JBL L100 Century, recreation
Copyright 2008-2014 Troels Gravesen


The JBL Kit is doing well and it's time to assemble my own "L100" from my eBay purchases. A true L100 box replicate it won't be because the 123A bass driver can do better compared to the original 44 litre vented box. As can seen in the JBL-Kit file, box simulation is not an easy exercise due to the driver's low electrical and mechanical damping, i.e. high Qe and Qm. We have a 3-4 dB bump in the 50-100 Hz region, thus cab volume has been increased and the vent is gone, being replaced by two Variovents to produce an aperiodic system.

The JBL Heritage website is the place to go if you want more info on this classic speaker, and I've read the L100 box being characterised as a solid, well-built construction. It isn't. Well-built yes, but solid? It's a boom box made from chipboard with no bracing at all. Put your hands on the cab when played loud and you'll feel it's highly resonant likely to cause some midrange smearing and lack of detail. The JBL L26-3way project left no doubt about the advantages of adding braces. The performance at all frequences will benefit from a non-resonant cabinet.
An option is to rebuild the L100 cabs like the L26-3-way. A new front panel with drivers placed symmetrically is a must if you choose to do this.

Building new cabs was a pleasant deja-vu, a straight forward rectangular box like I did many times during the Seventies, only this time braces and bitumen pads were added. I've recently bought a cheap 1800 watts Chinese table saw and I should have done this years ago. I takes little space and after adjusting almost everything - you don't get a plug'n play unit for 300 US$ - plus installing a new high-quality 60-teeth blade, it cuts pre-veneered 22 mm MDF like butter, leaving razor sharp edges ready for gluing.

Actually these cabs are meant for more designs (SEAS W26-Classic), so 20 x 20 mm pine fillets were added to support rear and front panels, allowing various drivers, vents and Variovents. 22 mm pre-veneered MDF was used and side, top and bottom panels were cut 45 deg. to allow nice looking joints. Only drawback with pre-veneered MDF is that you have to take great care during the rest of the construction work to avoid nicks and cracks. The rear and the front is going to be painted black and if you think veneering is tedious, try painting! Lots of sanding, priming and several coats of paint. Takes quite some time.

Cabinet drawings

Inner dimensions given here are only guidelines.
Dimensions on braces have to be adjusted once the outer cabinet is in place.

Box simulation (once again)

123A bass driver in original 44 litre cab @ 30 Hz port tuning. We have a ~4 dB bump at 70 Hz = boom-bass.

123A driver in new 65 litre closed cab. The 70 Hz bump is 2 dB down.

Above is seen 123A driver in 44 litre original vented cab compared to 65 litre closed cab, x-over (kit) attached for both.
What the Variovents are going to do to this has to be seen. It can't be simulated properly. My guess is that
the bump will be further reduced and we'll have a little extra bass below 30 Hz; the latter I really don't think matters.

The acoustic vent

May 2014: The Variovent is out of production but you can still get the same thing from using a conventional port and stuff it lightly with acoustilux. I suggest a 68 x 200 mm port (Jantzen Audio). Cut it to 10 cm length. Now, the question is how much acoustilux should be added? I suggest for a start rolling a 10 x 20 cm piece of 30 mm acoustilux at insert it into the port. If you can do an impedance plot you add damping material until you get the same impedance profile as can be seen below. If you cannot perform measurements, you have to tune by ear. No damping will make a boomy bass peaking 4 dB at 63 Hz from a 65 litre cabinet and Fb = 30 Hz. Stuffing the port really hard (closed box) will reduce the bump by 2 dB. The full length - non-stuffed - port (20 cm) will make a Fb = 22 Hz. Try it out.

The Variovent consists of two grilles with some damping material stuck in between. Simple as that. The diameter, amount of damping material and the compression of the material determines the air flow properties = acoustic resistance. The late Gilbert Briggs (Wharfedale) made slits in the rear panel and added damping material over the slits. Works the same and costs nothing. But you have to be able to follow the impedance of the system to tune the amount of damping material needed.

