Dual opposed sealed JBL GTI MKI 15" sub build

This is my latest sub build I'm working on. I started it a few weeks and I'm almost done with it. It consists of two used JBL GTI MKI 15" drivers that look almost brand new, $470 shipped. The amp, I ordered today. It's also used (1-month old) Peavey IPR DSP3000, $425 shipped. The box is a 140L (internal space) 22"H x 22"W x 24"D dual opposed sealed birch ply box which cost me about $100 in materials. It has a cross-type bracing on all four sides of the box (very stiff) and painted to match my wall unit. I will use some 4-lbs of polyfill and a Speakon connector for the subs. The total cost of this sub build came out to $1,000.

I wanted quality drivers and the GTI's are from what I hear the real deal. Due to the fact that there are very little DIY drivers to pick from, it made my decision easier. I wanted a light and powerful amp w/DSP for ease of use, that's were the Peavey IPRS DSP3000 came in. Again, I've heard very good commnets of that amp. I had to decide between the Peavy and Behringer Inuke DSP3000 amp. The peavey seems to be the better amp, it gives true RMS power and cost some $200 more. Its power specs are 1500W x 2 at 2ohm and 1,000W x 2 at 4ohm. The subs are DVC 6ohm for a final load of 3ohm, so the Peavey amp "should" provide each driver with about 1,200W or so. But until both are tested side by side, it's hard to tell.

This sub is replacing my previous TC Sound LMS Ultra 15" sub w/PR's in a 200L box, tuned at 16hz. I had the SMS-1 for EQ (sold) and the Yamaha P7000S amp to power it (sold). If anyone is interested in the LMS Ultra driver and Acoustic Elegance 18-2100 PR's let me know, they are for sale (Ebay). This new sub project is cheaper than the old one. Is it better? we'll see. More to come with pics.

This sub will go in a 5,000cf+ room, perhaps letter I will add another one just like it. It will go in the same spot as the previous sub (right corner) which gives me a good overall response. Eventhough the room is big, the corner placement with block walls and croncrete floor gives me good bass gain down low. Also, I'm not too crazy on the GTI's logos on the cone but since the sub will go next to the sofa, I won't see them. The MKI is the same driver as the MKII, except the logo on the cone. The amp should arrive in a few days. I want to see what a light/efficient/powerful class-D amp can do for sub duty. I do have a question; Since the amp will be used in stereo mode, will both of its Speakon outputs be used? Do I need two Speakon connectors and two Speakon cables to connect the amp to the two drivers? Before, I've only used 12-gauge wire to connect the amp to the sub. Do the Speakon cables do a better job? What gauge cable should I get? Thanks. Pics will come next.

Correct, add the CSS SDX 15" driver to that list. I know, it never stops, does it?

True, but I don't have the space for a horn sub. I've had a single sealed sub, but never a dual opposed sub. I have the flexibility that in the future if I want to add more bass to my system, I'll add another dual opposed sub and still have a "reasonable" size sub in my room. If/when I get the second one, with the added bass/smoother room response, it should keep me from "upgrading" for quite a while.

Bossobass, thanks for the info.

I ordered some Speakon connectors/cables which should be here in a few days. The amp came in yesterday (Peavey IPR DSP3000). The amp is very light indeed. From some of the readings I've done, the fan noise is not too bad, I guess it can be replaced if not to my liking. Some are reporting the power is there, we'll see. I want to see what the DSP can do. Also, efficiency should be good (Class-D). Price is good and build quality looks o.k., not great. If the reliability is there then what's not to like? I would have prefered for the unit to be black. The amp will be wired in stereo mode at 3ohm per channel (the JBL GTI MKI subs are DVC6ohm each). The amp does 1,500w at 2ohm and 1,000w at 4ohm in stereo mode so at 3ohm it should deliver around 1,200w per channel, no?

This box (internal) size came out to 4.7cf, so each driver is getting some 2.35cf of space. I'll be adding some 4.5cf of polyfill. I used one 4' x 8' 9-ply birch ply from Lowes which looked good, better than the ones at Home Depo. I used a table saw for the cuts, jig saw to cut the driver holes and screws/glue to put the box together. I used some "decorative" wood where the drivers are going (front/back of the box). The front/back of the box have a double bafle to have the drivers flush mount. I gave it three coats of stain which matches the color of my wall unit. Also, the sub used four small rubber feet for support.

Finally, here are some pics, more to follow.

