JBL CBT 70J-1 Review (Constant Beam Transducer)

[Selah Audio]

@BenB I failed to address the issue you raised of side lobes. I did notice a significant one with the CBT-36 design. I think some of it may have been due to using auto EQ / correction with a DEQX unit trying to fill in a notch with the on-axis response.

 

[yourmando]

Yes, I am aware of the shading and the novel use of passive filter delays to help shape the beamwidth. I helped build the arrays in the attached picture, which I believe were the first large scale CBT arrays in the USA. It would be helpful to see how these measure at different distances in a home setting.

Yes indeed I’m aware of your good work!

 

[Selah Audio]

It's possible that the designer decreased the upper octave impedance to boost that range with the tweeters and help offset the decoupling as you move up in frequency. Even considering this, there's still a droop in the response but not as much of a concern for the typical pro audio application.

 

[Archaea]

I see some questions by folk asking why the a type of design isn’t more common among manufacturers. My guess is the complexity. Complexity adds cost and time, and potential for failure.

I mentioned in post 50 that since I had bought my original 13 used, I had 10 of the 13 speakers that had a failed 8 ohm sandbox resister. ( I also got one that appeared to be seriously overpowered and caught fire inside, I returned that one for a replacement)
Here is that saga. (Got it all figured out with amazing community support from people so much more knowledgeable than me)
https://www.avsforum.com/threads/jbl-crossover-issue-polarity-seeking-help.3007478/

Suffice to say there is a very complicated crossover, and there are well over a hundred screws in each speaker, complicated wiring harnesses, loads of drivers to test in each speaker (20 drivers per speaker), assembly, and testing of each speaker therefore more complicated. Think how much more complicated this design is compared to a traditional monopole two or three way speaker. Complicated to design, complicated to manufacture, complicated to assemble, complicated to solder, complicated to troubleshoot, expensive to warranty/support.

Crossover board:

pulled apart

Tweeters removed for testing one speaker that had a dissimilar high frequency response, and I had to work through troubleshooting that.
32 screws to take the speaker apart and get to the main crossover board, 8 screws holding in the crossover board, 16 screws holding in the tweeter PCB panel, 16 screws for the four 5" woofers, 64 screws for the sixteen 1" tweeters. (and a few more various screws to hold in the varying wiring harnesses.) I assume these are human assembled. That's a lot of time testing, and just screwing stuff just for one speaker.
The screws in the two boxes are JUST the screws for a single speaker's tweeter array alone!

Post 60 here documents a call to JBL support, and attests to how complicated troubleshooting this design is:
https://www.avsforum.com/threads/jb...ity-seeking-help.3007478/page-3#post-56996854

Given all this, it's kind of a minor wonder JBL can sell these at a MSRP as low as they do.

 

[test1223]

Very nice to have a look inside.

Inside they aren't that beautiful. I assume the box is made out of plastic or is it wood? Bassreflex with very little damping material in the pictures!? You almost certainly have some serious mid range leaking out of the ports if there isn't more material.
For home use I would close all reflex ports and put in a lot of damping material, since the spl capability is more than enough.

How many ways do you need to replace the passive crossover with an active crossover? Are two ways enough for one fixes vertical beam mode?

 

[Archaea]

Very nice to have a look inside.

Inside they aren't that beautiful. I assume the box is made out of plastic or is it wood? Bassreflex with very little damping material in the pictures!? You almost certainly have some serious mid range leaking out of the ports if there isn't more material.
For home use I would close all reflex ports and put in a lot of damping material, since the spl capability is more than enough.

How many ways do you need to replace the passive crossover with an active crossover? Are two ways enough for one fixes vertical beam mode?

These at their core would be two ways, but my understanding is there is shading = not all tweeters play at the same volume --- also there are different crossover paths because of switch settings. Music/Speech, and Broad/Narrow. (complicating the crossover)

They are plastic boxes, not wood.
They do have foam in the back of the case and on both ends of the case - basically everywhere the giant crossover board is not.

 

[DubbyMcDubs]

I could not see through the grill to pick a "reference axis" and wouldn't know where that would be anyway. So for measurements, I just picked a more or less center location on the flat part of the speaker.

