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.