JBL L100 Up-Grade Kit, response from builders
Copyright 2016 © Troels Gravesen


Hi Troels,
Thank you for you X-over design for the JBL L100 Century, it has increased the listening joy of these classic speakers tremendously. 
The speakers are now more homogenous, low bas is present at low volumes and the treble does not have this tiring sound, I am very satisfied with my speakers, still amazed at how well such an old set of drivers sound.
I decided to go all in with the restauration, applied new american walnut veneer, made a new foil cal, painted them plus mounted a new grille - the stands will have to do for now but tilting the speakers has helped a lot compared to having them placed on the two 25cm high “stands".
Best regards



Hello again Troels!
Just wanted to thank you for the greatness of the rich sound of my upgraded Jbl l100. Here is a New Photo of the speakers since we moved... up in the corner is a small pair of jbls that i bought in tokyo: ) but their sound is nowhere near your modification of the originals.
Fun thing also that JBL are themselves releasing a New fresh version of the Classic l100. Have you heard them? 
I have pondered buying the buchardt speakers that i heard on a hifi event 2 years ago.daniah brand. Great speakers too... but I like my jbls too much and will never let them go.
All the best and many thanks.


22-08-2018: JBL Kit Level 1

Hi Troels-
Finally built the kit - had a blast doing it and the JBLs sound simply fantastic!
Thanks very much,

11-09-2017 Response from Alex Germany
I just wanted to give you a feedback on the crossover kit i purchased earlier this year. I found the time to finish building and installing the crossovers and i am really pleased by the quality of the kit and instructions. Also the sound turned out absolutely awesome! It is so much fun listening to the speakers, you just want to play the music louder and louder every time. Thanks again for your support and this awesome kit.
Best regards,

JBL Upgrade Kit + new cabs, made by Brandon, US:


Response from Pawel

Dear Mr Gravesen,
Hope you are well.
I have upgraded my JBL L100 with Level 3 from Jantzen Audio and I am very pleased with the speakers and impressed  by the fact how well designed crossover can change the character of the speaker, actually I didn't like them to be honest, I bought them because of the opinion on internet, they were too harsh, they could not compete with my current speakers..but after upgrade... wow :)
Having above experience I believe building one of your designs is a great investment for the next years.
Today I have realized that I have 5 pairs of speakers at home. I am going to sell 4 them, the sad part of it is that I will miss some of them:
1) Dynaudio Audience 52SE 89dB - actually I won't miss this one, needs plenty of power, good speakers, that's it.(hope no harsh feelings - it's a Danish brand :) 
2) Vandersteen 1C - wonderful, airy, transparent, great sound stage 
3) Vandersteen 2C - even more wonderful than 1C, with deeper/lower bass, but I was missing something... not sure what
4) ProAc Studio 140 MK2 - transparent, dynamic, great base, good sensitivity 91dB, good trumpets, drums, pretty good vocals, but the highs... cymbals are so harsh!!! it's a noise not cymbals! awful!
I also think that they are not so well balanced: when I played Log Sweep 20-20k it was louder and louder... when with "number 5" it's the same loudness independently from the frequency (apart from extremes)
"number 5") JBL L100 with upgraded crossover Level 3  this I. Absolute masterpiece, so well balanced, a bit too much bass, too boomy (I know it's the cabinet) cymbals are much better than in ProAc. Pleasure to listen to music. However I miss the transparency from ProAc, they are slightly too muddy in comparison to ProAc and also have lower sensitivity than English brand speakers.
My amplifier provides 20-25W in 8 Ohm, 40W - 4Ohm. However I think it's better if the speaker would be 8 Ohms, it's easier to drive it.  It's Nelson Pass design, I  build by myself, minimalist class A Push Pull.(I have  matched semiconductors and source resistors to extreme) amazing amplifier. (currently building electrostatic amplifier based on this topology - simulations in LTSpice so far)
I don't use any equalizer, it's a source of phase shift distortions. I don't  use even preamplifier, digital signal goes directly to the power amplifier! Volume is controlled digitally by 28-bits discrete DAC module, also from Denmark :)
I listen from Rock to Jazz, now more often Jazz, my room is 20-22 sq meter, It would be great if speaker would have a good base, vocals, trumpets, Mr Getz Saxophone small nuances, great spatial performance, obviously super natural cymbals :) I wish if speakers in general would have as much details as headphones, especially electrostatic ones but it's probably impossible.
Important thing with this low power amplifiers is the  speaker sensitivity. It doesn't have to work with 8W Single Ended amplifier of course. But It's worth to mention that with lower power/higher sensitivity speaker - the amplifier provides signal with lower distortions.
Best Regards,

