JBL Century L100 Restoration. 4.2015 - By Greg Roberts Page Two. The first thing I did was to remove the drivers and store them away. Here's a picture of one of the beautifully made woofers. Tweeters and mid drivers Crossovers, if you can call them crossovers. JBL has an audience out there, eager to embrace these big speakers. There was genuine affection to be felt. However, any piece of audio equipment has to deliver the goods sonically, and I am very happy to report that the JBL L100 Classics do that with panache.

If you own a pair of JBL L100 Century speakers and want to refurbish or upgrade your JBL L100 Crossover for improved sound by using a modern crossover, I highly recommend the JBL L100 Century Upgrade Kit from Jantzen Audio designed by Troels Gravesen in Denmark. The L100 drivers are still as good today as when they were originally produced, but the sound can be greatly improved with modern crossover technology.

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1. The Murphy crossover mods do not totally change the sound of the L100. Rather, from my listening experience, it tames some of the rougher edges of it's sound. As I stated before, the woofer has no crossover at all, meaning it can and does extend up into the midrange frequencies.
2. New Management and the L100 Phenomenon. At precisely the moment when the 4310 was gaining its widespread acceptance, JBL experienced the transfer of ownership from Bill Thomas to the Jervis Corporation controlled by Sidney Harman.

JBL L100 Crossover Kit

Read the Troels Gravesen L100 Crossover article to learn more about this JBL L100 Crossover Up-Grade Kit, including detailed frequency analysis comparing before and after the upgrade, photos of the components used, and feedback from upgrade kit buyers like myself.

Click this link to Contact Jantzen Audio to inquire or purchase this JBL L100 Crossover Upgrade Kit.

I chose their JBL L100 Crossover Level 2 kit – which is the preferred version – because of the top level audio components it contains, such as Superior Z-caps and air core inductor coils. The crossover was designed specifically for the L100 to replace and improve upon the original JBL two capacitor/L-pad crossover. The problem with that cheap design is that – at certain frequency bands – all 3 speakers (woofer, mid, tweeter) are trying to reproduce the same sounds simultaneously. There is a lot of phase cancellation going on and an amplitude spike at 6.5 kHz. Despite JBL's stellar audio reputation, these speakers never really handled vocals properly because their crossover design was too simple. Rather than using filters, JBL allowed the speakers to roll-off naturally, which is why – at certain frequencies – all 3 speakers attempt to play the same sounds. This should not be happening. These issues are all addressed with the new JBL L100 Crossover Kit.

These '70's era L100's were boomy with lot's of 'tizz', which everyone seemed to like. However, it wasn't until the '90's with the advent of computer-aided audio analysis that better 1st and 2nd order crossover filters were being utilized and designed properly. Jantzen Audio has done this work in great detail through extensive analysis and design of their JBL L100 Crossover Upgrade Kit. Their crossover breathes new life into these excellent JBL audio drivers. You still get plenty of boom, mid and high end, but now with a properly designed crossover network that corrects those out-of-phase cancellation issues, as well as the rolloff and crossover points that previously allowed all 3 speakers to play the same frequencies. The result is a superb L100 speaker whose former muddiness has been replaced by clarity. This new JBL L100 Crossover results in a L100 speaker that JBL should have made!

My letter to Jantzen Audio after building and testing the crossover kit:

I was able to give the new Level 2 crossovers a thorough testing with my entire music collection, including my own recorded music (I'm a home studio musician). I also ran a sine wave frequency sweep and also analyzed discrete test tones near the crossover zones. The sweep volume seemed level to my ears.

During my first listening experience, I immediately noticed an improved clarity over the original L100 crossovers. I went through my entire music collection and was delighted to hear more clear vocals than I had ever heard since I bought them in the 70's. Back then, I thought these were the best consumer speakers around. I knew they were modeled after the JBL 4311 studio monitors, which I first heard when I worked part time in a studio. At least my L100's were part of the best audio system I had ever owned. And who could argue with JBL? Everyone wanted them! In the 90's, I had the 12 inch and mids refurbished. I thought they were shot and went to buy new complete speakers, but was surprised at their inferior volume and sound. So I called JBL and asked them for advice. An engineer highly recommended I have my L100 speakers refurbished, citing that in order to be competitive, JBL no longer made speakers like that.

Now almost 40 years since purchasing my L100s, I had to do something about the intermittent L-pads. That's how I discovered your crossover kit. After reading your web article, it was clear that you have done some serious analysis and fine tuning of the new crossover design. So for the very same reasons I bought the L100s and held on to them all these years – namely, the quest for decent sound – decided to take a leap of faith and invest in the Level 2 kit. The quality of the Level 2 components is impressive. The inductor air coils are like magic - it's hard to believe they make them so perfect. The final assembly looks cool and adds some weight! Worth every cent.

