View Full Version : Subwoofers out of 123A-1s?

Jim DiFalco
12-31-2003, 03:09 AM
Question on 123A-1 for subwoofer use?

I currently own a set of L-100s. The cabinets although solid are pretty beat up with veneer missing in a couple of places. The high frequency drivers have long since been replaced with a set of silk dome tweeters from Sound Design. (sound much nicer than the original HF drivers)

Iím giving consideration to taking the low frequency drivers out of the speakers and using them to build subwoofers. Now I realize that a 15 or 18-inch driver may be a better choice but I have the 123A-1s available and with an fs of 25 they seem to be quite capable of some decent low-end performance.

Using Martin Kingís MathCAD program Iíve modeled a transmission-line enclosure for these speakers with some very decent specs:

30 to 525Hz at 90.5 to 91db, 20Hz at 87db, 18Hz at 85db
Internal dimensions 14.5" wide x 23.5" deep x 44.5" long with a 5Ē dia. By 7Ē long port
Iíll be using an active 2nd or 4th order crossover (havenít decided which yet) and will be crossing the speakers around 80 to 120 Hz

Is there any reason that one of you who is more experienced and knowledgeable than I can see why I canít reasonably expect to use the 123A-1s in this manner (as subwoofers) and get decent performance out of them?

Since I will be driving the speakers to lower frequencies than I imagine JBL intended is there a higher likelihood that I can overdrive or damage the speakers at lower power levels than in the L-100 cabinets?

I would appreciate any assistance or comments.

Thanks in advance!

Jim DiFalco

Mr. Widget
12-31-2003, 09:40 AM
My seat of the pants guess is that if you demand realistic movie subwoofer levels from them, the woofers will exceed their 7.87mm Xmax. Most contemporary 12" subs have significantly greater Xmax.

A T/L cabinet is a fair amount of labor/time investment. If you are prepared to go ahead I would expect the woofers would cry out in distress before you permanently damaged them.

I think I would sell them and look for a pair of LE14s.

Jim DiFalco
01-01-2004, 04:27 AM
Mr Widget,

After checking other 12" subwoofers I see you are quite correct. Many have an xmax that is almost double that of the 123A's.

That's the information I was looking for. Thank you for the help.

Now to find some LE 14's.


01-01-2004, 02:09 PM
Kinda agree with Widget. With just a pair of these, it could get indeed dicey, on explosive LF transients.

However, if you could accumulate a SECOND pair, THEN things get interesting. With 4 12" cones capable of nearly 5/8" cone travel each, you can generate some pretty serious SPL.

I'd be hesitant of using these in a TL... they were designed to be ported-box woofers. Though, a significanly BIGGER box than the L100, would be needed for sub-bass... I'd say at least DOUBLE the cabinet volume (4 cubic feet or more), with a port tuning right around woofer resonance (25 Hz), should be pretty safe, with 4 drivers. That'd get an f3 of below 30 Hz, which should be decent.

Of course, for many people, the prospect of having 4- 4-cubic foot cabinets is daunting. :eek: Hence, the trend toward extremely high-excursion drivers, usable in tiny cabinets. You give up efficiency (one of those high-excursion monster-woofers may need up to 4 to 8 times the power to produce the same dB output as a 123A, at the same frequency), but with the massive power handling these things have and the number of incredibly inexpensive huge amps out there... it's the way of the world, these days...


Jim DiFalco
01-02-2004, 02:34 PM

I currently live in a condo and the vision of cramming 4 each 4+ cubic foot enclosures into it with my wife and I makes me think that the WAF may be a problem.

I'm using an amp with 35 watts of power so I prefer to stick with the more efficient drivers and that is one of the reasons why I like the JBL drivers.

Living in a smaller space is also why I don't feel the need for 15" - 18" subwoofers. I can rock the place pretty seriously with the L-100s as it is. I'm just looking for a little bit lower extension in the bass region.

The LE-14s sound good because they will work in smaller enclosures than the 15" - 18" subs and are more efficient than other subs available on the market. (at least what I've seen)

I was fortunate to own a set of L-200's in the early 70's. Loved the bass available from the 15" driver. I happened to be moving around a lot at that time and just got tired of moving them by myself. Regretted selling them tho as the L-100's don't even come close in quality of sound.

Thank you