Low Power 2.1 HT Build - Fostex BK12m + Sub

[chad1376]

It's time to retire my old setup:

Ariel MTM speakers I built about 15 years ago (woofer surrounds are rotted and I'm tired of them) + a small sub based on the Tang Bang WT-1427B (boomy and never integrated well with the system.)



I have no interest in surround sound or rattling the house while watching movies - I just want a simple/clean setup. For the new system, I'm building some small Fostex BK-12m Folded Horns based on the Fostex FE126En 4" Full Range.

http://www.madisoundspeakerstore.com/full-range-speaker-kits/fostex-bk-12m-folded-horn-kit-pair/

These will be powered by a 2A3 amp I'm currently in proccess of rebuilding (I have another half-hearted thread on the rebuild here somewhere). Until I get that done, I'll power them with a Dayton Audio DTA-100a Class-T Digital Mini Amplifier.

The current sub has become important as a bench for the kids to sit in front of the fire place. I'm still waffling on how I want to re-do this. Whatever I do, It will be wider, to work better as a bench. I also might use it as an integral speaker stand to elevate the Fosgates a little. I'm trying to decide if I should just build a better box for the WT-1427B, or maybe a little 6.5" horn sub, based on the Reed Exodus Anarchy 25hz Tapped Horn (with a Tang Band 6.5" sub).

http://www.avsforum.com/t/1238398/reed-exodus-anarchy-25hz-tapped-horn

Anyway..I started on the Fosgate Horns, and will post back with progress.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

A little progress yesterday - Cut the speaker baffles (I have a nice 4" hole saw that works perfect), provided some relief on the inside of the baffles and installed T-nuts. (sorry - no pics.)

I also scrapped the idea of the 6.5" subwoofer horn. The width of the enclosure would not fit below the fireplace doors when open. I decided to re-use the WT-1427B and build a simple quarter wave design, without getting into a ton of detailed modeling. The enclosure will take up the majority of the tiled area in front of the fire place. I ripped all the major plywood pieces yesterday, but have a lot of trimming before anything is picture-worthy.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

Stayed home with my sick kid today (Extra shop time!). Made some decent progress. I got the internal panels all glued up.



I provided some relief behind the speakers, but I don't think it's enough. I might go at it some more with a die grinder or rotary sander.



I'm also thinking I'll fill the bottom triangle void with some sand to add some stability to the speakers.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

* Hogged out the baffle with a die grinder (no pics). The grinder worked great and the speaker should "breathe" well.

* Glued in several layers of felt and installed some "Nu-Foam". I don't know how well it will work, but it was easy to work with.


*Got the first box glued and clamped up for good.



I decided to hold back on the subwoofer for now. I want to hear how the speakers sound, and try to design an approriate sub after they're broken in. I'm definately going to need to build some stands in the interim.

[martyh]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

Good stuff so far! Did you add the extra weight at the bottom as planned?

[[email protected]]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

I has the same thought a while back cause the grandkids liked to look at the books nearby. I reality it was a terrible build, but a learning experience especially the upholstery.

[generic]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

I like it. Please report back how you like the Fostex BK-12m Folded Horns.

I'm worried that you aren't leaving enough space behind the driver. Might want to try only one layer of Nu-Foam. Never heard of that, but either way, two layers seems like overkill and it might do more harm then good. You need space behind the driver.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

I like it. Please report back how you like the Fostex BK-12m Folded Horns.

I'm worried that you aren't leaving enough space behind the driver. Might want to try only one layer of Nu-Foam. Never heard of that, but either way, two layers seems like overkill and it might do more harm then good. You need space behind the driver.

Actually, I hadn't ever seen it before this weekend either. It's supposted to be an alternative to foam rubber, but it's just poly-fill sort of stuck togeather in a block. It's very light-weight and open. I actually only have a single "slab" in each box, but I agree, it looks like I might have too much, especially immediately behind the speaker. I might try to hog it out some before I install the speaker. If all-else fails, I'll rip it out and try something else.

Latest progress, glued on the final panel and flush trimmed the top/bottom. I plan going over all the edges with a flush bit, then a 1/2" roundover. Need to fill some plywood voids and do lots of final sanding.



I did fill the bottom triangle with sand, which does give the assemply a better weight balance. It's still tall for it's width, so whatever stand I make will need to have a wide base and a solid connection to the speaker - especially with kids and dogs in the house.

[Brian Steele]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

* Hogged out the baffle with a die grinder (no pics). The grinder worked great and the speaker should "breathe" well.

* Glued in several layers of felt and installed some "Nu-Foam". I don't know how well it will work, but it was easy to work with.

How did you come up with the dimensions for that rear-loaded horn? Did you use HornResp?

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

These are just from plans available on the Madisound web site.

I'd like to try my own horn design, but it would be a steep learning curve. Maybe for the sub, if I can't find a proven design that fits my space.

[Brian Steele]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

These are just from plans available on the Madisound web site.

I'd like to try my own horn design, but it would be a steep learning curve. Maybe for the sub, if I can't find a proven design that fits my space.

HornResp isn't that difficult to pick up, particularly if you're entering the parameters of an existing design into it, rather than trying to build a new one. In this case, it looks like you're building a rear-loaded horn, so basically what's involved in HornResp is to (1) enter the driver's parameters, (2) select the rear-loaded horn option, (3) break the existing horn into four segments, (4) enter the cross-sectional area and length along the horn for each segment (starting at S1 at the throat and ending at S5 at the mouth).

I like to create and use spreadsheets to generate the HornResp parameters based on cabinet dimensions (see attached for example), but that usually takes a LONG while to put together.

The reason I asked is that rear-loaded horns usually have a dip in their midbass response (where the driver's output is out of phase with the output from the horn). It would be interesting to see if Madisound came up with a design that minimized that dip. Modelling the design in HornResp can give an idea of how big this dip might be and where it would be located.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

I couldn't find any FR info for the box. Maybe I'll try to model it, as you suggested, to get a feel for what I'm bulding and get up to speed on the software. I wouldn't be suprized if there's a midbass hole. The speaker is really intended as a first forey into horn loaded full range, so I'm sure ease of contruction was the main priority in it's design.

The Fostex designed box for the driver uses an unusual port in the back chamber, and a rear horn, in an attempt eliminate the dip. The box was a little more complicated though, and I just wanted something I could bang out in a few weekends.

At the end of the day, these speakers will primarily be used just to watch TV, so a perfect FR is not critical to me, as long as they subjectively sound good. Of course if I "know" there is a dip, I'll "hear" the dip....On second thought, maybe I won't model it :p

[generic]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

The credit for the Fostex horn design goes to David Dlugos of Planet_10 Hi-Fi. Reviews suggest that they are very good. I want to build them, but I just haven't taken the plunge.

[chad1376]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

Getting closer..

I built some stands for the speaker - just a box to be filled with sand. I ruminated a bit about putting a small sub in each, but decided to stay with the idea of auditioning the speakers before I decide if a sub is needed (or wanted). The hole is just access to fill it up.



The speaker will mount to the stand with just 1-bolt through the bottom into the top of the stand. This will allow me to toe in the speakers, while keeping the stand square. It'll also make it easy to remove if I do go with a sub later. I'm sure having the horn mouth off of the floor will probably hurt the low end a little, but getting the speakers at ear height is more important to me.



Still have lots of filling/sanding to do. I'm just going to keep it simple and finish these with a clear poly.

[generic]

Re: Low Power 2.1 HT Build - Fostex BK12m + Sub

Just curious, what is your 2.1 bass management solution? What gear are you using?