to be in. Note, if you are applying things like B-6 bass boost, that comes off of efficiency. you are pumping more power into the sub in the 20-25Hz region. Then again, your room may have a resonance that requires removing some power.
The subwoofer channel is definitely one where I would err on the side of caution (have enough driver and amplifier power). 3dB is a doubling of power and subs are where things are moving the most.
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