JBL L100 Crossover Upgrade

What To Upgrade In JBL L100

After an extensive online research, I concluded that the biggest improvements can be made by upgrading the crossover, which according to numerous Internet sources, is the weakest link of the JBL L100. There are other potential improvements that can be made, like more rigid enclosures or replacing damping material with acoustic foam, however, these are unlikely to make as much difference as the crossover upgrade. For this reason, I decided to focus my efforts on upgrading the original crossovers. As can be seen from the schematics below, the original JBL L100 crossover is a rather simplistic design. There are no filters on the woofer, and there is a high-pass filter on the midrange and tweeter section. To match the lower sensitivity of the woofer, tweeter and midrange drivers are also damped with variable L-pads. Long story short, each JBL L100 crossover contains two electrolytic caps and two L-pads. Electrolytic caps are not great but they tend to stay within JBL’s specs. The L-pads however, tend to deteriorate badly, either causing drivers to not work at all, or distort when playing. Overall, the crossover design is rather too simple considering modern standards.
jbl l100 crossover upgrade 01 original crossover

JBL L100 Crossover Upgrade Options

There are 3 potential ways to go about improving JBL L100 crossover.

1. Troels Gravesen’s Crossover (TG) – cost: approx. £180 for a pair

We can replace the JBL L100 crossover with a new one designed by Troels Gravesen. For those who do not know, Mr Gravesen is an independent Danish speaker designer, with wealth of experience and amazing speaker designs. A lot of time and effort was put into design of this crossover. The whole process is well documented on Mr Gravesen’s webiste. Sadly, this is was a commercial project, which means that you cannot just look at the schematics and make the crossover yourself using the components that you want to use. The only way to get the schematics, is to purchase the kit from Jantzen Audio, which also includes all the components necessary to build two crossovers. It comes in 4 different levels (quality of components) with the cheapest one starting at approx. £180 (Level 4) and the most expensive being priced at £440 (Level 1). This is not great if you already have a lot of components in stock and just want to try it out.

2. Dennis Murphy’s Crossover (DM) – cost: approx. £100 for a pair

We can replace the original JBL L100 crossover with a new one designed by Dennis Murphy. Mr Murphy is an independent speaker designer from America. He is not very well known in Europe, however, number of his speaker constructions have a cult like status in the USA. The upgrading of the crossover, was done in conjunction with Swerd, who initiated and coordinated the whole project. All of this is well documented on Audioholics forum. Because both of these guys done it from passion for loudspeakers, they shared the schematics of the crossover on the Internet. This means that if you are interested in trying it and already have some components in stock, you can do it for a relatively small amount of money.

3. Like-For-Like Recap – cost: approx. £55 for a pair

We can stick to the original JBL L100 crossover topology but replace the capacitors with good quality poly caps and replace the L-pads with new ones. Rewire with an OFC cables isn’t a bad idea either.

Because of my inquisitive nature, and because (to my knowledge) nobody has done it before, I decided to try out all 3 upgrades and share my opinion regarding each one of them with fellow DIYers. Therefore, if you are considering upgrading crossovers in your beloved JBL L100, the write-up below may be of an interest to you.

JBL L100 Crossover Upgrade Process

To be able to asses how each option affected the sound, I’ve kept one of the speakers in its original condition, and conducted all of the listening tests in mono. To provide further insight, I measured the original speaker as well as each of the crossover upgrades. Due to low bracketing (limitations of my workshop), the measurement is truthful down to approximately 500Hz. I don’t use a top of the range measuring equipment, but what I use has a fairly linear response and provides a good indication of what is happening. Moreover, because I compare the measurements with each other, the differences between them will be accurate.

1. Troels Gravesen’s Crossover

I started my journey with the the JBL L100 crossover designed by Troels Gravesen. People who tried it, tend to be really impressed with it (see the feedback). The crossover is the most complicated out of all 3 upgrades and according to Mr. Gravesen’s simulations, provides relatively flat frequency response as well as great phase integration. The kit comes really well packaged, with all components individually wrapped in bubble wrap. It comes with full set of instructions, schematics and drawings, so even a person with no understanding of electronics will be able to put it together.

