expert help requested for Legend CB15+Alpha 6a cab build

[derridiandrift]

I have an 8 ohm Eminence Legend CB15 that I bought for a different purpose and is now a spare driver. I recently read this thread New cab build about building a nice cabinet out of 1/2 inch BB plywood for the same driver and it gave me the idea for a project to mess around with.

I'd like to build a 15/6 that is wide enough to hold an SVT and still small enough to put into the hatch of small-ish car. The Ampeg SVT15E is 24" wide, so I figure that's a good place to start.

The cabinet design that Eminence suggests for the CB15 is 3 cubic feet with two 4" diameter ports that are 5 1/2 inches long. At 24" wide I could build a cab about that volume using something like 24wx20hx15d with the baffle set back by about 1 1/2 inches from the front. I can fine-tune the exact numbers later, but that's the ballpark size of the box.

Then I figured maybe I could add an 8 ohm Alpha 6A to counteract the beaming properties of the 15, and to extend the range of the cab. But admittedly, I'm not terribly knowledgeable about this process. So here I am being a hack and taking massive liberties with designs that have been well thought through. Greenboy put an Alpha 6A into his cab; maybe I could do the same thing with my BS cab design. The Alpha 6A would go into a sealed enclosure not unlike the fEarful 15/6 design. I'd probably just copy the enclosure size for the Alpha6A from what Greenboy used and make the cab a bit deeper to take into account the volume lost from the vented part of the enclosure. Despite the fact that this is an utterly unscientific way to go about doing this, does this make any sense?

Next, I looked at the crossovers sold by Eminence and that's where my idea really runs out of gas. I'm a complete hack and I know absolutely nothing about how to design a two-way speaker system, but I know there are plenty of people here who could recommend something that will work reasonably well. Can somebody recommend which crossover to use, or even warn me of pitfalls resulting from my ignorance?

Please remember that I'm just looking for a decent 15/6 design, premised on the Legend CB15.

[MuzikMan]

I would go with a 2 way 1.6k hz crossover. The CB15 seems to have strongs mids if I'm reading the charts correctly. I don't think the Alpha 6A could keep up with it any lower than 1.6k.

[rpsands]

1. Alpha 6a is nowhere near hot enough for the B15. You will need two to keep up.

2. 1200hz or so is the highest acceptable xover for a 15 in most cases.

3. you'll do better off diving in and figuring out how to design crossovers, or just copying someone else's.

4. GB has a fEarful tube design for this purpose. use it !

[derridiandrift]

Originally Posted by rpsands

1. Alpha 6a is nowhere near hot enough for the B15. You will need two to keep up.

2. 1200hz or so is the highest acceptable xover for a 15 in most cases.

3. you'll do better off diving in and figuring out how to design crossovers, or just copying someone else's.

4. GB has a fEarful tube design for this purpose. use it !

Thanks for your ideas.

1. I was wondering about that when I looked at the specs chart. You've confirmed that suspicion for me.

2. Can you explain why, or point me in the direction of an explanation?

3. Can you point me to a crossover that I can copy? I'm not having much luck with the search function here on TB. Maybe you could point me to some information about learning how to design crossovers. I was under the impression that designing crossovers requires a solid understanding of electronics. My knowledge of electronics doesn't go much further than an ability to identify component names and use a soldering iron.

4. I can't afford the neo driver for a fEarful cab. If I had the money, I'd build one, but I don't.

I think my original post might have been unclear. I don't have much money to play around with. I don't have the knowledge to design a crossover. I own a driver that I'd like to build an enclosure for. If possible, it would be nice to incorporate a mid-driver enclosure to augment the design provided by Eminence, but if it's too complicated I'll just buy some plywood and glue and build up something similar to what George did here New cab build.

[rpsands]

Above 1200hz 15's are fully directional, so crossing over much higher is going to leave you with off-axis gaps in a very important region. If you're going to use a stock crossover I'd probably go with the 800 or 500hz models.