JBL generously provide the CLF files on their website for a number of models, including the CBT 70J, which displays the reference axis. Simply select the "Cabinet 3D representation" radio button on the top right of the form.

https://jblpro.com/zh/support_downloads/download_types/clffile

I use this viewer:

http://www.clfgroup.org/viewer.htm

 

[jdubs]

How about a pair of these with sealed subwoofers for music? Anyone have thoughts on that? These are quite a bit cheaper than JBL 708s that I've been considering for a long while now.

Thanks,
Jim

 

[Bartl007]

After owning 708i's, and CBT50's, (currently running M2 LCR with cbt50 surrounds)) , just go with 708s with no sub if just for music only. They are a much better speaker for music only purposes. I don't think you'll find the Bass lacking at all.

 

[jdubs]

Thanks for the feedback! How do you think the 708is are better for music? Just because of the inclusion of bass?

-Jim

 

[Bartl007]

Thanks for the feedback! How do you think the 708is are better for music? Just because of the inclusion of bass?

-Jim

More than just bass, better high frequency dispersion characteristics/wave guide, better drivers, low frequency extension, easier to setup in a domestic space, etc. If music is your priority, the 708 is a better tool for the job. If you are listening at significant distance, the output advantage of the cbt70J may be worth considering, but we're talking outdoor type distances ,10+ meters.

 

[jdubs]

More than just bass, better high frequency dispersion characteristics/wave guide, better drivers, low frequency extension, easier to setup in a domestic space, etc. If music is your priority, the 708 is a better tool for the job. If you are listening at significant distance, the output advantage of the cbt70J may be worth considering, but we're talking outdoor type distances ,10+ meters.

This is very helpful, thank you! I guess the way I'm thinking about it is that 708is are like $2,500 pair ('ish). These CBTs are $1,500 a pair (you can find them that cheap, if you look). Plus I could pair them with 2x SVS SB-1000 Pro subs for $950. So, for about the same price as the JBLs, I could get the CBTs with 2 subwoofers.

Sooooo...a decision for sure.

-Jim

 

[Bartl007]

This is very helpful, thank you! I guess the way I'm thinking about it is that 708is are like $2,500 pair ('ish). These CBTs are $1,500 a pair (you can find them that cheap, if you look). Plus I could pair them with 2x SVS SB-1000 Pro subs for $950. So, for about the same price as the JBLs, I could get the CBTs with 2 subwoofers.

Sooooo...a decision for sure.

-Jim

If it helps your decision, you could save about ~$400 on the 708i through northernsoundandlight.com

You'll of course need the correct DSP settings for them to sound like they should. I use the crown DCI/N series amps with good success (with them mounted remotely in an equipment rack)

Just make sure to create an account and login for best pricing

 

[BenB]

What about the tweeter? I looked the other day and it is no longer available.

Are these the same speakers that left Dennis Murphy speechless? I would love to see Amir or Erin measure your speakers.

It looks like the tweeter in the skylarks is available again. The midrange is back-ordered, but should be available by the end of the month.

 

[DLxP]

currently running M2 LCR with cbt50 surrounds

Are you still happy with the CBT50s as surrounds? I'm very seriously considering buying four of them, to function as side and rear surrounds.

I'd be very interested in your (and others') opinion. For info: I sit approx 3 feet from my right surround, 4 ft from right surround back; 8ft from my left surround and 8 ft left surround back. I would need to mount the CBT50s flush to the wall using the flush wall mount. I find that, when using conventional speakers, the right side dominates significantly, even when all speakers are level-matched in Dirac.

 

[Archaea]

Are you still happy with the CBT50s as surrounds? I'm very seriously considering buying four of them, to function as side and rear surrounds.

I'd be very interested in your (and others') opinion. For info: I sit approx 3 feet from my right surround, 4 ft from right surround back; 8ft from my left surround and 8 ft left surround back. I would need to mount the CBT50s flush to the wall using the flush wall mount. I find that, when using conventional speakers, the right side dominates significantly, even when all speakers are level-matched in Dirac.