Response from Joseph

Hi Troels. Thank you for your replies to my emails regarding the JBL crossover up-grade kit. I would like to share with you my experience with the new crossovers. First of all they sound AWESOME. After building the first crossover I tested all 3 drivers. I wasn't expecting the 123A to sound so different. I was amazed how much better it sounded. The bass fills the room, very smooth and warm, focused much more on bass, no longer playing to much into the mid frequencies. The Le-5 very similar, much smoother and balanced, not to loud and coming at you. The Le25 is quite different. After building the first crossover and testing it. I wasn't sure if that was how the Le25 was supposed to sound, it  seemed to be playing only the highest of highs and quiet. After building the 2nd. crossover and testing them, all 3 drivers sounded virtually identical with either crossover. With that being said, I figured,they must be right. My final thoughts. The speakers have gone through a transformation, They are no longer the radical rock and roll speakers of the 70's. They now produce a much more smooth and balanced sound which is what I was looking for in a studio monitor speaker. Thank you for your hard work and wish you much success.

Response from Stepan

Troels! I wrote you long ago regarding the jbl L-100 crossovers you created. The importance of correct placement of the speakers was huge, but now they play supremely! Many thanks for such a cool contribution to hifi sound and a wonderful tribute to the beuatiful 70's design of these speakers. Your crossoverdesign is a very big improvement to the jbl's sound. You're the man!
Thank you, Stefan

Response from Wendy/US

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 L-100 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 L-100'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 a new complete speakers, but was surprised at their inferior volume and sound. So I called JBL and asked them for advice. An engineer highly recommend I have my L-100 speakers refurbished, citing that in order to be competitive, JBL no longer made speakers like that.
Now almost 40 years since purchasing my L-100s, 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 L-100s 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 L-100'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 an 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 seem dull on the JBLs, yet I needed to listen on big speakers in order to fine time my mixes. It was not until I read your analysis of the original L-100 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 L-100'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 by pass 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, 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 L-100, 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 L-100s.
Wendy Dunham, Hopkins, MN  JBL L100 Crossover Upgrade - GALXYgirl - Wendy Dunham

Dear Mr. Gravesen.
I truly admire the work you have done to upgrade the JBL L100 Loudspeaker. It’s sort of like you’ve given an old man a new heart. I am retired now, but I worked in the audio/consumer electronics manufacturing industry for forty years – from 1960 – 2000. During the 1960’s I was Vice-President of Engineering Administration at JBL. I am not an engineer – but I did direct the L100 development program. L100 Design Engineer Ed May was a very dear and close friend. I coordinated the work of industrial design consultant, Arnold Wolf – who, with Doug Warner, was responsible for the visual design. Arnold later went on to become President of JBL and my boss. Larry Phillips created maybe the best audio promotion program ever: “Wednesday for Trombones, Thursday for Drums.” Years later, when I was introduced to Mark Levinson engineer, Tom Colangelo – Tom – smiling when he heard about my past JBL work experience, simply smiled and spoke that now famous marketing phrase. The best L100 story “never told” is that of JBL draftsman/audio enthusiast Carl Davis from Ottumwa, Iowa. Ed May told us all that microphone wind screen foam material would make an acceptable grille material. Arnold Wolf gave us stunning designs. The problem was that the foam industry was basically the packaging and filtering industry. Pressure cutting foam tolerance was about +/- .25 inch. No foam vendor could meet Arnold’s design requirements. Carl Davis, without degree, without assignment and working on weekends – was the first – in the entire industry – to discover that foam could be hot wire cut – and with the required precision. The L100 project had lingered more than six months in limbo while we struggled with the grille design problem. Meanwhile, the L100 market got off to a head start when studios began ordering 50 to 100 pairs of walnut 4310’s. We all new an unauthorized distribution process was under way. 
Thanks for giving me a reason to look back, Jim Barthell

