Physics behind L100

[Yasnyi-Sokol]

Hi, all,

I have all the pairs of three drivers (LE123A1, LE5-2 and Le20-1) and I am on the way to build a system for tube amplifier. And I have never heard to original L100 monitors.

What is the best way - to follow exactly L100 design (same enclosure dimentions and X-over), or to make the bigger box?

It may look as stupid question, but I am confused. The measurements show for LE123A1 Qts=0.5 (0.52 the second driver) and Vas=250 (220 the second) liters. Almost same data as from JBL specs.

However, original L100 are very small for these parameters. In a box of about 60 liters it will be Fs=57 Hz, Qts=1.18 (for tube amp even more). This is all very strange for me: how then can these speakers produce deep bass? The modern approach instead says the bass must be abuzzing and boomy. But many people say they love L100 monintors, and L100 were very sucessful speakers, they were studio monitors etc...

On the other hand, JBL ALpha SIII speakers with same drivers (and passive radiator) have more "approapriate" boxes - much bigger, and even X-over look more realistic - with Le bass unit compensation and flattering the increasing responce of LE5-2.

Too many questions for me.

Whould You please help:

(i) is it worth to follow exaclty the L100 design (same box, same driver position, X-over etc)?
(ii) is the bass in L100 really fantastic as people say? I can understand "there is a lot of bass from L100". But is it fast, clean, dynamic, accurate, not boomy?
(iii) what would be the best enclosure volume with this driver if the speaker size is only on the second place?


Thanks,

Alexander

[Zilch]

L100 are, and never were, studio monitors, nope.

You are correct about the physics; L100's do NOT have "deep bass."

The larger box would be more appropriate per contemporary standards, yes.

[Unless you actually WANT L100 voicing.... ]

[duaneage]

(i) is it worth to follow exaclty the L100 design (same box, same driver position, X-over etc)?
(ii) is the bass in L100 really fantastic as people say? I can understand "there is a lot of bass from L100". But is it fast, clean, dynamic, accurate, not boomy?
(iii) what would be the best enclosure volume with this driver if the speaker size is only on the second place?

If I may....


THe L100 was a success in the age of power rock and roll, it sounds pretty good with 70's music and plays really loud. It is not the best designed speaker from JBL, only the best marketed!

I think it would sound better with a decent crossover network, unlike the very simple L100 network that just protects the midrange and tweeter.

Cross it over at 1100 hz to the midrange with a simple coil (1.25 mH should do it) and use a Zobel on it to tame the impedance peak ( that needs to be determined by formulas, not sure what would work) and do some listening for testing.

A larger box would slightly extend the bass but would reduce that huge peak. You are free to make the cabinet bigger, there is no marketing department asking to have it be a bookshelf speaker.

I would follow the 4311 approach for driver layout. Keep the drivers close together if possible

[Yasnyi-Sokol]

Thank You for Your opinions.

Cross it over at 1100 hz to the midrange with a simple coil (1.25 mH should do it) and use a Zobel on it to tame the impedance peak..

That means S70AlphaIII X-over is almost perfect?
Alexander

[hapy._.face]

The LHF sage "Giskard" has offered good advice on this position many times before. Not a quote- not verbatim: When you are building from scratch- unless you are interested in historical accuracy, you are not bound to make the same mistakes the orginal design had. You are not limited to time, manufacturing constraints, or presentation. You can improve upon these things in many ways. However, you are potentially subjecting yourself to a whole new batch of mistakes. Enjoy the journey.

[GordonW]

An L100A driver setup will work very well in a roughly 2.5 cubic foot enclosure. Cures, pretty much, the "boomy" character, while extending the useful bass response to just below 30 Hz. The cabinet I used, were about 14.5" wide, 29" tall and 14" deep. The 14.5" baffle width was helpful, as it seemed to preserve the baffle-step behaviour of the original cabinet (which was 14.25" wide)... this made the midbass/midrange balance work out well, without requiring crossover modifications (the stock L100A network seemed to work just fine).

I don't remember the exact tuning frequency I used when I did just this, but it was a 4" diameter vent, around 8 to 9 inches long. Should be somewhere around 28 Hz, plus or minus a few Hz. Exact tuning may depend on baffle shape and size (depending on whether you make a "squat" bookshelf shape, or a "thin" tower, and such considerations)... but that should get you into the ballpark.

Larger enclosures can be used, quite successfully... in fact, the "optimum" (according to most box-calculation programs) enclosure size is over 6 cubic feet, which will give a frequency response, in theory, down to nearly 20 Hz! However, when using a box this large, you change the bass/midbass/midrange balance of the driver enough, that wholesale crossover revision, even to the possible extent of needing to completely change the crossover topology, might be necessary. Also, one other thing to keep in mind- with such a large cabinet, power handling becomes lessened... this is the main reason why, I tend to "compromise" with a cabinet of somewhere between 2 and 3 cubic feet...

Regards,
Gordon.

[duaneage]

The woofer in the L100 is well damped already which means it has a great deal of internal control. The box emphasizes the bass but is not soley responsible for producing it.You could mount the woofer on an infinate baffle and probably get good bass response although power handling would be less. The larger box and low tuning is the way to go. I would also go with some polyester fill inside the cabinet to further dampen the driver a bit.

Happy Face is right on the money when he says you don't have to copy the original design, you can probably do better. Just because a system was successful in the marketplace does not mean it was necessarily designed properly. Just look at Bose. Maybe not a great example You get the point.

With a large box and a 1.25 mH coil along with lower tuning you should be fine. Then you can start on the midrange and HF crossovers, which also can be improved.