Omni 15 and Omni Sub

[Frankenspeakers]

I don't see an issue with crossing the (aux)Subs at around 100 Hz or so. The trick would be to run an impedance curve in that area to see what the actual impedance at 100 Hz is. I would assume that it is still on the downhill slope from the tuning frequency. (which is all good because the cap & coil values get pretty large with lower impedances and rather pricey) Hey, if it gives the sound that the customers like, it's all to the good.

[Mikey]

The two 8 ohm drivers in parallel would be a 4 ohm load, but the two crossover components in the woofer circuit would have to be changed in order to accomodate the 4 ohm load while maintaining the 500hz crossover frequency ... the 20uf cap would have to be 40uf, and the 5mh coil would have to be 2.5mh. The logical way to accomplish going with/without the sub would be to use two 20uf caps and two 2.5mh coils ... 1 cap + 2 coils for 8 ohm woofer, 2 caps + 1 coil for 4 ohm woofer, with a switch to accomodate that.

You're over thinking it, the crossovers would be in each cab, you parallel connect their inputs so each is an 8 ohm circuit. It's no different than simply running two O15's in parallel, or any other cabs with internal passive crossovers.

Hmmm, yeah, that's true. In parallel, the caps would combine their values and the coils would be half of the value of one. The two 5mH coils would be about $10 more than two 2.5mH coils, but then you wouldn't have to be bothered with a switch. So, yeah, it makes sense to spend the extra $10.

[Mikey]

I don't see an issue with crossing the (aux)Subs at around 100 Hz or so. The trick would be to run an impedance curve in that area to see what the actual impedance at 100 Hz is. I would assume that it is still on the downhill slope from the tuning frequency. (which is all good because the cap & coil values get pretty large with lower impedances and rather pricey) Hey, if it gives the sound that the customers like, it's all to the good.

It could be done, but what would you gain by doing it? You wouldn't get any lower, and you'd lose the sensitivity gained by having all 4 sub sections covering everything up to 500hz.

In the original O15Sub thread, long ago, Bill said it really wouldn't make sense to use more than one O15Sub. The reasoning was that two fully horn-loaded subs would have more benefit. However, personally, when the thread was all said and done, I felt that a pair of O15Subs could also be a reasonable, viable, and even preferred option, in particular cases where there was absolutely no intent for expansion, such as a big bass guitar rig or a strictly-club PA for about 300 and under. My reason for that thinking was output versus pack space. About the closest match in frequency response and sensitivity to an O15Sub is a 30"w T39. A pair of 30"w T39s occupy over 26 cu ft of space, while a pair of O15Subs occupy under 16 cu ft of space. What you give-up for that 10+ cu ft of pack space is full horn loading and the capability to gain 3db by v-plating. Some people may very well be willing to make that trade (?).

Nine months later I have finally found the time to post some pix of my exploits ... I have used these cabs for DJ work over the summer

Sounds to me like it worked pretty good for the OP.

[Pete J]

Hey,


Mikey wrote:

"The O15 Sub was discussed at great length in a thread quite some time ago. Many pros and cons were weighed, and the general concensus was that it would be good as a stand-alone, such as for bass guitar or keys, but fully horn-loaded cabs win-out above that, for many somewhat obvious reasons. Judging from these pics, it looks like tarduare probably used the O15 Sub dimensions from that thread, which also includes one extra piece of wood to keep the horn depth the same while making the cab deeper."

I don't think this thread is available anymore -- if so can you provide a link?

I assume that to get to the dimensions used here, you would calculate the same inner volume as the plans, then decide on your height and width (here it's basically the height of the port and the same width as the plans) and then plug the depth of the back as needed to equal the original inner volume.

Is the extra board Mikey speaks of for the purpose of forming a new back wall to the port which makes the port opening to the larger cabinet the same size as in the original? If so, how far out into the cabinet should that extra board go. Is it OK for it to be just the width of the port opening, or should it extend deeper into the box.

Thanks

Pete

[Harley]

... you would calculate the same inner volume as the plans, then decide on your height and width (here it's basically the height of the port and the same width as the plans) and then plug the depth of the back as needed to equal the original inner volume. .... Is the extra board Mikey speaks of for the purpose of forming a new back wall to the port which makes the port opening to the larger cabinet the same size as in the original? If so, how far out into the cabinet should that extra board go. Is it OK for it to be just the width of the port opening, or should it extend deeper into the box.