The Variovent is a device sometimes causing much debate. What is an acoustic vent? Well, an acoustic vent allows some ventilation at low frequences and virtually none at higher frequences. So what's that supposed to be good for? Neville Thiele studied the Variovent in detail and concluded it didn't offer any advantage over a properly designed vented systems. The general advice on the use of acoustic vents is simple: Try it! Which offers little comfort to those addicted to math. But box calculation based on Thiele/Small data really only apply for speakers with a Qt = 0.35, which only counts for very few drivers. Based on experience we use speakers with high Qt in cabs too small and drivers with low Qt in cabs too large compared to strict box simulation.

Comparing the impedance peaks of a driver in free air and a perfectly closed and empty box should reveal the same peak height, so we have no loss in the closed box. Obviously the resonance frequency is higher. Adding damping material to the closed box we see a decline in peak height; now we have some loss. Adding an acoustic vent may produce a significant reduction in peak height depending on how open the vent is. So, the system Q-value is lowered (= more loss) from applying an acoustic vent although we really can't speak of a Q-value of an aperiodic system. Basically a closed box has 2nd order roll-off, a vented system a 4th order roll-off and the aperiodic system a 3rd order roll-off.

Pics from cabinet construction

Cabs assembled and waiting for bracing and front/rear panels. Addind bracing after assembling he cabs may seem odd, but if you don't have
CNC cut panels, accurate down to +/- 0.1 mm, you need some freedom before to adjust bracing, fillets and front/rear panels.

20 x 20 mm pine fillets are used to support front and rear panels.

Rear panel is devided into two section in order to allow Variovents and traditional ports.

The mid and the tweeter -

Both tweeter and mid will be flush mounted. Yes, it's trouble due to truncated mid frame and the square tweeter faceplate. But take a deep breath - and a couple of hours later it's done and they look so much better - and performance is enhaced too as we shall see later. The mid needs a 5 mm rebate and the tweeter 7 mm. The LE26 tweeter will be used in this construction but the LE25 (brown surround), LE25 (black surround) and LE25-2 (also black surround) will be tested too to see if there are any differences that will need crossover modifications.
As seen in the
LE26 article, the conveks plastic faceplate isn't good and the tweeter performs - technically - better if we turn the faceplate upside down. This doesn't look particularly good, so I going to make a new faceplate for the LE26 as described in the article. 4 mm thick as seen below. The tweeter faceplate is 3 mm, additional faceplate = 4 mm, thus 7 mm rebate as seen above. Alternatively a 5 mm polyester foam "faceplate" may be used. I suggest cutting a 108 x 108 mm piece of polyester foam to cover the rebate if this is your choice.

See, the LE5-2 really looks good this way.

The LE5-2 needs deep chamfering to allow free ventriation.

Mid cabs

Left: I used 12 mm baltic birch to make a cab of 130 x 130 x 75 mm inner dimensions.
Right: Testing driver rebates.


Left: 4 mm bitumen pads were applied to all internal panels and on top of this 40 mm egg crate foam.
Above the crossover two layers of 30 mm acoustilux.
Right: The Variovents and binding posts.

First time set-up with LE26 tweeters. No changes to the crossover. High value of tweeter shunt resistor, R1061, was chosen.


No change to the crossover from the up-grade kit.
The crossover allows the use of LE25, LE25-2 and LE26 tweeters.
As always: Do take time to try out attenuation of tweeter and mid-driver:
Mid: Try bypassing R 2011 and hear what happens.
Tweeter: Try the two options of R1061 and hear what happens.

Check out the various crossover kit levels here.


Left: Red = SPL @ 1m/2.8V. Black = nearfield response of Variovents. As expected the Variovents add a little to the 20-40 Hz output.
Right: Impedance of system. Now, this is an easy load. Minimum impedance is 5.0 ohm and phase angles are modest.

