Replacing the capacitors of the 4311 crossover is not a good idea. Here’s why:
The crossovers on the JBL 4311 and L100A were meant to be minimal, but they were simply not up to the job. It contained only 1st order high-pass filters at 1.5 kHz for the mid-range and 6 kHz for the tweeter. The woofer had no filter at all, and the mid-range lacked any low-pass filter. It was certainly simple, but it’s far too simple for those drivers. It’s a good example that vintage is not always better.
Look at the L100A speaker’s frequency response curve with the JBL crossover. It graphically shows just what “The West Coast Sound” is really about. Several features are prominent:
- A big ugly looking peak appears from 6 to 7 kHz. With the right music provoking those frequencies, it sounds downright ugly, hissy & sibilant. This might be caused by running the mid-range driver beyond its breakup frequency without a low-pass filter, this peak certainly took some work before it was tamed.
- There is also a general rising response as frequency increases, especially above 2 kHz, which contributes to the overall forward sound. This was easily corrected.
- Finally, destructive cancellations were seen resulting in deep troughs at 3.3 kHz and above 9 kHz, producing a prominent comb filter effect. These peaks and troughs are probably due to the unfortunate placement of the mid-range driver relative to the other drivers on the front baffle. The best correction for this would be building a new cabinet in which the 3 drivers are placed one above the other vertically. Short of that, some of these peaks & troughs can be corrected by some equalization incorporated into the crossover network.
Replacing the capacitors in your 4311 will only preserve those flaws of this speaker. Instead, you should consider building an entire new crossover that was designed by Dennis Murphy in 2006. It creates new crossovers (high & low-pass filters) for the woofer-to-mid at about 950 Hz, and mid-to-tweeter at 5,000 Hz. They will result in a much smoother frequency response with greater balance between lower & higher frequencies, as shown below.
See the attached PDF file for all the details about this new crossover. It is already 2 years old. Prices & availability of the crossover parts have changed since then. I'll have to work on getting that up to date.
The new crossover also eliminates those troublesome variable L-pads. Variable L-pads always suffer from corrosion. It doesn’t matter if you are able to completely clean & restore your old L-pads, or if you replaced them with new ones. Their corrosion would continue, resulting in further degraded performance. Better to get rid of them.