Don't take this wrong, but I fail to see the sense of trying to redesign the wheel.

The tubas come in adjustable widths without any redesign and do a fantastic job, so it's no sweat making them match the width of any top you'd want to use. They perform better than a Omni as a sub. They are as easy to make as an Omni - if not easier. So why stuff about.

The hours you spend redesigning the wheel would be better spent making some DR tops...

[Mikey]

I don't think this thread is available anymore -- if so can you provide a link?

Nope, it's long gone.

I assume that to get to the dimensions used here, you would calculate the same inner volume as the plans, then decide on your height and width (here it's basically the height of the port and the same width as the plans) and then plug the depth of the back as needed to equal the original inner volume.

You can use any h/w/d ratio you want, as long as you keep the horn and the volume the same as in the plans. No idea what you mean by "plug the depth of the back", so I guess that answer would be "no", just calculate the h/w/d to provide the proper volume.

Is the extra board Mikey speaks of for the purpose of forming a new back wall to the port which makes the port opening to the larger cabinet the same size as in the original? If so, how far out into the cabinet should that extra board go. Is it OK for it to be just the width of the port opening, or should it extend deeper into the box.

You guessed it, Pete. The extra board is to provide the "back wall" for the horn. Here's the illustration which was in that old thread, showing the extra board ...



As you can see, the board is 6", dimension given by Bill.

Obviously, the easiest way to do this mod is to make it 26.5"w to keep the horn it's simplest, then vary the other two dimensions "to taste" (retaining the original volume, of course).

An idea that I had, long after the thread, was that the entire top or back (whichever you prefer) could be removable instead of having an access panel. It would simplify the build, plus make driver and wiring installation easier than working through an access panel.

Bear in mind that when you do the design work of this mod, you must provide adequate bracing, similar to that in the plans, and possibly more for long dimensions. The "extra board" must also be worked into your bracing scheme. If you go with a removable top or back, you must brace that, too, to keep it from flexing and vibrating.

[Mikey]

The tubas ... perform better than a Omni as a sub

As I stated previously, About the closest match in frequency response and sensitivity to an O15Sub is a 30"w T39., and here are the SPL charts:





Notice that the O15Sub is still the clear winner, by as much as 3db, which means that you'd have to v-plate a pair of 30"w T39s to make them a closer match to a pair of O15Subs. As far as total output capability, if 3015LFs are in the O15Subs, what 12"er could you put in the T39s to keep up with them?

There are definately trade-offs, as I mentioned clearly in an earlier post, but they're definately reasonable trade-offs, worthy of consideration. Think about it ... what pair of cabinets could give them good competition in under 16 cu ft of pack space?

The tubas ... are as easy to make as an Omni - if not easier.

Tubas and Titans have a 5 difficulty factor, the O15 is a 3. Take away the mid horn, high freq section, and crossovers, and you're down to a 2. The O15Sub would only be a tad bit more difficult than a sealed box.

[Pete J]

Hey,

Thanks for the help and some good point/counterpoint. My sub will need to use a 15 - because I have one, and I've built an O15 before, which takes the difficulty level down by about a factor of 2. The sizing of the O15 subs in this post is exactly what I want to put under my OT 15s when I use them with subs. Mikey's reply answers my last question so I'll be getting started soon.

Next quest is where to get the money for an amp, X over, mixer board etc. I'm using a powered mixer now.

Any ideas??

Cheers ,

Pete

[bgavin]

Take away the mid horn, high freq section, and crossovers, and you're down to a 2. The O15Sub would only be a tad bit more difficult than a sealed box.

After building an O15LB, that is my conclusion also. I found the mid-horn to be the most troublesome part of the O15 build. The drawing above allows for a simpler back also. Making the false back stop taller than 6" does not hurt anything. Too short is when it becomes a problem.

[Harley]

... and here are the SPL charts.....Tubas and Titans have a 5 difficulty factor, the O15 is a 3. Take away the mid horn, high freq section, and crossovers, and you're down to a 2. The O15Sub would only be a tad bit more difficult than a sealed box.

So you've built both then and talk from experience..... ...and packed them up to travel to gigs in your car?