Response from Vidar, Sweden

Hi Troels!
Finally, after several years since I ordered the kit. The JBL L100 aperiodic cabinet is finished. The goal was to have a friend of mine to put a really nice car finish on them, but he had his hands full of cars. So I ended up roll them with a paint I had laying around. They are built in MDF according to Troels description. I had some problem with the balance in the sound, but that turned out to be a tuning and balance problem with my subbas pair. I currently using one subbas for each channel. A pair of Audio Pro B2-50, from the seventies - of coarse! The sound this old elements produce is rather amazing. To let a vinyl land on my old Thorens TD-125 and send the signal thru a Mcintosh and out via the JBL and Audio Pro is a real joy! Thanks!

I've already sent my thanks once, but I still shake my head.
Soooo much better......John Drozdoff, Vancouver, Canada

Hi Troels, I finished installing my JBL L100 crossover (plus cabinet damping upgrade) last night, in Vancouver, Canada.
I am thrilled! The sound is wonderful. Thanks for making the product available. John Drozdoff

Hello in Denmark! I went to parts express for the L100 x-over upgrade per your suggestion. I really was pretty happy with the sound of my 4311B monitors, with a few exceptions, and bought the upgrade kit not knowing quite what to expect.  I had a great time assembling the kits, and while the speakers were down, cut up the cabinets and did a gorgeuos can't tell I was there job of mirror imaging the components.
Perhaps  most of the credit needs to be given to the crossover design and superior parts, but I now have a system in which the speakers have just disappeared.  I can see them, but wow, not there, just layers and layers of vocals and instruments. I now have a wall of music! !!
I have over the years been thru electrostats, Magnepans, big speakers, little speakers, and much in between, and nothing ever this pleasing overall.  Thank you Mr. Graveson, and staff,  I am as amazed as I am delighted with your talents.  There are true surprises out there, and anyone having skeptism or price issues, should talk to me about your offerings. {It did open up the mids and highs per the instructions, and somehow none of my amps open these speakers up the way my Mac Mc2500 does. Must be some magic there, wow!}
Many many thanks and very best regards,  Randy Pierce, Colorado Springs, Co.

Victor/Hong Kong:
Hi Troels, Ihave ordered your modified crossover a year ago and the result is amazing! The mids and highs are more balance and transparency is significantly improved. It handles wide range of music and female vocals are magificant. I am also very happy that it also performs well for classical music. I love it very much and still enjoying the modified L100 now with my 100W hybid amp.
My L100 is bought from 2nd hand and the cabinet is nearly broken so I would like to have it renewed according to your lovely new cabinet design.
Since I am not good at wood work and with no tools, I would like to have the new cabinet built by carpenters instead of myself. I am not sure the size/diameters/rebates of the speaker units  are and not sure to tell the carpenter the exact outer and inner diameters and the rebates. I try to measure it myself but I am not sure if it is correct or not and I dont want to have it wrong. My carpenters are far away from my house and it is difficult for me to take the speakers to him and let him try and test the diameters and size of the rebates.
I have tried to search it from internet or JBL vintage forums but no infomation for the diameters/size are found. Would you please let me know how large should I tell my carpenter that the tweeters/mid/bass driver outer and inner diameters/rebates for correct cutting? Since both tweeter/mid and bass will be flush mounted, the exact size of the truncated mid frame and the square tweeter faceplate is important.
Thank you again for your excellent design and I look forward to your reply soon. Best regards, Victor/Hong Kong

Thanks Troels !
I found my mistake. The JBL are rocking now. I think the sound is definitely better in the upper midrange and the fantastic JBL bass is still present. Listening to my Gainsbourg LPs is a pleasure. Your crossovers are really worth the price and the work. Regards, Gilles

mail #1:
YAHOO!!!!!!!! I owe you dinner!!!!!
First- after using my big 4333's for years I did not realize how much they lacked midrange- only makes sense as there is not a mid component in the cabinet but..... the low end on those L100's- how ya do that from a 12" cone????? So smooth- and so nice to have mids back (so important on vocals) I took the glass out and put that poly stuff in too!!! I do not miss those big 4333's at all- I am in love with my reborn L100's- This week i am making the stands you suggested on the 5 degree tilt too- Pictures when all done going to Troels . Are you coming to the east coast for vacation this year- I hope so- How else can you cash in a free dinner here in beautiful Pennsylvania!!!!!!! See Hershey, PA see the chocolate factory, broadway show in NYC, etc., etc- c'mon go for it!!!! Its only money- hahahaha- We'll put ya up- you just need airfare to PHL!!!