The result is a pair of L100's like I have never, ever heard them. The transition to this new sound is as refreshing as my long ago transition from toy stereo to a Hi-Fi component audio system with JBLs. But if I had never heard what this new crossover could do, I would not have ever realized what I was missing all these years! I have read one comment that a new crossover could not be better than JBL's original engineering design, but such remarks seem more an attitude than based on fact. Clearly this new crossover is a serious audiophile upgrade that get's it right. The sound is more transparent and clear. The overall muddiness, especially with vocals, is gone. As a recording musician, I have noticed that JBL muddiness compared to my near-field monitors. My music seemed dull on the JBLs, yet I needed to listen on big speakers in order to fine tune my mixes. It was not until I read your analysis of the original L100 crossovers that I finally understood the simple two cap design was contributing to the muddiness from phase cancellation and some frequencies driving all 3 speakers. Now that mess is gone! I also noticed that, for the first time, my L100's play well with in my 5:1 audio video set up. They aren't firing on all speakers with the same frequencies. They take a tiny tad more power now for the same level, but now they are level and balanced. Plus if you want more bite out of the mids, you explain how to bypass one resistor. It's also nice that you provide a choice of two resistors for the high frequency drivers. I went with the higher drive resistor, probably because I'm older and/or all the damping in my living room.

So, to that naysayer who didn't believe anyone could design a better crossover than in the original JBL L100, you stand corrected! Anyone considering this upgrade who has a discerning ear, understands clarity, or is considered an audiophile, you will not be disappointed. This breathes new life into the L100s.

Contact me if you have any questions about my experience building this upgrade kit.

Wendy

JBL, L100 Century Speakers, Crossover, Upgrade Kit, Troels Gravensen, Jantzen Audio

• Last updated on .

The original crossovers are not really crossovers at all. They consist of two capacitors, one for the mid and one for the tweeter, and two potentiometers to control the output of the mid and the tweeter. The midrange capacitor is a 8uFd and the tweeter a 13uFd, which at first glance is the opposite of what you would expect. Normally the mid would have a larger value hi-pass cap. The mid has no low-pass on the upper end (bandpass), and the woofer has no filter on it at all. Not a conventional crossover to say the least.

I'm not going to publish too many of the tests here, nor will I describe the testing procedure in detail. Instead I will boil it down and give you the end results. This whole procedure took more than twenty hours, over a four day period.

One thing to note - the components used in these speakers sound a lot better than they look in tests. I wasn't surprised by this, as it is consistent with my experiences where a good test does not necessarily correlate to a good sound, and vice versa.

Here's a simple test showing the frequency response on the woofer with no filter. I should note here that the frequency response of my testing in this room will not be accurate in the low bass. To accurately test bass response, it takes a much more sophisticated setup than I have available to me. Nonetheless, this is still useful information, and something that I can work with.

Here's a test showing the midrange frequency response with a 8uFd new capacitor, no attenuation.

Jbl L100 Crossover Schematic

Here's the tweeter with a 13uFd new capacitor, no attenuation.

Here are the three components together.

I was surprised to find that there was a LOT of overlap between the mid and the tweeter. In fact, using the original crossover design, the tweeter has more output at 900Hz than the midrange. Another interesting note - the midrange and tweeter tested exactly the same with the new caps as they did with the old caps. However, the speaker sounded better with the new caps and L-pad resistors (same design as the old crossover) than it did with the old crossover.

I began calculating and trying different filters on each of the components in an attempt to smooth out the response, and to optimize the bandwidth of each. After many hours of work, I had some very good looking test results. Smoother response, a better low-pass on the woofer, a better hi-pass on the tweeter - some very nice looking slopes that cross-over one another exactly how you'd draw it up. I had a test crossover built with various components sitting on top of the test speaker, alligator clips holding everything together.

And how did it sound with music? Worse than the old original crossovers!

Just too many crossover parts I thought, but actually, after some investigation, I figured out that it was really the midrange that I had screwed up by pushing it too low, causing an increase in distortion. I'm giving you the short version here. There were hours and hours of trial and error, testing and listening - all coming back to the fact that the JBL engineers had it pretty well sorted out to begin with.

So it was day four of working on the crossovers, and I had a huge amount of data from all my trials, and well over 100 tests. What to do with all of it? I kept working on it through the day and finally ended up with two small changes to the crossover design that made improvements both in tests and in listening. The first change rolls off the objectionable 900Hz peak in the tweeter, leaving it sounding more like a tweeter and less like another midrange. The second change smooths out the upper midrange peak in the midrange driver, flattening the response and taming the sharp, edginess that I detected in the upper midrange.

Here's the new crossover

The new crossover is certainly an improvement over the old, and all it took was a couple of small changes and some new parts. Gone are the tiny, thread-sized wires on the original potentiometers that carried the entire mid and tweeter signals, gone are the terrible connections inside the pots between two corroded plates scraping together, gone are the old caps.

The new crossover gives us new caps, nice connections throughout, nice wiring, L-pad resistors for attenuation rather than the pots, and a couple of small design changes that help smooth out the response.

To mount the crossovers in the cabinet, I chose to drill one hole in the back center, where a bolt would be inserted through the back and through the crossover board to hold it in place. I also drilled out the binding post holes to accept the new binding posts.

New wiring

Resistor L-pad modules to set different levels for the mid and tweet.

Here they are with the components installed and new foam rings for the tweeters

And with the Cane-style grill cloth

8/14 More finished photos to come

13 hours labor to put cloth on frames, build crossovers, install components, install new binding posts, install crossovers, install foam rings, make new wiring. Materials for grill cloth, crossovers, binding posts, wiring, foam rings $158.

Add 1 hour for packing the speakers and $10 in packing materials

Project cost totals

20 hours crossover design time - no charge

Labor is 41:50 - adjust to 39 hours @ $60 per hour = $2,340

Materials $343

Total = $2,683

Plus shipping cost TBD

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Jbl L100 Crossover Kit

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