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Author Topic: Amplifier power for JBL ASB7128
Mark Strube
Master Film Handler

Posts: 306
From: Milwaukee, WI, United States
Registered: Feb 2007


 - posted 12-19-2017 01:44 PM      Profile for Mark Strube   Author's Homepage   Email Mark Strube   Send New Private Message       Edit/Delete Post 
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: 12209
From: Annapolis, MD
Registered: Dec 1999


 - posted 12-19-2017 04:12 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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Jay Wyatt
Film Handler

Posts: 29
From: Los Angeles, CA
Registered: Mar 2015


 - posted 12-19-2017 10:41 PM      Profile for Jay Wyatt     Send New Private Message       Edit/Delete Post 
Mark,

Post your room dimensions and someone can crunch the numbers for you. I'm surprised the person from JBL didn't bother to ask. Also, do you plan on coupling or spacing the subs apart?

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Dave Macaulay
Film God

Posts: 2135
From: Toronto, Canada
Registered: Apr 2001


 - posted 12-20-2017 06:43 AM      Profile for Dave Macaulay   Email Dave Macaulay   Send New Private Message       Edit/Delete Post 
The absolute power rating is for 20Hz-20kHz pink noise, and specifies the thermal capacity of the voicecoil with that signal.
A subwoofer does not get wideband pink noise, the real "will it self destruct" limit is cone excursion. I think Harmon probably knows what the unit can take in SW application. An overpowered amp is not a big hazard unless the speaker is overpowered by it, but there's usually no excellent reason to pay for an overpowered amp. Clipping of the amp will sound awful but should not damage a SW, the problem is with full range speakers' HF drivers getting blown by the huge increase in harmonic distortion during clipping. Most modern cinema amps have a "soft clip" circuit that reduces gain to avoid producing a squared wave clipped signal.
Over excursion in a SW driver sounds like a machine gun firing during loud subwoofer passages. That will eventually destroy the driver - delaminate the coil, damage the coil support tube, or cause the tube to go out of the gap and get stuck there.
Whether you have the best speaker for your application and if you have enough drivers/amps to get enough LF acoustic energy into the room volume is another topic.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12209
From: Annapolis, MD
Registered: Dec 1999


 - posted 12-20-2017 07:19 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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: 306
From: Milwaukee, WI, United States
Registered: Feb 2007


 - posted 12-21-2017 04:43 PM      Profile for Mark Strube   Author's Homepage   Email Mark Strube   Send New Private Message       Edit/Delete Post 
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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Scott Norwood
Film God

Posts: 7991
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99


 - posted 12-21-2017 05:10 PM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
So, doing math....

2/3 back from the screen in an 80-foot cinema is about 53', which is about 16 meters. Using the calculator at this page, two of these speakers at 4600w each should give you 115dB SPL.

Except that the speaker is only rated for 4000 watts for two hours.

Now, Steve can tell me where I went wrong on the math and what to do about the power rating issue.

I am guessing that, for practical purposes, this can be made to work (since no movie uses the subwoofer at full power for two hours at a time). In an ideal world, you might be in the market for another sub.

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Mark Strube
Master Film Handler

Posts: 306
From: Milwaukee, WI, United States
Registered: Feb 2007


 - posted 12-21-2017 07:35 PM      Profile for Mark Strube   Author's Homepage   Email Mark Strube   Send New Private Message       Edit/Delete Post 
In an ideal world, perhaps we try to sell the ASB's and pick up a bunch of QSC 7218's?

The amount we paid for the ASB's new would get us 6 of the QSC's with money left over. [Roll Eyes]

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12209
From: Annapolis, MD
Registered: Dec 1999


 - posted 12-22-2017 06:45 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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Scott Norwood
Film God

Posts: 7991
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99


 - posted 12-23-2017 04:26 PM      Profile for Scott Norwood   Author's Homepage   Email Scott Norwood   Send New Private Message       Edit/Delete Post 
quote: Steve Guttag
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.
...
quote: Steve Guttag

If the subwoofer can play 25-120Hz, that allows for a relaxation of about 3dB of the single-number SPL.

Thanks for the correction. Out of curiousity, how does one arrive at these efficiency and SPL figures?

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Marco Giustini
Film God

Posts: 2545
From: Reading, UK
Registered: Nov 2007


 - posted 12-23-2017 04:41 PM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
My question is: why did JBL keep pointing out that those speakers are 800W AES then?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12209
From: Annapolis, MD
Registered: Dec 1999


 - posted 12-23-2017 06:16 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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