The James B. Lansing L100 Century loudspeaker, the consumer version of the 4310 studio monitor, became the largest selling loudspeaker model of any company in the Seventies and more than 125,000 pairs were sold. By the end of the decade, recording studios in the United States used more of JBL’s monitors than all other brands’ monitors combined. Due to the materials used, the L100 drivers are as good today as when they were produced, but the overall sound can be vastly improved by modern crossover technology. Jantzen Audio is proud to offer an up-grade kit for this magnificient loudspeaker.

August 2006 - February 2007.

While enjoying the sound from newly renovated JBL L26 Decades, I might as well start writing about the one and only, JBL L100 Century. A month ago one of my friends threw in these speakers, having bought them from an old fellow for 300 DKK (50 US $) - a total give-away. All drivers in mint condition and the cabs with minor scratches. Tweeter foam had gone but can easily be replaced from eBay sellers.
I won't go into historical details on the L100 as so much can be found at: http://audioheritage.org/html/projectmay/pmintro.htm, and in particular about the L100 here: http://audioheritage.org/html/profiles/jbl/l100.htm
A rare opportunity to measure what the L100 did back then where I could only dream of owing a pair of these speakers from "over there". I don't recall the retail price, but they were very expensive. Having recently experienced the SEAS 503 kit, I now realise the 503 kit was actually a better speaker at that time. Quite similar in size and power handling, the 503 had a better midrange and a better crossover and the 33FWK bass drivers may beat the 123A drivers. But the SEAS drivers didn't have the gorgeous looks of the JBL drivers, the white-coated cone of the bass driver and the exquisite finish of the LE5-2 driver. I've always had a week spot for the LE5-2 due to how well it is built. Take a look at how the dust cap is glued to the cone and the fine touch of paint around the edges. It takes great skills to make such fine work. However, the LE5-2 is not an easy driver as we shall see later.

This first part of the my L100 story will not engage in a new crossover, merely try to find out what the L100 was in terms of driver performance and crossover construction. And we might as well take a look at the original crossover right away, because it's simplicity itself: 8 uF to the mid and 3 uF to the tweeter. That's all! Both drivers connected via an L-Pad for attenuation. Mid connected with inverted polarity. Having a 12" bass driver running full range is a bold choice and a lot of work must have gone into designing the 123A driver to produce a smooth roll off. The LE25 is a better tweeter than we might think, maybe not from measuring performance, but well equalised, it can deliver some excellent treble integrating well with midrange, actually better than many modern domes in my opinion. I've recently bought some LE tweeters on eBay: LE20, LE25 and LE26. Read here: http://www.troelsgravesen.dk/JBL_LE20-25.htm, http://www.troelsgravesen.dk/JBL_LE26.htm

Download this article: 1.5 MB pdf file.

Modelled response of drivers where the drivers are attenuated with two resistors in order to recreate the measured performance. Not that easy, but this is close. Phase tracking between bass and mid is better that we might expect, but the tweeter is very much living its own life in the upper midrange.
When I reverse the polarity of mid and tweeter in the LspCAD, things at least get worse. It's not that bad.

New crossover design criteria

A lot of work has gone into designing a new crossover and numerous simulations and actual crossover constructions have been tried in order to try taming the LE5-2 middriver and provide an overall balanced sound from the L100.
What I wanted was to maintain the basic virtues of the L100, i.e. the 123A bass driver working - almost - full range, thus a 1st order filter was soon in place. To maintain 1st order filtering as high up in frequency as possible, the LE5-2 had an LCR circuit to flatten the rise in impedance at Fs, allowing the high-pass section to be one single capacitor. To cope with amplitude and phase between mid and tweeter, the mid needs a 2nd order low-pass section and the LE25 needs a 3rd order filter at 5 kHz to get amplitude and phase in place. See graphs below.

Three significant things have happened here: First of all, the LE5-2 middriver no longer peaks at 6.5 kHz. Secondly the mid-section was constructed in such a way that the common variation in driver impedance (LE5-2) only produce minor deviation from target response. Last but not least, both the tweeter and mid now produce a smooth roll-off below points of crossover.
Should you want to increase mid and tweeter levels, you may bypass R2011 (mid) and increase R1061 (tweeter) from 4R7 to 5R6. Both 4R7 and 5R6 are supplied with the kit for you to esperoiment.

The complete kit (a pair) includes:

1. 180 x 280 x 12 mm birch plywood boards for terminals and crossover.
2. Terminals.
3. Two extra resistors for tweeter attenuation.
4. Soldering tag strips.
5. All crossover components: Baked non-resonant coils, all polypropylene capacitors, MOX resistors and cables for connecting drivers.
6. Kit instruction containing crossover schematics, a short version of this page and large sharp photos of the assembled crossover board.

Not supplied is silicone glue, screws and damping material. Damping material takes a lot of volume and it's not such a good idea to ship air around the world. It's highly recommended to replace the old damping material. I've used soft 40 mm polyurethane foam and a sheet of MDM3. It changed the overall sound a lot - and for the better.
The kit price is set at a level where you cannot buy similar quality components at a lower price from any supplier.

Contact Jantzen Audio to kit price incl shipping and payment instructions: contact@jantzen-audio.com

The kit.

Left: L100 frequency response from new crossover. The dip at 1.3 kHz is caused by the small front panel. Some of the LE25 tweeters display a peak at 12 kHz. Some do not. However, I must say the treble from the renovated L100 is remarkably smooth. If your L25 tweeters are intact, do not discard. They're excellent tweeters - only need a proper crossover.
Right: Summed frequency response, red, and response of individual drivers from new crossover. All drivers now display smooth roll-off characteristics.

Left:The litmus test: Reverse mid polarity. Reading taken a 1 m distance at middriver height. Due to placement of drivers, this reading may somewhat depend on placement of microphone. In my own recreation of the L100 I'll have a standard vertical placement of drivers with mid and tweeter slightly off-set.
Right: System impedance from vented cabinet. This is an overall easy load on our amplifiers. 5 ohms minimum impedance and the negative phase angle at 70 Hz is where impedance is high.

Final remarks to the L100:

I dare say it's quite a different speaker compared to the previous "two-caps" set-up. Classical music is a delight, vocals are handled very well and should you throw a Saturday evening party, give it a 150 wpc amp and let it rock! It runs well from my 20 wpc PSE valve amps, though not at very high levels.

Thanks to Sven for lending me his speakers for this up-grade. It's been quite a journey and only thing remaining is this: Should I make some radical new cabs for my own drivers and what should it be? The high-Qt bass driver might fit well into a TL cab providing a smoother balance in the 50-100 Hz range. This could be done without changing the crossover at all. An approx. 80 liter TL floor-stander, hmm....

One last thing: The L100s need stands, at least 45 cm in height to bring the 123A drivers off the floor. Remember, the 123A is handling most of the midrange and a 5 deg. tilt is doing even better. I used the stands I made for the L26-3W.

Final-final...

Some comments on L100 placement: The new crossover is tuned to a placement like most modern speakers and due to the high-Qt bass driver we already have some +3 dB in the 50-100 Hz range. If the first option below is the only way you can place your speakers, don't up-grade. We need the bass driver up and out. From the new crossover, the 123A bass driver still handles most of the midrange, thus 40 cm from floor level is not a good idea. I recall studios using the 4311 and 4312 upside-down to get the 123A at ear-height.

This is a no-no with regard to placement. Too close to the floor and front wall and too close to the corner (for right speaker).
Based on simulation you will have +9 dB at 60-70 Hz. Way too boomy.

This German up-grader got it right: Free of nearby boundaries and a much smoother response.

Enjoy!