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Ed Zeppeli
12-10-2007, 09:16 PM
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
12-10-2007, 10:10 PM
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
12-10-2007, 10:47 PM
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
12-10-2007, 10:47 PM
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/showthread.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
12-10-2007, 11:08 PM
Thanks Zilch for running that! I'm scratching my head now on this one...

Zilch
12-10-2007, 11:18 PM
What are you using for a lowpass filter?

Ed Zeppeli
12-10-2007, 11:20 PM
What are you using for a lowpass filter?

The built in one from the Harman Kardon AVR 645.

Zilch
12-10-2007, 11:23 PM
That should be fine, I would think.

Wait for others to see this thread and comment.

Send Gordon a PM with a link to it, and see if he has any suggestions to offer.

Edit: Also see his post #15 et. seq. here:

http://audioheritage.org/vbulletin/showthread.php?t=6413

Ed Zeppeli
12-10-2007, 11:30 PM
thanks, I will.

BMWCCA
12-11-2007, 06:48 AM
Wow! I guess I stand corrected by the apparent impression here of the quality of the 128H-1 in use as a sub. I particularly liked the passive-radiator vent solution. Seems to validate the design of the L150/A. Makes me wonder what kind of sub you could make out of the smaller 127H-1 since many here seem to prefer the performance of the 4410A to the 4412A in a box that size.

Sorry for the skepticism (and for missing the 4208 in your first post). Carry on! :applaud:


BTW: For you 4208 fans in the San Diego area http://sandiego.craigslist.org/msg/504364701.html

grumpy
12-11-2007, 07:53 AM
Alternate placement (given the comments about bass in the other rooms)
and level matching might help if you leave them as subs.
Having them colocated with the 4208's and raising the crossover (not really using
them as subs) might be interesting as well.

John W
12-11-2007, 09:22 AM
Have you tried listening to the subs with the ports sealed?

I was reading this article on subwoofers:

http://sound.westhost.com/subcon.htm

Section 5 – Soundwave Vs. Pressure Mode raised a lot of questions in my mind about using ported subwoofers in small rooms. I need to play around with some of my own boxes to compare the differences.

hjames
12-11-2007, 09:56 AM
Have you tried listening to the subs with the ports sealed?


Thats easy enough to try ...
Break out the Zilch Plugs!! (available from your local Home Despot) ...

Ed Zeppeli
12-11-2007, 10:04 AM
I currently have the subs underneath the 4208s, which I realize isn't necessarily ideal. I'll have to throw one of those beasts onto my listening chair and crawl/walk around the room to perhaps find the ideal spot.

I haven't plugged the ports, no. That's something easy to try.


Warren

Robh3606
12-11-2007, 10:39 AM
I have a pair 4208's in my office at work. It's small 10X12 so close to a small bedroom in size. They do quite well in here without subs but certainly could benefit from them.

First thing I would check is phase on all 4 cabinets. Make sure they are all in phase. One cabinet out can drive you nuts.

How are you doing the bass management??

The low frequency drivers could have some mutual coupling going on if you have the 4208's stacked on top through the crossover range. Could be giving you a hump and hidding what's really there. It could mask any LF extension you may be getting and make it undefined and boomy sounding.

Simple rule of thumb is to place your speakers for best bass and drop the subs there. Then set them up to image the best and drop them there on stands. Try to minimize the LF reinforcement the room provides to the 4208's. Try to use the subs only from say 60-80hz and down.

Hope this helps

Rob:)

Ed Zeppeli
12-12-2007, 10:20 PM
Sooooooo, I got around to trying some of the suggestions here.

I removed one of the subs, put the amp in bridged mono and put it on the sofa. I found some affordable speaker stands to separate the subs from the 4208s as well. Also, I bumped the bass levels in my H/K AVR 635 and set the mains to the Small Speaker setting and crossed them over at 80Hz. The sub ended up in a corner right across from my L main as its final resting place.

I'm happy to report that it has never sounded better! So thanks very much to all who helped w/ suggestions and inspired me to spend a little time on this.

Cheers,

Warren

Robh3606
12-12-2007, 10:32 PM
Hello Warren

Glad it worked out and you are happy

Rob:)