You know Mikey, if I take what you are advocating here then why is anybody bothering with Titans or Tubas in the first place? Take it one step further, it almost seems that it's mandatory to get hold of Bill's plans and then redesign them...

[Mikey]

Harley, I haven't built either one, but it doesn't take a rocket scientist to see the difference. I've built an AT, which, like the Titans, has a winding horn path, and it was considerably more difficult than any sealed, ported, or open-backed cabs I've built. The O15Sub only has eight boards and bracing. Be honest, do you really think that you could build a pair of T39s in the same time that you could build a pair of O15Subs, even with your experience in building T39s?

This sub is a "whole 'nother animal" than Tubas and Titans. The differences have been clearly explained and purposely beaten to death, so that the positives, negatives, and limitations are perfectly clear to everyone who may consider building them. It was never intended or suggested that it's a replacement, only that it's a possible option for some applications, for some people.

The idea originally came-about when I was looking at the O15 SPL chart, and noticed that the bottom end looked damn good, but was sort-of "hidden" in the full range chart. I downloaded the chart, imported it into PSP, cropped it to leave only the bass end, then stretched it to get a clearer look at it. Upon this discovery, I posted my find, and it became the long-lost O15Sub thread. The chart which I posted in this thread is that exact chart, linked from my Photobucket folder.

This mod isn't a gross bastardization of Bill's design. It's just omission of the mid and high sections, allowing just the bass section to be used. The only actual "mod" is the possibility of adding the "extra board", so that it can be variable in dimension ratios. This isn't reinvention of the wheel, it's the same wheel. There's been no "redesign". Bill actively and (of course) constructively participated in that thread, and he's the one who gave the dimension for the "extra board". People come-up with all sorts of building "mods", tricks, and slight improvements for Tubas and Titans all the time, so what's wrong with the same for the O15?

[Bill Fitzmaurice]

People come-up with all sorts of building "mods", tricks, and slight improvements for Tubas and Titans all the time, so what's wrong with the same for the O15?

Nothing, so long as you don't assume that inch for inch an O15 sub will beat Tubas or Titans. One might just about equal them, but add more to the pile and the advantage swings to the horn, and the bigger the pile the bigger the horn's advantage.

[Tim A]

...don't assume that inch for inch an O15 sub will beat Tubas or Titans. One might just about equal them, but add more to the pile and the advantage swings to the horn, and the bigger the pile the bigger the horn's advantage.

And that's the key. I think there is a spot for the Omni 'subs', but it is limited. As a stand alone unit or a pair they make sense. As part of a modular system the concept falls apart. If the user wishes to have a system that will be usable in any venue by adding more cabs, the Tuba or Titan are still the best choices.

[Mikey]

People come-up with all sorts of building "mods", tricks, and slight improvements for Tubas and Titans all the time, so what's wrong with the same for the O15?

Nothing, so long as you don't assume that inch for inch an O15 sub will beat Tubas or Titans. One might just about equal them, but add more to the pile and the advantage swings to the horn, and the bigger the pile the bigger the horn's advantage.

Yup, and I made sure that the limitations and +/- factors were covered in this thread.

I actually cringe when the subject comes up, because I end-up taking some heat for it every time. The very reason that I post when the subject comes up is NOT to promote it, but rather to make sure that the pluses and minuses from the original thread are included in subsequent threads, so that people understand the limitations. It's not the greatest thing since sliced bread, it's just a possible option for some.

[tarduare]

I apologize for not replying sooner, too much work.
I built the O15 subs only to be used with the O15 TB's. I saw the old thread drawing with addition of a 6" piece of wood and I thought these would be ideal for my situation.
A super easy build (1to 2 on the scale)to increase the bass response and elevate the tallboys to ear level by stacking the subs underneath.
I used a 9 mH coil (from the plans)to low pass and run the cabs in parallel.
For DJ work this is ideal-wheel them in, stack them up and plug them in.

I consider these an application specific, quick bass fix which in no way compares to the versatility and efficiency of folded horns. I have since built an Autotuba (per plans, MCM driver) and I am really impressed with the performance from an 8 inch driver - imagine a 15. I intend to build a pair of OT12 this winter and I am entertaining the idea of recycling the 3015LFs from the Omni Subs into a pair of horns, but which ones. T48s use the 3015s but probably would not work well indoors for recorded music. Suggestions anyone.