Step response. With a 5 deg. tilt the bass and mid are close to being time-coherent (1st order filter and tilt)

Right: 20 dB cumulative spectral decay of 2007 set-up (LE25 tweeter). Right: 2008/LE26 set-up.
I wanted to see if the heavy bracing and panel damping would decrease resonances in the midrange and in
the 400-2000 Hz region something has happened.

40dB CSD 2007 (left) vs. 2008 (right).
Now, 40 dB really is too much to show, but the CLIO doesn't allow 30 dB, which would be more appropriate.
What is clear is that the vastly enhanced midrange from the new braced cabinet is not fully revealed in these CSD presentations.
Measurements are tricky business and basically tells terribly little about the sonic performance of a speaker.

Bass performance from new cabinet

Making plans for my own L100s I had two things in mind: Transmission line and the Variovents. Both would suit the high Qt 125A bass driver. A transmission line would require at least a 100 litre cabinet and I wasn't prepared for this, thus the Variovents. They did very well with the L26-3-way that I wanted to use them again.

The bass performance bears little resemblance of the former boomy bass reflex loaded 123A driver. It's deep, firm, tight, solid and the Variovents allow a shallower roll-off and despite the 50 Hz system resonance.
The renovated L100 handles bass noticeable different from vented designs of similar size, e.g. Ekta Grande. Running test warble tones at 20, 30, 40 and 50 Hz produce a smooth steadily increase in response and I think the best way to describe the bass quality is that it is doesn't change character over the whole bass region, 30-160 Hz, except for amplitude in the lower octave.

The sound

The JBL L100 Century loudspeaker was a worthy representative of the Seventies tizz-and-boom tradition. It had both. Loads of bass from a high Qt driver in a too small vented cabinet and loads of tizz from the 6.5 kHz peaking of the LE5-2 middriver. Very charming - and impressive - at first "hear", but tiresome in the long run. Placing these three high-quality drivers in a vibrant chip-board cabinet only made things worse as the bass driver was meant to handle most of ever so important midrange. The result was a smeared midrange garnished with an aggressive treble from all three drivers trying to do the same thing, playing treble. No wonder later JBL monitors had more sophisticated crossovers, usually 2nd order.

There's one thing that adds a specific character to this speaker: Having a 12" driver handling the midrange up to around 1-1.5 kHz. It's kind of studio sound. Professional studios - and cinemas for that matter - often use huge driver for midrange. Why? To reduce distortion obviously. We're so sensitive to distortion of the human voice and a proper designed 10-12" driver can do things here that no 6" can ever do. Due to the ingenious flat cone geometry of the 123A driver, this goes better than for most 12" drivers. I guess the heavy coating of the 123A driver is part of the secret, reducing potential cone break-up to acceptable levels. The price to pay was reduced efficiency. The calculated efficiency of the 123A driver is around 89 dB, well in accordance with actual measurements.

Playing Nils Lofgren, Acoustic Live, leaves no doubt that we really do need a 12" driver to fully enjoy his powerful guitar play.

More impressions to come.

The L100 Crossover Up-Grade Kit

The complete kit (a pair) includes:

1. Terminals.
2. Two extra resistors for tweeter attenuation.
3. Soldering tag strips.
4. All crossover components: Baked non-resonant coils, all polypropylene Cross Cap capacitors, MOX resistors and cables for connecting drivers.
5. Kit instruction containing crossover schematics, a short version of first article and large sharp photos of the assembled crossover board.

Not supplied is silicone glue, screws and damping material. It's highly recommended to replace the old damping material.
The kit price is set at a level where you cannot buy similar quality components at a lower price from any supplier.

Orders at: contact@jantzen-audio.com
When placing an order, please include full name and shipping address.

Kit instruction.

Download kit prices, pfd file

Response from people buying the up-grade kit