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New crossover design for L-100A
My first speakers were a pair JBL L-100As that I bought in 1973. I always enjoyed them and still use them today. I have recently built totally new crossovers for them that correct some of the flaws of these historic 3-ways and improve their sound. No doubt, some here might consider that as a sin. I hope others will be interested in the details.
Even by today's standards, they do amazingly well for a 3-way speaker with only 2 crossover components. Their relatively high sensitivity generates an incredible attack giving music an energy and presence that few other speakers could reproduce then or today.
A few years ago, I began playing with DIY speaker building. I was originally interested in learning what features are important in making a speaker sound good. To make a long story short, it’s all in the crossover, and to a lesser extent, cabinet design. A well designed crossover can make average or even poor drivers sound decent, and a well designed crossover combined with genuinely good drivers can make for a truly excellent speaker. Other exotic or expensive tweaks that we so often hear about all make much smaller differences – if they are audible at all – in comparison to the big improvements from a good crossover.
I eventually hit upon a DIY design that is my favorite, the CAOW1, a small 2-way speaker designed by Dennis Murphy (http://murphyblaster.com/) that combines a 5¼" midwoofer (SEAS CA15RLY) with a ¾" dome tweeter (Hiquphon OW1). Although I didn’t realize it at the time, I was carefully avoiding any DIY 3-way design that might compete with my JBLs. After I built the CAOW1s, I found that I preferred listening to music over them. Except for their obvious lack of deep bass below 50 Hz, they sound much more balanced and are more satisfying for listening. Not surprisingly, their frequency response curve is flat. The JBLs, my first love, sat in silence, except for movies. They just didn’t do it for me any more. I occasionally cranked them up to get a taste of their wonderful bass attack, but they sounded wrong in the critical midrange frequencies. But before completely giving up on them, I decided to test the idea that it’s all in the crossover.
The original L-100A crossover is a good example that vintage is not always better. It contains only 1st order high-pass filters at 1.5 kHz for the midrange and 6 kHz for the tweeter. The woofer had no filter at all, and the midrange lacked any low-pass filter. It is certainly simple, but as we’ll see, it’s far too simple.
Note that some JBL drivers at that time were made with the opposite absolute polarity compared to what most manufacturers do today. On my 123A-1 woofers the red terminals were positive. I did not directly see the terminals of the other two drivers, but the midrange had black and white wires attached with the black wire positive, and the tweeter had red and black wires with red the positive. It was easy to find the polarity of a driver by using a 1.5 volt AA battery. If the plus terminals of the battery and driver were wired together, the speaker cone would pop forward.
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New L-100A crossover - part 2
At this point, I asked Dennis Murphy, my crossover designer friend, if he was interested in this vintage make over project. His eager response generated most of the details below. I am most grateful for his expert help and enthusiastic guidance.
With his first look at the speakers he measured their frequency response curve. The curve (below) graphically shows just what “The West Coast Sound” means. Several features are prominent:
- A big ugly peak appears from 6 to 7 kHz. Perhaps caused by the unfiltered breakup of the midrange driver, this peak certainly would have to be tamed.
- A general rising response as frequency increases, especially above 2 kHz, that probably contributes to the L-100’s forward sound. This probably can be easily corrected.
- Destructive cancellations were seen resulting in deep troughs at 3.3 kHz and above 9 kHz, producing a prominent comb filter effect. This is probably due to the unfortunate placement of the midrange driver relative to the tweeter and woofer on the front baffle.
The prominent rise and fall of frequency response below 200 Hz may be the product of room reflections and their resulting standing waves and cancellations. It may not be directly due to the speaker.
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New L-100A crossovers - part 3
Further testing revealed that none of the individual drivers seemed all that bad. In fact, Dennis commented, “Those paper drivers are much better behaved at higher frequencies than many modern drivers.” A decent solution might be found without swapping in a different driver as I had originally expected. The unfiltered frequency response of the woofer is shown below, followed by the midrange, and finally by the tweeter.
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New L-100A crossover - part 4
Dennis used his years of experience and his crossover design software to work up a new crossover. While listening to it via his crossover emulation software, he sent me this rather provoking email:
“It sounds freaking great to me. The tweeter isn’t state of the art, but it gets the job done…”
For those who don’t know Dennis Murphy, he avoids colloquial exaggerations ands is usually rather understated. When he gets excited, I sit up and take notice.
The predicted frequency response with the redesigned crossover is shown below, first with proper driver polarity and then below that with the midrange’s polarity reversed. The latter curve shows the new crossover points, and demonstrates that the drivers are in phase around the crossover frequencies when the polarities of the connections are correct.
The woofer-mid crossover, at ~950 Hz, involves Linkwitz-Riley 4th order crossover slopes. The mid-tweeter crossover, at 5 kHz is also LR 4th order. The glaring 6-7 kHz peak is essentially eliminated, and the high frequency comb filter cancellations are also gone! According to Dennis,
“It took more than adding a low pass filter for the midrange driver above 5 kHz because the big peak was not caused by driver break-up. It’s actually an additive diffraction artifact caused by the wide baffle and the goofy layout of the drivers. Getting rid of it wasn’t easy, and certainly wouldn’t have been possible using the design technology of the '70s.”
The profile from 10 to 20 kHz remains uneven, and is probably the best the tweeter can do – looking just like the unfiltered tweeter response curve.
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New L-100A crossover - part 5
Dennis built a test version of the new crossover which produced the following directly measured frequency response curve. Below that is the schematic of the crossover.
I listened to it and quickly knew that I would be building them soon. If the original sound of the JBL L-100A was the "West Coast Sound", then I call this new modification the "Coast to Coast Sound".
Building the crossovers was straight forward. With the woofer removed, it was nice having an 11" diameter hole in the cabinet. All the crossover parts mounted on a 10"×7" pegboard fit in easily. I mounted the pegboard on the inside bottom of the cabinet using 1" tall plastic spacers and wood screws so that the pegboard wouldn’t crush the fiberglass cabinet lining. I also replaced the original JBL binding posts with some nice but inexpensive gold-plated brass binding posts ($1.25 each at Madisound).
How do they sound? In a word, excellent! My very first impression when I installed the first crossover, was that it made the speakers a lot less sensitive. I had expected that, but it was still quite noticeable. But despite that loss of about 5 dB, the new crossover did not suck the life out of those JBLs.
I spent about a week listening to one speaker with the new crossover, comparing it to the other speaker with the old crossover, adjusting for the change in volume. Both my wife and I agreed that the new crossover was a clear winner. I tried a wide variety of music that I knew well, including some music where I actually liked the effect that the bright JBL upper-midrange had. The new crossovers eliminate the glare and brightness that I thought I had gotten used to after all these years. The listener fatigue is gone, but the JBL excitement was still there. I remembered that years ago, I used to play with the bass and treble controls, and fiddle with the variable L-pads on the speakers, adjusting midrange and tweeter levels, trying without success to control that ear-fatigue-inducing brightness. The new crossover does it much better. It really amazes me how much better speakers sound when the frequency response curve is flat. You haven't really heard one of these old book-shelf JBL speakers until you have heard it with a proper crossover.
All the parts for two new crossovers cost me about $130. If any one is interested in a parts list, email or PM me with your email address and I'll send it to you.
New crossover design for L-100A
Neato! :applaud:
http://murphyblaster.com/content.php?f=cabinets.html
Interesting... kind of goes with what JBL has done but against what others have argued.