When I built the crossover I conducted the frequency response measurement first. When I compared the graph with the original JBL L100 crossover, I could not believe how well Mr Gravesen’s crossover measured. Most of the frequency response stayed in the +/- 3dB range. I was so excited, that I rushed home to listen to it. And this is where things started going down. When I plugged it in for the first time, I thought that I’ve loosen one of the connectors on the way from the workshop. The speaker sounded muffled. I’ve triple checked everything including polarity of the drivers, and it turned out that everything is connected properly. This is how this crossover sounds. However, sometimes first impressions can be misleading, so I conducted thorough listening test using different types of music as well as different volume levels. I’m sad to say this but, to my ears, this crossover sucks life out of the JBL L100. This is especially noticeable when listening at low to medium volume levels. It is strange because you can tell that it provides more balanced sound than the original crossover, but it constantly leave me with a feeling that it is lacking midrange and treble. I though that this maybe because, the original crossover is rather forward sounding, so I stopped listening to the original speaker, and only listed to the upgraded one, with a hope, that my feeling of lack of midrange and treble will go away – after all, the measurements suggest that there is enough treble and midrange there. Unfortunately, my feeling did not go away, and despite me trying really hard to find something that I would like about this crossover, I didn’t. It makes music less exiting and the voices don’t sound as good as in the original JBL L100 crossover. It also gives an impression that there is a bit too much upper bass. I even prefer the separation of the original crossover, that seems to push the voices in front of the speakers, whereas Mr Gravesen’s crossover, seems to bring all the sounds together. If I was to find something that was good about it… well, if you listen to your music very loud, then the Mr Gravesen’s crossover will make the JBL L100 very listenable and smooth, whereas the original crossover will make your ears bleed. Even when I played the speaker to my wife, she said that is sounds like someone wrapped a duvet around it… The kit comes with spare resistors which allow you too boost the treble and midrange without spoiling the phase integration. I’ve tried this but it did not make much difference to my ears (at least not enough to make me want to permanently use these crossovers).

2. Dennis Murphy’s Crossover

The next step of my journey was the JBL L100 crossover designed by Dennis Murphy. This time, when I built the crossover, I commenced the listening tests before I measured anything. I, of course, triple checked everything to ensure that the crossover is connected correctly. When I played it for the first time, my impression was that it sounds a bit muffled, but my ears quickly adjusted to the new, well balanced sound. When compared directly with original crossover, it sounds much smoother and a lot less bright. It removes a lot of the flaws of the original crossover, but it also takes away a lot of good things that original crossover was capable of. The vocals don’t seem to be as realistic and the guitars are a lot more polite – which to me isn’t necessary a good thing. Encouraged with a relatively good sound, I measured the crossover, and discovered that it provides a relatively flat frequency response, but with slightly more treble than the one designed by Troels Gravesen.
If I had to chose between the TG and DM design, I would definitely stick to DM topology. However, I was not fully pleased with the DM design, because I takes away too much of the original L100 sound. So we are ending up with a vintage speaker, that tries to sound like a modern speaker. It does it fairly well but you can get modern speakers that sound better than that. This leads to a question – why bother changing it?

3. Like-For-Like Recap

The last step of my journey, was to see if I can make any improvements to the sound without spoiling the original charm of the speaker (read: recap & rewire crossover and replace the L-pads).
This is the easiest and cheapest upgrade that one can make, and I can already tell you that it is well worth doing. The biggest sound improvement comes from replacing the L-pads with new ones. The old potentiometers don’t age very well and cause a lot of audible issues like distortion as well as and dips in the frequency response (see the graph below). Additional, but less significant gain in sound quality is due to replacing the original and slightly out of tolerance capacitors with new polypropylene ones. Finally, replacing the wires with good quality OFC ones is not going to do any harm, but I doubt it will make an audible difference. All of these changes, restore the speakers to their original glory and maintain their character. Have a look at the frequency response above to see the effects of these upgrades. They are far from being perfect but they sound how they were designed to sound – and I like it!


Both the Mr. Gravesen’s and Mr. Murphy’s JBL L100 crossover designs make the speakers a lot more polite, but if I was to choose one, it would definitely be the DM design. It not only sounds better to my ears, but you can make it from whatever components you like, as the schematics are freely available on the internet. However, IMHO neither of these designs make the speaker better. Of course, they make it measure really well, and they attempt to make it sound like a modern constructions – but they don’t. There are number of modern speakers capable of outperforming ‘upgraded’ JBL L100. Which brings me to my point – I like the JBLs for what they are and accept them with their faults. I would also like to make a point that I’m using the speakers for enjoyment, not for studio monitoring. Ultimately, does it really matter that the original speaker has +9db hump in the treble area? As long as it puts a smile on my face when I listen to it, I don’t care. Are the JBL L100 great speakers in their original format? No, far from it, but I use these as a ‘window’ to get the sound form 1970s. It is similar to driving a classic car from that era – it is not going to drive as well as modern cars, but despite this, it can still be a very enjoyable experience.

Therefore, if you want to improve your beloved JBL L100, my advice would be to stick to the original crossover topology, replace the capacitors with better quality ones and replace the variable L-pads with new ones; or even better, replace them with permanent L-pads (resistors).

One final note – there are number of people that are really happy with both Mr. Gravesen’s and Mr. Murphy’s crossovers, and I am not tying to discourage anyone from building these. I am simply sharing my subjective opinion, which clearly shows that we all have different preferences and we all like different things.

Completed: November 2016 | Published: July 2017

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