Crossover design is more about reading the manual - there is an excellent tool out there called 'passive crossover designer' that pretty much just requires reading and time to figure out from what I can tell. I will be using it next time I have the need to design a crossover - just not there yet. You can post your results and I bet someone will look at it and provide some input - lots of guys know their stuff around here. It will be worth your time!

If you've not got a lot of money, the Bill Fitzmaurice cabs are the way I would go for a standalone box. Try Jack 112's with eminence betas in them, maybe.

[will33]

Ya you don't want to cross above 800 - 1k or so due to the dispersion of the 15. I wouldn't do it at 500 either. It'd put unneeded stress on the mid, the 15 can play down there just fine. I'd use the 800 if you're going to use a prebuilt xover.

As far as the alpha6 not being hot enough, that depends on how you want the cab to sound. I biamp one with a couple different woofer combinations that run 97-98db and I like it fine. Biased a little towards the bass/lowmids but not that much. The alpha runs 95ish db from 1k to 5k. I like the overall "tonality" of it. If you want a lot of mids you could try the alpha8mr.

Your cb15 is a good driver, I'd use it. Especially since you already have one and the woofer is the most expensive part. Neo prices being what they they don't seem worth it to me anymore, especially if it means having to buy a bigger amp to make use of all their excursion.

[derridiandrift]

Originally Posted by will33

Neo prices being what they they don't seem worth it to me anymore, especially if it means having to buy a bigger amp to make use of all their excursion.

I'm of the same mind. Had I clued-in to the fEarful thing last year, I'd probably have one right now.

[MuzikMan]

The LA6-CBMR would be loud enough to cross over at about 1k hz. It is not difficult to build a simple 2-Way 2nd order butterworth crossover, which is nothing more than a second order low pass filter and a second order high pass filter. Use air core inductors and audio grade capicitors.

Second Order Passive Crossovers, 12 dB per Octave

2-Way Crossover Designer / Calculator

Parts-Express.com Is Your Speaker Building Supplies Store: Raw Speaker Drivers, Speaker Kits, Plate Amplifiers, Cabinets & Coverings, Crossover Components & Assembled Crossovers, Speaker Repair, Design Software

[staindbass]

i wouldent use a crossover on the 15 at all. it sucks up some dynamics and power. (unless its a 10-12 gauge coil) most have skinny wire for the coil. a good quality crossover weighs a few pounds. i think a 1000 hz crossover @ 12 or 18 db per octave would work well for the 6".

[Blues Bass 2]

I think MuzikMan might be on the right track . I've had a couple cabinets with that mid driver crossed over at 800Hz and they sounded pretty smooth in the mids to me . I've heard some say that the LA6-CBMR distorts at lower crossover points but I haven't heard that myself . Maybe I don't know what to listen for . The Eminence third order 800Hz crossover would probably work all right with it too .
http://www.parts-express.com/pe/show...number=290-632

[will33]

You want to lowpass the 15 or it'll be fighting/phasing/cancelling with the mid in the frequencies where they overlap. Also make an impedance dip to 4ohms in that same area. Can use a 14ga. ironcore for that, erse makes some you can get at partsexpress.

2nd order HP is fine for the regular alpha6. I'd use 3rd order in either if the closedbacks for more protection. Their fs/bottom of their range is up close to 500hz. If the xover is too low and/or not steep enough they can distort when you put more power to from getting stressed/kinda farting out at the bottom of their range like a woofer does.

Emi's prebuilts are 2nd order LP/3rd order HP already. Just look at the impedance curves of both drivers and make sure they're at least close to 8 ohms at the xover frequency. Rolling your own means you can set them specifically for the drivers being used.

[will33]

To be clear I'm no expert at this, just a tinkerer floating around somewhere in the lower half of the learning curve.

[will33]

Hey try something. Look up the impedance curves of the cb15 and the kappalite3015lf. IIRC they're both around 9-9.5 ohms at 700-800hz. If that's true greenboy's "cheap but good" xover might work for you, search that. It's an easy build only 4 parts. That's for a regular alpha6 not the closedback mids. Hopefully a pro can straighten us out here if we're getting too far astray.

