Let me introduce myself. My name is Lee, I moved to Southern NJ from England about 10 years ago, bringing with me all my possesions, including my HiFi and my 1970 Ford Capri. I have always been into HiFi and speakers in particular, I grew up around a pair of the original Mission 700s that my dad had, and a pair of KEF 104aBs that my mom had. I remember my dad and I walking to HiFi shows at a big hotel that overlooked Blackheath. Even to my young ears, the sounds of a pair of Mission 770s, powered by matching Mission amps, fed by a Linn Sondek LP12 turntable was truly awesome.
I recently came across a pair of JBL L100s. The cabinets looked pretty good apart from the usual scratches and rotted foam grilles, the drivers were free from damage apart from a pushed in tweeter dome, so I bought them.
When I got them home and hooked them up, I was impressed by their sound, particularly their low frequency response. If you've ever heard the music of Level 42, the early stuff before they got all poppy, you'll know all about Mark King's slap-bass technique. Try looking on YouTube for a track called "43", not the live version, there's a part about 3 minutes in where I swore the JBLs were going to blow me through the wall into my back yard. My current speakers are what I call my "bass monsters", I built these 20 years ago with SEAs drivers and matching crossover, with a dome tweeter, 8" and 10" woofers (the 10" runs through a subwoofer crossover). Until now, I had yet to find a pair of speakers with as good a bass response. Comparing the two, the foward response of the JBLs became more apparent, it seemed like the midrange and treble was more "shouty" for want of a better word. I decided to restore them, but give them some upgrades at the same time. I wanted to try one of the different crossover designs out there, but at the same time be able to switch back to the original crossover if I wanted that vintage sound back. I also wanted to do things like add some decent binding posts on the rear, make some new grilles, maybe even relocate that one grille peg that everyone who's seen them asks why it doesn't line up with the others. If this was a car, it would be a restomod...
The first problem I found was one of the woofers would make a scraping sound if you tapped the cone ("don't tap it then" should be the answer). It seemed to work OK playing music, but I didn't want it to become a problem later on, so I took the woofer out. Well, that was the plan, but the woofer wouldn't come out at first, so I removed the midrange and tweeter to see if I could get access to the back. I managed to squeeze my hand through the tweeter hole and eventually get the woofer out by pushing on the frame edge with my fingers. It turns out the voice coil had become detached from the spider. What makes these woofers great is you can remove the magnet to make repairs, and once I got the magnet off, I found a date stamp inside of September 18 1974.
After removing the dust cap, I re-attached the voice coil to the spider using some black glue purchased from Orange County Speaker. After some trial and error, I found a piece of glossy cardboard that was part of a magazine cover to use as a shim between the coil and the magnet pole. Since the spider wasn't in contact with the coil like it should be, once I got the glue around the edge of the coil, I used a coffee mug to push down on the cone, which brought the coil back in contact with the spider. Once this was done, and given a few days to dry, I tested the woofer using some music with long, low bass sweeps, and it seems to work perfectly! I reattached the dust cap using the same glue, and then gave it a couple of coats of black "Wet Look" speaker paint from Parts Express. Looks good doesn't it?
I then turned my attention to the crossover. After looking around on the 'net, it looked like Dennis Murphy's crossover design was the way to go. I wanted to be able to switch back to the original spec crossover if I need to, so I modified the design to include three switches to switch between the Murphy and JBL spec crossovers. As you probably know, the midrange on this speaker (well, mine anyway, and it appears to be commonplace) is wired out of phase. The woofer and tweeter both have their negative terminals connected to the negative input, whereas the midrange has it's positive terminal connected to the negative input. I was looking at Parts Express, and they stock a heavy duty 4 pole Double Throw switch that I could use for the mid, and use two DPDT switches for the woofer and tweeter. Two of the poles on the midrange switch would be to switch between crossovers, the other would be to reverse the polarity when using the JBL crossover. I could have used a 6 pole switch for everything, but finding one with a high current rating was a problem. Not that I planned on doing this, but you could in theory listen to the speaker with the JBL crossover on the woofer, and the Murphy crossover on the midrange and tweeter, or any other combination. Here you can see my modified schematic. It doesn't include the L-pads, but these will be used on the JBL crossover section.
Using parts from Parts Express and Erse Audio, I was able to build my crossovers. I did try using a big 5x9" circuit board from Parts Express, but it proved too small, it was mainly the big 25w resistors I was using. So I used a piece of scrap hardwood I had lying around the house, and hard-wired everything on the back. My next plan is to remake the JBL crossovers with some new capacitors and L-pads. I will then mount a 5x7" dished plate on the back of the speaker cabinet which will hold some chunky binding posts, and the three switches to switch between crossovers.
Watch this space for more updates...