Your walls are not symmetrical? I suspect you can fix that with some absorption treatment. It worked for me. My room isn’t symmetrical and even though I level matched the speakers with a calibrated mic, the side with the drywall closer to my seating position always sounded slightly louder when watching movies or listening to music. The other side of the room is far more open. I applied 3.5” foam to the closest wall and the concern completely disappeared -- I no longer notice the issue whatsoever. Here’s a picture. So while I agree CBTs make EXCELLENT surround speakers providing very uniform coverage that doesn't blow out the closest seat, and has less SPL fall off at distance compared to traditional speakers - - I don’t think CBTs alone will fix that problem, at least they didn’t for me. I suspect your problem is not the level matching or the speakers themselves, but the reflected sound coming off the closer wall from the speakers on the opposite wall, and level matching won't fix that. I needed to absorb some of the reflected sound off my closest wall. The foam I used worked better than the 2” and 4” DIY rockwool panels I tried. The foam’s shape allows for more and less absorption over any given space depending on where the reflected sound hits - since it varies in thickness. (Which is what you want vs. just flat OC 703 or rockwool panels absorbing uniformly)/ I tried panels first, then went to foam. I’m happier with foam. I don't see how anyone would be happier with panels, having tried both, unless you just prefer the cosmetics of panels.
https://www.avsforum.com/attachments/img_1602-jpg.2615364/

This is the seller I purchased my foam from. I don’t know them, but I was happy with it and told him so. Through our correspondence I agreed he could use my room photo in his eBay marketing so that’s why you see it there. I am not the seller, nor do I have any financial gain or relationship with the seller. I’m just a satisfied customer. The foam solved my problem, and perfectly to my ears/senses. On my wall is just a single $100 order of the foam he sells and I still have a few pieces left over.
I’ve been considering buying another set of it for my back wall to complete out my room sound treatment, but am undecided, because I think I like a dead front wall and a live back wall based on the various rooms I've visited and their sound treatment plans. My front wall is already treated. My wall not pictured is open enough I don't think it matters (and not a straight wall anyway, there is a cutout for a computer desk, the carpeted path upstairs, and a it opens up a lot further to another room in the back. I can tell a difference in absorption level as I walk between front and back of my HT area in just speaking aloud, so the foam clearly works. The back, untreated is more live, but I suspect I somewhat like that for cinema surround purposes. If you use acoustic foam skip Locktite adhesive spray. It just doesn’t hold the foam long term and my foam kept falling off after multiple months with that stuff. Frustrating and messy. 3M Super 77 adhesive spray is the stuff that has worked long term. It hasn’t given way yet, and every piece that has fallen off from locktite adhesive has stayed strong from the 3M spray adhesive.
https://www.ebay.com/itm/3-1-2-Thic...=p2349624.m2548.l6249&mkrid=711-127632-2357-0

 

[Bartl007]

Are you still happy with the CBT50s as surrounds? I'm very seriously considering buying four of them, to function as side and rear surrounds.

I'd be very interested in your (and others') opinion. For info: I sit approx 3 feet from my right surround, 4 ft from right surround back; 8ft from my left surround and 8 ft left surround back. I would need to mount the CBT50s flush to the wall using the flush wall mount. I find that, when using conventional speakers, the right side dominates significantly, even when all speakers are level-matched in Dirac.

I am still very pleased with my cbt50 surrounds and use them in a somewhat assymetric space as well with great results.

Also, the flush mount wall bracket works great and can be purchased from the same vendor as the speakers (referenced earlier in this thread)

Regarding level matching right and left side surrounds, make sure to check your level and delay settings on your AVR to ensure its not getting things wrong and don't forget to experiment.

Dirac is great, but it consistently sets my right main channel a few milliseconds off from the left main channel which shifts my center image and requires manual adjustment to correct, so don't be afraid to play with things a little.

 

[jdubs]

Where would one cross these to subs at?

 

[Bartl007]

If I remember correctly, I think dirac crosses my cbt-50's at ~90Hz. here's my in room response curves for my 4 ear level speakers as averaged over my 9 dirac measurement points (showing LF extension) Hope it helps for those considering these

 

[jdubs]

Thanks! Guessing that the CBT-70s could maybe be pushed a little bit more (80hz)?

-Jim