I forgot to mention that the other driver that you see there besides the two JBL's is the TC Sound LMS ULtra 15". The depth is almost the same but the Ultra weights as much as the two JBL's put together.

In the dual opposed design, besides almost no vibration to deal with, the sensitivity goes up some 3db due to the dual drivers, correct? Each driver has a sensitivity of 88.2db, it goes up to 91.2db with duals? Less power to make them go louder, no? I guess that's another positive of a dual opposed sealed sub.

A few more pics.

Finally, all the cables I needed came in. The two Speakon 10', 12ga are big. I had installed one of the Speakon connectors in the bottom of the sub but will have to install them on the side now. The reason? These Speakon cables stick out too much, as Bosso mentioned, at least 2"-3". I will have to fill the hole I made in the bottom of the box. Hopefully, I'll be able to test the sub before this coming weekend as everything I need is in place.

Good question, I believe that with 800W, it does around 103db at 20hz and 118db by 80hz in my 4.7cf box (2.35cf per driver). So with 1,600W for both drivers, it should be around 109db at 20hz and 124db at 80hz.

If you or anyone wants to model it, total SPL and xmax, I would appreciate it. I'll have about 1,200W per driver and hopefully it won't bottom the driver (20mm xmax) in the box I'm using. Also, since I'm using about 4.5lbs of polyfill, that might make the box look bigger, perhaps more like 5.5cf (2.75cf per driver)?

Thanks. I thought another dual opposed sub would add 6db's. I guess 5db's is close enough.

pbc, thanks for the sub modeling. You also have some nice "damn" sub drivers in the AE's. You did the modeling with 1,600W and the sub's xmax came close to their limit of 20mm xmax (19mm at 10hz). My amp should produce around 2,400W for both subs, a 1/2 increase of power over your model, which in turn might make the subs do an extra 1.5db's in SPL, but could make the subs go over their rated 20mm xmax, right?

I guess I can always dial down the amp power a bit or perhaps just line the sub enclosure walls with polyfill (2-lbs or so) instead of stuffing it with 4.5lbs of polyfill I was planning on doing to try and make the sub box seemed bigger than its actual size of 4.7cf, which will reduce the sub's power handling. Any ideas of what I should do?

LTD02, I read the article, interesting. Basically if the sub uses all the power needed to reach xmax, say 800W at 10hz in a 2.5cf box and whatever extra power the amp provides, say 1200W (which is 400W above the sub's handling capabilities is eliminated or dissipated by the JBL), correct? What happens to that extra 400W of power in the mid-bass where the sub's xmax is not reached? Do you gain an extra 1-1.5db's extra in those frequencies?

pbc, the Peavey amp will be used in stereo mode. The amp should produce around 1,200W p/channel with my 3ohm subs. It does 1,500W at 2ohm and about 900W at 4ohm. Halfway of those numbers which is were my sub is at is how I came up with 1,200W which should be close to the amp's real output.

Like you mentioned, when you calibrate the sub with the speakers and you lower the receiver's volume to maybe protect the sub from being overdriven past its xmax, you lower the volume on the speakers also. It's probably something you might not want, specially if you're used to listening to a higher volume, say reference (volume-0) when watching movies.

Another thing I was thinking to lower the amp's volume or impact is to place a high pass filter (subsonic filter) at a certain low frequency, say 10hz, to protect the driver from going past its rated xmax. I'm thinking that since quite a few amps have a roll-off below 10hz and there's not a lot of bass frequencies below 10hz anyways on most movies, that filter might not be needed. The less filters added, the better, specially in a sealed sub as I understand it. Anyways, as I mentioned above, I think my sub will handle that extra power without having to lower the volume or set a low pass filter. I think that I will do a full poly fill on my sub (1lb per square foot), unless someone convinces me otherwise, say to just line the walls.

On the second point, as I understand it, if you due a full box fill (1-lb per square foot) it will extend and increase a little the low bass response, lower the sub's QTS by making the box seem larger. On the other hand, making the sub box seem larger will lower its power handling down low (xmax will be reached with less power) and if I'm not mistaken, reduce the sensitivity a bit in the mid-bass. Lowering the sub's ouput 1-2db's in the mid-bass and raising it 1-2db's in the low bass should help the sub produce a smoother response, not taking into account the room's issues such as room gain, peaks/nulls which might need eq., bass traps, etc. Someone please jump in if I'm wrong somewhere.