I have a pair of pristine, JBL L86s I bought new in 1987. And while it may not seem worth the effort, I'd like to rebuild the crossovers.
Optimally, I'd like to rebuild the units with higher quality parts, and even make upgrade changes, if possible.
My problem is, I'm not sure where to start. Additionally, I've been unable to find any discussions of this model speaker anywhere on the forums I monitor. I did, however, receive this (see attached) advice from an extremely knowledgeable designer, but he was just making general suggestions, not offering feedback regarding results he personally experienced.
Is it simply a matter of matching values and ordering new components? Or are there other considerations to be weighed? Any help would be greatly appreciated.
Here is a
link for the original schematic of the L86 crossover. It looks like the schematic you attached.
I haven't heard that model speaker, but its design looks as if it might be a good one. Certainly by 1987, JBL had made significant improvements in speaker and crossover design over what it had produced during the 60s and 70s. In the past, I was familiar with the infamous L100 and its poorly designed crossover. I was involved in a
successful design for a completely new crossover for it. But the L86 original crossover seems to be the only design available.
I'd like to address the idea that you can improve the sound quality of speakers merely by rebuilding a crossover with 'higher quality parts'. For the most part, that's false advice. First of all, inductor coils don't go bad with use or time. Resistors also don't go bad, unless they completely fail because of enough overheating to char or burn them. Capacitors, however, have been the subject of great amounts of chatter among speaker fans – that capacitors made by different makers and different materials generate large differences in sound quality. Most of that info is false.
What does matter is that a crossover design keeps the speaker drivers within the audio range where they best perform. It's important that the parts used have values as close as possible to what is specified in the design. How close is good enough? For most audio crossovers, within 10% is good enough. A capacitor such as C3, the first one in the mid-range circuit, has a value of 14 µF. If you factor in a tolerance of ±10%, anything from 12.6 to 15.4 µF will be good enough.
The origin of the capacitor sound quality myth comes from the widespread use, in the past, of cheap non-polar electrolytic (NPE) capacitors. There are 2 problems with them. First, many of them were so cheaply made that, when new, they varied by more than ±10% from their printed label. And second, the capacitance values of NPE capacitors made with non-solid or 'wet' electrolyte (aluminum or tantalum oxides or salts which bind small amounts of water) are known to drift over time and with use. If they once were within specification, with time, they can drift out of spec.
Other capacitors made with rolls of thin plastic (mylar or polypropylene) coated with a very thin layer of a metal conductor (usually aluminum) do not suffer from this problem. They are called by a variety of names, usually film or metalized polypropylene (MPP) capacitors.
As a result,
if and only if, you have aged NPE caps (more than ~30 years old) in your crossovers made with materials that are known to drift out of spec, you probably will benefit from replacing them. Otherwise, don't bother. The idea that various different kinds of film or MPP capacitors impart an audibly different sound quality to speakers is nonsense widely repeated on the internet by people who clearly do not understand what they're talking about. I strongly advise that you ignore it. It's like the myth that speaker sound quality can be improved by choices in speaker cables, usually more expensive speaker cables.
You may wonder why the L86 schematic shows two capacitors where one would do. For example, look at C3, where there is a 14 µF cap in parallel with a much smaller 0.01 µF cap (often the smaller cap is called a bypass cap). Obviously this was not done to achieve a capacitance of 14.01 µF. It was based on a misguided belief in the capacitor sound quality myth. The idea was to use a cheap 14 µF capacitor combined with a smaller expensive film capacitor. The hope was that the 'sound quality' of the smaller bypass cap would dominate the poor sound quality of the larger cap. It's a nice idea if you like clever sounding tweaks, but there is simply no evidence to support the idea that it results in improved sound quality. It's another myth.
I don't know enough to address the idea of using charge-biased capacitors in audio crossovers. It was introduced in the 80s by JBL (and others?), but never did catch on in the industry, or among most DIY speaker builders. The extra cost and complexity may have been the main reason. I've never heard a speaker with that type of crossover so I cannot comment about its sound quality.