Mail #2:
Made the stands to 5 degree tilt   (maple)  as per your suggestion over the weekend-stained and finished!!!! your KOOL  crossover in  -I'm in audio heaven!!!!!!! THANKs!  Brian.

#1 mail:
Hello Troels, Today I finished building the JBL L100 Recreation I bought from you. I am very pleased with the result, but there is a problem with the second 123A. The bass "knarzt" - how can i say it in English? When playing music, it makes an ugly noise, and if I move the membrane forward and back without playing music, it makes a noise like "krrzztt". Can you imagine what the problem is? Do you have any advice for me what to do - how can I repair it? So far, even with that annoying fault, this speaker provides a very pleasant sound, and is so easy to listen to. I am looking forward to the moment when the bass is repaired, so I can enjoy it even more. With this mail I will send you some pictures of the building process. Regards, Volker

I suggested Volker that the unit had been fastened too tight as the voice coil gap is very narrow and tightening the screws too hard might bend the chassis and cause voice coil rubbing - and this turned out to be the case.
#2 mail:
Hello Troels, Now I know why the voice coil is rubbing. It was my mistake. Like you said, I tightened the screws very hard, and somehow twisted the chassis.
What a pity. Didn't think of something like that. When I took it out, the voice coil was still rubbing. So I ruined the chassis. Sorry, Troels. I will have to look after a new 123A. Do you happen to have a  third one you can spare? Pity, pity, pity. Regards, Volker

I suggested Volker trying to bend back the chassis by screwing the unit onto an plane surface and add small spacers here and there to see where the pull/perssure should be.
#3 mail:
Dear Troels, I did as you proposed, put the chassis on a straight piece of MPX and tried to bend it back into shape, so that no more voice coil rubbing happens. After an hour or more I finally managed to achieve a free cone movement except the last few mm of excursion. Then I put the chassis bach into the enclosure, and it works without any flaw with music now. These speakers play music in a way I like very much. It is an ease in the sound that makes it a lot of fun to listen to. In a time to come, when our children (5 and 7 years now) have grown up a little more, I hope to spend more time listening to music. Thank you very much for development, and for your help during the building process. Volker

Thanks Volker for your feedback and I'm pleased it all worked out. The lesson learned here is valuable for other builders so I have added all your notes. The cabs you have built look just great! Baltic birch - my favourite material.

Troels, I want to thank you for the crossover kits I received.  I just got around to installing them, and have to say they have made a significant improvement regarding the performance of my L100s. I initially installed one of the crossovers to compare the upgraded speaker against the original, and the improvement is significant! The tweeters sound much, much better. That's not the only improvement, but that's the most noticeable.  I found while comparing the two speakers the treble on the original struck me as garbled, and non-defined. I hope that there are others out there that buy your crossover and realize the potential of these speakers. 
I am impressed! Thanks, John.

Hello Troels, I thought I'd get back to you as I've now had a chance to re-test the crossovers and I owe you an apology - they're wonderful. However the issue causing the "bad sound" was neither the speakers nor the crossovers but my amplifier!! - I took the speakers to my local expert and he tested them on another amplifier and wow they sounded great, so we tested my amplifier and to say it is ruined is an understatement - the technician said it should go into the scrap bin.  I've therefore invested in a new amp and the result is fantastic.
So, thank you for a wonderful product and of course your expert advice. Kind regards Stephen.

Hello, I was very pleased with the crossover kit for the JBL L100 and I want to inform if you have a kit for the pioneer HPM 100 speaker system.
Thanks, B. Stesses/Belgium.