View Full Version : What to do with 6 123A?

08-17-2004, 01:24 PM
OK, I have gone cold turkey from eBay. But my brief period of craziness has resulted in my owning 6 123A 12" woofers.

Now, I can't do this:

But how about a single cabinet with 6 123A and perhaps 6 100w plate amps?

Anyone have an idea how big the cab would have to be? Any other suggestions or idea would be welcome.

Or am I out of my mind?

08-17-2004, 01:38 PM
Hey, Don...

GordonW posts quite warmly on the 123A, so maybe he'll pitch-up here, soon.

According to a post on this Advice Needed (http://audioheritage.csdco.com/vbulletin/showthread.php?s=&threadid=34&perpage=15&highlight=123a&pagenumber=2) Thread last-month he plans on mounting a 123A in a "proper box", of 6 ft3 with a PR. Never heard (sic) the results of that...

08-17-2004, 08:26 PM
I wouldn't do that. The boxes needed would be to big to extend and flatten the response. You could EQ them I quess with the plate amp to flatten them out a bit but if you did you will still have to deal the the other side of the "hump" so your topside crossover point would be limited. Watch the x-max on them.


Play with them in this software program and see for yourself what box size you need for flat alignment and what it looks like in 1.5 cu.ft The paramaters are already loaded in the program just scroll through the drivers until you hit the JBL list.


08-17-2004, 09:22 PM
Couple of data points:

- Be aware, that the 123A is really a SIX ohm woofer, in real life. So, some care is needed in connecting them, so that they don't overload an amp with low impedence.

- The X-max on a 123A is fairly large- as in about 7mm. So, with a number of them working together, you should get PRODIGIOUS bass output.

This leads me to an idea- how about an isobaric sealed box, with three "clamshell" pairs of 123As? My calculations show, that with a HEAVILY STUFFED box (as in FULL of dacron polyfill and/or fiberglass insulation), a 6 cubic foot box (about 36" tall, say 20" wide and 18" deep or so, would be in the ballpark) will give a Q of .7 and an f3 of 42 Hz. With the bass boost in the sub amp, you could probably get good output to below 30 Hz, I'd expect. The program I'm using, predicts that in-room (with the box near a boundary such as a corner), you should be able to get 110 dB at 30 Hz max output out of such a setup...

Now, if you REALLY wanted to crack plaster, put the three clamshell isobaric pairs into a 10 cubic foot ported box, tuned to just under 19 Hz (two 4" diameter ports 12.5" long would do it). This will give an f3 of 27 Hz, and a predicted max output level of 121 dB down to near 27 Hz (!), in-room (near-boundary placement of the box, again). Of course, this would need an amp that could produce 300 watts into a 4 ohm load (wire the woofers in two parallel triples, then series the two triples to get (6/3)*2=4 ohms)...


08-18-2004, 06:55 AM

Please explain "three "clamshell" pairs ". Do you mean bolting a pair of woofers face to face?

And is this the electrical hookup you mean?

08-18-2004, 09:46 PM
Yep, "clamshell" means mounted face-to-face, otherwise known as "isobaric" mounting... like the picture below.

And yes, your diagram is correct, to achieve a 4 ohm load for the composite total of the 6 drivers. If, OTOH, you wanted a load closer to 8 ohms, you could make three parallel pairs, and wire them in series (the opposite configuration to your picture), which would give a 9 ohm load.


08-18-2004, 09:50 PM
BTW: Other than it being seemingly "wasteful" of drivers, there are technical advantages to isobaric mounting. One primary benefit, is that it cancels out non-symmetric suspension-generated waveform errors... asymmetries in the output waveform caused by differences in suspension stiffness and magnet motor behavior in the outward vs. inward voice coil travel directions, are "averaged" and cancelled out in a clamshell installation. Bottom line, slightly more distortion-free bass.


09-04-2004, 11:17 AM
Are you saying the out of phase driver is mounted on the exterior of the cabinet ? Or is it in it's own box ?