Home Theatre Sub out of 128H-1

[Ed Zeppeli]

Hi,

I've just discovered this forum and it looks like I could learn a lot around here.

I have two 128H woofers that I would like to make into some decent subs for a home theatre and stereo application. They will be used in conjunction with a pair of 4208 studio monitors.

On my previous attempt at making these into subs I seem to have failed. The cabinets I built are solid 3/4" MDF, braced and damped with WHD inside dimensions of 16 1/4"x20"x14" for a total of 2.6 cubic feet. The port is 3" ID by 6.25" long. I can't remember if I used a hybrid of JBLs enclosure guide combined with some information gleaned from various books on speaker design to come up with my cabinets but there's definitely something wrong with how they perform. Muddy would be kind of an understatement. There seems to be no definition of notes and oddly the bass sounds more 'present' in other rooms of the house. I'm driving them with a 300 watt ADCOM GFA-555II.

Looking at it now, the ports may be the problem but I'm not sure how to rectify it and if I can salvage these cabinets or if I should start from scratch.

I know these drivers are capable of more and would love to realize the goal I had back in '98 when I first built the boxes.

If anyone has any input, tips or suggestions I'd appreciate a nudge in the right direction.

Thanks,

Warren

[BMWCCA]

If you wanted a sub, you probably should have chosen a real LF speaker instead of the 128H. Don't get me wrong; I love the 128H/H-1 and have several of them in use (six, to be exact), but I'd never call them subs in any application. Your cabinet design is not much bigger than my 4412As or my L112s and I never considered them subs. What are your main speakers if these are your subs?

[Ed Zeppeli]

My main speakers are the budget-pro 4208 monitors.

I guess I thought this would make a good sub, especially after reading JBL's Enclosure guide where it even mentions it as a candidate for "hi-fi low end or small subwoofer." I also thought that with tuning down to 30hz on their box design it would cover the bottom spectrum well.

JBL's own recommended box size is only 2.5 cu ft. Do you think that's too small? I did do a search here and noticed a few guys coming up w/ nearly 3.5 cu ft. in some of their designs but wasn't sure if that was for an isobarik chamber or something other than vented box.

[Zilch]

You are tuned to 29 Hz, which is about right, according to Bass Box 6 Pro.

That should give you useable bass down to 26 Hz (-10 dB).

Gordon alluded to a different alignment for 128H in this thread, but I never figured out what he meant:

http://audioheritage.org/vbulletin/s...ead.php?t=3922

In 3.8 cuft tuned to 23 Hz, you'll get down the 21 Hz, but lose 2/3 of your power handling capacity down there.

Use "Advanced search" on GordonW with 128H as the subject. He talks about various alignments in several different threads....

[Ed Zeppeli]

Thanks Zilch for running that! I'm scratching my head now on this one...

[Zilch]

What are you using for a lowpass filter?

[Ed Zeppeli]

What are you using for a lowpass filter?

The built in one from the Harman Kardon AVR 645.