[rpsands]

I want to say you also need to look at the coil inductance (which is very very different) but I'm really not sure.

BoxSim just had an english release so it might be worth plugging the numbers into there and seeing what effect GB's crossover has on the CB158.

[1n3]

Originally Posted by MuzikMan

The LA6-CBMR would be loud enough to cross over at about 1k hz.

Seems so. The Beta 8a would also be a closer match. The LA6-CBMR has the advantage of not needing an enclosure.

But I can imagine the Alpha 6 being strong enough for some. If you normally cut high mids and treble, you might like that response curve. Even if you aren't that guy, a 3db twist of the right tone controls might get you where you want to be.

Originally Posted by rpsands

I want to say you also need to look at the coil inductance (which is very very different) but I'm really not sure.

I would think so. Need to bone up on crossovers one of these days. But I assume a sophisticated design would have to consider speaker parameters (LCR).

[derridiandrift]

Originally Posted by will33

Hey try something. Look up the impedance curves of the cb15 and the kappalite3015lf. IIRC they're both around 9-9.5 ohms at 700-800hz. If that's true greenboy's "cheap but good" xover might work for you, search that. It's an easy build only 4 parts. That's for a regular alpha6 not the closedback mids. Hopefully a pro can straighten us out here if we're getting too far astray.

It looks like the Legend CB158 is between 13.5 and 14.5 ohms between 700 and 800Hz. The Kappalite 3015lf looks to be 11.5 to 12.5 ohms in the same range. I have no idea whether that's close enough to use the "cheap but good" crossover. I suspect not.

[DukeLeJeune]

I presume we're pretty much limited to off-the-shelf crossovers, and/or adapting an existing crossover (like the one for the fEarful).

If you're using a closed-back midrange, I'd go with the 1.6 kHz Eminence crossover instead of the 800 Hz one. The reason being, a closed-back mid will have a strong impedance peak at its resonant frequency, and unless you have a custom-tailored crossover the driver will be inadequately protected in that region and may well distort. I'd rather have a little bit of woofer beaming than have the mid distorting.

The open-back mids are more forgiving of a lower crossover frequency, so you can go with 800 Hz for an open-back mid (assuming you put it in its own sub-enclosure).

The fEarful crossover that will33 mentioned is a candidate for use with the Alpha 6a. As a rough guess, I'd probably try a 2.7 mH inductor and 20 uF capacitor on the woofer, instead of the 3.0 mH and 30 uF specified. The reason is, that fEarful crossover was designed to suppress the screaming peak of the 3015LF at 1.2 kHz, and the CB158 doesn't need to be filtered so aggressively. I'm also fudging for the somewhat different impedance curves. You could get a 20 uF cap and a 5 uF cap, and try both 20 uF and 25 uF in the circuit (20 uF in parallel with 5 uF = 25 uF), to see if one sounds better than the other.

[derridiandrift]

[quote=MuzikMan;11001079]It is not difficult to build a simple 2-Way 2nd order butterworth crossover, which is nothing more than a second order low pass filter and a second order high pass filter.

2-Way Crossover Designer / Calculator

Wow! There must be more to it than that.

[MuzikMan]

Originally Posted by derridiandrift

It looks like the Legend CB158 is between 13.5 and 14.5 ohms between 700 and 800Hz. The Kappalite 3015lf looks to be 11.5 to 12.5 ohms in the same range. I have no idea whether that's close enough to use the "cheap but good" crossover. I suspect not.

Either way it would be closer than the presumed 8 ohm impedance of an "off the shelf" design. It is worth noting that as components heat their impedance changes. A passive crossover network is imperfect no matter how much research is done. There has to be a point where it is "close enough".

[billfitzmaurice]

Originally Posted by derridiandrift

2-Way Crossover Designer / Calculator

Wow! There must be more to it than that.

There is. That will give you a basic useable result, but that's all. A really good crossover will take longer to arrive at than any other factor in the cabinet design and building process. For that matter it takes longer than all the other factors combined.